fix: optimize iter a bit

Signed-off-by: Dr. Carsten Leue <carsten.leue@de.ibm.com>

.github/workflows/build.yml

@@ -28,7 +28,7 @@ jobs:
       fail-fast: false  # Continue with other versions if one fails
     steps:
       # full checkout for semantic-release
-      - uses: actions/checkout@08eba0b27e820071cde6df949e0beb9ba4906955 # v4.3.0
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
         with:
           fetch-depth: 0
       - name: Set up Go ${{ matrix.go-version }}
@@ -41,6 +41,14 @@ jobs:
           go mod tidy
           go test -v -race -coverprofile=coverage.txt -covermode=atomic ./...
       
+      - name: Upload coverage to Coveralls
+        uses: coverallsapp/github-action@v2
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          file: ./coverage.txt
+          flag-name: v1-go-${{ matrix.go-version }}
+          parallel: true
+      
       # - name: Upload coverage to Codecov
       #   uses: codecov/codecov-action@v5
       #   with:
@@ -58,7 +66,7 @@ jobs:
       matrix:
         go-version: ['1.24.x', '1.25.x']
     steps:
-      - uses: actions/checkout@08eba0b27e820071cde6df949e0beb9ba4906955 # v4.3.0
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
         with:
           fetch-depth: 0
       - name: Set up Go ${{ matrix.go-version }}
@@ -72,6 +80,14 @@ jobs:
           go mod tidy
           go test -v -race -coverprofile=coverage.txt -covermode=atomic ./...
       
+      - name: Upload coverage to Coveralls
+        uses: coverallsapp/github-action@v2
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          file: ./v2/coverage.txt
+          flag-name: v2-go-${{ matrix.go-version }}
+          parallel: true
+      
       # - name: Upload coverage to Codecov
       #   uses: codecov/codecov-action@v5
       #   with:
@@ -82,9 +98,22 @@ jobs:
       #   env:
       #     CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
 
+  coveralls-finish:
+    name: Finish Coveralls
+    needs:
+      - build-v1
+      - build-v2
+    runs-on: ubuntu-latest
+    steps:
+      - name: Coveralls Finished
+        uses: coverallsapp/github-action@v2
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          parallel-finished: true
+
   release:
     name: Release
-    needs: 
+    needs:
       - build-v1
       - build-v2
     if: github.repository == 'IBM/fp-go' && github.event_name != 'pull_request'
@@ -97,7 +126,7 @@ jobs:
     steps:
       # full checkout for semantic-release
       - name: Full checkout
-        uses: actions/checkout@08eba0b27e820071cde6df949e0beb9ba4906955 # v4.3.0
+        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
         with:
           fetch-depth: 0
 

.gitignore

@@ -2,4 +2,5 @@ fp-go.exe
 fp-go
 main.exe
 build/
-.idea
+.idea
+*.exe

README.md

@@ -1,207 +1,312 @@
-# Functional programming library for golang
+# fp-go: Functional Programming Library for Go
 
-**🚧 Work in progress! 🚧** Despite major version 1 because of <https://github.com/semantic-release/semantic-release/issues/1507>. Trying to not make breaking changes, but devil is in the details.
+[![Go Reference](https://pkg.go.dev/badge/github.com/IBM/fp-go.svg)](https://pkg.go.dev/github.com/IBM/fp-go)
+[![Coverage Status](https://coveralls.io/repos/github/IBM/fp-go/badge.svg?branch=main)](https://coveralls.io/github/IBM/fp-go?branch=main)
+
+**🚧 Work in progress! 🚧** Despite major version 1 (due to [semantic-release limitations](https://github.com/semantic-release/semantic-release/issues/1507)), we're working to minimize breaking changes.
 
 ![logo](resources/images/logo.png)
 
-This library is strongly influenced by the awesome [fp-ts](https://github.com/gcanti/fp-ts).
+A comprehensive functional programming library for Go, strongly influenced by the excellent [fp-ts](https://github.com/gcanti/fp-ts) library for TypeScript.
 
-## Getting started
+## 📚 Table of Contents
+
+- [Getting Started](#-getting-started)
+- [Design Goals](#-design-goals)
+- [Core Concepts](#-core-concepts)
+- [Comparison to Idiomatic Go](#comparison-to-idiomatic-go)
+- [Implementation Notes](#implementation-notes)
+- [Common Operations](#common-operations)
+- [Resources](#-resources)
+
+## 🚀 Getting Started
+
+### Installation
 
 ```bash
 go get github.com/IBM/fp-go
 ```
 
-Refer to the [samples](./samples/).
+### Quick Example
 
-Find API documentation [here](https://pkg.go.dev/github.com/IBM/fp-go)
+```go
+import (
+    "errors"
+    "github.com/IBM/fp-go/either"
+    "github.com/IBM/fp-go/function"
+)
 
-## Design Goal
+// Pure function that can fail
+func divide(a, b int) either.Either[error, int] {
+    if b == 0 {
+        return either.Left[int](errors.New("division by zero"))
+    }
+    return either.Right[error](a / b)
+}
 
-This library aims to provide a set of data types and functions that make it easy and fun to write maintainable and testable code in golang. It encourages the following patterns:
+// Compose operations safely
+result := function.Pipe2(
+    divide(10, 2),
+    either.Map(func(x int) int { return x * 2 }),
+    either.GetOrElse(func() int { return 0 }),
+)
+// result = 10
+```
 
-- write many small, testable and pure functions, i.e. functions that produce output only depending on their input and that do not execute side effects
-- offer helpers to isolate side effects into lazily executed functions (IO)
-- expose a consistent set of composition to create new functions from existing ones
-  - for each data type there exists a small set of composition functions
-  - these functions are called the same across all data types, so you only have to learn a small number of function names
-  - the semantic of functions of the same name is consistent across all data types
+### Resources
 
-### How does this play with the [🧘🏽 Zen Of Go](https://the-zen-of-go.netlify.app/)?
+- 📖 [API Documentation](https://pkg.go.dev/github.com/IBM/fp-go)
+- 💡 [Code Samples](./samples/)
+- 🆕 [V2 Documentation](./v2/README.md) (requires Go 1.24+)
 
-#### 🧘🏽 Each package fulfills a single purpose
+## 🎯 Design Goals
 
-✔️ Each of the top level packages (e.g. Option, Either, ReaderIOEither, ...) fulfills the purpose of defining the respective data type and implementing the set of common operations for this data type.
+This library aims to provide a set of data types and functions that make it easy and fun to write maintainable and testable code in Go. It encourages the following patterns:
 
-#### 🧘🏽 Handle errors explicitly
+### Core Principles
 
-✔️ The library makes a clear distinction between that operations that cannot fail by design and operations that can fail. Failure is represented via the `Either` type and errors are handled explicitly by using `Either`'s monadic set of operations.
+- **Pure Functions**: Write many small, testable, and pure functions that produce output only depending on their input and execute no side effects
+- **Side Effect Isolation**: Isolate side effects into lazily executed functions using the `IO` monad
+- **Consistent Composition**: Expose a consistent set of composition functions across all data types
+  - Each data type has a small set of composition functions
+  - Functions are named consistently across all data types
+  - Semantics of same-named functions are consistent across data types
 
-#### 🧘🏽 Return early rather than nesting deeply
+### 🧘🏽 Alignment with the Zen of Go
 
-✔️ We recommend to implement simple, small functions that implement one feature and that would typically not invoke other functions. Interaction with other functions is done by function composition and the composition makes sure to run one function after the other. In the error case the `Either` monad makes sure to skip the error path.
+This library respects and aligns with [The Zen of Go](https://the-zen-of-go.netlify.app/):
 
-#### 🧘🏽 Leave concurrency to the caller
+| Principle | Alignment | Explanation |
+|-----------|-----------|-------------|
+| 🧘🏽 Each package fulfills a single purpose | ✔️ | Each top-level package (Option, Either, ReaderIOEither, etc.) defines one data type and its operations |
+| 🧘🏽 Handle errors explicitly | ✔️ | Clear distinction between operations that can/cannot fail; failures represented via `Either` type |
+| 🧘🏽 Return early rather than nesting deeply | ✔️ | Small, focused functions composed together; `Either` monad handles error paths automatically |
+| 🧘🏽 Leave concurrency to the caller | ✔️ | Pure functions are synchronous; I/O operations are asynchronous by default |
+| 🧘🏽 Before you launch a goroutine, know when it will stop | 🤷🏽 | Library doesn't start goroutines; Task monad supports cancellation via context |
+| 🧘🏽 Avoid package level state | ✔️ | No package-level state anywhere |
+| 🧘🏽 Simplicity matters | ✔️ | Small, consistent interface across data types; focus on business logic |
+| 🧘🏽 Write tests to lock in behaviour | 🟡 | Programming pattern encourages testing; library has growing test coverage |
+| 🧘🏽 If you think it's slow, first prove it with a benchmark | ✔️ | Performance claims should be backed by benchmarks |
+| 🧘🏽 Moderation is a virtue | ✔️ | No custom goroutines or expensive synchronization; atomic counters for coordination |
+| 🧘🏽 Maintainability counts | ✔️ | Small, concise operations; pure functions with clear type signatures |
 
-✔️ All pure are synchronous by default. The I/O operations are asynchronous per default.
+## 💡 Core Concepts
 
-#### 🧘🏽 Before you launch a goroutine, know when it will stop
+### Data Types
 
-🤷🏽 This is left to the user of the library since the library itself will not start goroutines on its own. The Task monad offers support for cancellation via the golang context, though.
+The library provides several key functional data types:
 
-#### 🧘🏽 Avoid package level state
+- **`Option[A]`**: Represents an optional value (Some or None)
+- **`Either[E, A]`**: Represents a value that can be one of two types (Left for errors, Right for success)
+- **`IO[A]`**: Represents a lazy computation that produces a value
+- **`IOEither[E, A]`**: Represents a lazy computation that can fail
+- **`Reader[R, A]`**: Represents a computation that depends on an environment
+- **`ReaderIOEither[R, E, A]`**: Combines Reader, IO, and Either for effectful computations with dependencies
+- **`Task[A]`**: Represents an asynchronous computation
+- **`State[S, A]`**: Represents a stateful computation
 
-✔️ No package level state anywhere, this would be a significant anti-pattern
+### Monadic Operations
 
-#### 🧘🏽 Simplicity matters
+All data types support common monadic operations:
 
-✔️ The library is simple in the sense that it offers a small, consistent interface to a variety of data types. Users can concentrate on implementing business logic rather than dealing with low level data structures.
-
-#### 🧘🏽 Write tests to lock in the behaviour of your package’s API
-
-🟡 The programming pattern suggested by this library encourages writing test cases. The library itself also has a growing number of tests, but not enough, yet. TBD
-
-#### 🧘🏽 If you think it’s slow, first prove it with a benchmark
-
-✔️ Absolutely. If you think the function composition offered by this library is too slow, please provide a benchmark.
-
-#### 🧘🏽 Moderation is a virtue
-
-✔️ The library does not implement its own goroutines and also does not require any expensive synchronization primitives. Coordination of IO operations is implemented via atomic counters without additional primitives.
-
-#### 🧘🏽 Maintainability counts
-
-✔️ Code that consumes this library is easy to maintain because of the small and concise set of operations exposed. Also the suggested programming paradigm to decompose an application into small functions increases maintainability, because these functions are easy to understand and if they are pure, it's often sufficient to look at the type signature to understand the purpose.
-
-The library itself also comprises many small functions, but it's admittedly harder to maintain than code that uses it. However this asymmetry is intended because it offloads complexity from users into a central component.
+- **`Map`**: Transform the value inside a context
+- **`Chain`** (FlatMap): Transform and flatten nested contexts
+- **`Ap`**: Apply a function in a context to a value in a context
+- **`Of`**: Wrap a value in a context
+- **`Fold`**: Extract a value from a context
 
 ## Comparison to Idiomatic Go
 
-In this section we discuss how the functional APIs differ from idiomatic go function signatures and how to convert back and forth.
+This section explains how functional APIs differ from idiomatic Go and how to convert between them.
 
-### Pure functions
+### Pure Functions
 
-Pure functions are functions that take input parameters and that compute an output without changing any global state and without mutating the input parameters. They will always return the same output for the same input.
+Pure functions take input parameters and compute output without changing global state or mutating inputs. They always return the same output for the same input.
 
 #### Without Errors
 
-If your pure function does not return an error, the idiomatic signature is just fine and no changes are required.
+If your pure function doesn't return an error, the idiomatic signature works as-is:
+
+```go
+func add(a, b int) int {
+    return a + b
+}
+```
 
 #### With Errors
 
-If your pure function can return an error, then it will have a `(T, error)` return value in idiomatic go. In functional style the return value is [Either[error, T]](https://pkg.go.dev/github.com/IBM/fp-go/either) because function composition is easier with such a return type. Use the `EitherizeXXX` methods in ["github.com/IBM/fp-go/either"](https://pkg.go.dev/github.com/IBM/fp-go/either) to convert from idiomatic to functional style and `UneitherizeXXX` to convert from functional to idiomatic style.
+**Idiomatic Go:**
+```go
+func divide(a, b int) (int, error) {
+    if b == 0 {
+        return 0, errors.New("division by zero")
+    }
+    return a / b, nil
+}
+```
 
-### Effectful functions
+**Functional Style:**
+```go
+func divide(a, b int) either.Either[error, int] {
+    if b == 0 {
+        return either.Left[int](errors.New("division by zero"))
+    }
+    return either.Right[error](a / b)
+}
+```
 
-An effectful function (or function with a side effect) is one that changes data outside the scope of the function or that does not always produce the same output for the same input (because it depends on some external, mutable state). There is no special way in idiomatic go to identify such a function other than documentation. In functional style we represent them as functions that do not take an input but that produce an output. The base type for these functions is [IO[T]](https://pkg.go.dev/github.com/IBM/fp-go/io) because in many cases such functions represent `I/O` operations.
+**Conversion:**
+- Use `either.EitherizeXXX` to convert from idiomatic to functional style
+- Use `either.UneitherizeXXX` to convert from functional to idiomatic style
+
+### Effectful Functions
+
+An effectful function changes data outside its scope or doesn't always produce the same output for the same input.
 
 #### Without Errors
 
-If your effectful function does not return an error, the functional signature is [IO[T]](https://pkg.go.dev/github.com/IBM/fp-go/io)
+**Functional signature:** `IO[T]`
+
+```go
+func getCurrentTime() io.IO[time.Time] {
+    return func() time.Time {
+        return time.Now()
+    }
+}
+```
 
 #### With Errors
 
-If your effectful function can return an error, the functional signature is [IOEither[error, T]](https://pkg.go.dev/github.com/IBM/fp-go/ioeither). Use `EitherizeXXX` from ["github.com/IBM/fp-go/ioeither"](https://pkg.go.dev/github.com/IBM/fp-go/ioeither) to convert an idiomatic go function to functional style.
+**Functional signature:** `IOEither[error, T]`
+
+```go
+func readFile(path string) ioeither.IOEither[error, []byte] {
+    return func() either.Either[error, []byte] {
+        data, err := os.ReadFile(path)
+        if err != nil {
+            return either.Left[[]byte](err)
+        }
+        return either.Right[error](data)
+    }
+}
+```
+
+**Conversion:**
+- Use `ioeither.EitherizeXXX` to convert idiomatic Go functions to functional style
 
 ### Go Context
 
-Functions that take a [context](https://pkg.go.dev/context) are per definition effectful because they depend on the context parameter that is designed to be mutable (it can e.g. be used to cancel a running operation). Furthermore in idiomatic go the parameter is typically passed as the first parameter to a function.
+Functions that take a `context.Context` are effectful because they depend on mutable context.
 
-In functional style we isolate the [context](https://pkg.go.dev/context) and represent the nature of the effectful function as an [IOEither[error, T]](https://pkg.go.dev/github.com/IBM/fp-go/ioeither). The resulting type is [ReaderIOEither[T]](https://pkg.go.dev/github.com/IBM/fp-go/context/readerioeither), a function taking a [context](https://pkg.go.dev/context) that returns a function without parameters returning an [Either[error, T]](https://pkg.go.dev/github.com/IBM/fp-go/either). Use the `EitherizeXXX` methods from ["github.com/IBM/fp-go/context/readerioeither"](https://pkg.go.dev/github.com/IBM/fp-go/context/readerioeither) to convert an idiomatic go function with a [context](https://pkg.go.dev/context) to functional style.
+**Idiomatic Go:**
+```go
+func fetchData(ctx context.Context, url string) ([]byte, error) {
+    // implementation
+}
+```
+
+**Functional Style:**
+```go
+func fetchData(url string) readerioeither.ReaderIOEither[context.Context, error, []byte] {
+    return func(ctx context.Context) ioeither.IOEither[error, []byte] {
+        return func() either.Either[error, []byte] {
+            // implementation
+        }
+    }
+}
+```
+
+**Conversion:**
+- Use `readerioeither.EitherizeXXX` to convert idiomatic Go functions with context to functional style
 
 ## Implementation Notes
 
 ### Generics
 
-All monadic operations are implemented via generics, i.e. they offer a type safe way to compose operations. This allows for convenient IDE support and also gives confidence about the correctness of the composition at compile time.
+All monadic operations use Go generics for type safety:
 
-Downside is that this will result in different versions of each operation per type, these versions are generated by the golang compiler at build time (unlike type erasure in languages such as Java of TypeScript). This might lead to large binaries for codebases with many different types. If this is a concern, you can always implement type erasure on top, i.e. use the monadic operations with the `any` type as if generics were not supported. You loose type safety, but this might result in smaller binaries.
+- ✅ **Pros**: Type-safe composition, IDE support, compile-time correctness
+- ⚠️ **Cons**: May result in larger binaries (different versions per type)
+- 💡 **Tip**: For binary size concerns, use type erasure with `any` type
 
 ### Ordering of Generic Type Parameters
 
-In go we need to specify all type parameters of a function on the global function definition, even if the function returns a higher order function and some of the type parameters are only applicable to the higher order function. So the following is not possible:
+Go requires all type parameters on the global function definition. Parameters that cannot be auto-detected come first:
 
 ```go
-func Map[A, B any](f func(A) B) [R, E any]func(fa ReaderIOEither[R, E, A]) ReaderIOEither[R, E, B]
+// Map: B cannot be auto-detected, so it comes first
+func Map[R, E, A, B any](f func(A) B) func(ReaderIOEither[R, E, A]) ReaderIOEither[R, E, B]
+
+// Ap: B cannot be auto-detected from the argument
+func Ap[B, R, E, A any](fa ReaderIOEither[R, E, A]) func(ReaderIOEither[R, E, func(A) B]) ReaderIOEither[R, E, B]
 ```
 
-Note that the parameters `R` and `E` are not needed by the first level of `Map` but only by the resulting higher order function. Instead we need to specify the following:
+This ordering maximizes type inference where possible.
 
-```go
-func Map[R, E, A, B any](f func(A) B) func(fa ReaderIOEither[R, E, A]) ReaderIOEither[R, E, B]
-```
+### Use of the ~ Operator
 
-which overspecifies `Map` on the global scope. As a result the go compiler will not be able to auto-detect these parameters, it can only auto detect `A` and `B` since they appear in the argument of `Map`. We need to explicitly pass values for these type parameters when `Map` is being used.
-
-Because of this limitation the order of parameters on a function matters. We want to make sure that we define those parameters that cannot be auto-detected, first, and the parameters that can be auto-detected, last. This can lead to inconsistencies in parameter ordering, but we believe that the gain in convenience is worth it. The parameter order of `Ap` is e.g. different from that of `Map`:
-
-```go
-func Ap[B, R, E, A any](fa ReaderIOEither[R, E, A]) func(fab ReaderIOEither[R, E, func(A) B]) ReaderIOEither[R, E, B]
-```
-
-because `R`, `E` and `A` can be determined from the argument to `Ap` but `B` cannot.
-
-### Use of the [~ Operator](https://go.googlesource.com/proposal/+/master/design/47781-parameterized-go-ast.md)
-
-The FP library attempts to be easy to consume and one aspect of this is the definition of higher level type definitions instead of having to use their low level equivalent. It is e.g. more convenient and readable to use
-
-```go
-ReaderIOEither[R, E, A]
-```
-
-than
-
-```go
-func(R) func() Either.Either[E, A]
-```
-
-although both are logically equivalent. At the time of this writing the go type system does not support generic type aliases, only generic type definition, i.e. it is not possible to write:
-
-```go
-type ReaderIOEither[R, E, A any] = RD.Reader[R, IOE.IOEither[E, A]]
-```
-
-only
+Go doesn't support generic type aliases (until Go 1.24), only type definitions. The `~` operator allows generic implementations to work with compatible types:
 
 ```go
 type ReaderIOEither[R, E, A any] RD.Reader[R, IOE.IOEither[E, A]]
 ```
 
-This makes a big difference, because in the second case the type `ReaderIOEither[R, E, A any]` is considered a completely new type, not compatible to its right hand side, so it's not just a shortcut but a fully new type.
+**Generic Subpackages:**
+- Each higher-level type has a `generic` subpackage with fully generic implementations
+- These are for library extensions, not end-users
+- Main packages specialize generic implementations for convenience
 
-From the implementation perspective however there is no reason to restrict the implementation to the new type, it can be generic for all compatible types. The way to express this in go is the [~](https://go.googlesource.com/proposal/+/master/design/47781-parameterized-go-ast.md) operator. This comes with some quite complicated type declarations in some cases, which undermines the goal of the library to be easy to use.
+### Higher Kinded Types (HKT)
 
-For that reason there exist sub-packages called `Generic` for all higher level types. These packages contain the fully generic implementation of the operations, preferring abstraction over usability. These packages are not meant to be used by end-users but are meant to be used by library extensions. The implementation for the convenient higher level types specializes the generic implementation for the particular higher level type, i.e. this layer does not contain any business logic but only *type magic*.
+Go doesn't support HKT natively. This library addresses this by:
 
-### Higher Kinded Types
+- Introducing HKTs as individual types (e.g., `HKTA` for `HKT[A]`)
+- Implementing generic algorithms in the `internal` package
+- Keeping complexity hidden from end-users
 
-Go does not support higher kinded types (HKT). Such types occur if a generic type itself is parametrized by another generic type. Example:
+## Common Operations
 
-The `Map` operation for `ReaderIOEither` is defined as:
+### Map/Chain/Ap/Flap
+
+| Operator | Parameter        | Monad           | Result   | Use Case |
+| -------- | ---------------- | --------------- | -------- | -------- |
+| Map      | `func(A) B`      | `HKT[A]`        | `HKT[B]` | Transform value in context |
+| Chain    | `func(A) HKT[B]` | `HKT[A]`        | `HKT[B]` | Transform and flatten |
+| Ap       | `HKT[A]`         | `HKT[func(A)B]` | `HKT[B]` | Apply function in context |
+| Flap     | `A`              | `HKT[func(A)B]` | `HKT[B]` | Apply value to function in context |
+
+### Example: Chaining Operations
 
 ```go
-func Map[R, E, A, B any](f func(A) B) func(fa ReaderIOEither[R, E, A]) ReaderIOEither[R, E, B]
+import (
+    "github.com/IBM/fp-go/either"
+    "github.com/IBM/fp-go/function"
+)
+
+result := function.Pipe3(
+    either.Right[error](10),
+    either.Map(func(x int) int { return x * 2 }),
+    either.Chain(func(x int) either.Either[error, int] {
+        if x > 15 {
+            return either.Right[error](x)
+        }
+        return either.Left[int](errors.New("too small"))
+    }),
+    either.GetOrElse(func() int { return 0 }),
+)
 ```
 
-and in fact the equivalent operations for all other monads follow the same pattern, we could try to introduce a new type for `ReaderIOEither` (without a parameter) as a HKT, e.g. like so (made-up syntax, does not work in go):
+## 📚 Resources
 
-```go
-func Map[HKT, R, E, A, B any](f func(A) B) func(HKT[R, E, A]) HKT[R, E, B]
-```
+- [API Documentation](https://pkg.go.dev/github.com/IBM/fp-go)
+- [Code Samples](./samples/)
+- [V2 Documentation](./v2/README.md) - New features in Go 1.24+
+- [fp-ts](https://github.com/gcanti/fp-ts) - Original TypeScript inspiration
 
-this would be the completely generic method signature for all possible monads. In particular in many cases it is possible to compose functions independent of the concrete knowledge of the actual `HKT`. From the perspective of a library this is the ideal situation because then a particular algorithm only has to be implemented and tested once.
+## 🤝 Contributing
 
-This FP library addresses this by introducing the HKTs as individual types, e.g. `HKT[A]` would be represented as a new generic type `HKTA`. This loses the correlation to the type `A` but allows to implement generic algorithms, at the price of readability.
+Contributions are welcome! Please feel free to submit issues or pull requests.
 
-For that reason these implementations are kept in the `internal` package. These are meant to be used by the library itself or by extensions, not by end users.
+## 📄 License
 
-## Map/Ap/Flap
-
-The following table lists the relationship between some selected operators
-
-| Operator | Parameter        | Monad           | Result   |
-| -------- | ---------------- | --------------- | -------- |
-| Map      | `func(A) B`      | `HKT[A]`        | `HKT[B]` |
-| Chain    | `func(A) HKT[B]` | `HKT[A]`        | `HKT[B]` |
-| Ap       | `HKT[A]`         | `HKT[func(A)B]` | `HKT[B]` |
-| Flap     | `A`              | `HKT[func(A)B]` | `HKT[B]` |
+This project is licensed under the Apache License 2.0 - see the LICENSE file for details.

either/core.go

@@ -21,8 +21,8 @@ import (
 
 type (
 	either struct {
-		isLeft bool
 		value  any
+		isLeft bool
 	}
 
 	// Either defines a data structure that logically holds either an E or an A. The flag discriminates the cases
@@ -31,7 +31,7 @@ type (
 
 // String prints some debug info for the object
 //
-// go:noinline
+//go:noinline
 func eitherString(s *either) string {
 	if s.isLeft {
 		return fmt.Sprintf("Left[%T](%v)", s.value, s.value)
@@ -41,7 +41,7 @@ func eitherString(s *either) string {
 
 // Format prints some debug info for the object
 //
-// go:noinline
+//go:noinline
 func eitherFormat(e *either, f fmt.State, c rune) {
 	switch c {
 	case 's':
@@ -73,12 +73,12 @@ func IsRight[E, A any](val Either[E, A]) bool {
 
 // Left creates a new instance of an [Either] representing the left value.
 func Left[A, E any](value E) Either[E, A] {
-	return Either[E, A]{true, value}
+	return Either[E, A]{value, true}
 }
 
 // Right creates a new instance of an [Either] representing the right value.
 func Right[E, A any](value A) Either[E, A] {
-	return Either[E, A]{false, value}
+	return Either[E, A]{value, false}
 }
 
 // MonadFold extracts the values from an [Either] by invoking the [onLeft] callback or the [onRight] callback depending on the case
@@ -94,8 +94,7 @@ func Unwrap[E, A any](ma Either[E, A]) (A, E) {
 	if ma.isLeft {
 		var a A
 		return a, ma.value.(E)
-	} else {
-		var e E
-		return ma.value.(A), e
 	}
+	var e E
+	return ma.value.(A), e
 }

internal/array/array.go

@@ -17,6 +17,37 @@ package array
 
 func Slice[GA ~[]A, A any](low, high int) func(as GA) GA {
 	return func(as GA) GA {
+		length := len(as)
+
+		// Handle negative indices - count backward from the end
+		if low < 0 {
+			low = length + low
+			if low < 0 {
+				low = 0
+			}
+		}
+		if high < 0 {
+			high = length + high
+			if high < 0 {
+				high = 0
+			}
+		}
+
+		// Start index > array length: return empty array
+		if low > length {
+			return Empty[GA, A]()
+		}
+
+		// End index > array length: slice to the end
+		if high > length {
+			high = length
+		}
+
+		// Start >= end: return empty array
+		if low >= high {
+			return Empty[GA, A]()
+		}
+
 		return as[low:high]
 	}
 }

ioeither/generic/ioeither.go

@@ -369,6 +369,11 @@ func ToIOOption[GA ~func() O.Option[A], GEA ~func() ET.Either[E, A], E, A any](i
 	)
 }
 
+// OrElse returns the original IOEither if it is a Right, otherwise it applies the given function to the error and returns the result.
+func OrElse[GA ~func() ET.Either[E, A], E, A any](onLeft func(E) GA) func(GA) GA {
+	return eithert.OrElse(IO.MonadChain[GA, GA, ET.Either[E, A], ET.Either[E, A]], IO.Of[GA, ET.Either[E, A]], onLeft)
+}
+
 func FromIOOption[GEA ~func() ET.Either[E, A], GA ~func() O.Option[A], E, A any](onNone func() E) func(ioo GA) GEA {
 	return IO.Map[GA, GEA](ET.FromOption[A](onNone))
 }

ioeither/ioeither.go

@@ -266,6 +266,11 @@ func Alt[E, A any](second L.Lazy[IOEither[E, A]]) func(IOEither[E, A]) IOEither[
 	return G.Alt(second)
 }
 
+// OrElse returns the original IOEither if it is a Right, otherwise it applies the given function to the error and returns the result.
+func OrElse[E, A any](onLeft func(E) IOEither[E, A]) func(IOEither[E, A]) IOEither[E, A] {
+	return G.OrElse[IOEither[E, A]](onLeft)
+}
+
 func MonadFlap[E, B, A any](fab IOEither[E, func(A) B], a A) IOEither[E, B] {
 	return G.MonadFlap[IOEither[E, func(A) B], IOEither[E, B]](fab, a)
 }

ioeither/ioeither_test.go

@@ -134,3 +134,44 @@ func TestApSecond(t *testing.T) {
 
 	assert.Equal(t, E.Of[error]("b"), x())
 }
+
+func TestOrElse(t *testing.T) {
+	// Test that OrElse recovers from a Left
+	recover := OrElse(func(err string) IOEither[string, int] {
+		return Right[string](42)
+	})
+
+	// When input is Left, should recover
+	leftResult := F.Pipe1(
+		Left[int]("error"),
+		recover,
+	)
+	assert.Equal(t, E.Right[string](42), leftResult())
+
+	// When input is Right, should pass through unchanged
+	rightResult := F.Pipe1(
+		Right[string](100),
+		recover,
+	)
+	assert.Equal(t, E.Right[string](100), rightResult())
+
+	// Test that OrElse can also return a Left (propagate different error)
+	recoverOrFail := OrElse(func(err string) IOEither[string, int] {
+		if err == "recoverable" {
+			return Right[string](0)
+		}
+		return Left[int]("unrecoverable: " + err)
+	})
+
+	recoverable := F.Pipe1(
+		Left[int]("recoverable"),
+		recoverOrFail,
+	)
+	assert.Equal(t, E.Right[string](0), recoverable())
+
+	unrecoverable := F.Pipe1(
+		Left[int]("fatal"),
+		recoverOrFail,
+	)
+	assert.Equal(t, E.Left[int]("unrecoverable: fatal"), unrecoverable())
+}

option/core.go

@@ -35,7 +35,7 @@ type Option[A any] struct {
 
 // optString prints some debug info for the object
 //
-// go:noinline
+//go:noinline
 func optString(isSome bool, value any) string {
 	if isSome {
 		return fmt.Sprintf("Some[%T](%v)", value, value)
@@ -45,7 +45,7 @@ func optString(isSome bool, value any) string {
 
 // optFormat prints some debug info for the object
 //
-// go:noinline
+//go:noinline
 func optFormat(isSome bool, value any, f fmt.State, c rune) {
 	switch c {
 	case 's':
@@ -78,7 +78,7 @@ func (s Option[A]) MarshalJSON() ([]byte, error) {
 
 // optUnmarshalJSON unmarshals the [Option] from a JSON string
 //
-// go:noinline
+//go:noinline
 func optUnmarshalJSON(isSome *bool, value any, data []byte) error {
 	// decode the value
 	if bytes.Equal(data, jsonNull) {

pair/pair.go

@@ -34,14 +34,14 @@ type (
 
 // String prints some debug info for the object
 //
-// go:noinline
+//go:noinline
 func pairString(s *pair) string {
 	return fmt.Sprintf("Pair[%T, %T](%v, %v)", s.h, s.t, s.h, s.t)
 }
 
 // Format prints some debug info for the object
 //
-// go:noinline
+//go:noinline
 func pairFormat(e *pair, f fmt.State, c rune) {
 	switch c {
 	case 's':

renovate.json

@@ -1,11 +1,20 @@
 {
     "$schema": "https://docs.renovatebot.com/renovate-schema.json",
     "extends": [
-        "config:base",
+        "config:recommended",
         ":dependencyDashboard"
     ],
     "rangeStrategy": "bump",
     "packageRules": [
+        {
+            "matchDatasources": [
+                "golang-version"
+            ],
+            "matchPackageNames": [
+                "go"
+            ],
+            "enabled": false
+        },
         {
             "matchManagers": [
                 "gomod"
@@ -25,15 +34,6 @@
             ],
             "automerge": true,
             "groupName": "go dependencies"
-        },
-        {
-            "matchPackageNames": [
-                "conventional-changelog-conventionalcommits"
-            ],
-            "matchUpdateTypes": [
-                "major"
-            ],
-            "enabled": false
         }
     ]
 }

v2/.claude/settings.local.json

@@ -0,0 +1,17 @@
+{
+  "permissions": {
+    "allow": [
+      "Bash(ls -la \"c:\\d\\fp-go\\v2\\internal\\monad\"\" && ls -la \"c:dfp-gov2internalapplicative\"\")",
+      "Bash(ls -la \"c:\\d\\fp-go\\v2\\internal\\chain\"\" && ls -la \"c:dfp-gov2internalfunctor\"\")",
+      "Bash(go build:*)",
+      "Bash(go test:*)",
+      "Bash(go doc:*)",
+      "Bash(go tool cover:*)",
+      "Bash(sort:*)",
+      "Bash(tee:*)",
+      "Bash(find:*)"
+    ],
+    "deny": [],
+    "ask": []
+  }
+}

v2/BENCHMARK_COMPARISON.md

@@ -0,0 +1,482 @@
+# Benchmark Comparison: Idiomatic vs Standard Either/Result
+
+**Date:** 2025-11-18
+**System:** AMD Ryzen 7 PRO 7840U w/ Radeon 780M Graphics (16 cores)
+**Go Version:** go1.23+
+
+This document provides a detailed performance comparison between the optimized `either` package and the `idiomatic/result` package after recent optimizations to the either package.
+
+## Executive Summary
+
+After optimizations to the `either` package, the performance characteristics have changed significantly:
+
+### Key Findings
+
+1. **Constructors & Predicates**: Both packages now perform comparably (~1-2 ns/op) with **zero heap allocations**
+2. **Zero-allocation insight**: The `Either` struct (24 bytes) does NOT escape to heap - Go returns it by value on the stack
+3. **Core Operations**: Idiomatic package has a **consistent advantage** of 1.2x - 2.3x for most operations
+4. **Complex Operations**: Idiomatic package shows **massive advantages**:
+   - ChainFirst (Right): **32.4x faster** (87.6 ns → 2.7 ns, 72 B → 0 B)
+   - Pipeline operations: **2-3x faster** with lower allocations
+5. **All simple operations**: Both maintain **zero heap allocations** (0 B/op, 0 allocs/op)
+
+### Winner by Category
+
+| Category | Winner | Reason |
+|----------|--------|--------|
+| Constructors | **TIE** | Both ~1.3-1.8 ns/op |
+| Predicates | **TIE** | Both ~1.2-1.5 ns/op |
+| Simple Transformations | **Idiomatic** | 1.2-2x faster |
+| Monadic Operations | **Idiomatic** | 1.2-2.3x faster |
+| Complex Chains | **Idiomatic** | 32x faster, zero allocs |
+| Pipelines | **Idiomatic** | 2-2.4x faster, fewer allocs |
+| Extraction | **Idiomatic** | 6x faster (GetOrElse) |
+
+## Detailed Benchmark Results
+
+### Constructor Operations
+
+| Operation | Either (ns/op) | Idiomatic (ns/op) | Speedup | Either Allocs | Idio Allocs |
+|-----------|----------------|-------------------|---------|---------------|-------------|
+| Left      | 1.76           | **1.35**          | **1.3x** ✓ | 0 B/op | 0 B/op |
+| Right     | 1.38           | 1.43              | 1.0x    | 0 B/op | 0 B/op |
+| Of        | 1.68           | **1.22**          | **1.4x** ✓ | 0 B/op | 0 B/op |
+
+**Analysis:** Both packages perform extremely well with **zero heap allocations**. Idiomatic has a slight edge on Left and Of.
+
+**Important Clarification: Neither Package Escapes to Heap**
+
+A common misconception is that struct-based Either escapes to heap while tuples stay on stack. The benchmarks prove this is FALSE:
+
+```go
+// Either package - NO heap allocation
+type Either[E, A any] struct {
+    r      A           // 8 bytes
+    l      E           // 8 bytes
+    isLeft bool        // 1 byte + 7 padding
+}                      // Total: 24 bytes
+
+func Of[E, A any](value A) Either[E, A] {
+    return Right[E](value)  // Returns 24-byte struct BY VALUE
+}
+
+// Benchmark result: 0 B/op, 0 allocs/op ✓
+```
+
+**Why Either doesn't escape:**
+1. **Small struct** - At 24 bytes, it's below Go's escape threshold (~64 bytes)
+2. **Return by value** - Go returns small structs on the stack
+3. **Inlining** - The `//go:inline` directive eliminates function overhead
+4. **No pointers** - No pointer escapes in normal usage
+
+**Idiomatic package:**
+```go
+// Returns native tuple - always stack allocated
+func Right[A any](a A) (A, error) {
+    return a, nil  // 16 bytes total (8 + 8)
+}
+
+// Benchmark result: 0 B/op, 0 allocs/op ✓
+```
+
+**Both achieve zero allocations** - the performance difference comes from other factors like function composition overhead, not from constructor allocations.
+
+### Predicate Operations
+
+| Operation | Either (ns/op) | Idiomatic (ns/op) | Speedup | Either Allocs | Idio Allocs |
+|-----------|----------------|-------------------|---------|---------------|-------------|
+| IsLeft    | 1.45           | **1.35**          | **1.1x** ✓ | 0 B/op | 0 B/op |
+| IsRight   | 1.47           | 1.51              | 1.0x    | 0 B/op | 0 B/op |
+
+**Analysis:** Virtually identical performance. The optimizations brought them to parity.
+
+### Fold Operations
+
+| Operation | Either (ns/op) | Idiomatic (ns/op) | Speedup | Either Allocs | Idio Allocs |
+|-----------|----------------|-------------------|---------|---------------|-------------|
+| MonadFold (Right) | 2.71   | -                 | -       | 0 B/op | - |
+| MonadFold (Left)  | 2.26   | -                 | -       | 0 B/op | - |
+| Fold (Right)      | 4.03   | **2.75**          | **1.5x** ✓ | 0 B/op | 0 B/op |
+| Fold (Left)       | 3.69   | **2.40**          | **1.5x** ✓ | 0 B/op | 0 B/op |
+
+**Analysis:** Idiomatic package is 1.5x faster for curried Fold operations.
+
+### Unwrap Operations
+
+| Operation | Either (ns/op) | Idiomatic (ns/op) | Note |
+|-----------|----------------|-------------------|------|
+| Unwrap (Right)      | 1.27  | N/A | Either-specific |
+| Unwrap (Left)       | 1.24  | N/A | Either-specific |
+| UnwrapError (Right) | 1.27  | N/A | Either-specific |
+| UnwrapError (Left)  | 1.27  | N/A | Either-specific |
+| ToError (Right)     | N/A   | 1.40 | Idiomatic-specific |
+| ToError (Left)      | N/A   | 1.84 | Idiomatic-specific |
+
+**Analysis:** Both provide fast unwrapping. Idiomatic's tuple return is naturally unwrapped.
+
+### Map Operations
+
+| Operation | Either (ns/op) | Idiomatic (ns/op) | Speedup | Either Allocs | Idio Allocs |
+|-----------|----------------|-------------------|---------|---------------|-------------|
+| MonadMap (Right)  | 2.96   | -                | -       | 0 B/op | - |
+| MonadMap (Left)   | 1.99   | -                | -       | 0 B/op | - |
+| Map (Right)       | 5.13   | **4.34**         | **1.2x** ✓ | 0 B/op | 0 B/op |
+| Map (Left)        | 4.19   | **2.48**         | **1.7x** ✓ | 0 B/op | 0 B/op |
+| MapLeft (Right)   | 3.93   | **2.22**         | **1.8x** ✓ | 0 B/op | 0 B/op |
+| MapLeft (Left)    | 7.22   | **3.51**         | **2.1x** ✓ | 0 B/op | 0 B/op |
+
+**Analysis:** Idiomatic is consistently faster across all Map variants, especially for error path (Left).
+
+### BiMap Operations
+
+| Operation | Either (ns/op) | Idiomatic (ns/op) | Speedup | Either Allocs | Idio Allocs |
+|-----------|----------------|-------------------|---------|---------------|-------------|
+| BiMap (Right)  | 16.79  | **3.82**         | **4.4x** ✓ | 0 B/op | 0 B/op |
+| BiMap (Left)   | 11.47  | **3.47**         | **3.3x** ✓ | 0 B/op | 0 B/op |
+
+**Analysis:** Idiomatic package shows significant advantage for BiMap operations (3-4x faster).
+
+### Chain (Monadic Bind) Operations
+
+| Operation | Either (ns/op) | Idiomatic (ns/op) | Speedup | Either Allocs | Idio Allocs |
+|-----------|----------------|-------------------|---------|---------------|-------------|
+| MonadChain (Right) | 2.89  | -                | -       | 0 B/op | - |
+| MonadChain (Left)  | 2.03  | -                | -       | 0 B/op | - |
+| Chain (Right)      | 5.44  | **2.34**         | **2.3x** ✓ | 0 B/op | 0 B/op |
+| Chain (Left)       | 4.44  | **2.53**         | **1.8x** ✓ | 0 B/op | 0 B/op |
+| ChainFirst (Right) | 87.62 | **2.71**         | **32.4x** ✓✓✓ | 72 B, 3 allocs | 0 B, 0 allocs |
+| ChainFirst (Left)  | 3.94  | **2.48**         | **1.6x** ✓ | 0 B/op | 0 B/op |
+
+**Analysis:**
+- Idiomatic is 2x faster for standard Chain operations
+- **ChainFirst shows the most dramatic difference**: 32.4x faster with zero allocations vs 72 bytes!
+
+### Flatten Operations
+
+| Operation | Either (ns/op) | Idiomatic (ns/op) | Note |
+|-----------|----------------|-------------------|------|
+| Flatten (Right) | 8.73  | N/A | Either-specific nested structure |
+| Flatten (Left)  | 8.86  | N/A | Either-specific nested structure |
+
+**Analysis:** Flatten is specific to Either's nested structure handling.
+
+### Applicative Operations
+
+| Operation | Either (ns/op) | Idiomatic (ns/op) | Speedup | Either Allocs | Idio Allocs |
+|-----------|----------------|-------------------|---------|---------------|-------------|
+| MonadAp (RR)  | 3.81  | -                | -       | 0 B/op | - |
+| MonadAp (RL)  | 3.07  | -                | -       | 0 B/op | - |
+| MonadAp (LR)  | 3.08  | -                | -       | 0 B/op | - |
+| Ap (RR)       | 6.99  | -                | -       | 0 B/op | - |
+
+**Analysis:** MonadAp is fast in Either. Idiomatic package doesn't expose direct Ap benchmarks.
+
+### Alternative Operations
+
+| Operation | Either (ns/op) | Idiomatic (ns/op) | Speedup | Either Allocs | Idio Allocs |
+|-----------|----------------|-------------------|---------|---------------|-------------|
+| Alt (RR)       | 5.72  | **2.40**         | **2.4x** ✓ | 0 B/op | 0 B/op |
+| Alt (LR)       | 4.89  | **2.39**         | **2.0x** ✓ | 0 B/op | 0 B/op |
+| OrElse (Right) | 5.28  | **2.40**         | **2.2x** ✓ | 0 B/op | 0 B/op |
+| OrElse (Left)  | 3.99  | **2.42**         | **1.6x** ✓ | 0 B/op | 0 B/op |
+
+**Analysis:** Idiomatic package is consistently 2x faster for alternative operations.
+
+### GetOrElse Operations
+
+| Operation | Either (ns/op) | Idiomatic (ns/op) | Speedup | Either Allocs | Idio Allocs |
+|-----------|----------------|-------------------|---------|---------------|-------------|
+| GetOrElse (Right) | 9.01  | **1.49**         | **6.1x** ✓✓ | 0 B/op | 0 B/op |
+| GetOrElse (Left)  | 6.35  | **2.08**         | **3.1x** ✓✓ | 0 B/op | 0 B/op |
+
+**Analysis:** Idiomatic package shows dramatic advantage for value extraction (3-6x faster).
+
+### TryCatch Operations
+
+| Operation | Either (ns/op) | Idiomatic (ns/op) | Note |
+|-----------|----------------|-------------------|------|
+| TryCatch (Success)       | 2.39  | N/A | Either-specific |
+| TryCatch (Error)         | 3.40  | N/A | Either-specific |
+| TryCatchError (Success)  | 3.32  | N/A | Either-specific |
+| TryCatchError (Error)    | 6.44  | N/A | Either-specific |
+
+**Analysis:** TryCatch/TryCatchError are Either-specific for wrapping (value, error) tuples.
+
+### Other Operations
+
+| Operation | Either (ns/op) | Idiomatic (ns/op) | Speedup | Either Allocs | Idio Allocs |
+|-----------|----------------|-------------------|---------|---------------|-------------|
+| Swap (Right)      | 2.30  | -                | -       | 0 B/op | - |
+| Swap (Left)       | 3.05  | -                | -       | 0 B/op | - |
+| MapTo (Right)     | -     | 1.60             | -       | -      | 0 B/op |
+| MapTo (Left)      | -     | 1.73             | -       | -      | 0 B/op |
+| ChainTo (Right)   | -     | 2.66             | -       | -      | 0 B/op |
+| ChainTo (Left)    | -     | 2.85             | -       | -      | 0 B/op |
+| Reduce (Right)    | -     | 2.34             | -       | -      | 0 B/op |
+| Reduce (Left)     | -     | 1.40             | -       | -      | 0 B/op |
+| Flap (Right)      | -     | 3.86             | -       | -      | 0 B/op |
+| Flap (Left)       | -     | 2.58             | -       | -      | 0 B/op |
+
+### FromPredicate Operations
+
+| Operation | Either (ns/op) | Idiomatic (ns/op) | Speedup | Either Allocs | Idio Allocs |
+|-----------|----------------|-------------------|---------|---------------|-------------|
+| FromPredicate (Pass) | -     | 3.38  | -       | -      | 0 B/op |
+| FromPredicate (Fail) | -     | 5.03  | -       | -      | 0 B/op |
+
+**Analysis:** FromPredicate in idiomatic shows good performance for validation patterns.
+
+### Option Conversion
+
+| Operation | Either (ns/op) | Idiomatic (ns/op) | Speedup | Either Allocs | Idio Allocs |
+|-----------|----------------|-------------------|---------|---------------|-------------|
+| ToOption (Right)   | -     | 1.17  | -       | -      | 0 B/op |
+| ToOption (Left)    | -     | 1.21  | -       | -      | 0 B/op |
+| FromOption (Some)  | -     | 2.68  | -       | -      | 0 B/op |
+| FromOption (None)  | -     | 3.72  | -       | -      | 0 B/op |
+
+**Analysis:** Very fast conversion between Result and Option in idiomatic package.
+
+## Pipeline Benchmarks
+
+These benchmarks measure realistic composition scenarios using F.Pipe.
+
+### Simple Map Pipeline
+
+| Operation | Either (ns/op) | Idiomatic (ns/op) | Speedup | Either Allocs | Idio Allocs |
+|-----------|----------------|-------------------|---------|---------------|-------------|
+| Pipeline Map (Right) | 112.7  | **46.5**  | **2.4x** ✓ | 72 B, 3 allocs | 48 B, 2 allocs |
+| Pipeline Map (Left)  | 116.8  | **47.2**  | **2.5x** ✓ | 72 B, 3 allocs | 48 B, 2 allocs |
+
+### Chain Pipeline
+
+| Operation | Either (ns/op) | Idiomatic (ns/op) | Speedup | Either Allocs | Idio Allocs |
+|-----------|----------------|-------------------|---------|---------------|-------------|
+| Pipeline Chain (Right) | 74.4  | **26.1**  | **2.9x** ✓ | 48 B, 2 allocs | 24 B, 1 allocs |
+| Pipeline Chain (Left)  | 86.4  | **25.7**  | **3.4x** ✓ | 48 B, 2 allocs | 24 B, 1 allocs |
+
+### Complex Pipeline (Map → Chain → Map)
+
+| Operation | Either (ns/op) | Idiomatic (ns/op) | Speedup | Either Allocs | Idio Allocs |
+|-----------|----------------|-------------------|---------|---------------|-------------|
+| Complex (Right) | 279.8  | **116.3**  | **2.4x** ✓ | 192 B, 8 allocs | 120 B, 5 allocs |
+| Complex (Left)  | 288.1  | **115.8**  | **2.5x** ✓ | 192 B, 8 allocs | 120 B, 5 allocs |
+
+**Analysis:**
+- Idiomatic package shows **2-3.4x speedup** for realistic pipelines
+- Significantly fewer allocations in all pipeline scenarios
+- The gap widens as pipelines become more complex
+
+## Array/Collection Operations
+
+### TraverseArray
+
+| Operation | Either (ns/op) | Idiomatic (ns/op) | Note |
+|-----------|----------------|-------------------|------|
+| TraverseArray (Success) | -     | 32.3  | 48 B, 1 alloc |
+| TraverseArray (Error)   | -     | 28.3  | 48 B, 1 alloc |
+
+**Analysis:** Idiomatic package provides efficient array traversal with minimal allocations.
+
+## Validation (ApV)
+
+### ApV Operations
+
+| Operation | Either (ns/op) | Idiomatic (ns/op) | Speedup | Either Allocs | Idio Allocs |
+|-----------|----------------|-------------------|---------|---------------|-------------|
+| ApV (BothRight) | -     | 1.17   | -       | -      | 0 B/op |
+| ApV (BothLeft)  | -     | 141.5  | -       | -      | 48 B, 2 allocs |
+
+**Analysis:** Idiomatic's validation applicative shows fast success path, with allocations only when accumulating errors.
+
+## String Formatting
+
+| Operation | Either (ns/op) | Idiomatic (ns/op) | Speedup | Either Allocs | Idio Allocs |
+|-----------|----------------|-------------------|---------|---------------|-------------|
+| String/ToString (Right) | 139.9  | **81.8**  | **1.7x** ✓ | 16 B, 1 alloc | 16 B, 1 alloc |
+| String/ToString (Left)  | 161.6  | **72.7**  | **2.2x** ✓ | 48 B, 1 alloc | 24 B, 1 alloc |
+
+**Analysis:** Idiomatic package formats strings faster with fewer allocations for Left values.
+
+## Do-Notation
+
+| Operation | Either (ns/op) | Idiomatic (ns/op) | Note |
+|-----------|----------------|-------------------|------|
+| Do        | 2.03  | -                | Either-specific |
+| Bind      | 153.4 | -                | 96 B, 4 allocs |
+| Let       | 33.5  | -                | 16 B, 1 alloc |
+
+**Analysis:** Do-notation is specific to Either package for monadic composition patterns.
+
+## Summary Statistics
+
+### Simple Operations (< 10 ns/op)
+
+**Either Package:**
+- Count: 24 operations
+- Average: 3.2 ns/op
+- Range: 1.24 - 9.01 ns/op
+
+**Idiomatic Package:**
+- Count: 36 operations
+- Average: 2.1 ns/op
+- Range: 1.17 - 5.03 ns/op
+
+**Winner:** Idiomatic (1.5x faster average)
+
+### Complex Operations (Pipelines, allocations)
+
+**Either Package:**
+- Pipeline Map: 112.7 ns/op (72 B, 3 allocs)
+- Pipeline Chain: 74.4 ns/op (48 B, 2 allocs)
+- Complex: 279.8 ns/op (192 B, 8 allocs)
+- ChainFirst: 87.6 ns/op (72 B, 3 allocs)
+
+**Idiomatic Package:**
+- Pipeline Map: 46.5 ns/op (48 B, 2 allocs)
+- Pipeline Chain: 26.1 ns/op (24 B, 1 allocs)
+- Complex: 116.3 ns/op (120 B, 5 allocs)
+- ChainFirst: 2.71 ns/op (0 B, 0 allocs)
+
+**Winner:** Idiomatic (2-32x faster, significantly fewer allocations)
+
+### Allocation Analysis
+
+**Either Package:**
+- Zero-allocation operations: Most simple operations
+- Operations with allocations: Pipelines, Bind, Do-notation, ChainFirst
+
+**Idiomatic Package:**
+- Zero-allocation operations: Almost all operations except pipelines and validation
+- Significantly fewer allocations in pipeline scenarios
+- ChainFirst: **Zero allocations** (vs 72 B in Either)
+
+## Performance Characteristics
+
+### Where Either Package Excels
+
+1. **Comparable to Idiomatic**: After optimizations, Either matches Idiomatic for constructors and predicates
+2. **Feature Richness**: More operations (Do-notation, Bind, Let, Flatten, Swap)
+3. **Type Flexibility**: Full Either[E, A] with custom error types
+
+### Where Idiomatic Package Excels
+
+1. **Core Operations**: 1.2-2.3x faster for Map, Chain, Fold
+2. **Complex Operations**: 32x faster for ChainFirst
+3. **Pipelines**: 2-3.4x faster with fewer allocations
+4. **Extraction**: 3-6x faster for GetOrElse
+5. **Alternative**: 2x faster for Alt/OrElse
+6. **BiMap**: 3-4x faster
+7. **Consistency**: More predictable performance profile
+
+## Real-World Performance Impact
+
+### Hot Path Example (1 million operations)
+
+```go
+// Map operation (very common)
+// Either:    5.13 ns/op × 1M = 5.13 ms
+// Idiomatic: 4.34 ns/op × 1M = 4.34 ms
+// Savings: 0.79 ms per million operations
+
+// Chain operation (common in pipelines)
+// Either:    5.44 ns/op × 1M = 5.44 ms
+// Idiomatic: 2.34 ns/op × 1M = 2.34 ms
+// Savings: 3.10 ms per million operations
+
+// Pipeline Complex (realistic composition)
+// Either:    279.8 ns/op × 1M = 279.8 ms
+// Idiomatic: 116.3 ns/op × 1M = 116.3 ms
+// Savings: 163.5 ms per million operations
+```
+
+### Memory Impact
+
+For 1 million ChainFirst operations:
+- Either: 72 MB allocated
+- Idiomatic: 0 MB allocated
+- **Savings: 72 MB + reduced GC pressure**
+
+## Recommendations
+
+### Use Idiomatic Package When:
+
+1. **Performance is Critical**
+   - Hot paths in your application
+   - High-throughput services (>10k req/s)
+   - Complex operation chains
+   - Memory-constrained environments
+
+2. **Natural Go Integration**
+   - Working with stdlib (value, error) patterns
+   - Team familiar with Go idioms
+   - Simple migration from existing code
+   - Want zero-cost abstractions
+
+3. **Pipeline-Heavy Code**
+   - 2-3.4x faster pipelines
+   - Significantly fewer allocations
+   - Better CPU cache utilization
+
+### Use Either Package When:
+
+1. **Feature Requirements**
+   - Need custom error types (Either[E, A])
+   - Using Do-notation for complex compositions
+   - Need Flatten, Swap, or other Either-specific operations
+   - Porting from FP languages (Scala, Haskell)
+
+2. **Type Safety Over Performance**
+   - Explicit Either semantics
+   - Algebraic data type guarantees
+   - Teaching/learning FP concepts
+
+3. **Moderate Performance Needs**
+   - After optimizations, Either is quite fast
+   - Difference matters only at high scale
+   - Code clarity > micro-optimizations
+
+### Hybrid Approach
+
+```go
+// Use Either for complex type safety
+import "github.com/IBM/fp-go/v2/either"
+type ValidationError struct { Field, Message string }
+validated := either.Either[ValidationError, Input]{...}
+
+// Convert to Idiomatic for hot path
+import "github.com/IBM/fp-go/v2/idiomatic/result"
+value, err := either.UnwrapError(either.MapLeft(toError)(validated))
+processed, err := result.Chain(hotPathProcessing)(value, err)
+```
+
+## Conclusion
+
+After optimizations to the Either package:
+
+1. **Both packages achieve zero heap allocations for constructors** - The Either struct (24 bytes) does NOT escape to heap
+2. **Simple operations** are now **comparable** between both packages (~1-2 ns/op, 0 B/op)
+3. **Core transformations** favor Idiomatic by **1.2-2.3x**
+4. **Complex operations** heavily favor Idiomatic by **2-32x**
+5. **Memory efficiency** strongly favors Idiomatic (especially ChainFirst: 72 B → 0 B)
+6. **Real-world pipelines** show **2-3.4x speedup** with Idiomatic
+
+### Key Insight: No Heap Escape Myth
+
+A critical finding: **Both packages avoid heap allocations for simple operations.** The Either struct is small enough (24 bytes) that Go returns it by value on the stack, not the heap. The `0 B/op, 0 allocs/op` benchmarks confirm this.
+
+The performance differences come from:
+- **Function composition overhead** in complex operations
+- **Currying and closure creation** in pipelines
+- **Tuple simplicity** vs struct field access
+
+Not from constructor allocations—both are equally efficient there.
+
+### Final Verdict
+
+The idiomatic package provides a compelling performance advantage for production workloads while maintaining zero-cost functional programming abstractions. The Either package remains excellent for type safety, feature richness, and scenarios where explicit Either[E, A] semantics are valuable.
+
+**Bottom Line:**
+- For **high-performance Go services**: idiomatic package is the clear winner (1.2-32x faster)
+- For **type-safe, feature-rich FP**: Either package is excellent (comparable simple ops, more features)
+- **Both avoid heap allocations** for constructors—choose based on your performance vs features trade-off

v2/CHAINING_PERFORMANCE_ANALYSIS.md

@@ -0,0 +1,344 @@
+# Deep Chaining Performance Analysis
+
+## Executive Summary
+
+The **only remaining performance gap** between `v2/option` and `idiomatic/option` is in **deep chaining operations** (multiple sequential transformations). This document demonstrates the problem, explains the root cause, and provides recommendations.
+
+## Benchmark Results
+
+### v2/option (Struct-based)
+```
+BenchmarkChain_3Steps      8.17 ns/op    0 allocs
+BenchmarkChain_5Steps     16.57 ns/op    0 allocs
+BenchmarkChain_10Steps    47.01 ns/op    0 allocs
+BenchmarkMap_5Steps        0.28 ns/op    0 allocs ⚡
+```
+
+### idiomatic/option (Tuple-based)
+```
+BenchmarkChain_3Steps      0.22 ns/op    0 allocs ⚡
+BenchmarkChain_5Steps      0.22 ns/op    0 allocs ⚡
+BenchmarkChain_10Steps     0.21 ns/op    0 allocs ⚡
+BenchmarkMap_5Steps        0.22 ns/op    0 allocs ⚡
+```
+
+### Performance Comparison
+
+| Steps | v2/option | idiomatic/option | Slowdown |
+|-------|-----------|------------------|----------|
+| 3 | 8.17 ns | 0.22 ns | **37x slower** |
+| 5 | 16.57 ns | 0.22 ns | **75x slower** |
+| 10 | 47.01 ns | 0.21 ns | **224x slower** |
+
+**Key Finding**: The performance gap **increases linearly** with chain depth!
+
+---
+
+## Visual Example: The Problem
+
+### Scenario: Processing User Input
+
+```go
+// Process user input through multiple validation steps
+input := "42"
+
+// v2/option - Nested MonadChain
+result := MonadChain(
+    MonadChain(
+        MonadChain(
+            Some(input),
+            validateNotEmpty,  // Step 1
+        ),
+        parseToInt,           // Step 2
+    ),
+    validateRange,            // Step 3
+)
+```
+
+### What Happens Under the Hood
+
+#### v2/option (Struct Construction Overhead)
+
+```go
+// Step 0: Initial value
+Some(input)
+// Creates: Option[string]{value: "42", isSome: true}
+// Memory: HEAP allocation
+
+// Step 1: Validate not empty
+MonadChain(opt, validateNotEmpty)
+// Input:  Option[string]{value: "42", isSome: true}   ← Read from heap
+// Output: Option[string]{value: "42", isSome: true}   ← NEW heap allocation
+// Memory: 2 heap allocations
+
+// Step 2: Parse to int
+MonadChain(opt, parseToInt)
+// Input:  Option[string]{value: "42", isSome: true}   ← Read from heap
+// Output: Option[int]{value: 42, isSome: true}        ← NEW heap allocation
+// Memory: 3 heap allocations
+
+// Step 3: Validate range
+MonadChain(opt, validateRange)
+// Input:  Option[int]{value: 42, isSome: true}        ← Read from heap
+// Output: Option[int]{value: 42, isSome: true}        ← NEW heap allocation
+// Memory: 4 heap allocations TOTAL
+
+// Each step:
+// 1. Reads Option struct from memory
+// 2. Checks isSome field
+// 3. Calls function
+// 4. Creates NEW Option struct
+// 5. Writes to memory
+```
+
+#### idiomatic/option (Zero Allocation)
+
+```go
+// Step 0: Initial value
+s, ok := Some(input)
+// Creates: ("42", true)
+// Memory: STACK only (registers)
+
+// Step 1: Validate not empty
+v1, ok1 := Chain(validateNotEmpty)(s, ok)
+// Input:  ("42", true)          ← Values in registers
+// Output: ("42", true)          ← Values in registers
+// Memory: ZERO allocations
+
+// Step 2: Parse to int
+v2, ok2 := Chain(parseToInt)(v1, ok1)
+// Input:  ("42", true)          ← Values in registers
+// Output: (42, true)            ← Values in registers
+// Memory: ZERO allocations
+
+// Step 3: Validate range
+v3, ok3 := Chain(validateRange)(v2, ok2)
+// Input:  (42, true)            ← Values in registers
+// Output: (42, true)            ← Values in registers
+// Memory: ZERO allocations TOTAL
+
+// Each step:
+// 1. Reads values from registers (no memory access!)
+// 2. Checks bool flag
+// 3. Calls function
+// 4. Returns new tuple (stays in registers)
+// 5. Compiler optimizes everything away!
+```
+
+---
+
+## Assembly-Level Difference
+
+### v2/option - Struct Overhead
+
+```asm
+; Every chain step does:
+MOV  RAX, [heap_ptr]        ; Load struct from heap
+TEST BYTE [RAX+8], 1        ; Check isSome field
+JZ   none_case              ; Branch if None
+MOV  RDI, [RAX]             ; Load value from struct
+CALL transform_func         ; Call the function
+CALL malloc                 ; Allocate new struct ⚠️
+MOV  [new_ptr], result      ; Store result
+MOV  [new_ptr+8], 1         ; Set isSome = true
+```
+
+### idiomatic/option - Optimized Away
+
+```asm
+; All steps compiled to:
+MOV  EAX, 42                ; The final result!
+; Everything else optimized away! ⚡
+```
+
+**Compiler insight**: With tuples, the Go compiler can:
+1. **Inline everything** - No function call overhead
+2. **Eliminate branches** - Constant propagation removes `if ok` checks
+3. **Use registers only** - Values never touch memory
+4. **Dead code elimination** - Removes unnecessary operations
+
+---
+
+## Real-World Example with Timings
+
+### Example: User Registration Validation Chain
+
+```go
+// Validate: email → trim → lowercase → check format → check uniqueness
+```
+
+#### v2/option Performance
+
+```go
+func ValidateEmail_v2(email string) Option[string] {
+    return MonadChain(
+        MonadChain(
+            MonadChain(
+                MonadChain(
+                    Some(email),
+                    trimWhitespace,      // ~2 ns
+                ),
+                toLowerCase,             // ~2 ns
+            ),
+            validateFormat,              // ~2 ns
+        ),
+        checkUniqueness,                 // ~2 ns
+    )
+}
+// Total: ~8-16 ns (matches our 5-step benchmark: 16.57 ns)
+```
+
+#### idiomatic/option Performance
+
+```go
+func ValidateEmail_idiomatic(email string) (string, bool) {
+    v1, ok1 := Chain(trimWhitespace)(email, true)
+    v2, ok2 := Chain(toLowerCase)(v1, ok1)
+    v3, ok3 := Chain(validateFormat)(v2, ok2)
+    return Chain(checkUniqueness)(v3, ok3)
+}
+// Total: ~0.22 ns (entire chain optimized to single operation!)
+```
+
+**Impact**: For 1 million validations:
+- v2/option: 16.57 ms
+- idiomatic/option: 0.22 ms
+- **Difference: 75x faster = saved 16.35 ms**
+
+---
+
+## Why Map is Fast in v2/option
+
+Interestingly, `Map` (pure transformations) is **much faster** than `Chain`:
+
+```
+v2/option:
+- BenchmarkChain_5Steps:  16.57 ns
+- BenchmarkMap_5Steps:     0.28 ns  ← 59x FASTER!
+```
+
+**Reason**: Map transformations can be **inlined and fused** by the compiler:
+
+```go
+// This:
+Map(f5)(Map(f4)(Map(f3)(Map(f2)(Map(f1)(opt)))))
+
+// Becomes (after compiler optimization):
+Some(f5(f4(f3(f2(f1(value))))))  // Single struct construction!
+
+// While Chain cannot be optimized the same way:
+MonadChain(MonadChain(...))  // Must construct at each step
+```
+
+---
+
+## When Does This Matter?
+
+### ⚠️ **Rarely Critical** (99% of use cases)
+
+Even 10-step chains only cost **47 nanoseconds**. For context:
+- Database query: **~1,000,000 ns** (1 ms)
+- HTTP request: **~10,000,000 ns** (10 ms)
+- File I/O: **~100,000 ns** (0.1 ms)
+
+**The 47 ns overhead is negligible compared to real I/O operations.**
+
+### ⚡ **Can Matter** (High-throughput scenarios)
+
+1. **In-memory data processing pipelines**
+   ```go
+   // Processing 10 million records with 5-step validation
+   v2/option:        165 ms
+   idiomatic/option:   2 ms
+   Difference:       163 ms saved ⚡
+   ```
+
+2. **Real-time stream processing**
+   - Processing 100k events/second with chained transformations
+   - 16.57 ns × 100,000 = 1.66 ms vs 0.22 ns × 100,000 = 0.022 ms
+   - Can affect throughput for high-frequency trading, gaming, etc.
+
+3. **Tight inner loops with chained logic**
+   ```go
+   for i := 0; i < 1_000_000; i++ {
+       result := Chain(f1).Chain(f2).Chain(f3).Chain(f4)(data[i])
+   }
+   // v2/option: 16 ms
+   // idiomatic: 0.22 ms
+   ```
+
+---
+
+## Root Cause Summary
+
+| Aspect | v2/option | idiomatic/option | Why? |
+|--------|-----------|------------------|------|
+| **Intermediate values** | `Option[T]` struct | `(T, bool)` tuple | Struct requires memory, tuple can use registers |
+| **Memory allocation** | 1 per step | 0 total | Heap vs stack |
+| **Compiler optimization** | Limited | Aggressive | Structs block inlining |
+| **Cache impact** | Heap reads | Register-only | Memory bandwidth saved |
+| **Branch prediction** | Struct checks | Optimized away | Compiler removes branches |
+
+---
+
+## Recommendations
+
+### ✅ **Use v2/option When:**
+- I/O-bound operations (database, network, files)
+- User-facing applications (latency dominated by I/O)
+- Need JSON marshaling, TryCatch, SequenceArray
+- Chain depth < 5 steps (overhead < 20 ns - negligible)
+- Code clarity > microsecond performance
+
+### ✅ **Use idiomatic/option When:**
+- CPU-bound data processing
+- High-throughput stream processing
+- Tight inner loops with chaining
+- In-memory analytics
+- Performance-critical paths
+- Chain depth > 5 steps
+
+### ✅ **Mitigation for v2/option:**
+
+If you need v2/option but want better chain performance:
+
+1. **Use Map instead of Chain** when possible:
+   ```go
+   // Bad (16.57 ns):
+   MonadChain(MonadChain(MonadChain(opt, f1), f2), f3)
+
+   // Good (0.28 ns):
+   Map(f3)(Map(f2)(Map(f1)(opt)))
+   ```
+
+2. **Batch operations**:
+   ```go
+   // Instead of chaining many steps:
+   validate := func(x T) Option[T] {
+       // Combine multiple checks in one function
+       if check1(x) && check2(x) && check3(x) {
+           return Some(transform(x))
+       }
+       return None[T]()
+   }
+   ```
+
+3. **Profile first**:
+   - Only optimize hot paths
+   - 47 ns is often acceptable
+   - Don't premature optimize
+
+---
+
+## Conclusion
+
+**The deep chaining performance gap is:**
+- ✅ **Real and measurable** (37-224x slower)
+- ✅ **Well understood** (struct construction overhead)
+- ⚠️ **Rarely critical** (nanosecond differences usually don't matter)
+- ✅ **Easy to work around** (use Map, batch operations)
+- ✅ **Worth it for the API benefits** (JSON, methods, helpers)
+
+**For 99% of applications, v2/option's performance is excellent.** The gap only matters in specialized high-throughput scenarios where you should probably use idiomatic/option anyway.
+
+The optimizations already applied (`//go:inline`, direct field access) brought v2/option to **competitive parity** for all practical purposes. The remaining gap is a **fundamental design trade-off**, not a fixable bug.

v2/DESIGN.md

@@ -0,0 +1,574 @@
+# Design Decisions
+
+This document explains the key design decisions and principles behind fp-go's API design.
+
+## Table of Contents
+
+- [Data Last Principle](#data-last-principle)
+- [Kleisli and Operator Types](#kleisli-and-operator-types)
+- [Monadic Operations Comparison](#monadic-operations-comparison)
+- [Type Parameter Ordering](#type-parameter-ordering)
+- [Generic Type Aliases](#generic-type-aliases)
+
+## Data Last Principle
+
+fp-go follows the **"data last"** principle, where the data being operated on is always the last parameter in a function. This design choice enables powerful function composition and partial application patterns.
+
+### What is "Data Last"?
+
+In the "data last" style, functions are structured so that:
+1. Configuration parameters come first
+2. The data to be transformed comes last
+
+This is the opposite of the traditional object-oriented style where the data (receiver) comes first.
+
+### Why "Data Last"?
+
+The "data last" principle enables:
+
+1. **Natural Currying**: Functions can be partially applied to create specialized transformations
+2. **Function Composition**: Operations can be composed before applying them to data
+3. **Point-Free Style**: Write transformations without explicitly mentioning the data
+4. **Reusability**: Create reusable transformation pipelines
+
+### Examples
+
+#### Basic Transformation
+
+```go
+// Data last style (fp-go)
+double := array.Map(number.Mul(2))
+result := double([]int{1, 2, 3}) // [2, 4, 6]
+
+// Compare with data first style (traditional)
+result := array.Map([]int{1, 2, 3}, number.Mul(2))
+```
+
+#### Function Composition
+
+```go
+import (
+    A "github.com/IBM/fp-go/v2/array"
+    F "github.com/IBM/fp-go/v2/function"
+    N "github.com/IBM/fp-go/v2/number"
+)
+
+// Create a pipeline of transformations
+pipeline := F.Flow3(
+    A.Filter(N.MoreThan(0)),     // Keep positive numbers
+    A.Map(N.Mul(2)),                                  // Double each number
+    A.Reduce(func(acc, x int) int { return acc + x }, 0), // Sum them up
+)
+
+// Apply the pipeline to different data
+result1 := pipeline([]int{-1, 2, 3, -4, 5})  // (2 + 3 + 5) * 2 = 20
+result2 := pipeline([]int{1, 2, 3})          // (1 + 2 + 3) * 2 = 12
+```
+
+#### Partial Application
+
+```go
+import (
+    O "github.com/IBM/fp-go/v2/option"
+)
+
+// Create specialized functions by partial application
+getOrZero := O.GetOrElse(func() int { return 0 })
+getOrEmpty := O.GetOrElse(func() string { return "" })
+
+// Use them with different data
+value1 := getOrZero(O.Some(42))        // 42
+value2 := getOrZero(O.None[int]())     // 0
+
+text1 := getOrEmpty(O.Some("hello"))   // "hello"
+text2 := getOrEmpty(O.None[string]())  // ""
+```
+
+#### Building Reusable Transformations
+
+```go
+import (
+    E "github.com/IBM/fp-go/v2/either"
+    O "github.com/IBM/fp-go/v2/option"
+)
+
+// Create a reusable validation pipeline
+type User struct {
+    Name  string
+    Email string
+    Age   int
+}
+
+validateAge := E.FromPredicate(
+    func(u User) bool { return u.Age >= 18 },
+    func(u User) error { return errors.New("must be 18 or older") },
+)
+
+validateEmail := E.FromPredicate(
+    func(u User) bool { return strings.Contains(u.Email, "@") },
+    func(u User) error { return errors.New("invalid email") },
+)
+
+// Compose validators
+validateUser := F.Flow2(
+    validateAge,
+    E.Chain(validateEmail),
+)
+
+// Apply to different users
+result1 := validateUser(User{Name: "Alice", Email: "alice@example.com", Age: 25})
+result2 := validateUser(User{Name: "Bob", Email: "invalid", Age: 30})
+```
+
+#### Monadic Operations
+
+```go
+import (
+    O "github.com/IBM/fp-go/v2/option"
+)
+
+// Data last enables clean monadic chains
+parseAndDouble := F.Flow2(
+    O.FromPredicate(func(s string) bool { return s != "" }),
+    O.Chain(func(s string) O.Option[int] {
+        n, err := strconv.Atoi(s)
+        if err != nil {
+            return O.None[int]()
+        }
+        return O.Some(n * 2)
+    }),
+)
+
+result1 := parseAndDouble("21")  // Some(42)
+result2 := parseAndDouble("")    // None
+result3 := parseAndDouble("abc") // None
+```
+
+### Monadic vs Non-Monadic Forms
+
+fp-go provides two forms for most operations:
+
+1. **Curried form** (data last): Returns a function that can be composed
+2. **Monadic form** (data first): Takes all parameters at once
+
+```go
+// Curried form - data last, returns a function
+Map[A, B any](f func(A) B) func(Option[A]) Option[B]
+
+// Monadic form - data first, direct execution
+MonadMap[A, B any](fa Option[A], f func(A) B) Option[B]
+```
+
+**When to use each:**
+
+- **Curried form**: When building pipelines, composing functions, or creating reusable transformations
+- **Monadic form**: When you have all parameters available and want direct execution
+
+```go
+// Curried form - building a pipeline
+transform := F.Flow3(
+    O.Map(strings.ToUpper),
+    O.Filter(func(s string) bool { return len(s) > 3 }),
+    O.GetOrElse(func() string { return "DEFAULT" }),
+)
+result := transform(O.Some("hello"))
+
+// Monadic form - direct execution
+result := O.MonadMap(O.Some("hello"), strings.ToUpper)
+```
+
+### Further Reading on Data-Last Pattern
+
+The data-last currying pattern is well-documented in the functional programming community:
+
+- [Mostly Adequate Guide - Ch. 4: Currying](https://mostly-adequate.gitbook.io/mostly-adequate-guide/ch04) - Excellent introduction with clear examples
+- [Curry and Function Composition](https://medium.com/javascript-scene/curry-and-function-composition-2c208d774983) by Eric Elliott
+- [fp-ts Issue #1238](https://github.com/gcanti/fp-ts/issues/1238) - Real-world examples of data-last refactoring
+
+## Kleisli and Operator Types
+
+fp-go uses consistent type aliases across all monads to make code more recognizable and composable. These types provide a common vocabulary that works across different monadic contexts.
+
+### Type Definitions
+
+```go
+// Kleisli arrow - a function that returns a monadic value
+type Kleisli[A, B any] = func(A) M[B]
+
+// Operator - a function that transforms a monadic value
+type Operator[A, B any] = func(M[A]) M[B]
+```
+
+Where `M` represents the specific monad (Option, Either, IO, etc.).
+
+### Why These Types Matter
+
+1. **Consistency**: The same type names appear across all monads
+2. **Recognizability**: Experienced functional programmers immediately understand the intent
+3. **Composability**: Functions with these types compose naturally
+4. **Documentation**: Type signatures clearly communicate the operation's behavior
+
+### Examples Across Monads
+
+#### Option Monad
+
+```go
+// option/option.go
+type Kleisli[A, B any] = func(A) Option[B]
+type Operator[A, B any] = func(Option[A]) Option[B]
+
+// Chain uses Kleisli
+func Chain[A, B any](f Kleisli[A, B]) Operator[A, B]
+
+// Map returns an Operator
+func Map[A, B any](f func(A) B) Operator[A, B]
+```
+
+#### Either Monad
+
+```go
+// either/either.go
+type Kleisli[E, A, B any] = func(A) Either[E, B]
+type Operator[E, A, B any] = func(Either[E, A]) Either[E, B]
+
+// Chain uses Kleisli
+func Chain[E, A, B any](f Kleisli[E, A, B]) Operator[E, A, B]
+
+// Map returns an Operator
+func Map[E, A, B any](f func(A) B) Operator[E, A, B]
+```
+
+#### IO Monad
+
+```go
+// io/io.go
+type Kleisli[A, B any] = func(A) IO[B]
+type Operator[A, B any] = func(IO[A]) IO[B]
+
+// Chain uses Kleisli
+func Chain[A, B any](f Kleisli[A, B]) Operator[A, B]
+
+// Map returns an Operator
+func Map[A, B any](f func(A) B) Operator[A, B]
+```
+
+#### Array (List Monad)
+
+```go
+// array/array.go
+type Kleisli[A, B any] = func(A) []B
+type Operator[A, B any] = func([]A) []B
+
+// Chain uses Kleisli
+func Chain[A, B any](f Kleisli[A, B]) Operator[A, B]
+
+// Map returns an Operator
+func Map[A, B any](f func(A) B) Operator[A, B]
+```
+
+### Pattern Recognition
+
+Once you learn these patterns in one monad, you can apply them to all monads:
+
+```go
+// The pattern is always the same, just the monad changes
+
+// Option
+validateAge := option.Chain(func(user User) option.Option[User] {
+    if user.Age >= 18 {
+        return option.Some(user)
+    }
+    return option.None[User]()
+})
+
+// Either
+validateAge := either.Chain(func(user User) either.Either[error, User] {
+    if user.Age >= 18 {
+        return either.Right[error](user)
+    }
+    return either.Left[User](errors.New("too young"))
+})
+
+// IO
+validateAge := io.Chain(func(user User) io.IO[User] {
+    return io.Of(user) // Always succeeds in IO
+})
+
+// Array
+validateAge := array.Chain(func(user User) []User {
+    if user.Age >= 18 {
+        return []User{user}
+    }
+    return []User{} // Empty array = failure
+})
+```
+
+### Composing Kleisli Arrows
+
+Kleisli arrows compose naturally using monadic composition:
+
+```go
+import (
+    O "github.com/IBM/fp-go/v2/option"
+    F "github.com/IBM/fp-go/v2/function"
+)
+
+// Define Kleisli arrows
+parseAge := func(s string) O.Option[int] {
+    n, err := strconv.Atoi(s)
+    if err != nil {
+        return O.None[int]()
+    }
+    return O.Some(n)
+}
+
+validateAge := func(age int) O.Option[int] {
+    if age >= 18 {
+        return O.Some(age)
+    }
+    return O.None[int]()
+}
+
+formatAge := func(age int) O.Option[string] {
+    return O.Some(fmt.Sprintf("Age: %d", age))
+}
+
+// Compose them using Flow and Chain
+pipeline := F.Flow3(
+    parseAge,
+    O.Chain(validateAge),
+    O.Chain(formatAge),
+)
+
+result := pipeline("25") // Some("Age: 25")
+result := pipeline("15") // None (too young)
+result := pipeline("abc") // None (parse error)
+```
+
+### Building Reusable Operators
+
+Operators can be created once and reused across your codebase:
+
+```go
+import (
+    E "github.com/IBM/fp-go/v2/either"
+)
+
+// Create reusable operators
+type ValidationError struct {
+    Field   string
+    Message string
+}
+
+// Reusable validation operators
+validateNonEmpty := E.Chain(func(s string) E.Either[ValidationError, string] {
+    if s == "" {
+        return E.Left[string](ValidationError{
+            Field:   "input",
+            Message: "cannot be empty",
+        })
+    }
+    return E.Right[ValidationError](s)
+})
+
+validateEmail := E.Chain(func(s string) E.Either[ValidationError, string] {
+    if !strings.Contains(s, "@") {
+        return E.Left[string](ValidationError{
+            Field:   "email",
+            Message: "invalid format",
+        })
+    }
+    return E.Right[ValidationError](s)
+})
+
+// Compose operators
+validateEmailInput := F.Flow2(
+    validateNonEmpty,
+    validateEmail,
+)
+
+// Use across your application
+result1 := validateEmailInput(E.Right[ValidationError]("user@example.com"))
+result2 := validateEmailInput(E.Right[ValidationError](""))
+result3 := validateEmailInput(E.Right[ValidationError]("invalid"))
+```
+
+### Benefits of Consistent Naming
+
+1. **Cross-monad understanding**: Learn once, apply everywhere
+2. **Easier refactoring**: Changing monads requires minimal code changes
+3. **Better tooling**: IDEs can provide better suggestions
+4. **Team communication**: Shared vocabulary across the team
+5. **Library integration**: Third-party libraries follow the same patterns
+
+### Identity Monad - The Simplest Case
+
+The Identity monad shows these types in their simplest form:
+
+```go
+// identity/doc.go
+type Operator[A, B any] = func(A) B
+
+// In Identity, there's no wrapping, so:
+// - Kleisli[A, B] is just func(A) B
+// - Operator[A, B] is just func(A) B
+// They're the same because Identity adds no context
+```
+
+This demonstrates that these type aliases represent fundamental functional programming concepts, not just arbitrary naming conventions.
+
+
+## Monadic Operations Comparison
+
+fp-go's monadic operations are inspired by functional programming languages and libraries. Here's how they compare:
+
+| fp-go | fp-ts | Haskell | Scala | Description |
+|-------|-------|---------|-------|-------------|
+| `Map` | `map` | `fmap` | `map` | Functor mapping - transforms the value inside a context |
+| `Chain` | `chain` | `>>=` (bind) | `flatMap` | Monadic bind - chains computations that return wrapped values |
+| `Ap` | `ap` | `<*>` | `ap` | Applicative apply - applies a wrapped function to a wrapped value |
+| `Of` | `of` | `return`/`pure` | `pure` | Lifts a pure value into a monadic context |
+| `Fold` | `fold` | `either` | `fold` | Eliminates the context by providing handlers for each case |
+| `Filter` | `filter` | `mfilter` | `filter` | Keeps values that satisfy a predicate |
+| `Flatten` | `flatten` | `join` | `flatten` | Removes one level of nesting |
+| `ChainFirst` | `chainFirst` | `>>` (then) | `tap` | Chains for side effects, keeping the original value |
+| `Alt` | `alt` | `<\|>` | `orElse` | Provides an alternative value if the first fails |
+| `GetOrElse` | `getOrElse` | `fromMaybe` | `getOrElse` | Extracts the value or provides a default |
+| `FromPredicate` | `fromPredicate` | `guard` | `filter` | Creates a monadic value based on a predicate |
+| `Sequence` | `sequence` | `sequence` | `sequence` | Transforms a collection of effects into an effect of a collection |
+| `Traverse` | `traverse` | `traverse` | `traverse` | Maps and sequences in one operation |
+| `Reduce` | `reduce` | `foldl` | `foldLeft` | Folds a structure from left to right |
+| `ReduceRight` | `reduceRight` | `foldr` | `foldRight` | Folds a structure from right to left |
+
+### Key Differences from Other Languages
+
+#### Naming Conventions
+
+- **Go conventions**: fp-go uses PascalCase for exported functions (e.g., `Map`, `Chain`) following Go's naming conventions
+- **Type parameters first**: Non-inferrable type parameters come first (e.g., `Ap[B, E, A any]`)
+- **Monadic prefix**: Direct execution forms use the `Monad` prefix (e.g., `MonadMap`, `MonadChain`)
+
+#### Type System
+
+```go
+// fp-go (explicit type parameters when needed)
+result := option.Map(transform)(value)
+result := option.Map[string, int](transform)(value) // explicit when inference fails
+
+// Haskell (type inference)
+result = fmap transform value
+
+// Scala (type inference with method syntax)
+result = value.map(transform)
+
+// fp-ts (TypeScript type inference)
+const result = pipe(value, map(transform))
+```
+
+#### Currying
+
+```go
+// fp-go - explicit currying with data last
+double := array.Map(number.Mul(2))
+result := double(numbers)
+
+// Haskell - automatic currying
+double = fmap (*2)
+result = double numbers
+
+// Scala - method syntax
+result = numbers.map(_ * 2)
+```
+
+## Type Parameter Ordering
+
+fp-go v2 uses a specific ordering for type parameters to maximize type inference:
+
+### Rule: Non-Inferrable Parameters First
+
+Type parameters that **cannot be inferred** from function arguments come first. This allows the Go compiler to infer as many types as possible.
+
+```go
+// Ap - B cannot be inferred from arguments, so it comes first
+func Ap[B, E, A any](fa Either[E, A]) func(Either[E, func(A) B]) Either[E, B]
+
+// Usage - only B needs to be specified
+result := either.Ap[string](value)(funcInEither)
+```
+
+### Examples
+
+```go
+// Map - all types can be inferred from arguments
+func Map[E, A, B any](f func(A) B) func(Either[E, A]) Either[E, B]
+// Usage - no type parameters needed
+result := either.Map(transform)(value)
+
+// Chain - all types can be inferred
+func Chain[E, A, B any](f func(A) Either[E, B]) func(Either[E, A]) Either[E, B]
+// Usage - no type parameters needed
+result := either.Chain(validator)(value)
+
+// Of - E cannot be inferred, comes first
+func Of[E, A any](value A) Either[E, A]
+// Usage - only E needs to be specified
+result := either.Of[error](42)
+```
+
+### Benefits
+
+1. **Less verbose code**: Most operations don't require explicit type parameters
+2. **Better IDE support**: Type inference provides better autocomplete
+3. **Clearer intent**: Only specify types that can't be inferred
+
+## Generic Type Aliases
+
+fp-go v2 leverages Go 1.24's generic type aliases for cleaner type definitions:
+
+```go
+// V2 - using generic type alias (requires Go 1.24+)
+type ReaderIOEither[R, E, A any] = RD.Reader[R, IOE.IOEither[E, A]]
+
+// V1 - using type definition (Go 1.18+)
+type ReaderIOEither[R, E, A any] RD.Reader[R, IOE.IOEither[E, A]]
+```
+
+### Benefits
+
+1. **True aliases**: The type is interchangeable with its definition
+2. **No namespace imports needed**: Can use types directly without package prefixes
+3. **Simpler codebase**: Eliminates the need for `generic` subpackages
+4. **Better composability**: Types compose more naturally
+
+### Migration Pattern
+
+```go
+// Define project-wide aliases once
+package types
+
+import (
+    "github.com/IBM/fp-go/v2/option"
+    "github.com/IBM/fp-go/v2/result"
+    "github.com/IBM/fp-go/v2/ioresult"
+)
+
+type Option[A any] = option.Option[A]
+type Result[A any] = result.Result[A]
+type IOResult[A any] = ioresult.IOResult[A]
+
+// Use throughout your codebase
+package myapp
+
+import "myproject/types"
+
+func process(input string) types.Result[types.Option[int]] {
+    // implementation
+}
+```
+
+---
+
+For more information, see:
+- [README.md](./README.md) - Overview and quick start
+- [API Documentation](https://pkg.go.dev/github.com/IBM/fp-go/v2) - Complete API reference
+- [Samples](./samples/) - Practical examples

v2/EXAMPLE_TESTS_PROGRESS.md

@@ -0,0 +1,212 @@
+# Example Tests Progress
+
+This document tracks the progress of converting documentation examples into executable example test files.
+
+## Overview
+
+The codebase has 300+ documentation examples across many packages. This document tracks which packages have been completed and which still need work.
+
+## Completed Packages
+
+### Core Packages
+- [x] **result** - Created `examples_bind_test.go`, `examples_curry_test.go`, `examples_apply_test.go`
+  - Files: `bind.go` (10 examples), `curry.go` (5 examples), `apply.go` (2 examples)
+  - Status: ✅ 17 tests passing
+
+### Utility Packages
+- [x] **pair** - Created `examples_test.go`
+  - Files: `pair.go` (14 examples)
+  - Status: ✅ 14 tests passing
+
+- [x] **tuple** - Created `examples_test.go`
+  - Files: `tuple.go` (6 examples)
+  - Status: ✅ 6 tests passing
+
+### Type Class Packages
+- [x] **semigroup** - Created `examples_test.go`
+  - Files: `semigroup.go` (7 examples)
+  - Status: ✅ 7 tests passing
+
+### Utility Packages (continued)
+- [x] **predicate** - Created `examples_test.go`
+  - Files: `bool.go` (3 examples), `contramap.go` (1 example)
+  - Status: ✅ 4 tests passing
+
+### Context Reader Packages
+- [x] **idiomatic/context/readerresult** - Created `examples_reader_test.go`, `examples_bind_test.go`
+  - Files: `reader.go` (8 examples), `bind.go` (14 examples)
+  - Status: ✅ 22 tests passing
+
+## Summary Statistics
+- **Total Example Tests Created**: 74
+- **Total Packages Completed**: 7 (result, pair, tuple, semigroup, predicate, idiomatic/context/readerresult)
+- **All Tests Status**: ✅ PASSING
+
+### Breakdown by Package
+- **result**: 21 tests (bind: 10, curry: 5, apply: 2, array: 4)
+- **pair**: 14 tests
+- **tuple**: 6 tests
+- **semigroup**: 7 tests
+- **predicate**: 4 tests
+- **idiomatic/context/readerresult**: 22 tests (reader: 8, bind: 14)
+
+## Packages with Existing Examples
+
+These packages already have some example test files:
+- result (has `examples_create_test.go`, `examples_extract_test.go`)
+- option (has `examples_create_test.go`, `examples_extract_test.go`)
+- either (has `examples_create_test.go`, `examples_extract_test.go`)
+- ioeither (has `examples_create_test.go`, `examples_do_test.go`, `examples_extract_test.go`)
+- ioresult (has `examples_create_test.go`, `examples_do_test.go`, `examples_extract_test.go`)
+- lazy (has `example_lazy_test.go`)
+- array (has `examples_basic_test.go`, `examples_sort_test.go`, `example_any_test.go`, `example_find_test.go`)
+- readerioeither (has `traverse_example_test.go`)
+- context/readerioresult (has `flip_example_test.go`)
+
+## Packages Needing Example Tests
+
+### Core Packages (High Priority)
+- [ ] **result** - Additional files need examples:
+  - `apply.go` (2 examples)
+  - `array.go` (7 examples)
+  - `core.go` (6 examples)
+  - `either.go` (26 examples)
+  - `eq.go` (2 examples)
+  - `functor.go` (1 example)
+  
+- [ ] **option** - Additional files need examples
+- [ ] **either** - Additional files need examples
+
+### Reader Packages (High Priority)
+- [ ] **reader** - Many examples in:
+  - `array.go` (12 examples)
+  - `bind.go` (10 examples)
+  - `curry.go` (8 examples)
+  - `flip.go` (2 examples)
+  - `reader.go` (21 examples)
+
+- [ ] **readeroption** - Examples in:
+  - `array.go` (3 examples)
+  - `bind.go` (7 examples)
+  - `curry.go` (5 examples)
+  - `flip.go` (2 examples)
+  - `from.go` (4 examples)
+  - `reader.go` (18 examples)
+  - `sequence.go` (4 examples)
+
+- [ ] **readerresult** - Examples in:
+  - `array.go` (3 examples)
+  - `bind.go` (24 examples)
+  - `curry.go` (7 examples)
+  - `flip.go` (2 examples)
+  - `from.go` (4 examples)
+  - `monoid.go` (3 examples)
+
+- [ ] **readereither** - Examples in:
+  - `array.go` (3 examples)
+  - `bind.go` (7 examples)
+  - `flip.go` (3 examples)
+
+- [ ] **readerio** - Examples in:
+  - `array.go` (3 examples)
+  - `bind.go` (7 examples)
+  - `flip.go` (2 examples)
+  - `logging.go` (4 examples)
+  - `reader.go` (30 examples)
+
+- [ ] **readerioeither** - Examples in:
+  - `bind.go` (7 examples)
+  - `flip.go` (1 example)
+
+- [ ] **readerioresult** - Examples in:
+  - `array.go` (8 examples)
+  - `bind.go` (24 examples)
+
+### State Packages
+- [ ] **statereaderioeither** - Examples in:
+  - `bind.go` (5 examples)
+  - `resource.go` (1 example)
+  - `state.go` (13 examples)
+
+### Utility Packages
+- [ ] **lazy** - Additional examples in:
+  - `apply.go` (2 examples)
+  - `bind.go` (7 examples)
+  - `lazy.go` (10 examples)
+  - `sequence.go` (4 examples)
+  - `traverse.go` (2 examples)
+
+- [ ] **pair** - Additional examples in:
+  - `monad.go` (12 examples)
+  - `pair.go` (remaining ~20 examples)
+
+- [ ] **tuple** - Examples in:
+  - `tuple.go` (6 examples)
+
+- [ ] **predicate** - Examples in:
+  - `bool.go` (3 examples)
+  - `contramap.go` (1 example)
+  - `monoid.go` (4 examples)
+
+- [ ] **retry** - Examples in:
+  - `retry.go` (7 examples)
+
+- [ ] **logging** - Examples in:
+  - `logger.go` (5 examples)
+
+### Collection Packages
+- [ ] **record** - Examples in:
+  - `bind.go` (3 examples)
+
+### Type Class Packages
+- [ ] **semigroup** - Examples in:
+  - `alt.go` (1 example)
+  - `apply.go` (1 example)
+  - `array.go` (4 examples)
+  - `semigroup.go` (7 examples)
+
+- [ ] **ord** - Examples in:
+  - `ord.go` (1 example)
+
+## Strategy for Completion
+
+1. **Prioritize by usage**: Focus on core packages (result, option, either) first
+2. **Group by package**: Complete all examples for one package before moving to next
+3. **Test incrementally**: Run tests after each file to catch errors early
+4. **Follow patterns**: Use existing example test files as templates
+5. **Document as you go**: Update this file with progress
+
+## Example Test File Template
+
+```go
+// Copyright header...
+
+package packagename_test
+
+import (
+	"fmt"
+	PKG "github.com/IBM/fp-go/v2/packagename"
+)
+
+func ExampleFunctionName() {
+	// Copy example from doc comment
+	// Ensure it compiles and produces correct output
+	fmt.Println(result)
+	// Output:
+	// expected output
+}
+```
+
+## Notes
+
+- Use `F.Constant1[error](defaultValue)` for GetOrElse in result package
+- Use `F.Pipe1` instead of `F.Pipe2` when only one transformation
+- Check function signatures carefully for type parameters
+- Some functions like `BiMap` are capitalized differently than in docs
+- **Prefer `R.Eitherize1(func)` over manual error handling** - converts `func(T) (R, error)` to `func(T) Result[R]`
+  - Example: Use `R.Eitherize1(strconv.Atoi)` instead of manual if/else error checking
+- **Add Go documentation comments to all example functions** - Each example should have a comment explaining what it demonstrates
+- **Idiomatic vs Non-Idiomatic packages**:
+  - Non-idiomatic (e.g., `result`): Uses `Result[A]` type (Either monad)
+  - Idiomatic (e.g., `idiomatic/result`): Uses `(A, error)` tuples (Go-style)
+  - Context readers use non-idiomatic `Result[A]` internally

v2/IDIOMATIC_COMPARISON.md

@@ -0,0 +1,816 @@
+# Idiomatic vs Standard Package Comparison
+
+> **Latest Update:** 2025-11-18 - Updated with fresh benchmarks after `either` package optimizations
+
+This document provides a comprehensive comparison between the `idiomatic` packages and the standard fp-go packages (`result` and `option`).
+
+**See also:** [BENCHMARK_COMPARISON.md](./BENCHMARK_COMPARISON.md) for detailed performance analysis.
+
+## Table of Contents
+
+1. [Overview](#overview)
+2. [Design Differences](#design-differences)
+3. [Performance Comparison](#performance-comparison)
+4. [API Comparison](#api-comparison)
+5. [When to Use Each](#when-to-use-each)
+
+## Overview
+
+The fp-go library provides two approaches to functional programming patterns in Go:
+
+- **Standard Packages** (`result`, `either`, `option`): Use struct wrappers for algebraic data types
+- **Idiomatic Packages** (`idiomatic/result`, `idiomatic/option`): Use native Go tuples for the same patterns
+
+### Key Insight
+
+After recent optimizations to the `either` package, both approaches now offer excellent performance:
+
+- **Simple operations** (~1-5 ns/op): Both packages perform comparably
+- **Core transformations**: Idiomatic is **1.2-2.3x faster**
+- **Complex operations**: Idiomatic is **2-32x faster** with significantly fewer allocations
+- **Real-world pipelines**: Idiomatic shows **2-3.4x speedup**
+
+The idiomatic packages provide:
+- Consistently better performance across most operations
+- Zero allocations for complex operations (ChainFirst: 72 B → 0 B)
+- More familiar Go idioms
+- Seamless integration with existing Go code
+
+## Design Differences
+
+### Data Representation
+
+#### Standard Result Package
+
+```go
+// Uses Either[error, A] which is a struct wrapper
+type Result[A any] = Either[error, A]
+type Either[E, A any] struct {
+    r      A
+    l      E
+    isLeft bool
+}
+
+// Creating values - ZERO heap allocations (struct returned by value)
+success := result.Right[error](42)  // Returns Either struct by value (0 B/op)
+failure := result.Left[int](err)    // Returns Either struct by value (0 B/op)
+
+// Benchmarks confirm:
+// BenchmarkRight-16    871258489    1.384 ns/op    0 B/op    0 allocs/op
+// BenchmarkLeft-16     683089270    1.761 ns/op    0 B/op    0 allocs/op
+```
+
+#### Idiomatic Result Package
+
+```go
+// Uses native Go tuples (value, error)
+type Kleisli[A, B any]  = func(A) (B, error)
+type Operator[A, B any] = func(A, error) (B, error)
+
+// Creating values - ZERO allocations (tuples on stack)
+success := result.Right(42)         // Returns (42, nil) - 0 B/op
+failure := result.Left[int](err)    // Returns (0, err) - 0 B/op
+
+// Benchmarks confirm:
+// BenchmarkRight-16    789879016    1.427 ns/op    0 B/op    0 allocs/op
+// BenchmarkLeft-16     895412131    1.349 ns/op    0 B/op    0 allocs/op
+```
+
+### Type Signatures
+
+#### Standard Result
+
+```go
+// Functions take and return Result[T] structs
+func Map[A, B any](f func(A) B) func(Result[A]) Result[B]
+func Chain[A, B any](f Kleisli[A, B]) func(Result[A]) Result[B]
+func Fold[A, B any](onLeft func(error) B, onRight func(A) B) func(Result[A]) B
+
+// Usage requires wrapping/unwrapping
+result := result.Right[error](42)
+mapped := result.Map(double)(result)
+value, err := result.UnwrapError(mapped)
+```
+
+#### Idiomatic Result
+
+```go
+// Functions work directly with tuples
+func Map[A, B any](f func(A) B) func(A, error) (B, error)
+func Chain[A, B any](f Kleisli[A, B]) func(A, error) (B, error)
+func Fold[A, B any](onLeft func(error) B, onRight func(A) B) func(A, error) B
+
+// Usage works naturally with Go's error handling
+value, err := result.Right(42)
+value, err = result.Map(double)(value, err)
+// Can use directly: if err != nil { ... }
+```
+
+### Memory Layout
+
+#### Standard Result (struct-based)
+
+```
+Either[error, int] struct (returned by value):
+┌─────────────────────┐
+│ r: int (8B)         │  Stack allocation: 24 bytes
+│ l: error (8B)       │  NO heap allocation when returned by value
+│ isLeft: bool (1B)   │  Benchmarks show 0 B/op, 0 allocs/op
+│ padding (7B)        │
+└─────────────────────┘
+
+Key insight: Go returns small structs (<= ~64 bytes) by value on the stack.
+The Either struct (24 bytes) does NOT escape to heap in normal usage.
+```
+
+#### Idiomatic Result (tuple-based)
+
+```
+(int, error) tuple:
+┌─────────────────────┐
+│ int: 8 bytes        │  Stack allocation: 16 bytes
+│ error: 8 bytes      │  NO heap allocation
+└─────────────────────┘
+
+Both approaches achieve zero heap allocations for constructor operations!
+```
+
+### Why Both Have Zero Allocations
+
+Both packages avoid heap allocations for simple operations:
+
+**Standard Either/Result:**
+- `Either` struct is small (24 bytes)
+- Go returns by value on the stack
+- Inlining eliminates function call overhead
+- Result: `0 B/op, 0 allocs/op`
+
+**Idiomatic Result:**
+- Tuples are native Go multi-value returns
+- Always on stack, never heap
+- Even simpler than structs
+- Result: `0 B/op, 0 allocs/op`
+
+**When Either WOULD escape to heap:**
+```go
+// Taking address of local Either
+func bad1() *Either[error, int] {
+    e := Right[error](42)
+    return &e  // ESCAPES: pointer to local
+}
+
+// Storing in interface
+func bad2() interface{} {
+    return Right[error](42)  // ESCAPES: interface boxing
+}
+
+// Closure capture with pointer receiver
+func bad3() func() Either[error, int] {
+    e := Right[error](42)
+    return func() Either[error, int] {
+        return e  // May escape depending on usage
+    }
+}
+```
+
+In normal functional composition (Map, Chain, Fold), neither package causes heap allocations for simple operations.
+
+## Performance Comparison
+
+> **Latest benchmarks:** 2025-11-18 after `either` package optimizations
+>
+> For detailed analysis, see [BENCHMARK_COMPARISON.md](./BENCHMARK_COMPARISON.md)
+
+### Quick Summary (Either vs Idiomatic)
+
+Both packages now show **excellent performance** after optimizations:
+
+| Category | Either | Idiomatic | Winner | Speedup |
+|----------|--------|-----------|--------|---------|
+| **Constructors** | 1.4-1.8 ns/op | 1.2-1.4 ns/op | **TIE** | ~1.0-1.3x |
+| **Predicates** | 1.5 ns/op | 1.3-1.5 ns/op | **TIE** | ~1.0x |
+| **Map Operations** | 4.2-7.2 ns/op | 2.5-4.3 ns/op | **Idiomatic** | 1.2-2.1x |
+| **Chain Operations** | 4.4-5.4 ns/op | 2.3-2.5 ns/op | **Idiomatic** | 1.8-2.3x |
+| **ChainFirst** | **87.6 ns/op** (72 B) | **2.7 ns/op** (0 B) | **Idiomatic** | **32.4x** ✓✓✓ |
+| **BiMap** | 11.5-16.8 ns/op | 3.5-3.8 ns/op | **Idiomatic** | 3.3-4.4x |
+| **Alt/OrElse** | 4.0-5.7 ns/op | 2.4 ns/op | **Idiomatic** | 1.6-2.4x |
+| **GetOrElse** | 6.3-9.0 ns/op | 1.5-2.1 ns/op | **Idiomatic** | 3.1-6.1x |
+| **Pipelines** | 75-280 ns/op | 26-116 ns/op | **Idiomatic** | 2.4-3.4x |
+
+### Constructor Operations
+
+| Operation | Either (ns/op) | Idiomatic (ns/op) | Speedup | Winner |
+|-----------|----------------|-------------------|---------|--------|
+| Left      | 1.76           | **1.35**          | 1.3x    | Idiomatic ✓ |
+| Right     | 1.38           | 1.43              | ~1.0x   | Tie |
+| Of        | 1.68           | **1.22**          | 1.4x    | Idiomatic ✓ |
+
+**Analysis:** After optimizations, both packages have comparable constructor performance.
+
+### Core Transformation Operations
+
+| Operation        | Either (ns/op) | Idiomatic (ns/op) | Speedup | Winner |
+|------------------|----------------|-------------------|---------|--------|
+| Map (Right)      | 5.13           | **4.34**          | 1.2x    | Idiomatic ✓ |
+| Map (Left)       | 4.19           | **2.48**          | 1.7x    | Idiomatic ✓ |
+| MapLeft (Right)  | 3.93           | **2.22**          | 1.8x    | Idiomatic ✓ |
+| MapLeft (Left)   | 7.22           | **3.51**          | 2.1x    | Idiomatic ✓ |
+| Chain (Right)    | 5.44           | **2.34**          | 2.3x    | Idiomatic ✓ |
+| Chain (Left)     | 4.44           | **2.53**          | 1.8x    | Idiomatic ✓ |
+
+### Complex Operations - The Big Difference
+
+| Operation             | Either (ns/op) | Idiomatic (ns/op) | Speedup | Either Allocs | Idio Allocs |
+|-----------------------|----------------|-------------------|---------|---------------|-------------|
+| **ChainFirst (Right)** | **87.62**     | **2.71**          | **32.4x** ✓✓✓ | 72 B, 3 allocs | **0 B, 0 allocs** |
+| ChainFirst (Left)     | 3.94           | 2.48              | 1.6x    | 0 B | 0 B |
+| BiMap (Right)         | 16.79          | **3.82**          | 4.4x    | 0 B | 0 B |
+| BiMap (Left)          | 11.47          | **3.47**          | 3.3x    | 0 B | 0 B |
+
+**Critical Insight:** ChainFirst shows the most dramatic difference - **32x faster** with **zero allocations** in idiomatic.
+
+### Pipeline Benchmarks (Real-World Scenarios)
+
+| Operation | Either (ns/op) | Idiomatic (ns/op) | Speedup | Either Allocs | Idio Allocs |
+|-----------|----------------|-------------------|---------|---------------|-------------|
+| Pipeline Map (Right)    | 112.7  | **46.5**  | **2.4x** ✓ | 72 B, 3 allocs | 48 B, 2 allocs |
+| Pipeline Chain (Right)  | 74.4   | **26.1**  | **2.9x** ✓ | 48 B, 2 allocs | 24 B, 1 alloc |
+| Pipeline Complex (Right)| 279.8  | **116.3** | **2.4x** ✓ | 192 B, 8 allocs | 120 B, 5 allocs |
+
+**Analysis:** In realistic composition scenarios, idiomatic is consistently 2-3x faster with fewer allocations.
+
+### Extraction Operations
+
+| Operation | Either (ns/op) | Idiomatic (ns/op) | Speedup | Winner |
+|-----------|----------------|-------------------|---------|--------|
+| GetOrElse (Right) | 9.01   | **1.49**  | **6.1x** ✓✓ | Idiomatic |
+| GetOrElse (Left)  | 6.35   | **2.08**  | **3.1x** ✓✓ | Idiomatic |
+| Alt (Right)       | 5.72   | **2.40**  | **2.4x** ✓  | Idiomatic |
+| Alt (Left)        | 4.89   | **2.39**  | **2.0x** ✓  | Idiomatic |
+| Fold (Right)      | 4.03   | **2.75**  | **1.5x** ✓  | Idiomatic |
+| Fold (Left)       | 3.69   | **2.40**  | **1.5x** ✓  | Idiomatic |
+
+**Analysis:** Idiomatic shows significant advantages (1.5-6x) for value extraction operations.
+
+### Key Findings After Optimizations
+
+1. **Both packages are now fast** - Simple operations are in the 1-5 ns/op range for both
+2. **Idiomatic leads in most operations** - 1.2-2.3x faster for common transformations
+3. **ChainFirst is the standout** - 32x faster with zero allocations in idiomatic
+4. **Pipelines favor idiomatic** - 2-3.4x faster in realistic composition scenarios
+5. **Memory efficiency** - Idiomatic consistently uses fewer allocations
+
+### Performance Summary
+
+**Idiomatic Advantages:**
+- **Core operations**: 1.2-2.3x faster for Map, Chain, Fold
+- **Complex operations**: 3-32x faster with zero allocations
+- **Pipelines**: 2-3.4x faster with significantly fewer allocations
+- **Extraction**: 1.5-6x faster for GetOrElse, Alt, Fold
+- **Consistency**: Predictable, fast performance across all operations
+
+**Either Advantages:**
+- **Comparable performance**: After optimizations, matches idiomatic for simple operations
+- **Feature richness**: More operations (Do-notation, Bind, Let, Flatten, Swap)
+- **Type flexibility**: Full Either[E, A] with custom error types
+- **Zero allocations**: Most simple operations have zero allocations
+
+## API Comparison
+
+### Creating Values
+
+#### Standard Result
+
+```go
+import "github.com/IBM/fp-go/v2/result"
+
+// Create success/failure
+success := result.Right[error](42)
+failure := result.Left[int](errors.New("oops"))
+
+// Type annotation required
+var r result.Result[int] = result.Right[error](42)
+```
+
+#### Idiomatic Result
+
+```go
+import "github.com/IBM/fp-go/v2/idiomatic/result"
+
+// Create success/failure (more concise)
+success := result.Right(42)           // (42, nil)
+failure := result.Left[int](errors.New("oops"))  // (0, error)
+
+// Native Go pattern
+value, err := result.Right(42)
+if err != nil {
+    // handle error
+}
+```
+
+### Transforming Values
+
+#### Standard Result
+
+```go
+// Map transforms the success value
+double := result.Map(N.Mul(2))
+result := double(result.Right[error](21))  // Right(42)
+
+// Chain sequences operations
+validate := result.Chain(func(x int) result.Result[int] {
+    if x > 0 {
+        return result.Right[error](x * 2)
+    }
+    return result.Left[int](errors.New("negative"))
+})
+```
+
+#### Idiomatic Result
+
+```go
+// Map transforms the success value
+double := result.Map(N.Mul(2))
+value, err := double(21, nil)  // (42, nil)
+
+// Chain sequences operations
+validate := result.Chain(func(x int) (int, error) {
+    if x > 0 {
+        return x * 2, nil
+    }
+    return 0, errors.New("negative")
+})
+```
+
+### Pattern Matching
+
+#### Standard Result
+
+```go
+// Fold extracts the value
+output := result.Fold(
+    func(err error) string { return "Error: " + err.Error() },
+    func(n int) string { return fmt.Sprintf("Value: %d", n) },
+)(myResult)
+
+// GetOrElse with default
+value := result.GetOrElse(func(err error) int { return 0 })(myResult)
+```
+
+#### Idiomatic Result
+
+```go
+// Fold extracts the value (same API, different input)
+output := result.Fold(
+    func(err error) string { return "Error: " + err.Error() },
+    func(n int) string { return fmt.Sprintf("Value: %d", n) },
+)(value, err)
+
+// GetOrElse with default
+value := result.GetOrElse(func(err error) int { return 0 })(value, err)
+
+// Or use native Go pattern
+if err != nil {
+    value = 0
+}
+```
+
+### Integration with Existing Code
+
+#### Standard Result
+
+```go
+// Converting from (value, error) to Result
+func doSomething() (int, error) {
+    return 42, nil
+}
+
+result := result.TryCatchError(doSomething())
+
+// Converting back to (value, error)
+value, err := result.UnwrapError(result)
+```
+
+#### Idiomatic Result
+
+```go
+// Direct compatibility with (value, error)
+func doSomething() (int, error) {
+    return 42, nil
+}
+
+// No conversion needed!
+value, err := doSomething()
+value, err = result.Map(double)(value, err)
+```
+
+### Pipeline Composition
+
+#### Standard Result
+
+```go
+import F "github.com/IBM/fp-go/v2/function"
+
+output := F.Pipe3(
+    result.Right[error](10),
+    result.Map(double),
+    result.Chain(validate),
+    result.Map(format),
+)
+
+// Need to unwrap at the end
+value, err := result.UnwrapError(output)
+```
+
+#### Idiomatic Result
+
+```go
+import F "github.com/IBM/fp-go/v2/function"
+
+value, err := F.Pipe3(
+    result.Right(10),
+    result.Map(double),
+    result.Chain(validate),
+    result.Map(format),
+)
+
+// Already in (value, error) form
+if err != nil {
+    // handle error
+}
+```
+
+## Detailed Design Comparison
+
+### Type System
+
+#### Standard Result
+
+**Strengths:**
+- Full algebraic data type semantics
+- Explicit Either[E, A] allows custom error types
+- Type-safe by construction
+- Clear separation of error and success channels
+
+**Weaknesses:**
+- Requires wrapper structs (memory overhead)
+- Less familiar to Go developers
+- Needs conversion functions for Go's standard library
+- More verbose type annotations
+
+#### Idiomatic Result
+
+**Strengths:**
+- Native Go idioms (value, error) pattern
+- Zero wrapper overhead
+- Seamless stdlib integration
+- Familiar to all Go developers
+- Terser syntax
+
+**Weaknesses:**
+- Error type fixed to `error`
+- Less explicit about Either semantics
+- Cannot use custom error types without conversion
+- Slightly less type-safe (can accidentally ignore bool/error)
+
+### Monad Laws
+
+Both packages satisfy the monad laws, but enforce them differently:
+
+#### Standard Result
+
+```go
+// Left identity: return a >>= f  ≡  f a
+assert.Equal(
+    result.Chain(f)(result.Of(a)),
+    f(a),
+)
+
+// Right identity: m >>= return  ≡  m
+assert.Equal(
+    result.Chain(result.Of[int])(m),
+    m,
+)
+
+// Associativity: (m >>= f) >>= g  ≡  m >>= (\x -> f x >>= g)
+assert.Equal(
+    result.Chain(g)(result.Chain(f)(m)),
+    result.Chain(func(x int) result.Result[int] {
+        return result.Chain(g)(f(x))
+    })(m),
+)
+```
+
+#### Idiomatic Result
+
+```go
+// Same laws, different syntax
+// Left identity
+a, aerr := result.Of(val)
+b, berr := result.Chain(f)(a, aerr)
+c, cerr := f(val)
+assert.Equal((b, berr), (c, cerr))
+
+// Right identity
+value, err := m()
+identity := result.Chain(result.Of[int])
+assert.Equal(identity(value, err), (value, err))
+
+// Associativity (same structure, tuple-based)
+```
+
+### Error Handling Philosophy
+
+#### Standard Result
+
+```go
+// Explicit error handling through types
+func processUser(id int) result.Result[User] {
+    user := fetchUser(id)  // Returns Result[User]
+
+    return F.Pipe2(
+        user,
+        result.Chain(validateUser),
+        result.Chain(enrichUser),
+    )
+}
+
+// Must explicitly unwrap
+user, err := result.UnwrapError(processUser(42))
+if err != nil {
+    log.Error(err)
+}
+```
+
+#### Idiomatic Result
+
+```go
+// Natural Go error handling
+func processUser(id int) (User, error) {
+    user, err := fetchUser(id)  // Returns (User, error)
+
+    return F.Pipe2(
+        (user, err),
+        result.Chain(validateUser),
+        result.Chain(enrichUser),
+    )
+}
+
+// Already in Go form
+user, err := processUser(42)
+if err != nil {
+    log.Error(err)
+}
+```
+
+### Composition Patterns
+
+#### Standard Result
+
+```go
+// Applicative composition
+import A "github.com/IBM/fp-go/v2/apply"
+
+type Config struct {
+    Host string
+    Port int
+    DB   string
+}
+
+config := A.SequenceT3(
+    result.FromPredicate(validHost, hostError)(host),
+    result.FromPredicate(validPort, portError)(port),
+    result.FromPredicate(validDB, dbError)(db),
+)(func(h string, p int, d string) Config {
+    return Config{h, p, d}
+})
+```
+
+#### Idiomatic Result
+
+```go
+// Direct tuple composition
+config, err := func() (Config, error) {
+    host, err := result.FromPredicate(validHost, hostError)(host)
+    if err != nil {
+        return Config{}, err
+    }
+
+    port, err := result.FromPredicate(validPort, portError)(port)
+    if err != nil {
+        return Config{}, err
+    }
+
+    db, err := result.FromPredicate(validDB, dbError)(db)
+    if err != nil {
+        return Config{}, err
+    }
+
+    return Config{host, port, db}, nil
+}()
+```
+
+## When to Use Each
+
+### Use Idiomatic Result When (Recommended for Most Cases):
+
+1. **Performance Matters** ⭐
+   - Any production service (web servers, APIs, microservices)
+   - Hot paths and high-throughput scenarios (>1000 req/s)
+   - Complex operation chains (**32x faster** ChainFirst)
+   - Real-world pipelines (**2-3x faster**)
+   - Memory-constrained environments (zero allocations)
+   - Want **1.2-6x speedup** across most operations
+
+2. **Go Integration** ⭐⭐
+   - Working with existing Go codebases
+   - Interfacing with standard library (native (value, error))
+   - Team familiar with Go, new to FP
+   - Want zero-cost functional abstractions
+   - Seamless error handling patterns
+
+3. **Pragmatic Functional Programming**
+   - Value performance AND functional patterns
+   - Prefer Go idioms over FP terminology
+   - Simpler function signatures
+   - Lower cognitive overhead
+   - Production-ready patterns
+
+4. **Real-World Applications**
+   - Web servers, REST APIs, gRPC services
+   - CLI tools and command-line applications
+   - Data processing pipelines
+   - Any latency-sensitive application
+   - Systems with tight performance budgets
+
+**Performance Gains:** Use idiomatic for 1.2-32x speedup depending on operation, with consistently lower allocations.
+
+### Use Standard Either/Result When:
+
+1. **Type Safety & Flexibility**
+   - Need explicit Either[E, A] with **custom error types**
+   - Building domain-specific error hierarchies
+   - Want to distinguish different error categories at type level
+   - Type system enforcement is critical
+
+2. **Advanced FP Features**
+   - Using Do-notation for complex monadic compositions
+   - Need operations like Flatten, Swap, Bind, Let
+   - Leveraging advanced type classes (Semigroup, Monoid)
+   - Want the complete FP toolkit
+
+3. **FP Expertise & Education**
+   - Porting code from other FP languages (Scala, Haskell)
+   - Teaching functional programming concepts
+   - Team has strong FP background
+   - Explicit algebraic data types preferred
+   - Code review benefits from FP terminology
+
+4. **Performance is Acceptable**
+   - After optimizations, Either is **quite fast** (1-5 ns/op for simple operations)
+   - Difference matters mainly at high scale (millions of operations)
+   - Code clarity > micro-optimizations
+   - Simple operations dominate your workload
+
+**Note:** Either package is now performant enough for most use cases. Choose it for features, not performance concerns.
+
+### Hybrid Approach
+
+You can use both packages together:
+
+```go
+import (
+    stdResult "github.com/IBM/fp-go/v2/result"
+    "github.com/IBM/fp-go/v2/idiomatic/result"
+)
+
+// Use standard for complex types
+type ValidationError struct {
+    Field string
+    Error string
+}
+
+func validateInput(input string) stdResult.Either[ValidationError, Input] {
+    // ... validation logic
+}
+
+// Convert to idiomatic for performance
+func processInput(input string) (Output, error) {
+    validated := validateInput(input)
+    value, err := stdResult.UnwrapError(
+        stdResult.MapLeft(toError)(validated),
+    )
+
+    // Use idiomatic for hot path
+    return result.Chain(heavyProcessing)(value, err)
+}
+```
+
+## Migration Guide
+
+### From Standard to Idiomatic
+
+```go
+// Before (standard)
+import "github.com/IBM/fp-go/v2/result"
+
+func process(x int) result.Result[int] {
+    return F.Pipe2(
+        result.Right[error](x),
+        result.Map(double),
+        result.Chain(validate),
+    )
+}
+
+// After (idiomatic)
+import "github.com/IBM/fp-go/v2/idiomatic/result"
+
+func process(x int) (int, error) {
+    return F.Pipe2(
+        result.Right(x),
+        result.Map(double),
+        result.Chain(validate),
+    )
+}
+```
+
+### Key Changes
+
+1. **Type signatures**: `Result[T]` → `(T, error)`
+2. **Kleisli**: `func(A) Result[B]` → `func(A) (B, error)`
+3. **Operator**: `func(Result[A]) Result[B]` → `func(A, error) (B, error)`
+4. **Return values**: Function calls return tuples, not wrapped values
+5. **Pattern matching**: Same Fold/GetOrElse API, different inputs
+
+## Conclusion
+
+### Performance Summary (After Either Optimizations)
+
+The latest benchmark results show a clear pattern:
+
+**Both packages are now fast**, but idiomatic consistently leads:
+
+- **Constructors & Predicates**: Both ~1-2 ns/op (essentially tied)
+- **Core transformations**: Idiomatic **1.2-2.3x faster** (Map, Chain, Fold)
+- **Complex operations**: Idiomatic **3-32x faster** (BiMap, ChainFirst)
+- **Pipelines**: Idiomatic **2-3.4x faster** with fewer allocations
+- **Extraction**: Idiomatic **1.5-6x faster** (GetOrElse, Alt)
+
+**Key Insight:** The idiomatic package delivers **consistently better performance** across the board while maintaining zero-cost abstractions. The Either package is now fast enough for most use cases, but idiomatic is the performance winner.
+
+### Updated Recommendation Matrix
+
+| Scenario | Recommendation | Reason |
+|----------|---------------|--------|
+| **New Go project** | **Idiomatic** ⭐ | Natural Go patterns, 1.2-6x faster, better integration |
+| **Production services** | **Idiomatic** ⭐⭐ | 2-3x faster pipelines, zero allocations, proven performance |
+| **Performance critical** | **Idiomatic** ⭐⭐⭐ | 32x faster complex ops, minimal allocations |
+| **Microservices/APIs** | **Idiomatic** ⭐⭐ | High throughput, familiar patterns, better performance |
+| **CLI Tools** | **Idiomatic** ⭐ | Low overhead, Go idioms, fast |
+| Custom error types | Standard/Either | Need Either[E, A] with domain types |
+| Learning FP | Standard/Either | Clearer ADT semantics, educational |
+| FP-heavy codebase | Standard/Either | Consistency, Do-notation, full FP toolkit |
+| Library/Framework | Either way | Both are good; choose based on API style |
+
+### Real-World Impact
+
+For a service handling 10,000 requests/second with typical pipeline operations:
+
+```
+Either package:     280 ns/op × 10M req/day = 2,800 seconds = 46.7 minutes
+Idiomatic package:  116 ns/op × 10M req/day = 1,160 seconds = 19.3 minutes
+Time saved: 27.4 minutes of CPU time per day
+```
+
+At scale, this translates to:
+- Lower latency (2-3x faster response times for FP operations)
+- Reduced CPU usage (fewer cores needed)
+- Lower memory pressure (significantly fewer allocations)
+- Better resource utilization
+
+### Final Recommendation
+
+**For most Go projects:** Use **idiomatic packages**
+- 1.2-32x faster across operations
+- Native Go idioms
+- Zero-cost abstractions
+- Production-proven performance
+- Easier integration
+
+**For specialized needs:** Use **standard Either/Result**
+- Need custom error types Either[E, A]
+- Want Do-notation and advanced FP features
+- Porting from FP languages
+- Educational/learning context
+- FP-heavy existing codebase
+
+### Bottom Line
+
+After optimizations, both packages are excellent:
+
+- **Either/Result**: Fast enough for most use cases, feature-rich, type-safe
+- **Idiomatic**: **Faster in practice** (1.2-32x), native Go, zero-cost FP
+
+The idiomatic packages now represent the **best of both worlds**: full functional programming capabilities with Go's native performance and idioms. Unless you specifically need Either[E, A]'s custom error types or advanced FP features, **idiomatic is the recommended choice** for production Go services.
+
+Both maintain the core benefits of functional programming—choose based on whether you prioritize performance & Go integration (idiomatic) or type flexibility & FP features (either).

v2/IDIOMATIC_READERIORESULT_TODO.md

@@ -0,0 +1,174 @@
+# Idiomatic ReadIOResult Functions - Implementation Plan
+
+## Overview
+
+This document outlines the idiomatic functions that should be added to the `readerioresult` package to support Go's native `(value, error)` pattern, similar to what was implemented for `readerresult`.
+
+## Key Concepts
+
+The idiomatic package `github.com/IBM/fp-go/v2/idiomatic/readerioresult` defines:
+- `ReaderIOResult[R, A]` as `func(R) func() (A, error)` (idiomatic style)
+- This contrasts with `readerioresult.ReaderIOResult[R, A]` which is `Reader[R, IOResult[A]]` (functional style)
+
+## Functions to Add
+
+### In `readerioresult/reader.go`
+
+Add helper functions at the top:
+```go
+func fromReaderIOResultKleisliI[R, A, B any](f RIORI.Kleisli[R, A, B]) Kleisli[R, A, B] {
+	return function.Flow2(f, FromReaderIOResultI[R, B])
+}
+
+func fromIOResultKleisliI[A, B any](f IORI.Kleisli[A, B]) ioresult.Kleisli[A, B] {
+	return ioresult.Eitherize1(f)
+}
+```
+
+### Core Conversion Functions
+
+1. **FromResultI** - Lift `(value, error)` to ReaderIOResult
+   ```go
+   func FromResultI[R, A any](a A, err error) ReaderIOResult[R, A]
+   ```
+
+2. **FromIOResultI** - Lift idiomatic IOResult to functional
+   ```go
+   func FromIOResultI[R, A any](ioe func() (A, error)) ReaderIOResult[R, A]
+   ```
+
+3. **FromReaderIOResultI** - Convert idiomatic ReaderIOResult to functional
+   ```go
+   func FromReaderIOResultI[R, A any](rr RIORI.ReaderIOResult[R, A]) ReaderIOResult[R, A]
+   ```
+
+### Chain Functions
+
+4. **MonadChainI** / **ChainI** - Chain with idiomatic Kleisli
+   ```go
+   func MonadChainI[R, A, B any](ma ReaderIOResult[R, A], f RIORI.Kleisli[R, A, B]) ReaderIOResult[R, B]
+   func ChainI[R, A, B any](f RIORI.Kleisli[R, A, B]) Operator[R, A, B]
+   ```
+
+5. **MonadChainEitherIK** / **ChainEitherIK** - Chain with idiomatic Result functions
+   ```go
+   func MonadChainEitherIK[R, A, B any](ma ReaderIOResult[R, A], f func(A) (B, error)) ReaderIOResult[R, B]
+   func ChainEitherIK[R, A, B any](f func(A) (B, error)) Operator[R, A, B]
+   ```
+
+6. **MonadChainIOResultIK** / **ChainIOResultIK** - Chain with idiomatic IOResult
+   ```go
+   func MonadChainIOResultIK[R, A, B any](ma ReaderIOResult[R, A], f func(A) func() (B, error)) ReaderIOResult[R, B]
+   func ChainIOResultIK[R, A, B any](f func(A) func() (B, error)) Operator[R, A, B]
+   ```
+
+### Applicative Functions
+
+7. **MonadApI** / **ApI** - Apply with idiomatic value
+   ```go
+   func MonadApI[B, R, A any](fab ReaderIOResult[R, func(A) B], fa RIORI.ReaderIOResult[R, A]) ReaderIOResult[R, B]
+   func ApI[B, R, A any](fa RIORI.ReaderIOResult[R, A]) Operator[R, func(A) B, B]
+   ```
+
+### Error Handling Functions
+
+8. **OrElseI** - Fallback with idiomatic computation
+   ```go
+   func OrElseI[R, A any](onLeft RIORI.Kleisli[R, error, A]) Operator[R, A, A]
+   ```
+
+9. **MonadAltI** / **AltI** - Alternative with idiomatic computation
+   ```go
+   func MonadAltI[R, A any](first ReaderIOResult[R, A], second Lazy[RIORI.ReaderIOResult[R, A]]) ReaderIOResult[R, A]
+   func AltI[R, A any](second Lazy[RIORI.ReaderIOResult[R, A]]) Operator[R, A, A]
+   ```
+
+### Flatten Functions
+
+10. **FlattenI** - Flatten nested idiomatic ReaderIOResult
+    ```go
+    func FlattenI[R, A any](mma ReaderIOResult[R, RIORI.ReaderIOResult[R, A]]) ReaderIOResult[R, A]
+    ```
+
+### In `readerioresult/bind.go`
+
+11. **BindI** - Bind with idiomatic Kleisli
+    ```go
+    func BindI[R, S1, S2, T any](setter func(T) func(S1) S2, f RIORI.Kleisli[R, S1, T]) Operator[R, S1, S2]
+    ```
+
+12. **ApIS** - Apply idiomatic value to state
+    ```go
+    func ApIS[R, S1, S2, T any](setter func(T) func(S1) S2, fa RIORI.ReaderIOResult[R, T]) Operator[R, S1, S2]
+    ```
+
+13. **ApISL** - Apply idiomatic value using lens
+    ```go
+    func ApISL[R, S, T any](lens L.Lens[S, T], fa RIORI.ReaderIOResult[R, T]) Operator[R, S, S]
+    ```
+
+14. **BindIL** - Bind idiomatic with lens
+    ```go
+    func BindIL[R, S, T any](lens L.Lens[S, T], f RIORI.Kleisli[R, T, T]) Operator[R, S, S]
+    ```
+
+15. **BindEitherIK** / **BindResultIK** - Bind idiomatic Result
+    ```go
+    func BindEitherIK[R, S1, S2, T any](setter func(T) func(S1) S2, f func(S1) (T, error)) Operator[R, S1, S2]
+    func BindResultIK[R, S1, S2, T any](setter func(T) func(S1) S2, f func(S1) (T, error)) Operator[R, S1, S2]
+    ```
+
+16. **BindIOResultIK** - Bind idiomatic IOResult
+    ```go
+    func BindIOResultIK[R, S1, S2, T any](setter func(T) func(S1) S2, f func(S1) func() (T, error)) Operator[R, S1, S2]
+    ```
+
+17. **BindToEitherI** / **BindToResultI** - Initialize from idiomatic pair
+    ```go
+    func BindToEitherI[R, S1, T any](setter func(T) S1) func(T, error) ReaderIOResult[R, S1]
+    func BindToResultI[R, S1, T any](setter func(T) S1) func(T, error) ReaderIOResult[R, S1]
+    ```
+
+18. **BindToIOResultI** - Initialize from idiomatic IOResult
+    ```go
+    func BindToIOResultI[R, S1, T any](setter func(T) S1) func(func() (T, error)) ReaderIOResult[R, S1]
+    ```
+
+19. **ApEitherIS** / **ApResultIS** - Apply idiomatic pair to state
+    ```go
+    func ApEitherIS[R, S1, S2, T any](setter func(T) func(S1) S2) func(T, error) Operator[R, S1, S2]
+    func ApResultIS[R, S1, S2, T any](setter func(T) func(S1) S2) func(T, error) Operator[R, S1, S2]
+    ```
+
+20. **ApIOResultIS** - Apply idiomatic IOResult to state
+    ```go
+    func ApIOResultIS[R, S1, S2, T any](setter func(T) func(S1) S2, fa func() (T, error)) Operator[R, S1, S2]
+    ```
+
+## Testing Strategy
+
+Create `readerioresult/idiomatic_test.go` with:
+- Tests for each idiomatic function
+- Success and error cases
+- Integration tests showing real-world usage patterns
+- Parallel execution tests where applicable
+- Complex scenarios combining multiple idiomatic functions
+
+## Implementation Priority
+
+1. **High Priority** - Core conversion and chain functions (1-6)
+2. **Medium Priority** - Bind functions for do-notation (11-16)
+3. **Low Priority** - Advanced applicative and error handling (7-10, 17-20)
+
+## Benefits
+
+1. **Seamless Integration** - Mix Go idiomatic code with functional pipelines
+2. **Gradual Adoption** - Convert code incrementally from idiomatic to functional
+3. **Interoperability** - Work with existing Go libraries that return `(value, error)`
+4. **Consistency** - Mirrors the successful pattern from `readerresult`
+
+## References
+
+- See `readerresult` package for similar implementations
+- See `idiomatic/readerresult` for the idiomatic types
+- See `idiomatic/ioresult` for IO-level idiomatic patterns

v2/README.md

@@ -1,43 +1,510 @@
-# Functional programming library for golang V2
+# fp-go V2: Enhanced Functional Programming for Go 1.24+
 
-Go 1.24 introduces [generic type aliases](https://github.com/golang/go/issues/46477) which are leveraged by V2.
+[![Go Reference](https://pkg.go.dev/badge/github.com/IBM/fp-go/v2.svg)](https://pkg.go.dev/github.com/IBM/fp-go/v2)
+[![Coverage Status](https://coveralls.io/repos/github/IBM/fp-go/badge.svg?branch=main&flag=v2)](https://coveralls.io/github/IBM/fp-go?branch=main)
+[![Go Report Card](https://goreportcard.com/badge/github.com/IBM/fp-go/v2)](https://goreportcard.com/report/github.com/IBM/fp-go/v2)
 
-## ⚠️ Breaking Changes
+**fp-go** is a comprehensive functional programming library for Go, bringing type-safe functional patterns inspired by [fp-ts](https://gcanti.github.io/fp-ts/) to the Go ecosystem. Version 2 leverages [generic type aliases](https://github.com/golang/go/issues/46477) introduced in Go 1.24, providing a more ergonomic and streamlined API.
 
-- use of [generic type aliases](https://github.com/golang/go/issues/46477) which requires [go1.24](https://tip.golang.org/doc/go1.24)
-- order of generic type arguments adjusted such that types that _cannot_ be inferred by the method argument come first, e.g. in the `Ap` methods
-- monadic operations for `Pair` operate on the second argument, to be compatible with the [Haskell](https://hackage.haskell.org/package/TypeCompose-0.9.14/docs/Data-Pair.html) definition
+## 📚 Table of Contents
 
-## Simplifications
+- [Overview](#-overview)
+- [Features](#-features)
+- [Requirements](#-requirements)
+- [Installation](#-installation)
+- [Quick Start](#-quick-start)
+- [Breaking Changes](#️-breaking-changes)
+- [Key Improvements](#-key-improvements)
+- [Migration Guide](#-migration-guide)
+- [What's New](#-whats-new)
+- [Documentation](#-documentation)
+- [Contributing](#-contributing)
+- [License](#-license)
 
-- use type aliases to get rid of namespace imports for type declarations, e.g. instead of
+## 🎯 Overview
+
+fp-go brings the power of functional programming to Go with:
+
+- **Type-safe abstractions** - Monads, Functors, Applicatives, and more
+- **Composable operations** - Build complex logic from simple, reusable functions
+- **Error handling** - Elegant error management with `Either`, `Result`, and `IOEither`
+- **Lazy evaluation** - Control when and how computations execute
+- **Optics** - Powerful lens, prism, and traversal operations for immutable data manipulation
+
+## ✨ Features
+
+- 🔒 **Type Safety** - Leverage Go's generics for compile-time guarantees
+- 🧩 **Composability** - Chain operations naturally with functional composition
+- 📦 **Rich Type System** - `Option`, `Either`, `Result`, `IO`, `Reader`, and more
+- 🎯 **Practical** - Designed for real-world Go applications
+- 🚀 **Performance** - Zero-cost abstractions where possible
+- 📖 **Well-documented** - Comprehensive API documentation and examples
+- 🧪 **Battle-tested** - Extensive test coverage
+
+## 🔧 Requirements
+
+- **Go 1.24 or later** (for generic type alias support)
+
+## 📦 Installation
+
+```bash
+go get github.com/IBM/fp-go/v2
+```
+
+## 🚀 Quick Start
+
+### Working with Option
 
 ```go
+package main
+
 import (
-    ET "github.com/IBM/fp-go/v2/either"
+    "fmt"
+    "github.com/IBM/fp-go/v2/option"
+    N "github.com/IBM/fp-go/v2/number"
 )
 
-func doSth() ET.Either[error, string] {
-    ...
+func main() {
+    // Create an Option
+    some := option.Some(42)
+    none := option.None[int]()
+    
+    // Map over values
+    doubled := option.Map(N.Mul(2))(some)
+    fmt.Println(option.GetOrElse(0)(doubled)) // Output: 84
+    
+    // Chain operations
+    result := option.Chain(func(x int) option.Option[string] {
+        if x > 0 {
+            return option.Some(fmt.Sprintf("Positive: %d", x))
+        }
+        return option.None[string]()
+    })(some)
+    
+    fmt.Println(option.GetOrElse("No value")(result)) // Output: Positive: 42
 }
 ```
 
-you can declare your type once 
+### Error Handling with Result
 
+```go
+package main
+
+import (
+    "errors"
+    "fmt"
+    "github.com/IBM/fp-go/v2/result"
+)
+
+func divide(a, b int) result.Result[int] {
+    if b == 0 {
+        return result.Error[int](errors.New("division by zero"))
+    }
+    return result.Ok(a / b)
+}
+
+func main() {
+    res := divide(10, 2)
+    
+    // Pattern match on the result
+    result.Fold(
+        func(err error) { fmt.Println("Error:", err) },
+        func(val int) { fmt.Println("Result:", val) },
+    )(res)
+    // Output: Result: 5
+    
+    // Or use GetOrElse for a default value
+    value := result.GetOrElse(0)(divide(10, 0))
+    fmt.Println("Value:", value) // Output: Value: 0
+}
+```
+
+### Composing IO Operations
+
+```go
+package main
+
+import (
+    "fmt"
+    "github.com/IBM/fp-go/v2/io"
+)
+
+func main() {
+    // Define pure IO operations
+    readInput := io.MakeIO(func() string {
+        return "Hello, fp-go!"
+    })
+    
+    // Transform the result
+    uppercase := io.Map(func(s string) string {
+        return fmt.Sprintf(">>> %s <<<", s)
+    })(readInput)
+    
+    // Execute the IO operation
+    result := uppercase()
+    fmt.Println(result) // Output: >>> Hello, fp-go! <<<
+}
+```
+
+## ⚠️ Breaking Changes
+
+### From V1 to V2
+
+#### 1. Generic Type Aliases
+
+V2 uses [generic type aliases](https://github.com/golang/go/issues/46477) which require Go 1.24+. This is the most significant change and enables cleaner type definitions.
+
+**V1:**
+```go
+type ReaderIOEither[R, E, A any] RD.Reader[R, IOE.IOEither[E, A]]
+```
+
+**V2:**
+```go
+type ReaderIOEither[R, E, A any] = RD.Reader[R, IOE.IOEither[E, A]]
+```
+
+#### 2. Generic Type Parameter Ordering
+
+Type parameters that **cannot** be inferred from function arguments now come first, improving type inference.
+
+**V1:**
+```go
+// Ap in V1 - less intuitive ordering
+func Ap[R, E, A, B any](fa ReaderIOEither[R, E, A]) func(ReaderIOEither[R, E, func(A) B]) ReaderIOEither[R, E, B]
+```
+
+**V2:**
+```go
+// Ap in V2 - B comes first as it cannot be inferred
+func Ap[B, R, E, A any](fa ReaderIOEither[R, E, A]) func(ReaderIOEither[R, E, func(A) B]) ReaderIOEither[R, E, B]
+```
+
+This change allows the Go compiler to infer more types automatically, reducing the need for explicit type parameters.
+
+#### 3. Pair Monad Semantics
+
+Monadic operations for `Pair` now operate on the **second argument** to align with the [Haskell definition](https://hackage.haskell.org/package/TypeCompose-0.9.14/docs/Data-Pair.html).
+
+**V1:**
+```go
+// Operations on first element
+pair := MakePair(1, "hello")
+result := Map(N.Mul(2))(pair) // Pair(2, "hello")
+```
+
+**V2:**
+```go
+// Operations on second element (Haskell-compatible)
+pair := MakePair(1, "hello")
+result := Map(func(s string) string { return s + "!" })(pair) // Pair(1, "hello!")
+```
+
+#### 4. Endomorphism Compose Semantics
+
+The `Compose` function for endomorphisms now follows **mathematical function composition** (right-to-left execution), aligning with standard functional programming conventions.
+
+**V1:**
+```go
+// Compose executed left-to-right
+double := N.Mul(2)
+increment := N.Add(1)
+composed := Compose(double, increment)
+result := composed(5) // (5 * 2) + 1 = 11
+```
+
+**V2:**
+```go
+// Compose executes RIGHT-TO-LEFT (mathematical composition)
+double := N.Mul(2)
+increment := N.Add(1)
+composed := Compose(double, increment)
+result := composed(5) // (5 + 1) * 2 = 12
+
+// Use MonadChain for LEFT-TO-RIGHT execution
+chained := MonadChain(double, increment)
+result2 := chained(5) // (5 * 2) + 1 = 11
+```
+
+**Key Difference:**
+- `Compose(f, g)` now means `f ∘ g`, which applies `g` first, then `f` (right-to-left)
+- `MonadChain(f, g)` applies `f` first, then `g` (left-to-right)
+
+## ✨ Key Improvements
+
+### 1. Simplified Type Declarations
+
+Generic type aliases eliminate the need for namespace imports in type declarations.
+
+**V1 Approach:**
+```go
+import (
+    ET "github.com/IBM/fp-go/either"
+    OPT "github.com/IBM/fp-go/option"
+)
+
+func processData(input string) ET.Either[error, OPT.Option[int]] {
+    // implementation
+}
+```
+
+**V2 Approach:**
+```go
+import (
+    "github.com/IBM/fp-go/v2/result"
+    "github.com/IBM/fp-go/v2/option"
+)
+
+// Define type aliases once
+type Result[A any] = result.Result[A]
+type Option[A any] = option.Option[A]
+
+// Use them throughout your codebase
+func processData(input string) Result[Option[int]] {
+    // implementation
+}
+```
+
+### 2. No More `generic` Subpackages
+
+The library implementation no longer requires separate `generic` subpackages, making the codebase simpler and easier to understand.
+
+**V1 Structure:**
+```
+either/
+  either.go
+  generic/
+    either.go  // Generic implementation
+```
+
+**V2 Structure:**
+```
+either/
+  either.go  // Single, clean implementation
+```
+
+### 3. Better Type Inference
+
+The reordered type parameters allow the Go compiler to infer more types automatically:
+
+**V1:**
+```go
+// Often need explicit type parameters
+result := Map[Context, error, int, string](transform)(value)
+```
+
+**V2:**
+```go
+// Compiler can infer more types
+result := Map(transform)(value)  // Cleaner!
+```
+
+## 🚀 Migration Guide
+
+### Step 1: Update Go Version
+
+Ensure you're using Go 1.24 or later:
+
+```bash
+go version  # Should show go1.24 or higher
+```
+
+### Step 2: Update Import Paths
+
+Change all import paths from `github.com/IBM/fp-go` to `github.com/IBM/fp-go/v2`:
+
+**Before:**
+```go
+import (
+    "github.com/IBM/fp-go/either"
+    "github.com/IBM/fp-go/option"
+)
+```
+
+**After:**
+```go
+import (
+    "github.com/IBM/fp-go/v2/either"
+    "github.com/IBM/fp-go/v2/option"
+)
+```
+
+### Step 3: Remove `generic` Subpackage Imports
+
+If you were using generic subpackages, remove them:
+
+**Before:**
+```go
+import (
+    E "github.com/IBM/fp-go/either/generic"
+)
+```
+
+**After:**
 ```go
 import (
     "github.com/IBM/fp-go/v2/either"
 )
-
-type Either[A any] = either.Either[error, A]
 ```
 
-and then use it across your codebase
+### Step 4: Update Type Parameter Order
+
+Review functions like `Ap` where type parameter order has changed. The compiler will help identify these:
+
+**Before:**
+```go
+result := Ap[Context, error, int, string](value)(funcInContext)
+```
+
+**After:**
+```go
+result := Ap[string, Context, error, int](value)(funcInContext)
+// Or better yet, let the compiler infer:
+result := Ap(value)(funcInContext)
+```
+
+### Step 5: Update Pair Operations
+
+If you're using `Pair`, update operations to work on the second element:
+
+**Before (V1):**
+```go
+pair := MakePair(42, "data")
+// Map operates on first element
+result := Map(N.Mul(2))(pair)
+```
+
+**After (V2):**
+```go
+pair := MakePair(42, "data")
+// Map operates on second element
+result := Map(func(s string) string { return s + "!" })(pair)
+```
+
+### Step 6: Simplify Type Aliases
+
+Create project-wide type aliases for common patterns:
 
 ```go
-func doSth() Either[string] {
-    ...
+// types.go - Define once, use everywhere
+package myapp
+
+import (
+    "github.com/IBM/fp-go/v2/result"
+    "github.com/IBM/fp-go/v2/option"
+    "github.com/IBM/fp-go/v2/ioresult"
+)
+
+type Result[A any] = result.Result[A]
+type Option[A any] = option.Option[A]
+type IOResult[A any] = ioresult.IOResult[A]
+```
+
+## 🆕 What's New
+
+### Cleaner API Surface
+
+The elimination of `generic` subpackages means:
+- Fewer imports to manage
+- Simpler package structure
+- Easier to navigate documentation
+- More intuitive API
+
+### Example: Before and After
+
+**V1 Complex Example:**
+```go
+import (
+    ET "github.com/IBM/fp-go/either"
+    EG "github.com/IBM/fp-go/either/generic"
+    IOET "github.com/IBM/fp-go/ioeither"
+    IOEG "github.com/IBM/fp-go/ioeither/generic"
+)
+
+func process() IOET.IOEither[error, string] {
+    return IOEG.Map[error, int, string](
+        strconv.Itoa,
+    )(fetchData())
 }
 ```
 
-- library implementation does no longer need to use the `generic` subpackage, this simplifies reading and understanding of the code
+**V2 Simplified Example:**
+```go
+import (
+    "strconv"
+    "github.com/IBM/fp-go/v2/ioresult"
+)
+
+type IOResult[A any] = ioresult.IOResult[A]
+
+func process() IOResult[string] {
+    return ioresult.Map(
+        strconv.Itoa,
+    )(fetchData())
+}
+```
+
+## 📚 Documentation
+
+- **[API Documentation](https://pkg.go.dev/github.com/IBM/fp-go/v2)** - Complete API reference
+- **[Code Samples](./samples/)** - Practical examples and use cases
+- **[Go 1.24 Release Notes](https://tip.golang.org/doc/go1.24)** - Information about generic type aliases
+
+### Core Modules
+
+#### Standard Packages (Struct-based)
+- **Option** - Represent optional values without nil
+- **Either** - Type-safe error handling with left/right values
+- **Result** - Simplified Either with error as left type (recommended for error handling)
+- **IO** - Lazy evaluation and side effect management
+- **IOResult** - Combine IO with Result for error handling (recommended over IOEither)
+- **Reader** - Dependency injection pattern
+- **ReaderIOResult** - Combine Reader, IO, and Result for complex workflows
+- **Array** - Functional array operations
+- **Record** - Functional record/map operations
+- **Optics** - Lens, Prism, Optional, and Traversal for immutable updates
+
+#### Idiomatic Packages (Tuple-based, High Performance)
+- **idiomatic/option** - Option monad using native Go `(value, bool)` tuples
+- **idiomatic/result** - Result monad using native Go `(value, error)` tuples
+- **idiomatic/ioresult** - IOResult monad using `func() (value, error)` for IO operations
+- **idiomatic/readerresult** - Reader monad combined with Result pattern
+- **idiomatic/readerioresult** - Reader monad combined with IOResult pattern
+
+The idiomatic packages offer 2-10x performance improvements and zero allocations by using Go's native tuple patterns instead of struct wrappers. Use them for performance-critical code or when you prefer Go's native error handling style.
+
+## 🤔 Should I Migrate?
+
+**Migrate to V2 if:**
+- ✅ You can use Go 1.24+
+- ✅ You want cleaner, more maintainable code
+- ✅ You want better type inference
+- ✅ You're starting a new project
+
+**Stay on V1 if:**
+- ⚠️ You're locked to Go < 1.24
+- ⚠️ Migration effort outweighs benefits for your project
+- ⚠️ You need stability in production (V2 is newer)
+
+## 🤝 Contributing
+
+Contributions are welcome! Here's how you can help:
+
+1. **Report bugs** - Open an issue with a clear description and reproduction steps
+2. **Suggest features** - Share your ideas for improvements
+3. **Submit PRs** - Fix bugs or add features (please discuss major changes first)
+4. **Improve docs** - Help make the documentation clearer and more comprehensive
+
+Please read our contribution guidelines before submitting pull requests.
+
+## 🐛 Issues and Feedback
+
+Found a bug or have a suggestion? Please [open an issue](https://github.com/IBM/fp-go/issues) on GitHub.
+
+## 📄 License
+
+This project is licensed under the Apache License 2.0. See the [LICENSE](https://github.com/IBM/fp-go/blob/main/LICENSE) file for details.
+
+---
+
+**Made with ❤️ by IBM**

v2/array/any.go

@@ -29,6 +29,8 @@ import (
 //	    return i%2 == 0 && x%2 == 0
 //	})
 //	result := hasEvenAtEvenIndex([]int{1, 3, 4, 5}) // true (4 is at index 2)
+//
+//go:inline
 func AnyWithIndex[A any](pred func(int, A) bool) func([]A) bool {
 	return G.AnyWithIndex[[]A](pred)
 }
@@ -41,6 +43,8 @@ func AnyWithIndex[A any](pred func(int, A) bool) func([]A) bool {
 //
 //	hasEven := array.Any(func(x int) bool { return x%2 == 0 })
 //	result := hasEven([]int{1, 3, 4, 5}) // true
+//
+//go:inline
 func Any[A any](pred func(A) bool) func([]A) bool {
 	return G.Any[[]A](pred)
 }

v2/array/array.go

@@ -17,48 +17,57 @@ package array
 
 import (
 	G "github.com/IBM/fp-go/v2/array/generic"
-	EM "github.com/IBM/fp-go/v2/endomorphism"
 	F "github.com/IBM/fp-go/v2/function"
 	"github.com/IBM/fp-go/v2/internal/array"
 	M "github.com/IBM/fp-go/v2/monoid"
-	O "github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/option"
 	"github.com/IBM/fp-go/v2/tuple"
 )
 
 // From constructs an array from a set of variadic arguments
+//
+//go:inline
 func From[A any](data ...A) []A {
 	return G.From[[]A](data...)
 }
 
 // MakeBy returns a `Array` of length `n` with element `i` initialized with `f(i)`.
+//
+//go:inline
 func MakeBy[F ~func(int) A, A any](n int, f F) []A {
 	return G.MakeBy[[]A](n, f)
 }
 
 // Replicate creates a `Array` containing a value repeated the specified number of times.
+//
+//go:inline
 func Replicate[A any](n int, a A) []A {
 	return G.Replicate[[]A](n, a)
 }
 
 // MonadMap applies a function to each element of an array, returning a new array with the results.
 // This is the monadic version of Map that takes the array as the first parameter.
-func MonadMap[A, B any](as []A, f func(a A) B) []B {
+//
+//go:inline
+func MonadMap[A, B any](as []A, f func(A) B) []B {
 	return G.MonadMap[[]A, []B](as, f)
 }
 
 // MonadMapRef applies a function to a pointer to each element of an array, returning a new array with the results.
 // This is useful when you need to access elements by reference without copying.
-func MonadMapRef[A, B any](as []A, f func(a *A) B) []B {
+func MonadMapRef[A, B any](as []A, f func(*A) B) []B {
 	count := len(as)
 	bs := make([]B, count)
-	for i := count - 1; i >= 0; i-- {
+	for i := range count {
 		bs[i] = f(&as[i])
 	}
 	return bs
 }
 
 // MapWithIndex applies a function to each element and its index in an array, returning a new array with the results.
-func MapWithIndex[A, B any](f func(int, A) B) func([]A) []B {
+//
+//go:inline
+func MapWithIndex[A, B any](f func(int, A) B) Operator[A, B] {
 	return G.MapWithIndex[[]A, []B](f)
 }
 
@@ -67,86 +76,102 @@ func MapWithIndex[A, B any](f func(int, A) B) func([]A) []B {
 //
 // Example:
 //
-//	double := array.Map(func(x int) int { return x * 2 })
+//	double := array.Map(N.Mul(2))
 //	result := double([]int{1, 2, 3}) // [2, 4, 6]
-func Map[A, B any](f func(a A) B) func([]A) []B {
-	return G.Map[[]A, []B, A, B](f)
+//
+//go:inline
+func Map[A, B any](f func(A) B) Operator[A, B] {
+	return G.Map[[]A, []B](f)
 }
 
 // MapRef applies a function to a pointer to each element of an array, returning a new array with the results.
 // This is the curried version that returns a function.
-func MapRef[A, B any](f func(a *A) B) func([]A) []B {
+func MapRef[A, B any](f func(*A) B) Operator[A, B] {
 	return F.Bind2nd(MonadMapRef[A, B], f)
 }
 
-func filterRef[A any](fa []A, pred func(a *A) bool) []A {
-	var result []A
+func filterRef[A any](fa []A, pred func(*A) bool) []A {
 	count := len(fa)
-	for i := 0; i < count; i++ {
-		a := fa[i]
-		if pred(&a) {
-			result = append(result, a)
+	var result []A = make([]A, 0, count)
+	for i := range count {
+		a := &fa[i]
+		if pred(a) {
+			result = append(result, *a)
 		}
 	}
 	return result
 }
 
-func filterMapRef[A, B any](fa []A, pred func(a *A) bool, f func(a *A) B) []B {
-	var result []B
+func filterMapRef[A, B any](fa []A, pred func(*A) bool, f func(*A) B) []B {
 	count := len(fa)
-	for i := 0; i < count; i++ {
-		a := fa[i]
-		if pred(&a) {
-			result = append(result, f(&a))
+	var result []B = make([]B, 0, count)
+	for i := range count {
+		a := &fa[i]
+		if pred(a) {
+			result = append(result, f(a))
 		}
 	}
 	return result
 }
 
 // Filter returns a new array with all elements from the original array that match a predicate
-func Filter[A any](pred func(A) bool) EM.Endomorphism[[]A] {
+//
+//go:inline
+func Filter[A any](pred func(A) bool) Operator[A, A] {
 	return G.Filter[[]A](pred)
 }
 
 // FilterWithIndex returns a new array with all elements from the original array that match a predicate
-func FilterWithIndex[A any](pred func(int, A) bool) EM.Endomorphism[[]A] {
+//
+//go:inline
+func FilterWithIndex[A any](pred func(int, A) bool) Operator[A, A] {
 	return G.FilterWithIndex[[]A](pred)
 }
 
 // FilterRef returns a new array with all elements from the original array that match a predicate operating on pointers.
-func FilterRef[A any](pred func(*A) bool) EM.Endomorphism[[]A] {
+func FilterRef[A any](pred func(*A) bool) Operator[A, A] {
 	return F.Bind2nd(filterRef[A], pred)
 }
 
 // MonadFilterMap maps an array with a function that returns an Option and keeps only the Some values.
 // This is the monadic version that takes the array as the first parameter.
-func MonadFilterMap[A, B any](fa []A, f func(A) O.Option[B]) []B {
+//
+//go:inline
+func MonadFilterMap[A, B any](fa []A, f option.Kleisli[A, B]) []B {
 	return G.MonadFilterMap[[]A, []B](fa, f)
 }
 
 // MonadFilterMapWithIndex maps an array with a function that takes an index and returns an Option,
 // keeping only the Some values. This is the monadic version that takes the array as the first parameter.
-func MonadFilterMapWithIndex[A, B any](fa []A, f func(int, A) O.Option[B]) []B {
+//
+//go:inline
+func MonadFilterMapWithIndex[A, B any](fa []A, f func(int, A) Option[B]) []B {
 	return G.MonadFilterMapWithIndex[[]A, []B](fa, f)
 }
 
-// FilterMap maps an array with an iterating function that returns an [O.Option] and it keeps only the Some values discarding the Nones.
-func FilterMap[A, B any](f func(A) O.Option[B]) func([]A) []B {
+// FilterMap maps an array with an iterating function that returns an [Option] and it keeps only the Some values discarding the Nones.
+//
+//go:inline
+func FilterMap[A, B any](f option.Kleisli[A, B]) Operator[A, B] {
 	return G.FilterMap[[]A, []B](f)
 }
 
-// FilterMapWithIndex maps an array with an iterating function that returns an [O.Option] and it keeps only the Some values discarding the Nones.
-func FilterMapWithIndex[A, B any](f func(int, A) O.Option[B]) func([]A) []B {
+// FilterMapWithIndex maps an array with an iterating function that returns an [Option] and it keeps only the Some values discarding the Nones.
+//
+//go:inline
+func FilterMapWithIndex[A, B any](f func(int, A) Option[B]) Operator[A, B] {
 	return G.FilterMapWithIndex[[]A, []B](f)
 }
 
-// FilterChain maps an array with an iterating function that returns an [O.Option] of an array. It keeps only the Some values discarding the Nones and then flattens the result.
-func FilterChain[A, B any](f func(A) O.Option[[]B]) func([]A) []B {
+// FilterChain maps an array with an iterating function that returns an [Option] of an array. It keeps only the Some values discarding the Nones and then flattens the result.
+//
+//go:inline
+func FilterChain[A, B any](f option.Kleisli[A, []B]) Operator[A, B] {
 	return G.FilterChain[[]A](f)
 }
 
 // FilterMapRef filters an array using a predicate on pointers and maps the matching elements using a function on pointers.
-func FilterMapRef[A, B any](pred func(a *A) bool, f func(a *A) B) func([]A) []B {
+func FilterMapRef[A, B any](pred func(a *A) bool, f func(*A) B) Operator[A, B] {
 	return func(fa []A) []B {
 		return filterMapRef(fa, pred, f)
 	}
@@ -154,13 +179,13 @@ func FilterMapRef[A, B any](pred func(a *A) bool, f func(a *A) B) func([]A) []B
 
 func reduceRef[A, B any](fa []A, f func(B, *A) B, initial B) B {
 	current := initial
-	count := len(fa)
-	for i := 0; i < count; i++ {
+	for i := range len(fa) {
 		current = f(current, &fa[i])
 	}
 	return current
 }
 
+//go:inline
 func MonadReduce[A, B any](fa []A, f func(B, A) B, initial B) B {
 	return G.MonadReduce(fa, f, initial)
 }
@@ -171,23 +196,31 @@ func MonadReduce[A, B any](fa []A, f func(B, A) B, initial B) B {
 //
 //	sum := array.Reduce(func(acc, x int) int { return acc + x }, 0)
 //	result := sum([]int{1, 2, 3, 4, 5}) // 15
+//
+//go:inline
 func Reduce[A, B any](f func(B, A) B, initial B) func([]A) B {
 	return G.Reduce[[]A](f, initial)
 }
 
 // ReduceWithIndex folds an array from left to right with access to the index,
 // applying a function to accumulate a result.
+//
+//go:inline
 func ReduceWithIndex[A, B any](f func(int, B, A) B, initial B) func([]A) B {
 	return G.ReduceWithIndex[[]A](f, initial)
 }
 
 // ReduceRight folds an array from right to left, applying a function to accumulate a result.
+//
+//go:inline
 func ReduceRight[A, B any](f func(A, B) B, initial B) func([]A) B {
 	return G.ReduceRight[[]A](f, initial)
 }
 
 // ReduceRightWithIndex folds an array from right to left with access to the index,
 // applying a function to accumulate a result.
+//
+//go:inline
 func ReduceRightWithIndex[A, B any](f func(int, A, B) B, initial B) func([]A) B {
 	return G.ReduceRightWithIndex[[]A](f, initial)
 }
@@ -201,11 +234,15 @@ func ReduceRef[A, B any](f func(B, *A) B, initial B) func([]A) B {
 }
 
 // Append adds an element to the end of an array, returning a new array.
+//
+//go:inline
 func Append[A any](as []A, a A) []A {
 	return G.Append(as, a)
 }
 
 // IsEmpty checks if an array has no elements.
+//
+//go:inline
 func IsEmpty[A any](as []A) bool {
 	return G.IsEmpty(as)
 }
@@ -216,24 +253,32 @@ func IsNonEmpty[A any](as []A) bool {
 }
 
 // Empty returns an empty array of type A.
+//
+//go:inline
 func Empty[A any]() []A {
 	return G.Empty[[]A]()
 }
 
 // Zero returns an empty array of type A (alias for Empty).
+//
+//go:inline
 func Zero[A any]() []A {
 	return Empty[A]()
 }
 
 // Of constructs a single element array
+//
+//go:inline
 func Of[A any](a A) []A {
 	return G.Of[[]A](a)
 }
 
 // MonadChain applies a function that returns an array to each element and flattens the results.
 // This is the monadic version that takes the array as the first parameter (also known as FlatMap).
-func MonadChain[A, B any](fa []A, f func(a A) []B) []B {
-	return G.MonadChain[[]A, []B](fa, f)
+//
+//go:inline
+func MonadChain[A, B any](fa []A, f Kleisli[A, B]) []B {
+	return G.MonadChain(fa, f)
 }
 
 // Chain applies a function that returns an array to each element and flattens the results.
@@ -243,58 +288,76 @@ func MonadChain[A, B any](fa []A, f func(a A) []B) []B {
 //
 //	duplicate := array.Chain(func(x int) []int { return []int{x, x} })
 //	result := duplicate([]int{1, 2, 3}) // [1, 1, 2, 2, 3, 3]
-func Chain[A, B any](f func(A) []B) func([]A) []B {
-	return G.Chain[[]A, []B](f)
+//
+//go:inline
+func Chain[A, B any](f Kleisli[A, B]) Operator[A, B] {
+	return G.Chain[[]A](f)
 }
 
 // MonadAp applies an array of functions to an array of values, producing all combinations.
 // This is the monadic version that takes both arrays as parameters.
+//
+//go:inline
 func MonadAp[B, A any](fab []func(A) B, fa []A) []B {
 	return G.MonadAp[[]B](fab, fa)
 }
 
 // Ap applies an array of functions to an array of values, producing all combinations.
 // This is the curried version.
-func Ap[B, A any](fa []A) func([]func(A) B) []B {
+//
+//go:inline
+func Ap[B, A any](fa []A) Operator[func(A) B, B] {
 	return G.Ap[[]B, []func(A) B](fa)
 }
 
 // Match performs pattern matching on an array, calling onEmpty if empty or onNonEmpty if not.
+//
+//go:inline
 func Match[A, B any](onEmpty func() B, onNonEmpty func([]A) B) func([]A) B {
-	return G.Match[[]A](onEmpty, onNonEmpty)
+	return G.Match(onEmpty, onNonEmpty)
 }
 
 // MatchLeft performs pattern matching on an array, calling onEmpty if empty or onNonEmpty with head and tail if not.
+//
+//go:inline
 func MatchLeft[A, B any](onEmpty func() B, onNonEmpty func(A, []A) B) func([]A) B {
-	return G.MatchLeft[[]A](onEmpty, onNonEmpty)
+	return G.MatchLeft(onEmpty, onNonEmpty)
 }
 
 // Tail returns all elements except the first, wrapped in an Option.
 // Returns None if the array is empty.
-func Tail[A any](as []A) O.Option[[]A] {
+//
+//go:inline
+func Tail[A any](as []A) Option[[]A] {
 	return G.Tail(as)
 }
 
 // Head returns the first element of an array, wrapped in an Option.
 // Returns None if the array is empty.
-func Head[A any](as []A) O.Option[A] {
+//
+//go:inline
+func Head[A any](as []A) Option[A] {
 	return G.Head(as)
 }
 
 // First returns the first element of an array, wrapped in an Option (alias for Head).
 // Returns None if the array is empty.
-func First[A any](as []A) O.Option[A] {
+//
+//go:inline
+func First[A any](as []A) Option[A] {
 	return G.First(as)
 }
 
 // Last returns the last element of an array, wrapped in an Option.
 // Returns None if the array is empty.
-func Last[A any](as []A) O.Option[A] {
+//
+//go:inline
+func Last[A any](as []A) Option[A] {
 	return G.Last(as)
 }
 
 // PrependAll inserts a separator before each element of an array.
-func PrependAll[A any](middle A) EM.Endomorphism[[]A] {
+func PrependAll[A any](middle A) Operator[A, A] {
 	return func(as []A) []A {
 		count := len(as)
 		dst := count * 2
@@ -314,7 +377,7 @@ func PrependAll[A any](middle A) EM.Endomorphism[[]A] {
 // Example:
 //
 //	result := array.Intersperse(0)([]int{1, 2, 3}) // [1, 0, 2, 0, 3]
-func Intersperse[A any](middle A) EM.Endomorphism[[]A] {
+func Intersperse[A any](middle A) Operator[A, A] {
 	prepend := PrependAll(middle)
 	return func(as []A) []A {
 		if IsEmpty(as) {
@@ -327,7 +390,7 @@ func Intersperse[A any](middle A) EM.Endomorphism[[]A] {
 // Intercalate inserts a separator between elements and concatenates them using a Monoid.
 func Intercalate[A any](m M.Monoid[A]) func(A) func([]A) A {
 	return func(middle A) func([]A) A {
-		return Match(m.Empty, F.Flow2(Intersperse(middle), ConcatAll[A](m)))
+		return Match(m.Empty, F.Flow2(Intersperse(middle), ConcatAll(m)))
 	}
 }
 
@@ -336,28 +399,36 @@ func Intercalate[A any](m M.Monoid[A]) func(A) func([]A) A {
 // Example:
 //
 //	result := array.Flatten([][]int{{1, 2}, {3, 4}, {5}}) // [1, 2, 3, 4, 5]
+//
+//go:inline
 func Flatten[A any](mma [][]A) []A {
 	return G.Flatten(mma)
 }
 
 // Slice extracts a subarray from index low (inclusive) to high (exclusive).
-func Slice[A any](low, high int) func(as []A) []A {
+func Slice[A any](low, high int) Operator[A, A] {
 	return array.Slice[[]A](low, high)
 }
 
 // Lookup returns the element at the specified index, wrapped in an Option.
 // Returns None if the index is out of bounds.
-func Lookup[A any](idx int) func([]A) O.Option[A] {
+//
+//go:inline
+func Lookup[A any](idx int) func([]A) Option[A] {
 	return G.Lookup[[]A](idx)
 }
 
 // UpsertAt returns a function that inserts or updates an element at a specific index.
 // If the index is out of bounds, the element is appended.
-func UpsertAt[A any](a A) EM.Endomorphism[[]A] {
+//
+//go:inline
+func UpsertAt[A any](a A) Operator[A, A] {
 	return G.UpsertAt[[]A](a)
 }
 
 // Size returns the number of elements in an array.
+//
+//go:inline
 func Size[A any](as []A) int {
 	return G.Size(as)
 }
@@ -365,12 +436,16 @@ func Size[A any](as []A) int {
 // MonadPartition splits an array into two arrays based on a predicate.
 // The first array contains elements for which the predicate returns false,
 // the second contains elements for which it returns true.
+//
+//go:inline
 func MonadPartition[A any](as []A, pred func(A) bool) tuple.Tuple2[[]A, []A] {
 	return G.MonadPartition(as, pred)
 }
 
 // Partition creates two new arrays out of one, the left result contains the elements
 // for which the predicate returns false, the right one those for which the predicate returns true
+//
+//go:inline
 func Partition[A any](pred func(A) bool) func([]A) tuple.Tuple2[[]A, []A] {
 	return G.Partition[[]A](pred)
 }
@@ -391,53 +466,159 @@ func ConstNil[A any]() []A {
 }
 
 // SliceRight extracts a subarray from the specified start index to the end.
-func SliceRight[A any](start int) EM.Endomorphism[[]A] {
+//
+//go:inline
+func SliceRight[A any](start int) Operator[A, A] {
 	return G.SliceRight[[]A](start)
 }
 
 // Copy creates a shallow copy of the array
+//
+//go:inline
 func Copy[A any](b []A) []A {
 	return G.Copy(b)
 }
 
 // Clone creates a deep copy of the array using the provided endomorphism to clone the values
-func Clone[A any](f func(A) A) func(as []A) []A {
+//
+//go:inline
+func Clone[A any](f func(A) A) Operator[A, A] {
 	return G.Clone[[]A](f)
 }
 
 // FoldMap maps and folds an array. Map the Array passing each value to the iterating function. Then fold the results using the provided Monoid.
+//
+//go:inline
 func FoldMap[A, B any](m M.Monoid[B]) func(func(A) B) func([]A) B {
 	return G.FoldMap[[]A](m)
 }
 
 // FoldMapWithIndex maps and folds an array. Map the Array passing each value to the iterating function. Then fold the results using the provided Monoid.
+//
+//go:inline
 func FoldMapWithIndex[A, B any](m M.Monoid[B]) func(func(int, A) B) func([]A) B {
 	return G.FoldMapWithIndex[[]A](m)
 }
 
 // Fold folds the array using the provided Monoid.
+//
+//go:inline
 func Fold[A any](m M.Monoid[A]) func([]A) A {
 	return G.Fold[[]A](m)
 }
 
 // Push adds an element to the end of an array (alias for Append).
-func Push[A any](a A) EM.Endomorphism[[]A] {
-	return G.Push[EM.Endomorphism[[]A]](a)
+//
+//go:inline
+func Push[A any](a A) Operator[A, A] {
+	return G.Push[Operator[A, A]](a)
 }
 
 // MonadFlap applies a value to an array of functions, producing an array of results.
 // This is the monadic version that takes both parameters.
+//
+//go:inline
 func MonadFlap[B, A any](fab []func(A) B, a A) []B {
-	return G.MonadFlap[func(A) B, []func(A) B, []B, A, B](fab, a)
+	return G.MonadFlap[func(A) B, []func(A) B, []B](fab, a)
 }
 
 // Flap applies a value to an array of functions, producing an array of results.
 // This is the curried version.
-func Flap[B, A any](a A) func([]func(A) B) []B {
-	return G.Flap[func(A) B, []func(A) B, []B, A, B](a)
+//
+//go:inline
+func Flap[B, A any](a A) Operator[func(A) B, B] {
+	return G.Flap[func(A) B, []func(A) B, []B](a)
 }
 
 // Prepend adds an element to the beginning of an array, returning a new array.
-func Prepend[A any](head A) EM.Endomorphism[[]A] {
-	return G.Prepend[EM.Endomorphism[[]A]](head)
+//
+//go:inline
+func Prepend[A any](head A) Operator[A, A] {
+	return G.Prepend[Operator[A, A]](head)
+}
+
+// Reverse returns a new slice with elements in reverse order.
+// This function creates a new slice containing all elements from the input slice
+// in reverse order, without modifying the original slice.
+//
+// Type Parameters:
+//   - A: The type of elements in the slice
+//
+// Parameters:
+//   - as: The input slice to reverse
+//
+// Returns:
+//   - A new slice with elements in reverse order
+//
+// Behavior:
+//   - Creates a new slice with the same length as the input
+//   - Copies elements from the input slice in reverse order
+//   - Does not modify the original slice
+//   - Returns an empty slice if the input is empty
+//   - Returns a single-element slice unchanged if input has one element
+//
+// Example:
+//
+//	numbers := []int{1, 2, 3, 4, 5}
+//	reversed := array.Reverse(numbers)
+//	// reversed: []int{5, 4, 3, 2, 1}
+//	// numbers: []int{1, 2, 3, 4, 5} (unchanged)
+//
+// Example with strings:
+//
+//	words := []string{"hello", "world", "foo", "bar"}
+//	reversed := array.Reverse(words)
+//	// reversed: []string{"bar", "foo", "world", "hello"}
+//
+// Example with empty slice:
+//
+//	empty := []int{}
+//	reversed := array.Reverse(empty)
+//	// reversed: []int{} (empty slice)
+//
+// Example with single element:
+//
+//	single := []string{"only"}
+//	reversed := array.Reverse(single)
+//	// reversed: []string{"only"}
+//
+// Use cases:
+//   - Reversing the order of elements for display or processing
+//   - Implementing stack-like behavior (LIFO)
+//   - Processing data in reverse chronological order
+//   - Reversing transformation pipelines
+//   - Creating palindrome checks
+//   - Implementing undo/redo functionality
+//
+// Example with processing in reverse:
+//
+//	events := []string{"start", "middle", "end"}
+//	reversed := array.Reverse(events)
+//	// Process events in reverse order
+//	for _, event := range reversed {
+//	    fmt.Println(event) // Prints: "end", "middle", "start"
+//	}
+//
+// Example with functional composition:
+//
+//	numbers := []int{1, 2, 3, 4, 5}
+//	result := F.Pipe2(
+//	    numbers,
+//	    array.Map(N.Mul(2)),
+//	    array.Reverse,
+//	)
+//	// result: []int{10, 8, 6, 4, 2}
+//
+// Performance:
+//   - Time complexity: O(n) where n is the length of the slice
+//   - Space complexity: O(n) for the new slice
+//   - Does not allocate if the input slice is empty
+//
+// Note: This function is immutable - it does not modify the original slice.
+// If you need to reverse a slice in-place, consider using a different approach
+// or modifying the slice directly.
+//
+//go:inline
+func Reverse[A any](as []A) []A {
+	return G.Reverse(as)
 }

v2/array/array_extended_test.go

@@ -35,7 +35,7 @@ func TestReplicate(t *testing.T) {
 
 func TestMonadMap(t *testing.T) {
 	src := []int{1, 2, 3}
-	result := MonadMap(src, func(x int) int { return x * 2 })
+	result := MonadMap(src, N.Mul(2))
 	assert.Equal(t, []int{2, 4, 6}, result)
 }
 
@@ -173,8 +173,8 @@ func TestChain(t *testing.T) {
 
 func TestMonadAp(t *testing.T) {
 	fns := []func(int) int{
-		func(x int) int { return x * 2 },
-		func(x int) int { return x + 10 },
+		N.Mul(2),
+		N.Add(10),
 	}
 	values := []int{1, 2}
 	result := MonadAp(fns, values)
@@ -268,7 +268,7 @@ func TestCopy(t *testing.T) {
 
 func TestClone(t *testing.T) {
 	src := []int{1, 2, 3}
-	cloner := Clone(func(x int) int { return x * 2 })
+	cloner := Clone(N.Mul(2))
 	result := cloner(src)
 	assert.Equal(t, []int{2, 4, 6}, result)
 }

v2/array/array_test.go

@@ -22,6 +22,7 @@ import (
 
 	F "github.com/IBM/fp-go/v2/function"
 	"github.com/IBM/fp-go/v2/internal/utils"
+	N "github.com/IBM/fp-go/v2/number"
 	O "github.com/IBM/fp-go/v2/option"
 	S "github.com/IBM/fp-go/v2/string"
 	T "github.com/IBM/fp-go/v2/tuple"
@@ -97,7 +98,7 @@ func TestAp(t *testing.T) {
 				utils.Double,
 				utils.Triple,
 			},
-			Ap[int, int]([]int{1, 2, 3}),
+			Ap[int]([]int{1, 2, 3}),
 		),
 	)
 }
@@ -214,3 +215,262 @@ func ExampleFoldMap() {
 	// Output: ABC
 
 }
+
+// TestReverse tests the Reverse function
+func TestReverse(t *testing.T) {
+	t.Run("Reverse integers", func(t *testing.T) {
+		input := []int{1, 2, 3, 4, 5}
+		result := Reverse(input)
+		expected := []int{5, 4, 3, 2, 1}
+		assert.Equal(t, expected, result)
+	})
+
+	t.Run("Reverse strings", func(t *testing.T) {
+		input := []string{"hello", "world", "foo", "bar"}
+		result := Reverse(input)
+		expected := []string{"bar", "foo", "world", "hello"}
+		assert.Equal(t, expected, result)
+	})
+
+	t.Run("Reverse empty slice", func(t *testing.T) {
+		input := []int{}
+		result := Reverse(input)
+		assert.Equal(t, []int{}, result)
+	})
+
+	t.Run("Reverse single element", func(t *testing.T) {
+		input := []string{"only"}
+		result := Reverse(input)
+		assert.Equal(t, []string{"only"}, result)
+	})
+
+	t.Run("Reverse two elements", func(t *testing.T) {
+		input := []int{1, 2}
+		result := Reverse(input)
+		assert.Equal(t, []int{2, 1}, result)
+	})
+
+	t.Run("Does not modify original slice", func(t *testing.T) {
+		original := []int{1, 2, 3, 4, 5}
+		originalCopy := []int{1, 2, 3, 4, 5}
+		_ = Reverse(original)
+		assert.Equal(t, originalCopy, original)
+	})
+
+	t.Run("Reverse with floats", func(t *testing.T) {
+		input := []float64{1.1, 2.2, 3.3}
+		result := Reverse(input)
+		expected := []float64{3.3, 2.2, 1.1}
+		assert.Equal(t, expected, result)
+	})
+
+	t.Run("Reverse with structs", func(t *testing.T) {
+		type Person struct {
+			Name string
+			Age  int
+		}
+		input := []Person{
+			{"Alice", 30},
+			{"Bob", 25},
+			{"Charlie", 35},
+		}
+		result := Reverse(input)
+		expected := []Person{
+			{"Charlie", 35},
+			{"Bob", 25},
+			{"Alice", 30},
+		}
+		assert.Equal(t, expected, result)
+	})
+
+	t.Run("Reverse with pointers", func(t *testing.T) {
+		a, b, c := 1, 2, 3
+		input := []*int{&a, &b, &c}
+		result := Reverse(input)
+		assert.Equal(t, []*int{&c, &b, &a}, result)
+	})
+
+	t.Run("Double reverse returns original order", func(t *testing.T) {
+		original := []int{1, 2, 3, 4, 5}
+		reversed := Reverse(original)
+		doubleReversed := Reverse(reversed)
+		assert.Equal(t, original, doubleReversed)
+	})
+
+	t.Run("Reverse with large slice", func(t *testing.T) {
+		input := MakeBy(1000, F.Identity[int])
+		result := Reverse(input)
+
+		// Check first and last elements
+		assert.Equal(t, 999, result[0])
+		assert.Equal(t, 0, result[999])
+
+		// Check length
+		assert.Equal(t, 1000, len(result))
+	})
+
+	t.Run("Reverse palindrome", func(t *testing.T) {
+		input := []int{1, 2, 3, 2, 1}
+		result := Reverse(input)
+		assert.Equal(t, input, result)
+	})
+}
+
+// TestReverseComposition tests Reverse with other array operations
+func TestReverseComposition(t *testing.T) {
+	t.Run("Reverse after Map", func(t *testing.T) {
+		input := []int{1, 2, 3, 4, 5}
+		result := F.Pipe2(
+			input,
+			Map(N.Mul(2)),
+			Reverse[int],
+		)
+		expected := []int{10, 8, 6, 4, 2}
+		assert.Equal(t, expected, result)
+	})
+
+	t.Run("Map after Reverse", func(t *testing.T) {
+		input := []int{1, 2, 3, 4, 5}
+		result := F.Pipe2(
+			input,
+			Reverse[int],
+			Map(N.Mul(2)),
+		)
+		expected := []int{10, 8, 6, 4, 2}
+		assert.Equal(t, expected, result)
+	})
+
+	t.Run("Reverse with Filter", func(t *testing.T) {
+		input := []int{1, 2, 3, 4, 5, 6}
+		result := F.Pipe2(
+			input,
+			Filter(func(n int) bool { return n%2 == 0 }),
+			Reverse[int],
+		)
+		expected := []int{6, 4, 2}
+		assert.Equal(t, expected, result)
+	})
+
+	t.Run("Reverse with Reduce", func(t *testing.T) {
+		input := []string{"a", "b", "c"}
+		reversed := Reverse(input)
+		result := Reduce(func(acc, val string) string {
+			return acc + val
+		}, "")(reversed)
+		assert.Equal(t, "cba", result)
+	})
+
+	t.Run("Reverse with Flatten", func(t *testing.T) {
+		input := [][]int{{1, 2}, {3, 4}, {5, 6}}
+		result := F.Pipe2(
+			input,
+			Reverse[[]int],
+			Flatten[int],
+		)
+		expected := []int{5, 6, 3, 4, 1, 2}
+		assert.Equal(t, expected, result)
+	})
+}
+
+// TestReverseUseCases demonstrates practical use cases for Reverse
+func TestReverseUseCases(t *testing.T) {
+	t.Run("Process events in reverse chronological order", func(t *testing.T) {
+		events := []string{"2024-01-01", "2024-01-02", "2024-01-03"}
+		reversed := Reverse(events)
+
+		// Most recent first
+		assert.Equal(t, "2024-01-03", reversed[0])
+		assert.Equal(t, "2024-01-01", reversed[2])
+	})
+
+	t.Run("Implement stack behavior (LIFO)", func(t *testing.T) {
+		stack := []int{1, 2, 3, 4, 5}
+		reversed := Reverse(stack)
+
+		// Pop from reversed (LIFO)
+		assert.Equal(t, 5, reversed[0])
+		assert.Equal(t, 4, reversed[1])
+	})
+
+	t.Run("Reverse string characters", func(t *testing.T) {
+		chars := []rune("hello")
+		reversed := Reverse(chars)
+		result := string(reversed)
+		assert.Equal(t, "olleh", result)
+	})
+
+	t.Run("Check palindrome", func(t *testing.T) {
+		word := []rune("racecar")
+		reversed := Reverse(word)
+		assert.Equal(t, word, reversed)
+
+		notPalindrome := []rune("hello")
+		reversedNot := Reverse(notPalindrome)
+		assert.NotEqual(t, notPalindrome, reversedNot)
+	})
+
+	t.Run("Reverse transformation pipeline", func(t *testing.T) {
+		// Apply transformations in reverse order
+		numbers := []int{1, 2, 3}
+
+		// Normal: add 10, then multiply by 2
+		normal := F.Pipe2(
+			numbers,
+			Map(N.Add(10)),
+			Map(N.Mul(2)),
+		)
+
+		// Reversed order of operations
+		reversed := F.Pipe2(
+			numbers,
+			Map(N.Mul(2)),
+			Map(N.Add(10)),
+		)
+
+		assert.NotEqual(t, normal, reversed)
+		assert.Equal(t, []int{22, 24, 26}, normal)
+		assert.Equal(t, []int{12, 14, 16}, reversed)
+	})
+}
+
+// TestReverseProperties tests mathematical properties of Reverse
+func TestReverseProperties(t *testing.T) {
+	t.Run("Involution property: Reverse(Reverse(x)) == x", func(t *testing.T) {
+		testCases := [][]int{
+			{1, 2, 3, 4, 5},
+			{1},
+			{},
+			{1, 2},
+			{5, 4, 3, 2, 1},
+		}
+
+		for _, original := range testCases {
+			result := Reverse(Reverse(original))
+			assert.Equal(t, original, result)
+		}
+	})
+
+	t.Run("Length preservation: len(Reverse(x)) == len(x)", func(t *testing.T) {
+		testCases := [][]int{
+			{1, 2, 3, 4, 5},
+			{1},
+			{},
+			MakeBy(100, F.Identity[int]),
+		}
+
+		for _, input := range testCases {
+			result := Reverse(input)
+			assert.Equal(t, len(input), len(result))
+		}
+	})
+
+	t.Run("First element becomes last", func(t *testing.T) {
+		input := []int{1, 2, 3, 4, 5}
+		result := Reverse(input)
+
+		if len(input) > 0 {
+			assert.Equal(t, input[0], result[len(result)-1])
+			assert.Equal(t, input[len(input)-1], result[0])
+		}
+	})
+}

v2/array/bind.go

@@ -29,10 +29,12 @@ import (
 //	    Y int
 //	}
 //	result := array.Do(State{})
+//
+//go:inline
 func Do[S any](
 	empty S,
 ) []S {
-	return G.Do[[]S, S](empty)
+	return G.Do[[]S](empty)
 }
 
 // Bind attaches the result of a computation to a context S1 to produce a context S2.
@@ -50,11 +52,13 @@ func Do[S any](
 //	        func(s struct{}) []int { return []int{1, 2} },
 //	    ),
 //	)
+//
+//go:inline
 func Bind[S1, S2, T any](
 	setter func(T) func(S1) S2,
-	f func(S1) []T,
-) func([]S1) []S2 {
-	return G.Bind[[]S1, []S2, []T, S1, S2, T](setter, f)
+	f Kleisli[S1, T],
+) Operator[S1, S2] {
+	return G.Bind[[]S1, []S2](setter, f)
 }
 
 // Let attaches the result of a pure computation to a context S1 to produce a context S2.
@@ -70,11 +74,13 @@ func Bind[S1, S2, T any](
 //	    },
 //	    func(s struct{ X int }) int { return s.X * 2 },
 //	)
+//
+//go:inline
 func Let[S1, S2, T any](
 	setter func(T) func(S1) S2,
 	f func(S1) T,
-) func([]S1) []S2 {
-	return G.Let[[]S1, []S2, S1, S2, T](setter, f)
+) Operator[S1, S2] {
+	return G.Let[[]S1, []S2](setter, f)
 }
 
 // LetTo attaches a constant value to a context S1 to produce a context S2.
@@ -90,11 +96,13 @@ func Let[S1, S2, T any](
 //	    },
 //	    "constant",
 //	)
+//
+//go:inline
 func LetTo[S1, S2, T any](
 	setter func(T) func(S1) S2,
 	b T,
-) func([]S1) []S2 {
-	return G.LetTo[[]S1, []S2, S1, S2, T](setter, b)
+) Operator[S1, S2] {
+	return G.LetTo[[]S1, []S2](setter, b)
 }
 
 // BindTo initializes a new state S1 from a value T.
@@ -108,10 +116,12 @@ func LetTo[S1, S2, T any](
 //	        return struct{ X int }{x}
 //	    }),
 //	)
+//
+//go:inline
 func BindTo[S1, T any](
 	setter func(T) S1,
-) func([]T) []S1 {
-	return G.BindTo[[]S1, []T, S1, T](setter)
+) Operator[T, S1] {
+	return G.BindTo[[]S1, []T](setter)
 }
 
 // ApS attaches a value to a context S1 to produce a context S2 by considering
@@ -128,9 +138,11 @@ func BindTo[S1, T any](
 //	    },
 //	    []int{10, 20},
 //	)
+//
+//go:inline
 func ApS[S1, S2, T any](
 	setter func(T) func(S1) S2,
 	fa []T,
-) func([]S1) []S2 {
-	return G.ApS[[]S1, []S2, []T, S1, S2, T](setter, fa)
+) Operator[S1, S2] {
+	return G.ApS[[]S1, []S2](setter, fa)
 }

v2/array/doc.go

@@ -36,7 +36,7 @@
 //	generated := array.MakeBy(5, func(i int) int { return i * 2 })
 //
 //	// Transforming arrays
-//	doubled := array.Map(func(x int) int { return x * 2 })(arr)
+//	doubled := array.Map(N.Mul(2))(arr)
 //	filtered := array.Filter(func(x int) bool { return x > 2 })(arr)
 //
 //	// Combining arrays
@@ -50,7 +50,7 @@
 //	numbers := []int{1, 2, 3, 4, 5}
 //
 //	// Map transforms each element
-//	doubled := array.Map(func(x int) int { return x * 2 })(numbers)
+//	doubled := array.Map(N.Mul(2))(numbers)
 //	// Result: [2, 4, 6, 8, 10]
 //
 //	// Filter keeps elements matching a predicate

v2/array/eq.go

@@ -16,22 +16,11 @@
 package array
 
 import (
+	"slices"
+
 	E "github.com/IBM/fp-go/v2/eq"
 )
 
-func equals[T any](left []T, right []T, eq func(T, T) bool) bool {
-	if len(left) != len(right) {
-		return false
-	}
-	for i, v1 := range left {
-		v2 := right[i]
-		if !eq(v1, v2) {
-			return false
-		}
-	}
-	return true
-}
-
 // Eq creates an equality checker for arrays given an equality checker for elements.
 // Two arrays are considered equal if they have the same length and all corresponding
 // elements are equal according to the provided Eq instance.
@@ -46,6 +35,11 @@ func equals[T any](left []T, right []T, eq func(T, T) bool) bool {
 func Eq[T any](e E.Eq[T]) E.Eq[[]T] {
 	eq := e.Equals
 	return E.FromEquals(func(left, right []T) bool {
-		return equals(left, right, eq)
+		return slices.EqualFunc(left, right, eq)
 	})
 }
+
+//go:inline
+func StrictEquals[T comparable]() E.Eq[[]T] {
+	return E.FromEquals(slices.Equal[[]T])
+}

v2/array/examples_sort_test.go

@@ -87,6 +87,6 @@ func Example_sort() {
 	// [abc klm zyx]
 	// [zyx klm abc]
 	// [None[int] Some[int](42) Some[int](1337)]
-	// [{c {false 0}} {b {true 10}} {d {true 10}} {a {true 30}}]
+	// [{c {0 false}} {b {10 true}} {d {10 true}} {a {30 true}}]
 
 }

v2/array/find.go

@@ -17,7 +17,7 @@ package array
 
 import (
 	G "github.com/IBM/fp-go/v2/array/generic"
-	O "github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/option"
 )
 
 // FindFirst finds the first element which satisfies a predicate function.
@@ -28,7 +28,9 @@ import (
 //	findGreaterThan3 := array.FindFirst(func(x int) bool { return x > 3 })
 //	result := findGreaterThan3([]int{1, 2, 4, 5}) // Some(4)
 //	result2 := findGreaterThan3([]int{1, 2, 3}) // None
-func FindFirst[A any](pred func(A) bool) func([]A) O.Option[A] {
+//
+//go:inline
+func FindFirst[A any](pred func(A) bool) option.Kleisli[[]A, A] {
 	return G.FindFirst[[]A](pred)
 }
 
@@ -41,7 +43,9 @@ func FindFirst[A any](pred func(A) bool) func([]A) O.Option[A] {
 //	    return i%2 == 0 && x%2 == 0
 //	})
 //	result := findEvenAtEvenIndex([]int{1, 3, 4, 5}) // Some(4)
-func FindFirstWithIndex[A any](pred func(int, A) bool) func([]A) O.Option[A] {
+//
+//go:inline
+func FindFirstWithIndex[A any](pred func(int, A) bool) option.Kleisli[[]A, A] {
 	return G.FindFirstWithIndex[[]A](pred)
 }
 
@@ -59,13 +63,17 @@ func FindFirstWithIndex[A any](pred func(int, A) bool) func([]A) O.Option[A] {
 //	    return option.None[int]()
 //	})
 //	result := parseFirst([]string{"a", "42", "b"}) // Some(42)
-func FindFirstMap[A, B any](sel func(A) O.Option[B]) func([]A) O.Option[B] {
+//
+//go:inline
+func FindFirstMap[A, B any](sel option.Kleisli[A, B]) option.Kleisli[[]A, B] {
 	return G.FindFirstMap[[]A](sel)
 }
 
 // FindFirstMapWithIndex finds the first element for which the selector function returns Some.
 // The selector receives both the index and the element.
-func FindFirstMapWithIndex[A, B any](sel func(int, A) O.Option[B]) func([]A) O.Option[B] {
+//
+//go:inline
+func FindFirstMapWithIndex[A, B any](sel func(int, A) Option[B]) option.Kleisli[[]A, B] {
 	return G.FindFirstMapWithIndex[[]A](sel)
 }
 
@@ -76,24 +84,32 @@ func FindFirstMapWithIndex[A, B any](sel func(int, A) O.Option[B]) func([]A) O.O
 //
 //	findGreaterThan3 := array.FindLast(func(x int) bool { return x > 3 })
 //	result := findGreaterThan3([]int{1, 4, 2, 5}) // Some(5)
-func FindLast[A any](pred func(A) bool) func([]A) O.Option[A] {
+//
+//go:inline
+func FindLast[A any](pred func(A) bool) option.Kleisli[[]A, A] {
 	return G.FindLast[[]A](pred)
 }
 
 // FindLastWithIndex finds the last element which satisfies a predicate function that also receives the index.
 // Returns Some(element) if found, None if no element matches.
-func FindLastWithIndex[A any](pred func(int, A) bool) func([]A) O.Option[A] {
+//
+//go:inline
+func FindLastWithIndex[A any](pred func(int, A) bool) option.Kleisli[[]A, A] {
 	return G.FindLastWithIndex[[]A](pred)
 }
 
 // FindLastMap finds the last element for which the selector function returns Some.
 // This combines finding and mapping in a single operation, searching from the end.
-func FindLastMap[A, B any](sel func(A) O.Option[B]) func([]A) O.Option[B] {
+//
+//go:inline
+func FindLastMap[A, B any](sel option.Kleisli[A, B]) option.Kleisli[[]A, B] {
 	return G.FindLastMap[[]A](sel)
 }
 
 // FindLastMapWithIndex finds the last element for which the selector function returns Some.
 // The selector receives both the index and the element, searching from the end.
-func FindLastMapWithIndex[A, B any](sel func(int, A) O.Option[B]) func([]A) O.Option[B] {
+//
+//go:inline
+func FindLastMapWithIndex[A, B any](sel func(int, A) Option[B]) option.Kleisli[[]A, B] {
 	return G.FindLastMapWithIndex[[]A](sel)
 }

v2/array/generic/array.go

@@ -25,31 +25,33 @@ import (
 )
 
 // Of constructs a single element array
+//
+//go:inline
 func Of[GA ~[]A, A any](value A) GA {
-	return GA{value}
+	return array.Of[GA](value)
 }
 
 func Reduce[GA ~[]A, A, B any](f func(B, A) B, initial B) func(GA) B {
 	return func(as GA) B {
-		return MonadReduce[GA](as, f, initial)
+		return MonadReduce(as, f, initial)
 	}
 }
 
 func ReduceWithIndex[GA ~[]A, A, B any](f func(int, B, A) B, initial B) func(GA) B {
 	return func(as GA) B {
-		return MonadReduceWithIndex[GA](as, f, initial)
+		return MonadReduceWithIndex(as, f, initial)
 	}
 }
 
 func ReduceRight[GA ~[]A, A, B any](f func(A, B) B, initial B) func(GA) B {
 	return func(as GA) B {
-		return MonadReduceRight[GA](as, f, initial)
+		return MonadReduceRight(as, f, initial)
 	}
 }
 
 func ReduceRightWithIndex[GA ~[]A, A, B any](f func(int, A, B) B, initial B) func(GA) B {
 	return func(as GA) B {
-		return MonadReduceRightWithIndex[GA](as, f, initial)
+		return MonadReduceRightWithIndex(as, f, initial)
 	}
 }
 
@@ -82,7 +84,7 @@ func MakeBy[AS ~[]A, F ~func(int) A, A any](n int, f F) AS {
 	}
 	// run the generator function across the input
 	as := make(AS, n)
-	for i := n - 1; i >= 0; i-- {
+	for i := range n {
 		as[i] = f(i)
 	}
 	return as
@@ -138,22 +140,27 @@ func Empty[GA ~[]A, A any]() GA {
 	return array.Empty[GA]()
 }
 
+//go:inline
 func UpsertAt[GA ~[]A, A any](a A) func(GA) GA {
 	return array.UpsertAt[GA](a)
 }
 
+//go:inline
 func MonadMap[GA ~[]A, GB ~[]B, A, B any](as GA, f func(a A) B) GB {
 	return array.MonadMap[GA, GB](as, f)
 }
 
+//go:inline
 func Map[GA ~[]A, GB ~[]B, A, B any](f func(a A) B) func(GA) GB {
 	return array.Map[GA, GB](f)
 }
 
+//go:inline
 func MonadMapWithIndex[GA ~[]A, GB ~[]B, A, B any](as GA, f func(int, A) B) GB {
 	return array.MonadMapWithIndex[GA, GB](as, f)
 }
 
+//go:inline
 func MapWithIndex[GA ~[]A, GB ~[]B, A, B any](f func(int, A) B) func(GA) GB {
 	return F.Bind2nd(MonadMapWithIndex[GA, GB, A, B], f)
 }
@@ -165,10 +172,9 @@ func Size[GA ~[]A, A any](as GA) int {
 func filterMap[GA ~[]A, GB ~[]B, A, B any](fa GA, f func(A) O.Option[B]) GB {
 	result := make(GB, 0, len(fa))
 	for _, a := range fa {
-		O.Map(func(b B) B {
+		if b, ok := O.Unwrap(f(a)); ok {
 			result = append(result, b)
-			return b
-		})(f(a))
+		}
 	}
 	return result
 }
@@ -176,10 +182,9 @@ func filterMap[GA ~[]A, GB ~[]B, A, B any](fa GA, f func(A) O.Option[B]) GB {
 func filterMapWithIndex[GA ~[]A, GB ~[]B, A, B any](fa GA, f func(int, A) O.Option[B]) GB {
 	result := make(GB, 0, len(fa))
 	for i, a := range fa {
-		O.Map(func(b B) B {
+		if b, ok := O.Unwrap(f(i, a)); ok {
 			result = append(result, b)
-			return b
-		})(f(i, a))
+		}
 	}
 	return result
 }
@@ -296,16 +301,14 @@ func MatchLeft[AS ~[]A, A, B any](onEmpty func() B, onNonEmpty func(A, AS) B) fu
 	}
 }
 
-func Slice[AS ~[]A, A any](start int, end int) func(AS) AS {
-	return func(a AS) AS {
-		return a[start:end]
-	}
+//go:inline
+func Slice[AS ~[]A, A any](start, end int) func(AS) AS {
+	return array.Slice[AS](start, end)
 }
 
+//go:inline
 func SliceRight[AS ~[]A, A any](start int) func(AS) AS {
-	return func(a AS) AS {
-		return a[start:]
-	}
+	return array.SliceRight[AS](start)
 }
 
 func Copy[AS ~[]A, A any](b AS) AS {
@@ -363,6 +366,12 @@ func Flap[FAB ~func(A) B, GFAB ~[]FAB, GB ~[]B, A, B any](a A) func(GFAB) GB {
 	return FC.Flap(Map[GFAB, GB], a)
 }
 
+//go:inline
 func Prepend[ENDO ~func(AS) AS, AS []A, A any](head A) ENDO {
 	return array.Prepend[ENDO](head)
 }
+
+//go:inline
+func Reverse[GT ~[]T, T any](as GT) GT {
+	return array.Reverse(as)
+}

v2/array/generic/bind.go

@@ -21,14 +21,56 @@ import (
 	F "github.com/IBM/fp-go/v2/internal/functor"
 )
 
-// Bind creates an empty context of type [S] to be used with the [Bind] operation
+// Do creates an empty context of type [S] to be used with the [Bind] operation.
+// This is the starting point for do-notation style composition.
+//
+// Example:
+//
+//	type State struct {
+//	    X int
+//	    Y int
+//	}
+//	result := generic.Do[[]State, State](State{})
 func Do[GS ~[]S, S any](
 	empty S,
 ) GS {
 	return Of[GS](empty)
 }
 
-// Bind attaches the result of a computation to a context [S1] to produce a context [S2]
+// Bind attaches the result of a computation to a context [S1] to produce a context [S2].
+// This enables sequential composition where each step can depend on the results of previous steps.
+// For arrays, this produces the cartesian product where later steps can use values from earlier steps.
+//
+// The setter function takes the result of the computation and returns a function that
+// updates the context from S1 to S2.
+//
+// Example:
+//
+//	type State struct {
+//	    X int
+//	    Y int
+//	}
+//
+//	result := F.Pipe2(
+//	    generic.Do[[]State, State](State{}),
+//	    generic.Bind[[]State, []State, []int, State, State, int](
+//	        func(x int) func(State) State {
+//	            return func(s State) State { s.X = x; return s }
+//	        },
+//	        func(s State) []int {
+//	            return []int{1, 2, 3}
+//	        },
+//	    ),
+//	    generic.Bind[[]State, []State, []int, State, State, int](
+//	        func(y int) func(State) State {
+//	            return func(s State) State { s.Y = y; return s }
+//	        },
+//	        func(s State) []int {
+//	            // This can access s.X from the previous step
+//	            return []int{s.X * 10, s.X * 20}
+//	        },
+//	    ),
+//	) // Produces: {1,10}, {1,20}, {2,20}, {2,40}, {3,30}, {3,60}
 func Bind[GS1 ~[]S1, GS2 ~[]S2, GT ~[]T, S1, S2, T any](
 	setter func(T) func(S1) S2,
 	f func(S1) GT,
@@ -75,7 +117,39 @@ func BindTo[GS1 ~[]S1, GT ~[]T, S1, T any](
 	)
 }
 
-// ApS attaches a value to a context [S1] to produce a context [S2] by considering the context and the value concurrently
+// ApS attaches a value to a context [S1] to produce a context [S2] by considering
+// the context and the value concurrently (using Applicative rather than Monad).
+// This allows independent computations to be combined without one depending on the result of the other.
+//
+// Unlike Bind, which sequences operations, ApS can be used when operations are independent
+// and can conceptually run in parallel. For arrays, this produces the cartesian product.
+//
+// Example:
+//
+//	type State struct {
+//	    X int
+//	    Y string
+//	}
+//
+//	// These operations are independent and can be combined with ApS
+//	xValues := []int{1, 2}
+//	yValues := []string{"a", "b"}
+//
+//	result := F.Pipe2(
+//	    generic.Do[[]State, State](State{}),
+//	    generic.ApS[[]State, []State, []int, State, State, int](
+//	        func(x int) func(State) State {
+//	            return func(s State) State { s.X = x; return s }
+//	        },
+//	        xValues,
+//	    ),
+//	    generic.ApS[[]State, []State, []string, State, State, string](
+//	        func(y string) func(State) State {
+//	            return func(s State) State { s.Y = y; return s }
+//	        },
+//	        yValues,
+//	    ),
+//	) // [{1,"a"}, {1,"b"}, {2,"a"}, {2,"b"}]
 func ApS[GS1 ~[]S1, GS2 ~[]S2, GT ~[]T, S1, S2, T any](
 	setter func(T) func(S1) S2,
 	fa GT,

v2/array/generic/find.go

@@ -42,8 +42,7 @@ func FindFirst[AS ~[]A, PRED ~func(A) bool, A any](pred PRED) func(AS) O.Option[
 func FindFirstMapWithIndex[AS ~[]A, PRED ~func(int, A) O.Option[B], A, B any](pred PRED) func(AS) O.Option[B] {
 	none := O.None[B]()
 	return func(as AS) O.Option[B] {
-		count := len(as)
-		for i := 0; i < count; i++ {
+		for i := range len(as) {
 			out := pred(i, as[i])
 			if O.IsSome(out) {
 				return out

v2/array/generic/monad.go

@@ -22,19 +22,19 @@ import (
 type arrayMonad[A, B any, GA ~[]A, GB ~[]B, GAB ~[]func(A) B] struct{}
 
 func (o *arrayMonad[A, B, GA, GB, GAB]) Of(a A) GA {
-	return Of[GA, A](a)
+	return Of[GA](a)
 }
 
 func (o *arrayMonad[A, B, GA, GB, GAB]) Map(f func(A) B) func(GA) GB {
-	return Map[GA, GB, A, B](f)
+	return Map[GA, GB](f)
 }
 
 func (o *arrayMonad[A, B, GA, GB, GAB]) Chain(f func(A) GB) func(GA) GB {
-	return Chain[GA, GB, A, B](f)
+	return Chain[GA](f)
 }
 
 func (o *arrayMonad[A, B, GA, GB, GAB]) Ap(fa GA) func(GAB) GB {
-	return Ap[GB, GAB, GA, B, A](fa)
+	return Ap[GB, GAB](fa)
 }
 
 // Monad implements the monadic operations for an array

v2/array/generic/monoid.go

@@ -0,0 +1,34 @@
+package generic
+
+import (
+	"github.com/IBM/fp-go/v2/internal/array"
+	M "github.com/IBM/fp-go/v2/monoid"
+	S "github.com/IBM/fp-go/v2/semigroup"
+)
+
+// Monoid returns a Monoid instance for arrays.
+// The Monoid combines arrays through concatenation, with an empty array as the identity element.
+//
+// Example:
+//
+//	m := array.Monoid[int]()
+//	result := m.Concat([]int{1, 2}, []int{3, 4}) // [1, 2, 3, 4]
+//	empty := m.Empty() // []
+//
+//go:inline
+func Monoid[GT ~[]T, T any]() M.Monoid[GT] {
+	return M.MakeMonoid(array.Concat[GT], Empty[GT]())
+}
+
+// Semigroup returns a Semigroup instance for arrays.
+// The Semigroup combines arrays through concatenation.
+//
+// Example:
+//
+//	s := array.Semigroup[int]()
+//	result := s.Concat([]int{1, 2}, []int{3, 4}) // [1, 2, 3, 4]
+//
+//go:inline
+func Semigroup[GT ~[]T, T any]() S.Semigroup[GT] {
+	return S.MakeSemigroup(array.Concat[GT])
+}

v2/array/generic/zip.go

@@ -26,7 +26,7 @@ import (
 func ZipWith[AS ~[]A, BS ~[]B, CS ~[]C, FCT ~func(A, B) C, A, B, C any](fa AS, fb BS, f FCT) CS {
 	l := N.Min(len(fa), len(fb))
 	res := make(CS, l)
-	for i := l - 1; i >= 0; i-- {
+	for i := range l {
 		res[i] = f(fa[i], fb[i])
 	}
 	return res
@@ -43,7 +43,7 @@ func Unzip[AS ~[]A, BS ~[]B, CS ~[]T.Tuple2[A, B], A, B any](cs CS) T.Tuple2[AS,
 	l := len(cs)
 	as := make(AS, l)
 	bs := make(BS, l)
-	for i := l - 1; i >= 0; i-- {
+	for i := range l {
 		t := cs[i]
 		as[i] = t.F1
 		bs[i] = t.F2

v2/array/magma.go

@@ -33,6 +33,8 @@ import (
 //	// Concatenate all strings
 //	concatStrings := array.ConcatAll(monoid.MonoidString())
 //	result2 := concatStrings([]string{"Hello", " ", "World"}) // "Hello World"
+//
+//go:inline
 func ConcatAll[A any](m M.Monoid[A]) func([]A) A {
 	return Reduce(m.Concat, m.Empty())
 }

v2/array/misc_test.go

@@ -18,7 +18,6 @@ package array
 import (
 	"testing"
 
-	O "github.com/IBM/fp-go/v2/option"
 	OR "github.com/IBM/fp-go/v2/ord"
 	"github.com/stretchr/testify/assert"
 )
@@ -103,39 +102,6 @@ func TestSortByKey(t *testing.T) {
 	assert.Equal(t, "Charlie", result[2].Name)
 }
 
-func TestMonadTraverse(t *testing.T) {
-	result := MonadTraverse(
-		O.Of[[]int],
-		O.Map[[]int, func(int) []int],
-		O.Ap[[]int, int],
-		[]int{1, 3, 5},
-		func(n int) O.Option[int] {
-			if n%2 == 1 {
-				return O.Some(n * 2)
-			}
-			return O.None[int]()
-		},
-	)
-
-	assert.Equal(t, O.Some([]int{2, 6, 10}), result)
-
-	// Test with None case
-	result2 := MonadTraverse(
-		O.Of[[]int],
-		O.Map[[]int, func(int) []int],
-		O.Ap[[]int, int],
-		[]int{1, 2, 3},
-		func(n int) O.Option[int] {
-			if n%2 == 1 {
-				return O.Some(n * 2)
-			}
-			return O.None[int]()
-		},
-	)
-
-	assert.Equal(t, O.None[[]int](), result2)
-}
-
 func TestUniqByKey(t *testing.T) {
 	type Person struct {
 		Name string

v2/array/monad.go

@@ -34,6 +34,8 @@ import (
 //	    return []string{fmt.Sprintf("%d", x), fmt.Sprintf("%d!", x)}
 //	})
 //	// Result: ["1", "1!", "2", "2!", "3", "3!"]
+//
+//go:inline
 func Monad[A, B any]() monad.Monad[A, B, []A, []B, []func(A) B] {
 	return G.Monad[A, B, []A, []B, []func(A) B]()
 }

v2/array/monoid.go

@@ -16,27 +16,12 @@
 package array
 
 import (
+	G "github.com/IBM/fp-go/v2/array/generic"
 	"github.com/IBM/fp-go/v2/internal/array"
 	M "github.com/IBM/fp-go/v2/monoid"
 	S "github.com/IBM/fp-go/v2/semigroup"
 )
 
-func concat[T any](left, right []T) []T {
-	// some performance checks
-	ll := len(left)
-	if ll == 0 {
-		return right
-	}
-	lr := len(right)
-	if lr == 0 {
-		return left
-	}
-	// need to copy
-	buf := make([]T, ll+lr)
-	copy(buf[copy(buf, left):], right)
-	return buf
-}
-
 // Monoid returns a Monoid instance for arrays.
 // The Monoid combines arrays through concatenation, with an empty array as the identity element.
 //
@@ -45,8 +30,10 @@ func concat[T any](left, right []T) []T {
 //	m := array.Monoid[int]()
 //	result := m.Concat([]int{1, 2}, []int{3, 4}) // [1, 2, 3, 4]
 //	empty := m.Empty() // []
+//
+//go:inline
 func Monoid[T any]() M.Monoid[[]T] {
-	return M.MakeMonoid(concat[T], Empty[T]())
+	return G.Monoid[[]T]()
 }
 
 // Semigroup returns a Semigroup instance for arrays.
@@ -56,8 +43,10 @@ func Monoid[T any]() M.Monoid[[]T] {
 //
 //	s := array.Semigroup[int]()
 //	result := s.Concat([]int{1, 2}, []int{3, 4}) // [1, 2, 3, 4]
+//
+//go:inline
 func Semigroup[T any]() S.Semigroup[[]T] {
-	return S.MakeSemigroup(concat[T])
+	return G.Semigroup[[]T]()
 }
 
 func addLen[A any](count int, data []A) int {

v2/array/nonempty/array.go

@@ -18,14 +18,11 @@ package nonempty
 import (
 	G "github.com/IBM/fp-go/v2/array/generic"
 	EM "github.com/IBM/fp-go/v2/endomorphism"
-	F "github.com/IBM/fp-go/v2/function"
 	"github.com/IBM/fp-go/v2/internal/array"
+	"github.com/IBM/fp-go/v2/option"
 	S "github.com/IBM/fp-go/v2/semigroup"
 )
 
-// NonEmptyArray represents an array with at least one element
-type NonEmptyArray[A any] []A
-
 // Of constructs a single element array
 func Of[A any](first A) NonEmptyArray[A] {
 	return G.Of[NonEmptyArray[A]](first)
@@ -44,20 +41,24 @@ func From[A any](first A, data ...A) NonEmptyArray[A] {
 	return buffer
 }
 
+//go:inline
 func IsEmpty[A any](_ NonEmptyArray[A]) bool {
 	return false
 }
 
+//go:inline
 func IsNonEmpty[A any](_ NonEmptyArray[A]) bool {
 	return true
 }
 
+//go:inline
 func MonadMap[A, B any](as NonEmptyArray[A], f func(a A) B) NonEmptyArray[B] {
 	return G.MonadMap[NonEmptyArray[A], NonEmptyArray[B]](as, f)
 }
 
-func Map[A, B any](f func(a A) B) func(NonEmptyArray[A]) NonEmptyArray[B] {
-	return F.Bind2nd(MonadMap[A, B], f)
+//go:inline
+func Map[A, B any](f func(a A) B) Operator[A, B] {
+	return G.Map[NonEmptyArray[A], NonEmptyArray[B]](f)
 }
 
 func Reduce[A, B any](f func(B, A) B, initial B) func(NonEmptyArray[A]) B {
@@ -72,22 +73,27 @@ func ReduceRight[A, B any](f func(A, B) B, initial B) func(NonEmptyArray[A]) B {
 	}
 }
 
+//go:inline
 func Tail[A any](as NonEmptyArray[A]) []A {
 	return as[1:]
 }
 
+//go:inline
 func Head[A any](as NonEmptyArray[A]) A {
 	return as[0]
 }
 
+//go:inline
 func First[A any](as NonEmptyArray[A]) A {
 	return as[0]
 }
 
+//go:inline
 func Last[A any](as NonEmptyArray[A]) A {
 	return as[len(as)-1]
 }
 
+//go:inline
 func Size[A any](as NonEmptyArray[A]) int {
 	return G.Size(as)
 }
@@ -96,12 +102,12 @@ func Flatten[A any](mma NonEmptyArray[NonEmptyArray[A]]) NonEmptyArray[A] {
 	return G.Flatten(mma)
 }
 
-func MonadChain[A, B any](fa NonEmptyArray[A], f func(a A) NonEmptyArray[B]) NonEmptyArray[B] {
-	return G.MonadChain[NonEmptyArray[A], NonEmptyArray[B]](fa, f)
+func MonadChain[A, B any](fa NonEmptyArray[A], f Kleisli[A, B]) NonEmptyArray[B] {
+	return G.MonadChain(fa, f)
 }
 
-func Chain[A, B any](f func(A) NonEmptyArray[B]) func(NonEmptyArray[A]) NonEmptyArray[B] {
-	return G.Chain[NonEmptyArray[A], NonEmptyArray[B]](f)
+func Chain[A, B any](f func(A) NonEmptyArray[B]) Operator[A, B] {
+	return G.Chain[NonEmptyArray[A]](f)
 }
 
 func MonadAp[B, A any](fab NonEmptyArray[func(A) B], fa NonEmptyArray[A]) NonEmptyArray[B] {
@@ -134,3 +140,89 @@ func Fold[A any](s S.Semigroup[A]) func(NonEmptyArray[A]) A {
 func Prepend[A any](head A) EM.Endomorphism[NonEmptyArray[A]] {
 	return array.Prepend[EM.Endomorphism[NonEmptyArray[A]]](head)
 }
+
+// ToNonEmptyArray attempts to convert a regular slice into a NonEmptyArray.
+// This function provides a safe way to create a NonEmptyArray from a slice that might be empty,
+// returning an Option type to handle the case where the input slice is empty.
+//
+// Type Parameters:
+//   - A: The element type of the array
+//
+// Parameters:
+//   - as: A regular slice that may or may not be empty
+//
+// Returns:
+//   - Option[NonEmptyArray[A]]: Some(NonEmptyArray) if the input slice is non-empty, None if empty
+//
+// Behavior:
+//   - If the input slice is empty, returns None
+//   - If the input slice has at least one element, wraps it in Some and returns it as a NonEmptyArray
+//   - The conversion is a type cast, so no data is copied
+//
+// Example:
+//
+//	// Convert non-empty slice
+//	numbers := []int{1, 2, 3}
+//	result := ToNonEmptyArray(numbers)  // Some(NonEmptyArray[1, 2, 3])
+//
+//	// Convert empty slice
+//	empty := []int{}
+//	result := ToNonEmptyArray(empty)  // None
+//
+//	// Use with Option methods
+//	numbers := []int{1, 2, 3}
+//	result := ToNonEmptyArray(numbers)
+//	if O.IsSome(result) {
+//	    nea := O.GetOrElse(F.Constant(From(0)))(result)
+//	    head := Head(nea)  // 1
+//	}
+//
+// Use cases:
+//   - Safely converting user input or external data to NonEmptyArray
+//   - Validating that a collection has at least one element before processing
+//   - Converting results from functions that return regular slices
+//   - Ensuring type safety when working with collections that must not be empty
+//
+// Example with validation:
+//
+//	func processItems(items []string) Option[string] {
+//	    return F.Pipe2(
+//	        items,
+//	        ToNonEmptyArray[string],
+//	        O.Map(func(nea NonEmptyArray[string]) string {
+//	            return Head(nea)  // Safe to get head since we know it's non-empty
+//	        }),
+//	    )
+//	}
+//
+// Example with error handling:
+//
+//	items := []int{1, 2, 3}
+//	result := ToNonEmptyArray(items)
+//	switch {
+//	case O.IsSome(result):
+//	    nea := O.GetOrElse(F.Constant(From(0)))(result)
+//	    fmt.Println("First item:", Head(nea))
+//	case O.IsNone(result):
+//	    fmt.Println("Array is empty")
+//	}
+//
+// Example with chaining:
+//
+//	// Process only if non-empty
+//	result := F.Pipe3(
+//	    []int{1, 2, 3},
+//	    ToNonEmptyArray[int],
+//	    O.Map(Map(func(x int) int { return x * 2 })),
+//	    O.Map(Head[int]),
+//	)  // Some(2)
+//
+// Note: This function is particularly useful when working with APIs or functions
+// that return regular slices but you need the type-level guarantee that the
+// collection is non-empty for subsequent operations.
+func ToNonEmptyArray[A any](as []A) Option[NonEmptyArray[A]] {
+	if G.IsEmpty(as) {
+		return option.None[NonEmptyArray[A]]()
+	}
+	return option.Some(NonEmptyArray[A](as))
+}

v2/array/nonempty/array_test.go

@@ -0,0 +1,370 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package nonempty
+
+import (
+	"testing"
+
+	F "github.com/IBM/fp-go/v2/function"
+	O "github.com/IBM/fp-go/v2/option"
+	"github.com/stretchr/testify/assert"
+)
+
+// TestToNonEmptyArray tests the ToNonEmptyArray function
+func TestToNonEmptyArray(t *testing.T) {
+	t.Run("Convert non-empty slice of integers", func(t *testing.T) {
+		input := []int{1, 2, 3}
+		result := ToNonEmptyArray(input)
+
+		assert.True(t, O.IsSome(result))
+		nea := O.GetOrElse(F.Constant(From(0)))(result)
+		assert.Equal(t, 3, Size(nea))
+		assert.Equal(t, 1, Head(nea))
+		assert.Equal(t, 3, Last(nea))
+	})
+
+	t.Run("Convert empty slice returns None", func(t *testing.T) {
+		input := []int{}
+		result := ToNonEmptyArray(input)
+
+		assert.True(t, O.IsNone(result))
+	})
+
+	t.Run("Convert single element slice", func(t *testing.T) {
+		input := []string{"hello"}
+		result := ToNonEmptyArray(input)
+
+		assert.True(t, O.IsSome(result))
+		nea := O.GetOrElse(F.Constant(From("")))(result)
+		assert.Equal(t, 1, Size(nea))
+		assert.Equal(t, "hello", Head(nea))
+	})
+
+	t.Run("Convert non-empty slice of strings", func(t *testing.T) {
+		input := []string{"a", "b", "c", "d"}
+		result := ToNonEmptyArray(input)
+
+		assert.True(t, O.IsSome(result))
+		nea := O.GetOrElse(F.Constant(From("")))(result)
+		assert.Equal(t, 4, Size(nea))
+		assert.Equal(t, "a", Head(nea))
+		assert.Equal(t, "d", Last(nea))
+	})
+
+	t.Run("Convert nil slice returns None", func(t *testing.T) {
+		var input []int
+		result := ToNonEmptyArray(input)
+
+		assert.True(t, O.IsNone(result))
+	})
+
+	t.Run("Convert slice with struct elements", func(t *testing.T) {
+		type Person struct {
+			Name string
+			Age  int
+		}
+		input := []Person{
+			{Name: "Alice", Age: 30},
+			{Name: "Bob", Age: 25},
+		}
+		result := ToNonEmptyArray(input)
+
+		assert.True(t, O.IsSome(result))
+		nea := O.GetOrElse(F.Constant(From(Person{})))(result)
+		assert.Equal(t, 2, Size(nea))
+		assert.Equal(t, "Alice", Head(nea).Name)
+	})
+
+	t.Run("Convert slice with pointer elements", func(t *testing.T) {
+		val1, val2 := 10, 20
+		input := []*int{&val1, &val2}
+		result := ToNonEmptyArray(input)
+
+		assert.True(t, O.IsSome(result))
+		nea := O.GetOrElse(F.Constant(From[*int](nil)))(result)
+		assert.Equal(t, 2, Size(nea))
+		assert.Equal(t, 10, *Head(nea))
+	})
+
+	t.Run("Convert large slice", func(t *testing.T) {
+		input := make([]int, 1000)
+		for i := range input {
+			input[i] = i
+		}
+		result := ToNonEmptyArray(input)
+
+		assert.True(t, O.IsSome(result))
+		nea := O.GetOrElse(F.Constant(From(0)))(result)
+		assert.Equal(t, 1000, Size(nea))
+		assert.Equal(t, 0, Head(nea))
+		assert.Equal(t, 999, Last(nea))
+	})
+
+	t.Run("Convert slice with float64 elements", func(t *testing.T) {
+		input := []float64{1.5, 2.5, 3.5}
+		result := ToNonEmptyArray(input)
+
+		assert.True(t, O.IsSome(result))
+		nea := O.GetOrElse(F.Constant(From(0.0)))(result)
+		assert.Equal(t, 3, Size(nea))
+		assert.Equal(t, 1.5, Head(nea))
+	})
+
+	t.Run("Convert slice with boolean elements", func(t *testing.T) {
+		input := []bool{true, false, true}
+		result := ToNonEmptyArray(input)
+
+		assert.True(t, O.IsSome(result))
+		nea := O.GetOrElse(F.Constant(From(false)))(result)
+		assert.Equal(t, 3, Size(nea))
+		assert.True(t, Head(nea))
+	})
+}
+
+// TestToNonEmptyArrayWithOption tests ToNonEmptyArray with Option operations
+func TestToNonEmptyArrayWithOption(t *testing.T) {
+	t.Run("Chain with Map to process elements", func(t *testing.T) {
+		input := []int{1, 2, 3}
+		result := F.Pipe2(
+			input,
+			ToNonEmptyArray[int],
+			O.Map(Map(func(x int) int { return x * 2 })),
+		)
+
+		assert.True(t, O.IsSome(result))
+		nea := O.GetOrElse(F.Constant(From(0)))(result)
+		assert.Equal(t, 2, Head(nea))
+		assert.Equal(t, 6, Last(nea))
+	})
+
+	t.Run("Chain with Map to get head", func(t *testing.T) {
+		input := []string{"first", "second", "third"}
+		result := F.Pipe2(
+			input,
+			ToNonEmptyArray[string],
+			O.Map(Head[string]),
+		)
+
+		assert.True(t, O.IsSome(result))
+		value := O.GetOrElse(F.Constant(""))(result)
+		assert.Equal(t, "first", value)
+	})
+
+	t.Run("GetOrElse with default value for empty slice", func(t *testing.T) {
+		input := []int{}
+		defaultValue := From(42)
+		result := F.Pipe2(
+			input,
+			ToNonEmptyArray[int],
+			O.GetOrElse(F.Constant(defaultValue)),
+		)
+
+		assert.Equal(t, 1, Size(result))
+		assert.Equal(t, 42, Head(result))
+	})
+
+	t.Run("GetOrElse with default value for non-empty slice", func(t *testing.T) {
+		input := []int{1, 2, 3}
+		defaultValue := From(42)
+		result := F.Pipe2(
+			input,
+			ToNonEmptyArray[int],
+			O.GetOrElse(F.Constant(defaultValue)),
+		)
+
+		assert.Equal(t, 3, Size(result))
+		assert.Equal(t, 1, Head(result))
+	})
+
+	t.Run("Fold with Some case", func(t *testing.T) {
+		input := []int{1, 2, 3}
+		result := F.Pipe2(
+			input,
+			ToNonEmptyArray[int],
+			O.Fold(
+				F.Constant(0),
+				func(nea NonEmptyArray[int]) int { return Head(nea) },
+			),
+		)
+
+		assert.Equal(t, 1, result)
+	})
+
+	t.Run("Fold with None case", func(t *testing.T) {
+		input := []int{}
+		result := F.Pipe2(
+			input,
+			ToNonEmptyArray[int],
+			O.Fold(
+				F.Constant(-1),
+				func(nea NonEmptyArray[int]) int { return Head(nea) },
+			),
+		)
+
+		assert.Equal(t, -1, result)
+	})
+}
+
+// TestToNonEmptyArrayComposition tests composing ToNonEmptyArray with other operations
+func TestToNonEmptyArrayComposition(t *testing.T) {
+	t.Run("Compose with filter-like operation", func(t *testing.T) {
+		input := []int{1, 2, 3, 4, 5}
+		// Filter even numbers then convert
+		filtered := []int{}
+		for _, x := range input {
+			if x%2 == 0 {
+				filtered = append(filtered, x)
+			}
+		}
+		result := ToNonEmptyArray(filtered)
+
+		assert.True(t, O.IsSome(result))
+		nea := O.GetOrElse(F.Constant(From(0)))(result)
+		assert.Equal(t, 2, Size(nea))
+		assert.Equal(t, 2, Head(nea))
+	})
+
+	t.Run("Compose with map operation before conversion", func(t *testing.T) {
+		input := []int{1, 2, 3}
+		// Map then convert
+		mapped := make([]int, len(input))
+		for i, x := range input {
+			mapped[i] = x * 10
+		}
+		result := ToNonEmptyArray(mapped)
+
+		assert.True(t, O.IsSome(result))
+		nea := O.GetOrElse(F.Constant(From(0)))(result)
+		assert.Equal(t, 10, Head(nea))
+		assert.Equal(t, 30, Last(nea))
+	})
+
+	t.Run("Chain multiple Option operations", func(t *testing.T) {
+		input := []int{5, 10, 15}
+		result := F.Pipe3(
+			input,
+			ToNonEmptyArray[int],
+			O.Map(Map(func(x int) int { return x / 5 })),
+			O.Map(func(nea NonEmptyArray[int]) int {
+				return Head(nea) + Last(nea)
+			}),
+		)
+
+		assert.True(t, O.IsSome(result))
+		value := O.GetOrElse(F.Constant(0))(result)
+		assert.Equal(t, 4, value) // 1 + 3
+	})
+}
+
+// TestToNonEmptyArrayUseCases demonstrates practical use cases
+func TestToNonEmptyArrayUseCases(t *testing.T) {
+	t.Run("Validate user input has at least one item", func(t *testing.T) {
+		// Simulate user input
+		userInput := []string{"item1", "item2"}
+
+		result := ToNonEmptyArray(userInput)
+		if O.IsSome(result) {
+			nea := O.GetOrElse(F.Constant(From("")))(result)
+			firstItem := Head(nea)
+			assert.Equal(t, "item1", firstItem)
+		} else {
+			t.Fatal("Expected Some but got None")
+		}
+	})
+
+	t.Run("Process only non-empty collections", func(t *testing.T) {
+		processItems := func(items []int) Option[int] {
+			return F.Pipe2(
+				items,
+				ToNonEmptyArray[int],
+				O.Map(func(nea NonEmptyArray[int]) int {
+					// Safe to use Head since we know it's non-empty
+					return Head(nea) * 2
+				}),
+			)
+		}
+
+		result1 := processItems([]int{5, 10, 15})
+		assert.True(t, O.IsSome(result1))
+		assert.Equal(t, 10, O.GetOrElse(F.Constant(0))(result1))
+
+		result2 := processItems([]int{})
+		assert.True(t, O.IsNone(result2))
+	})
+
+	t.Run("Convert API response to NonEmptyArray", func(t *testing.T) {
+		// Simulate API response
+		type APIResponse struct {
+			Items []string
+		}
+
+		response := APIResponse{Items: []string{"data1", "data2", "data3"}}
+
+		result := F.Pipe2(
+			response.Items,
+			ToNonEmptyArray[string],
+			O.Map(func(nea NonEmptyArray[string]) string {
+				return "First item: " + Head(nea)
+			}),
+		)
+
+		assert.True(t, O.IsSome(result))
+		message := O.GetOrElse(F.Constant("No items"))(result)
+		assert.Equal(t, "First item: data1", message)
+	})
+
+	t.Run("Ensure collection is non-empty before processing", func(t *testing.T) {
+		calculateAverage := func(numbers []float64) Option[float64] {
+			return F.Pipe2(
+				numbers,
+				ToNonEmptyArray[float64],
+				O.Map(func(nea NonEmptyArray[float64]) float64 {
+					sum := 0.0
+					for _, n := range nea {
+						sum += n
+					}
+					return sum / float64(Size(nea))
+				}),
+			)
+		}
+
+		result1 := calculateAverage([]float64{10.0, 20.0, 30.0})
+		assert.True(t, O.IsSome(result1))
+		assert.Equal(t, 20.0, O.GetOrElse(F.Constant(0.0))(result1))
+
+		result2 := calculateAverage([]float64{})
+		assert.True(t, O.IsNone(result2))
+	})
+
+	t.Run("Safe head extraction with type guarantee", func(t *testing.T) {
+		getFirstOrDefault := func(items []string, defaultValue string) string {
+			return F.Pipe2(
+				items,
+				ToNonEmptyArray[string],
+				O.Fold(
+					F.Constant(defaultValue),
+					Head[string],
+				),
+			)
+		}
+
+		result1 := getFirstOrDefault([]string{"a", "b", "c"}, "default")
+		assert.Equal(t, "a", result1)
+
+		result2 := getFirstOrDefault([]string{}, "default")
+		assert.Equal(t, "default", result2)
+	})
+}

v2/array/nonempty/types.go

@@ -0,0 +1,20 @@
+package nonempty
+
+import "github.com/IBM/fp-go/v2/option"
+
+type (
+	// NonEmptyArray represents an array that is guaranteed to have at least one element.
+	// This provides compile-time safety for operations that require non-empty collections.
+	NonEmptyArray[A any] []A
+
+	// Kleisli represents a Kleisli arrow for the NonEmptyArray monad.
+	// It's a function from A to NonEmptyArray[B], used for composing operations that produce non-empty arrays.
+	Kleisli[A, B any] = func(A) NonEmptyArray[B]
+
+	// Operator represents a function that transforms one NonEmptyArray into another.
+	// It takes a NonEmptyArray[A] and produces a NonEmptyArray[B].
+	Operator[A, B any] = Kleisli[NonEmptyArray[A], B]
+
+	// Option represents an optional value that may or may not be present.
+	Option[A any] = option.Option[A]
+)

v2/array/sequence.go

@@ -16,10 +16,18 @@
 package array
 
 import (
-	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/internal/array"
+	M "github.com/IBM/fp-go/v2/monoid"
 	O "github.com/IBM/fp-go/v2/option"
 )
 
+func MonadSequence[HKTA, HKTRA any](
+	fof func(HKTA) HKTRA,
+	m M.Monoid[HKTRA],
+	ma []HKTA) HKTRA {
+	return array.MonadSequence(fof, m.Empty(), m.Concat, ma)
+}
+
 // Sequence takes an array where elements are HKT<A> (higher kinded type) and,
 // using an applicative of that HKT, returns an HKT of []A.
 //
@@ -55,16 +63,11 @@ import (
 //	    option.MonadAp[[]int, int],
 //	)
 //	result := seq(opts) // Some([1, 2, 3])
-func Sequence[A, HKTA, HKTRA, HKTFRA any](
-	_of func([]A) HKTRA,
-	_map func(HKTRA, func([]A) func(A) []A) HKTFRA,
-	_ap func(HKTFRA, HKTA) HKTRA,
+func Sequence[HKTA, HKTRA any](
+	fof func(HKTA) HKTRA,
+	m M.Monoid[HKTRA],
 ) func([]HKTA) HKTRA {
-	ca := F.Curry2(Append[A])
-	empty := _of(Empty[A]())
-	return Reduce(func(fas HKTRA, fa HKTA) HKTRA {
-		return _ap(_map(fas, ca), fa)
-	}, empty)
+	return array.Sequence[[]HKTA](fof, m.Empty(), m.Concat)
 }
 
 // ArrayOption returns a function to convert a sequence of options into an option of a sequence.
@@ -86,10 +89,10 @@ func Sequence[A, HKTA, HKTRA, HKTFRA any](
 //	    option.Some(3),
 //	}
 //	result2 := array.ArrayOption[int]()(opts2) // None
-func ArrayOption[A any]() func([]O.Option[A]) O.Option[[]A] {
-	return Sequence(
-		O.Of[[]A],
-		O.MonadMap[[]A, func(A) []A],
-		O.MonadAp[[]A, A],
+func ArrayOption[A any](ma []Option[A]) Option[[]A] {
+	return MonadSequence(
+		O.Map(Of[A]),
+		O.ApplicativeMonoid(Monoid[A]()),
+		ma,
 	)
 }

v2/array/sequence_test.go

@@ -24,8 +24,7 @@ import (
 )
 
 func TestSequenceOption(t *testing.T) {
-	seq := ArrayOption[int]()
 
-	assert.Equal(t, O.Of([]int{1, 3}), seq([]O.Option[int]{O.Of(1), O.Of(3)}))
-	assert.Equal(t, O.None[[]int](), seq([]O.Option[int]{O.Of(1), O.None[int]()}))
+	assert.Equal(t, O.Of([]int{1, 3}), ArrayOption([]O.Option[int]{O.Of(1), O.Of(3)}))
+	assert.Equal(t, O.None[[]int](), ArrayOption([]O.Option[int]{O.Of(1), O.None[int]()}))
 }

v2/array/slice_test.go

@@ -0,0 +1,406 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package array
+
+import (
+	"testing"
+
+	N "github.com/IBM/fp-go/v2/number"
+	"github.com/stretchr/testify/assert"
+)
+
+// TestSliceBasicCases tests normal slicing operations
+func TestSliceBasicCases(t *testing.T) {
+	data := []int{0, 1, 2, 3, 4, 5}
+
+	t.Run("normal slice from middle", func(t *testing.T) {
+		assert.Equal(t, []int{1, 2, 3}, Slice[int](1, 4)(data))
+	})
+
+	t.Run("slice from start", func(t *testing.T) {
+		assert.Equal(t, []int{0, 1, 2}, Slice[int](0, 3)(data))
+	})
+
+	t.Run("slice to end", func(t *testing.T) {
+		assert.Equal(t, []int{3, 4, 5}, Slice[int](3, 6)(data))
+	})
+
+	t.Run("slice single element", func(t *testing.T) {
+		assert.Equal(t, []int{2}, Slice[int](2, 3)(data))
+	})
+
+	t.Run("slice entire array", func(t *testing.T) {
+		assert.Equal(t, []int{0, 1, 2, 3, 4, 5}, Slice[int](0, 6)(data))
+	})
+}
+
+// TestSliceNegativeIndices tests negative index handling (counting from end)
+func TestSliceNegativeIndices(t *testing.T) {
+	data := []int{0, 1, 2, 3, 4, 5}
+
+	t.Run("negative start index", func(t *testing.T) {
+		// -2 means length + (-2) = 6 - 2 = 4
+		assert.Equal(t, []int{4, 5}, Slice[int](-2, 6)(data))
+	})
+
+	t.Run("negative end index", func(t *testing.T) {
+		// -2 means length + (-2) = 6 - 2 = 4
+		assert.Equal(t, []int{0, 1, 2, 3}, Slice[int](0, -2)(data))
+	})
+
+	t.Run("both negative indices", func(t *testing.T) {
+		// -4 = 2, -2 = 4
+		assert.Equal(t, []int{2, 3}, Slice[int](-4, -2)(data))
+	})
+
+	t.Run("negative index beyond array start", func(t *testing.T) {
+		// -10 would be -4, clamped to 0
+		assert.Equal(t, []int{0, 1, 2}, Slice[int](-10, 3)(data))
+	})
+
+	t.Run("negative end index beyond array start", func(t *testing.T) {
+		// -10 would be -4, clamped to 0
+		assert.Equal(t, []int{}, Slice[int](0, -10)(data))
+	})
+}
+
+// TestSliceEmptyArray tests slicing on empty arrays (totality proof)
+func TestSliceEmptyArray(t *testing.T) {
+	empty := []int{}
+
+	t.Run("slice empty array with zero indices", func(t *testing.T) {
+		assert.Equal(t, []int{}, Slice[int](0, 0)(empty))
+	})
+
+	t.Run("slice empty array with positive indices", func(t *testing.T) {
+		assert.Equal(t, []int{}, Slice[int](0, 5)(empty))
+	})
+
+	t.Run("slice empty array with negative indices", func(t *testing.T) {
+		assert.Equal(t, []int{}, Slice[int](-1, -1)(empty))
+	})
+
+	t.Run("slice empty array with mixed indices", func(t *testing.T) {
+		assert.Equal(t, []int{}, Slice[int](-5, 5)(empty))
+	})
+}
+
+// TestSliceOutOfBounds tests out-of-bounds scenarios (totality proof)
+func TestSliceOutOfBounds(t *testing.T) {
+	data := []int{0, 1, 2, 3, 4}
+
+	t.Run("start index beyond array length", func(t *testing.T) {
+		assert.Equal(t, []int{}, Slice[int](10, 15)(data))
+	})
+
+	t.Run("end index beyond array length", func(t *testing.T) {
+		assert.Equal(t, []int{2, 3, 4}, Slice[int](2, 100)(data))
+	})
+
+	t.Run("both indices beyond array length", func(t *testing.T) {
+		assert.Equal(t, []int{}, Slice[int](10, 20)(data))
+	})
+
+	t.Run("start equals array length", func(t *testing.T) {
+		assert.Equal(t, []int{}, Slice[int](5, 10)(data))
+	})
+
+	t.Run("end equals array length", func(t *testing.T) {
+		assert.Equal(t, []int{3, 4}, Slice[int](3, 5)(data))
+	})
+}
+
+// TestSliceInvalidRanges tests invalid range scenarios (totality proof)
+func TestSliceInvalidRanges(t *testing.T) {
+	data := []int{0, 1, 2, 3, 4}
+
+	t.Run("start equals end", func(t *testing.T) {
+		assert.Equal(t, []int{}, Slice[int](2, 2)(data))
+	})
+
+	t.Run("start greater than end", func(t *testing.T) {
+		assert.Equal(t, []int{}, Slice[int](4, 2)(data))
+	})
+
+	t.Run("start greater than end with negative indices", func(t *testing.T) {
+		// -1 = 4, -3 = 2
+		assert.Equal(t, []int{}, Slice[int](-1, -3)(data))
+	})
+
+	t.Run("zero range at start", func(t *testing.T) {
+		assert.Equal(t, []int{}, Slice[int](0, 0)(data))
+	})
+
+	t.Run("zero range at end", func(t *testing.T) {
+		assert.Equal(t, []int{}, Slice[int](5, 5)(data))
+	})
+}
+
+// TestSliceEdgeCases tests additional edge cases (totality proof)
+func TestSliceEdgeCases(t *testing.T) {
+	t.Run("single element array - slice all", func(t *testing.T) {
+		data := []int{42}
+		assert.Equal(t, []int{42}, Slice[int](0, 1)(data))
+	})
+
+	t.Run("single element array - slice none", func(t *testing.T) {
+		data := []int{42}
+		assert.Equal(t, []int{}, Slice[int](1, 1)(data))
+	})
+
+	t.Run("single element array - negative indices", func(t *testing.T) {
+		data := []int{42}
+		assert.Equal(t, []int{42}, Slice[int](-1, 1)(data))
+	})
+
+	t.Run("large array slice", func(t *testing.T) {
+		data := MakeBy(1000, func(i int) int { return i })
+		result := Slice[int](100, 200)(data)
+		assert.Equal(t, 100, len(result))
+		assert.Equal(t, 100, result[0])
+		assert.Equal(t, 199, result[99])
+	})
+}
+
+// TestSliceWithDifferentTypes tests that Slice works with different types (totality proof)
+func TestSliceWithDifferentTypes(t *testing.T) {
+	t.Run("string slice", func(t *testing.T) {
+		data := []string{"a", "b", "c", "d", "e"}
+		assert.Equal(t, []string{"b", "c", "d"}, Slice[string](1, 4)(data))
+	})
+
+	t.Run("float slice", func(t *testing.T) {
+		data := []float64{1.1, 2.2, 3.3, 4.4, 5.5}
+		assert.Equal(t, []float64{2.2, 3.3}, Slice[float64](1, 3)(data))
+	})
+
+	t.Run("bool slice", func(t *testing.T) {
+		data := []bool{true, false, true, false}
+		assert.Equal(t, []bool{false, true}, Slice[bool](1, 3)(data))
+	})
+
+	t.Run("struct slice", func(t *testing.T) {
+		type Point struct{ X, Y int }
+		data := []Point{{1, 2}, {3, 4}, {5, 6}}
+		assert.Equal(t, []Point{{3, 4}}, Slice[Point](1, 2)(data))
+	})
+
+	t.Run("pointer slice", func(t *testing.T) {
+		a, b, c := 1, 2, 3
+		data := []*int{&a, &b, &c}
+		result := Slice[*int](1, 3)(data)
+		assert.Equal(t, 2, len(result))
+		assert.Equal(t, 2, *result[0])
+		assert.Equal(t, 3, *result[1])
+	})
+}
+
+// TestSliceNilArray tests behavior with nil arrays (totality proof)
+func TestSliceNilArray(t *testing.T) {
+	var nilArray []int
+
+	t.Run("slice nil array with zero indices", func(t *testing.T) {
+		assert.Equal(t, []int{}, Slice[int](0, 0)(nilArray))
+	})
+
+	t.Run("slice nil array with positive indices", func(t *testing.T) {
+		assert.Equal(t, []int{}, Slice[int](0, 5)(nilArray))
+	})
+
+	t.Run("slice nil array with negative indices", func(t *testing.T) {
+		assert.Equal(t, []int{}, Slice[int](-1, 1)(nilArray))
+	})
+
+	t.Run("slice nil array with out of bounds indices", func(t *testing.T) {
+		assert.Equal(t, []int{}, Slice[int](10, 20)(nilArray))
+	})
+}
+
+// TestSliceComposition tests that Slice can be composed with other functions
+func TestSliceComposition(t *testing.T) {
+	data := []int{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
+
+	t.Run("compose multiple slices", func(t *testing.T) {
+		// First slice [2:8], then slice [1:4] of result
+		slice1 := Slice[int](2, 8)
+		slice2 := Slice[int](1, 4)
+		result := slice2(slice1(data))
+		// [2,3,4,5,6,7] -> [3,4,5]
+		assert.Equal(t, []int{3, 4, 5}, result)
+	})
+
+	t.Run("slice then map", func(t *testing.T) {
+		sliced := Slice[int](2, 5)(data)
+		mapped := Map(N.Mul(2))(sliced)
+		assert.Equal(t, []int{4, 6, 8}, mapped)
+	})
+
+	t.Run("slice then filter", func(t *testing.T) {
+		sliced := Slice[int](0, 6)(data)
+		filtered := Filter(func(x int) bool { return x%2 == 0 })(sliced)
+		assert.Equal(t, []int{0, 2, 4}, filtered)
+	})
+}
+
+// TestSliceImmutability tests that Slice doesn't modify the original array
+func TestSliceImmutability(t *testing.T) {
+	original := []int{0, 1, 2, 3, 4}
+	originalCopy := []int{0, 1, 2, 3, 4}
+
+	t.Run("slicing doesn't modify original", func(t *testing.T) {
+		result := Slice[int](1, 4)(original)
+		assert.Equal(t, []int{1, 2, 3}, result)
+		assert.Equal(t, originalCopy, original)
+	})
+
+	t.Run("slice shares underlying array with original", func(t *testing.T) {
+		// Note: Go's slice operation creates a view of the underlying array,
+		// not a deep copy. This is expected behavior and matches Go's built-in slice semantics.
+		result := Slice[int](1, 4)(original)
+		result[0] = 999
+		// The original array is affected because slices share the underlying array
+		assert.Equal(t, 999, original[1], "Slices share underlying array (expected Go behavior)")
+	})
+}
+
+// TestSliceTotality is a comprehensive test proving Slice is a total function
+// A total function is defined for all possible inputs and never panics
+func TestSliceTotality(t *testing.T) {
+	testCases := []struct {
+		name  string
+		data  []int
+		low   int
+		high  int
+		panic bool // Should always be false for a total function
+	}{
+		// Normal cases
+		{"normal range", []int{1, 2, 3, 4, 5}, 1, 3, false},
+		{"full range", []int{1, 2, 3}, 0, 3, false},
+		{"empty result", []int{1, 2, 3}, 1, 1, false},
+
+		// Edge cases with empty/nil arrays
+		{"empty array", []int{}, 0, 0, false},
+		{"empty array with indices", []int{}, 1, 5, false},
+		{"nil array", nil, 0, 5, false},
+
+		// Negative indices
+		{"negative low", []int{1, 2, 3, 4, 5}, -2, 5, false},
+		{"negative high", []int{1, 2, 3, 4, 5}, 0, -1, false},
+		{"both negative", []int{1, 2, 3, 4, 5}, -3, -1, false},
+		{"negative beyond bounds", []int{1, 2, 3}, -100, -50, false},
+
+		// Out of bounds
+		{"low beyond length", []int{1, 2, 3}, 10, 20, false},
+		{"high beyond length", []int{1, 2, 3}, 1, 100, false},
+		{"both beyond length", []int{1, 2, 3}, 10, 20, false},
+
+		// Invalid ranges
+		{"low equals high", []int{1, 2, 3}, 2, 2, false},
+		{"low greater than high", []int{1, 2, 3}, 3, 1, false},
+		{"negative invalid range", []int{1, 2, 3, 4, 5}, -1, -3, false},
+
+		// Extreme values
+		{"very large indices", []int{1, 2, 3}, 1000000, 2000000, false},
+		{"very negative indices", []int{1, 2, 3}, -1000000, -500000, false},
+		{"mixed extreme", []int{1, 2, 3}, -1000000, 1000000, false},
+
+		// Zero values
+		{"zero indices", []int{1, 2, 3}, 0, 0, false},
+		{"zero low", []int{1, 2, 3}, 0, 3, false},
+		{"zero high", []int{1, 2, 3}, 0, 0, false},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			// This test proves totality by ensuring no panic occurs
+			defer func() {
+				if r := recover(); r != nil {
+					if !tc.panic {
+						t.Errorf("Slice panicked unexpectedly: %v", r)
+					}
+				} else {
+					if tc.panic {
+						t.Errorf("Slice should have panicked but didn't")
+					}
+				}
+			}()
+
+			// Execute the function - if it's total, it will never panic
+			result := Slice[int](tc.low, tc.high)(tc.data)
+
+			// Additional verification: result should always be a valid slice
+			assert.NotNil(t, result, "Result should never be nil")
+			assert.True(t, len(result) >= 0, "Result length should be non-negative")
+		})
+	}
+}
+
+// TestSlicePropertyBased tests mathematical properties of Slice
+func TestSlicePropertyBased(t *testing.T) {
+	data := []int{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
+
+	t.Run("identity: Slice(0, len) returns copy of array", func(t *testing.T) {
+		result := Slice[int](0, len(data))(data)
+		assert.Equal(t, data, result)
+	})
+
+	t.Run("empty: Slice(i, i) always returns empty", func(t *testing.T) {
+		for i := 0; i <= len(data); i++ {
+			result := Slice[int](i, i)(data)
+			assert.Equal(t, []int{}, result)
+		}
+	})
+
+	t.Run("length property: len(Slice(i, j)) = max(0, min(j, len) - max(i, 0))", func(t *testing.T) {
+		testCases := []struct{ low, high, expected int }{
+			{0, 5, 5},
+			{2, 7, 5},
+			{5, 5, 0},
+			{3, 2, 0},   // invalid range
+			{-2, 10, 2}, // -2 becomes 8, so slice [8:10] has length 2
+			{0, 100, 10},
+		}
+
+		for _, tc := range testCases {
+			result := Slice[int](tc.low, tc.high)(data)
+			assert.Equal(t, tc.expected, len(result),
+				"Slice(%d, %d) should have length %d", tc.low, tc.high, tc.expected)
+		}
+	})
+
+	t.Run("concatenation: Slice(0,i) + Slice(i,len) = original", func(t *testing.T) {
+		for i := 0; i <= len(data); i++ {
+			left := Slice[int](0, i)(data)
+			right := Slice[int](i, len(data))(data)
+			concatenated := append(left, right...)
+			assert.Equal(t, data, concatenated)
+		}
+	})
+
+	t.Run("subset property: all elements in slice are in original", func(t *testing.T) {
+		result := Slice[int](2, 7)(data)
+		for _, elem := range result {
+			found := false
+			for _, orig := range data {
+				if elem == orig {
+					found = true
+					break
+				}
+			}
+			assert.True(t, found, "Element %d should be in original array", elem)
+		}
+	})
+}

v2/array/sort.go

@@ -30,7 +30,9 @@ import (
 //	numbers := []int{3, 1, 4, 1, 5, 9, 2, 6}
 //	sorted := array.Sort(ord.FromStrictCompare[int]())(numbers)
 //	// Result: [1, 1, 2, 3, 4, 5, 6, 9]
-func Sort[T any](ord O.Ord[T]) func(ma []T) []T {
+//
+//go:inline
+func Sort[T any](ord O.Ord[T]) Operator[T, T] {
 	return G.Sort[[]T](ord)
 }
 
@@ -58,7 +60,9 @@ func Sort[T any](ord O.Ord[T]) func(ma []T) []T {
 //	)
 //	sorted := sortByAge(people)
 //	// Result: [{"Bob", 25}, {"Alice", 30}, {"Charlie", 35}]
-func SortByKey[K, T any](ord O.Ord[K], f func(T) K) func(ma []T) []T {
+//
+//go:inline
+func SortByKey[K, T any](ord O.Ord[K], f func(T) K) Operator[T, T] {
 	return G.SortByKey[[]T](ord, f)
 }
 
@@ -87,6 +91,8 @@ func SortByKey[K, T any](ord O.Ord[K], f func(T) K) func(ma []T) []T {
 //	})
 //	sorted := sortByName(people)
 //	// Result: [{"Jones", "Bob"}, {"Smith", "Alice"}, {"Smith", "John"}]
-func SortBy[T any](ord []O.Ord[T]) func(ma []T) []T {
-	return G.SortBy[[]T, []O.Ord[T]](ord)
+//
+//go:inline
+func SortBy[T any](ord []O.Ord[T]) Operator[T, T] {
+	return G.SortBy[[]T](ord)
 }

v2/array/traverse.go

@@ -52,6 +52,8 @@ import (
 //
 //	result := parseAll([]string{"1", "2", "3"}) // Some([1, 2, 3])
 //	result2 := parseAll([]string{"1", "x", "3"}) // None
+//
+//go:inline
 func Traverse[A, B, HKTB, HKTAB, HKTRB any](
 	fof func([]B) HKTRB,
 	fmap func(func([]B) func(B) []B) func(HKTRB) HKTAB,
@@ -66,6 +68,8 @@ func Traverse[A, B, HKTB, HKTAB, HKTRB any](
 // into an effect of an array.
 //
 // This is useful when you want to apply the traverse operation directly without currying.
+//
+//go:inline
 func MonadTraverse[A, B, HKTB, HKTAB, HKTRB any](
 	fof func([]B) HKTRB,
 	fmap func(func([]B) func(B) []B) func(HKTRB) HKTAB,
@@ -76,3 +80,25 @@ func MonadTraverse[A, B, HKTB, HKTAB, HKTRB any](
 
 	return array.MonadTraverse(fof, fmap, fap, ta, f)
 }
+
+//go:inline
+func TraverseWithIndex[A, B, HKTB, HKTAB, HKTRB any](
+	fof func([]B) HKTRB,
+	fmap func(func([]B) func(B) []B) func(HKTRB) HKTAB,
+	fap func(HKTB) func(HKTAB) HKTRB,
+
+	f func(int, A) HKTB) func([]A) HKTRB {
+	return array.TraverseWithIndex[[]A](fof, fmap, fap, f)
+}
+
+//go:inline
+func MonadTraverseWithIndex[A, B, HKTB, HKTAB, HKTRB any](
+	fof func([]B) HKTRB,
+	fmap func(func([]B) func(B) []B) func(HKTRB) HKTAB,
+	fap func(HKTB) func(HKTAB) HKTRB,
+
+	ta []A,
+	f func(int, A) HKTB) HKTRB {
+
+	return array.MonadTraverseWithIndex(fof, fmap, fap, ta, f)
+}

v2/array/types.go

@@ -0,0 +1,16 @@
+package array
+
+import "github.com/IBM/fp-go/v2/option"
+
+type (
+	// Kleisli represents a Kleisli arrow for arrays.
+	// It's a function from A to []B, used for composing operations that produce arrays.
+	Kleisli[A, B any] = func(A) []B
+
+	// Operator represents a function that transforms one array into another.
+	// It takes a []A and produces a []B.
+	Operator[A, B any] = Kleisli[[]A, B]
+
+	// Option represents an optional value that may or may not be present.
+	Option[A any] = option.Option[A]
+)

v2/array/uniq.go

@@ -15,8 +15,10 @@ import (
 //
 //	strings := []string{"a", "b", "a", "c", "b"}
 //	unique2 := array.StrictUniq(strings) // ["a", "b", "c"]
+//
+//go:inline
 func StrictUniq[A comparable](as []A) []A {
-	return G.StrictUniq[[]A](as)
+	return G.StrictUniq(as)
 }
 
 // Uniq converts an array of arbitrary items into an array of unique items
@@ -42,6 +44,8 @@ func StrictUniq[A comparable](as []A) []A {
 //	uniqueByName := array.Uniq(func(p Person) string { return p.Name })
 //	result := uniqueByName(people)
 //	// Result: [{"Alice", 30}, {"Bob", 25}, {"Charlie", 30}]
-func Uniq[A any, K comparable](f func(A) K) func(as []A) []A {
+//
+//go:inline
+func Uniq[A any, K comparable](f func(A) K) Operator[A, A] {
 	return G.Uniq[[]A](f)
 }

v2/array/zip.go

@@ -33,8 +33,10 @@ import (
 //	    return fmt.Sprintf("%s is %d years old", name, age)
 //	})
 //	// Result: ["Alice is 30 years old", "Bob is 25 years old", "Charlie is 35 years old"]
+//
+//go:inline
 func ZipWith[FCT ~func(A, B) C, A, B, C any](fa []A, fb []B, f FCT) []C {
-	return G.ZipWith[[]A, []B, []C, FCT](fa, fb, f)
+	return G.ZipWith[[]A, []B, []C](fa, fb, f)
 }
 
 // Zip takes two arrays and returns an array of corresponding pairs (tuples).
@@ -51,6 +53,8 @@ func ZipWith[FCT ~func(A, B) C, A, B, C any](fa []A, fb []B, f FCT) []C {
 //	// With different lengths
 //	pairs2 := array.Zip([]int{1, 2})([]string{"a", "b", "c"})
 //	// Result: [(a, 1), (b, 2)]
+//
+//go:inline
 func Zip[A, B any](fb []B) func([]A) []T.Tuple2[A, B] {
 	return G.Zip[[]A, []B, []T.Tuple2[A, B]](fb)
 }
@@ -72,6 +76,8 @@ func Zip[A, B any](fb []B) func([]A) []T.Tuple2[A, B] {
 //	// Result: (["Alice", "Bob", "Charlie"], [30, 25, 35])
 //	names := result.Head  // ["Alice", "Bob", "Charlie"]
 //	ages := result.Tail   // [30, 25, 35]
+//
+//go:inline
 func Unzip[A, B any](cs []T.Tuple2[A, B]) T.Tuple2[[]A, []B] {
-	return G.Unzip[[]A, []B, []T.Tuple2[A, B]](cs)
+	return G.Unzip[[]A, []B](cs)
 }

v2/assert/assert.go

@@ -0,0 +1,710 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package assert provides functional assertion helpers for testing.
+//
+// This package wraps testify/assert functions in a Reader monad pattern,
+// allowing for composable and functional test assertions. Each assertion
+// returns a Reader that takes a *testing.T and performs the assertion.
+//
+// # Data Last Principle
+//
+// This package follows the "data last" functional programming principle, where
+// the data being operated on comes as the last parameter in a chain of function
+// applications. This design enables several powerful functional programming patterns:
+//
+//  1. **Partial Application**: You can create reusable assertion functions by providing
+//     configuration parameters first, leaving the data and testing context for later.
+//
+//  2. **Function Composition**: Assertions can be composed and combined before being
+//     applied to actual data.
+//
+//  3. **Point-Free Style**: You can pass assertion functions around without immediately
+//     providing the data they operate on.
+//
+// The general pattern is:
+//
+//	assert.Function(config)(data)(testingContext)
+//	               ↑        ↑     ↑
+//	            expected  actual  *testing.T (always last)
+//
+// For single-parameter assertions:
+//
+//	assert.Function(data)(testingContext)
+//	                ↑     ↑
+//	              actual  *testing.T (always last)
+//
+// Examples of "data last" in action:
+//
+//	// Multi-parameter: expected value → actual value → testing context
+//	assert.Equal(42)(result)(t)
+//	assert.ArrayContains(3)(numbers)(t)
+//
+//	// Single-parameter: data → testing context
+//	assert.NoError(err)(t)
+//	assert.ArrayNotEmpty(arr)(t)
+//
+//	// Partial application - create reusable assertions
+//	isPositive := assert.That(N.MoreThan(0))
+//	// Later, apply to different values:
+//	isPositive(42)(t)   // Passes
+//	isPositive(-5)(t)   // Fails
+//
+//	// Composition - combine assertions before applying data
+//	validateUser := func(u User) assert.Reader {
+//	    return assert.AllOf([]assert.Reader{
+//	        assert.Equal("Alice")(u.Name),
+//	        assert.That(func(age int) bool { return age >= 18 })(u.Age),
+//	    })
+//	}
+//	validateUser(user)(t)
+//
+// The package supports:
+//   - Equality and inequality assertions
+//   - Collection assertions (arrays, maps, strings)
+//   - Error handling assertions
+//   - Result type assertions
+//   - Custom predicate assertions
+//   - Composable test suites
+//
+// Example:
+//
+//	func TestExample(t *testing.T) {
+//	    value := 42
+//	    assert.Equal(42)(value)(t)  // Curried style
+//
+//	    // Composing multiple assertions
+//	    arr := []int{1, 2, 3}
+//	    assertions := assert.AllOf([]assert.Reader{
+//	        assert.ArrayNotEmpty(arr),
+//	        assert.ArrayLength[int](3)(arr),
+//	        assert.ArrayContains(2)(arr),
+//	    })
+//	    assertions(t)
+//	}
+package assert
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/IBM/fp-go/v2/boolean"
+	"github.com/IBM/fp-go/v2/eq"
+	"github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/reader"
+	"github.com/IBM/fp-go/v2/result"
+	"github.com/stretchr/testify/assert"
+)
+
+var (
+	// Eq is the equal predicate checking if objects are equal
+	Eq = eq.FromEquals(assert.ObjectsAreEqual)
+)
+
+// wrap1 is an internal helper function that wraps testify assertion functions
+// into the Reader monad pattern with curried parameters.
+//
+// It takes a testify assertion function and converts it into a curried function
+// that first takes an expected value, then an actual value, and finally returns
+// a Reader that performs the assertion when given a *testing.T.
+//
+// Parameters:
+//   - wrapped: The testify assertion function to wrap
+//   - expected: The expected value for comparison
+//   - msgAndArgs: Optional message and arguments for assertion failure
+//
+// Returns:
+//   - A Kleisli function that takes the actual value and returns a Reader
+func wrap1[T any](wrapped func(t assert.TestingT, expected, actual any, msgAndArgs ...any) bool, expected T, msgAndArgs ...any) Kleisli[T] {
+	return func(actual T) Reader {
+		return func(t *testing.T) bool {
+			return wrapped(t, expected, actual, msgAndArgs...)
+		}
+	}
+}
+
+// NotEqual tests if the expected and the actual values are not equal.
+//
+// This function follows the "data last" principle - you provide the expected value first,
+// then the actual value, and finally the testing.T context.
+//
+// Example:
+//
+//	func TestNotEqual(t *testing.T) {
+//	    value := 42
+//	    assert.NotEqual(10)(value)(t)  // Passes: 42 != 10
+//	    assert.NotEqual(42)(value)(t)  // Fails: 42 == 42
+//	}
+func NotEqual[T any](expected T) Kleisli[T] {
+	return wrap1(assert.NotEqual, expected)
+}
+
+// Equal tests if the expected and the actual values are equal.
+//
+// This is one of the most commonly used assertions. It follows the "data last" principle -
+// you provide the expected value first, then the actual value, and finally the testing.T context.
+//
+// Example:
+//
+//	func TestEqual(t *testing.T) {
+//	    result := 2 + 2
+//	    assert.Equal(4)(result)(t)  // Passes
+//
+//	    name := "Alice"
+//	    assert.Equal("Alice")(name)(t)  // Passes
+//
+//	    // Can be composed with other assertions
+//	    user := User{Name: "Bob", Age: 30}
+//	    assertions := assert.AllOf([]assert.Reader{
+//	        assert.Equal("Bob")(user.Name),
+//	        assert.Equal(30)(user.Age),
+//	    })
+//	    assertions(t)
+//	}
+func Equal[T any](expected T) Kleisli[T] {
+	return wrap1(assert.Equal, expected)
+}
+
+// ArrayNotEmpty checks if an array is not empty.
+//
+// Example:
+//
+//	func TestArrayNotEmpty(t *testing.T) {
+//	    numbers := []int{1, 2, 3}
+//	    assert.ArrayNotEmpty(numbers)(t)  // Passes
+//
+//	    empty := []int{}
+//	    assert.ArrayNotEmpty(empty)(t)  // Fails
+//	}
+func ArrayNotEmpty[T any](arr []T) Reader {
+	return func(t *testing.T) bool {
+		return assert.NotEmpty(t, arr)
+	}
+}
+
+// RecordNotEmpty checks if a map is not empty.
+//
+// Example:
+//
+//	func TestRecordNotEmpty(t *testing.T) {
+//	    config := map[string]int{"timeout": 30, "retries": 3}
+//	    assert.RecordNotEmpty(config)(t)  // Passes
+//
+//	    empty := map[string]int{}
+//	    assert.RecordNotEmpty(empty)(t)  // Fails
+//	}
+func RecordNotEmpty[K comparable, T any](mp map[K]T) Reader {
+	return func(t *testing.T) bool {
+		return assert.NotEmpty(t, mp)
+	}
+}
+
+// StringNotEmpty checks if a string is not empty.
+//
+// Example:
+//
+//	func TestStringNotEmpty(t *testing.T) {
+//	    message := "Hello, World!"
+//	    assert.StringNotEmpty(message)(t)  // Passes
+//
+//	    empty := ""
+//	    assert.StringNotEmpty(empty)(t)  // Fails
+//	}
+func StringNotEmpty(s string) Reader {
+	return func(t *testing.T) bool {
+		return assert.NotEmpty(t, s)
+	}
+}
+
+// ArrayLength tests if an array has the expected length.
+//
+// Example:
+//
+//	func TestArrayLength(t *testing.T) {
+//	    numbers := []int{1, 2, 3, 4, 5}
+//	    assert.ArrayLength[int](5)(numbers)(t)  // Passes
+//	    assert.ArrayLength[int](3)(numbers)(t)  // Fails
+//	}
+func ArrayLength[T any](expected int) Kleisli[[]T] {
+	return func(actual []T) Reader {
+		return func(t *testing.T) bool {
+			return assert.Len(t, actual, expected)
+		}
+	}
+}
+
+// RecordLength tests if a map has the expected length.
+//
+// Example:
+//
+//	func TestRecordLength(t *testing.T) {
+//	    config := map[string]string{"host": "localhost", "port": "8080"}
+//	    assert.RecordLength[string, string](2)(config)(t)  // Passes
+//	    assert.RecordLength[string, string](3)(config)(t)  // Fails
+//	}
+func RecordLength[K comparable, T any](expected int) Kleisli[map[K]T] {
+	return func(actual map[K]T) Reader {
+		return func(t *testing.T) bool {
+			return assert.Len(t, actual, expected)
+		}
+	}
+}
+
+// StringLength tests if a string has the expected length.
+//
+// Example:
+//
+//	func TestStringLength(t *testing.T) {
+//	    message := "Hello"
+//	    assert.StringLength[any, any](5)(message)(t)  // Passes
+//	    assert.StringLength[any, any](10)(message)(t)  // Fails
+//	}
+func StringLength[K comparable, T any](expected int) Kleisli[string] {
+	return func(actual string) Reader {
+		return func(t *testing.T) bool {
+			return assert.Len(t, actual, expected)
+		}
+	}
+}
+
+// NoError validates that there is no error.
+//
+// This is commonly used to assert that operations complete successfully.
+//
+// Example:
+//
+//	func TestNoError(t *testing.T) {
+//	    err := doSomething()
+//	    assert.NoError(err)(t)  // Passes if err is nil
+//
+//	    // Can be used with result types
+//	    result := result.TryCatch(func() (int, error) {
+//	        return 42, nil
+//	    })
+//	    assert.Success(result)(t)  // Uses NoError internally
+//	}
+func NoError(err error) Reader {
+	return func(t *testing.T) bool {
+		return assert.NoError(t, err)
+	}
+}
+
+// Error validates that there is an error.
+//
+// This is used to assert that operations fail as expected.
+//
+// Example:
+//
+//	func TestError(t *testing.T) {
+//	    err := validateInput("")
+//	    assert.Error(err)(t)  // Passes if err is not nil
+//
+//	    err2 := validateInput("valid")
+//	    assert.Error(err2)(t)  // Fails if err2 is nil
+//	}
+func Error(err error) Reader {
+	return func(t *testing.T) bool {
+		return assert.Error(t, err)
+	}
+}
+
+// Success checks if a [Result] represents success.
+//
+// This is a convenience function for testing Result types from the fp-go library.
+//
+// Example:
+//
+//	func TestSuccess(t *testing.T) {
+//	    res := result.Of[int](42)
+//	    assert.Success(res)(t)  // Passes
+//
+//	    failedRes := result.Error[int](errors.New("failed"))
+//	    assert.Success(failedRes)(t)  // Fails
+//	}
+func Success[T any](res Result[T]) Reader {
+	return NoError(result.ToError(res))
+}
+
+// Failure checks if a [Result] represents failure.
+//
+// This is a convenience function for testing Result types from the fp-go library.
+//
+// Example:
+//
+//	func TestFailure(t *testing.T) {
+//	    res := result.Error[int](errors.New("something went wrong"))
+//	    assert.Failure(res)(t)  // Passes
+//
+//	    successRes := result.Of[int](42)
+//	    assert.Failure(successRes)(t)  // Fails
+//	}
+func Failure[T any](res Result[T]) Reader {
+	return Error(result.ToError(res))
+}
+
+// ArrayContains tests if a value is contained in an array.
+//
+// Example:
+//
+//	func TestArrayContains(t *testing.T) {
+//	    numbers := []int{1, 2, 3, 4, 5}
+//	    assert.ArrayContains(3)(numbers)(t)  // Passes
+//	    assert.ArrayContains(10)(numbers)(t)  // Fails
+//
+//	    names := []string{"Alice", "Bob", "Charlie"}
+//	    assert.ArrayContains("Bob")(names)(t)  // Passes
+//	}
+func ArrayContains[T any](expected T) Kleisli[[]T] {
+	return func(actual []T) Reader {
+		return func(t *testing.T) bool {
+			return assert.Contains(t, actual, expected)
+		}
+	}
+}
+
+// ContainsKey tests if a key is contained in a map.
+//
+// Example:
+//
+//	func TestContainsKey(t *testing.T) {
+//	    config := map[string]int{"timeout": 30, "retries": 3}
+//	    assert.ContainsKey[int]("timeout")(config)(t)  // Passes
+//	    assert.ContainsKey[int]("maxSize")(config)(t)  // Fails
+//	}
+func ContainsKey[T any, K comparable](expected K) Kleisli[map[K]T] {
+	return func(actual map[K]T) Reader {
+		return func(t *testing.T) bool {
+			return assert.Contains(t, actual, expected)
+		}
+	}
+}
+
+// NotContainsKey tests if a key is not contained in a map.
+//
+// Example:
+//
+//	func TestNotContainsKey(t *testing.T) {
+//	    config := map[string]int{"timeout": 30, "retries": 3}
+//	    assert.NotContainsKey[int]("maxSize")(config)(t)  // Passes
+//	    assert.NotContainsKey[int]("timeout")(config)(t)  // Fails
+//	}
+func NotContainsKey[T any, K comparable](expected K) Kleisli[map[K]T] {
+	return func(actual map[K]T) Reader {
+		return func(t *testing.T) bool {
+			return assert.NotContains(t, actual, expected)
+		}
+	}
+}
+
+// That asserts that a particular predicate matches.
+//
+// This is a powerful function that allows you to create custom assertions using predicates.
+//
+// Example:
+//
+//	func TestThat(t *testing.T) {
+//	    // Test if a number is positive
+//	    isPositive := N.MoreThan(0)
+//	    assert.That(isPositive)(42)(t)  // Passes
+//	    assert.That(isPositive)(-5)(t)  // Fails
+//
+//	    // Test if a string is uppercase
+//	    isUppercase := func(s string) bool { return s == strings.ToUpper(s) }
+//	    assert.That(isUppercase)("HELLO")(t)  // Passes
+//	    assert.That(isUppercase)("Hello")(t)  // Fails
+//
+//	    // Can be combined with Local for property testing
+//	    type User struct { Age int }
+//	    ageIsAdult := assert.Local(func(u User) int { return u.Age })(
+//	        assert.That(func(age int) bool { return age >= 18 }),
+//	    )
+//	    user := User{Age: 25}
+//	    ageIsAdult(user)(t)  // Passes
+//	}
+func That[T any](pred Predicate[T]) Kleisli[T] {
+	return func(a T) Reader {
+		return func(t *testing.T) bool {
+			if pred(a) {
+				return true
+			}
+			return assert.Fail(t, fmt.Sprintf("Preficate %v does not match value %v", pred, a))
+		}
+	}
+}
+
+// AllOf combines multiple assertion Readers into a single Reader that passes
+// only if all assertions pass.
+//
+// This function uses boolean AND logic (MonoidAll) to combine the results of
+// all assertions. If any assertion fails, the combined assertion fails.
+//
+// This is useful for grouping related assertions together and ensuring all
+// conditions are met.
+//
+// Parameters:
+//   - readers: Array of assertion Readers to combine
+//
+// Returns:
+//   - A single Reader that performs all assertions and returns true only if all pass
+//
+// Example:
+//
+//	func TestUser(t *testing.T) {
+//	    user := User{Name: "Alice", Age: 30, Active: true}
+//	    assertions := assert.AllOf([]assert.Reader{
+//	        assert.Equal("Alice")(user.Name),
+//	        assert.Equal(30)(user.Age),
+//	        assert.Equal(true)(user.Active),
+//	    })
+//	    assertions(t)
+//	}
+//
+//go:inline
+func AllOf(readers []Reader) Reader {
+	return reader.MonadReduceArrayM(readers, boolean.MonoidAll)
+}
+
+// RunAll executes a map of named test cases, running each as a subtest.
+//
+// This function creates a Reader that runs multiple named test cases using
+// Go's t.Run for proper test isolation and reporting. Each test case is
+// executed as a separate subtest with its own name.
+//
+// The function returns true only if all subtests pass. This allows for
+// better test organization and clearer test output.
+//
+// Parameters:
+//   - testcases: Map of test names to assertion Readers
+//
+// Returns:
+//   - A Reader that executes all named test cases and returns true if all pass
+//
+// Example:
+//
+//	func TestMathOperations(t *testing.T) {
+//	    testcases := map[string]assert.Reader{
+//	        "addition":       assert.Equal(4)(2 + 2),
+//	        "multiplication": assert.Equal(6)(2 * 3),
+//	        "subtraction":    assert.Equal(1)(3 - 2),
+//	    }
+//	    assert.RunAll(testcases)(t)
+//	}
+//
+//go:inline
+func RunAll(testcases map[string]Reader) Reader {
+	return func(t *testing.T) bool {
+		current := true
+		for k, r := range testcases {
+			current = current && t.Run(k, func(t1 *testing.T) {
+				r(t1)
+			})
+		}
+		return current
+	}
+}
+
+// Local transforms a Reader that works on type R1 into a Reader that works on type R2,
+// by providing a function that converts R2 to R1. This allows you to focus a test on a
+// specific property or subset of a larger data structure.
+//
+// This is particularly useful when you have an assertion that operates on a specific field
+// or property, and you want to apply it to a complete object. Instead of extracting the
+// property and then asserting on it, you can transform the assertion to work directly
+// on the whole object.
+//
+// Parameters:
+//   - f: A function that extracts or transforms R2 into R1
+//
+// Returns:
+//   - A function that transforms a Reader[R1, Reader] into a Reader[R2, Reader]
+//
+// Example:
+//
+//	type User struct {
+//	    Name string
+//	    Age  int
+//	}
+//
+//	// Create an assertion that checks if age is positive
+//	ageIsPositive := assert.That(func(age int) bool { return age > 0 })
+//
+//	// Focus this assertion on the Age field of User
+//	userAgeIsPositive := assert.Local(func(u User) int { return u.Age })(ageIsPositive)
+//
+//	// Now we can test the whole User object
+//	user := User{Name: "Alice", Age: 30}
+//	userAgeIsPositive(user)(t)
+//
+//go:inline
+func Local[R1, R2 any](f func(R2) R1) func(Kleisli[R1]) Kleisli[R2] {
+	return reader.Local[Reader](f)
+}
+
+// LocalL is similar to Local but uses a Lens to focus on a specific property.
+// A Lens is a functional programming construct that provides a composable way to
+// focus on a part of a data structure.
+//
+// This function is particularly useful when you want to focus a test on a specific
+// field of a struct using a lens, making the code more declarative and composable.
+// Lenses are often code-generated or predefined for common data structures.
+//
+// Parameters:
+//   - l: A Lens that focuses from type S to type T
+//
+// Returns:
+//   - A function that transforms a Reader[T, Reader] into a Reader[S, Reader]
+//
+// Example:
+//
+//	type Person struct {
+//	    Name  string
+//	    Email string
+//	}
+//
+//	// Assume we have a lens that focuses on the Email field
+//	var emailLens = lens.Prop[Person, string]("Email")
+//
+//	// Create an assertion for email format
+//	validEmail := assert.That(func(email string) bool {
+//	    return strings.Contains(email, "@")
+//	})
+//
+//	// Focus this assertion on the Email property using a lens
+//	validPersonEmail := assert.LocalL(emailLens)(validEmail)
+//
+//	// Test a Person object
+//	person := Person{Name: "Bob", Email: "bob@example.com"}
+//	validPersonEmail(person)(t)
+//
+//go:inline
+func LocalL[S, T any](l Lens[S, T]) func(Kleisli[T]) Kleisli[S] {
+	return reader.Local[Reader](l.Get)
+}
+
+// fromOptionalGetter is an internal helper that creates an assertion Reader from
+// an optional getter function. It asserts that the optional value is present (Some).
+func fromOptionalGetter[S, T any](getter func(S) option.Option[T], msgAndArgs ...any) Kleisli[S] {
+	return func(s S) Reader {
+		return func(t *testing.T) bool {
+			return assert.True(t, option.IsSome(getter(s)), msgAndArgs...)
+		}
+	}
+}
+
+// FromOptional creates an assertion that checks if an Optional can successfully extract a value.
+// An Optional is an optic that represents an optional reference to a subpart of a data structure.
+//
+// This function is useful when you have an Optional optic and want to assert that the optional
+// value is present (Some) rather than absent (None). The assertion passes if the Optional's
+// GetOption returns Some, and fails if it returns None.
+//
+// This enables property-focused testing where you verify that a particular optional field or
+// sub-structure exists and is accessible.
+//
+// Parameters:
+//   - opt: An Optional optic that focuses from type S to type T
+//
+// Returns:
+//   - A Reader that asserts the optional value is present when applied to a value of type S
+//
+// Example:
+//
+//	type Config struct {
+//	    Database *DatabaseConfig  // Optional field
+//	}
+//
+//	type DatabaseConfig struct {
+//	    Host string
+//	    Port int
+//	}
+//
+//	// Create an Optional that focuses on the Database field
+//	dbOptional := optional.MakeOptional(
+//	    func(c Config) option.Option[*DatabaseConfig] {
+//	        if c.Database != nil {
+//	            return option.Some(c.Database)
+//	        }
+//	        return option.None[*DatabaseConfig]()
+//	    },
+//	    func(c Config, db *DatabaseConfig) Config {
+//	        c.Database = db
+//	        return c
+//	    },
+//	)
+//
+//	// Assert that the database config is present
+//	hasDatabaseConfig := assert.FromOptional(dbOptional)
+//
+//	config := Config{Database: &DatabaseConfig{Host: "localhost", Port: 5432}}
+//	hasDatabaseConfig(config)(t)  // Passes
+//
+//	emptyConfig := Config{Database: nil}
+//	hasDatabaseConfig(emptyConfig)(t)  // Fails
+//
+//go:inline
+func FromOptional[S, T any](opt Optional[S, T]) reader.Reader[S, Reader] {
+	return fromOptionalGetter(opt.GetOption, "Optional: %s", opt)
+}
+
+// FromPrism creates an assertion that checks if a Prism can successfully extract a value.
+// A Prism is an optic used to select part of a sum type (tagged union or variant).
+//
+// This function is useful when you have a Prism optic and want to assert that a value
+// matches a specific variant of a sum type. The assertion passes if the Prism's GetOption
+// returns Some (meaning the value is of the expected variant), and fails if it returns None
+// (meaning the value is a different variant).
+//
+// This enables variant-focused testing where you verify that a value is of a particular
+// type or matches a specific condition within a sum type.
+//
+// Parameters:
+//   - p: A Prism optic that focuses from type S to type T
+//
+// Returns:
+//   - A Reader that asserts the prism successfully extracts when applied to a value of type S
+//
+// Example:
+//
+//	type Result interface{ isResult() }
+//	type Success struct{ Value int }
+//	type Failure struct{ Error string }
+//
+//	func (Success) isResult() {}
+//	func (Failure) isResult() {}
+//
+//	// Create a Prism that focuses on Success variant
+//	successPrism := prism.MakePrism(
+//	    func(r Result) option.Option[int] {
+//	        if s, ok := r.(Success); ok {
+//	            return option.Some(s.Value)
+//	        }
+//	        return option.None[int]()
+//	    },
+//	    func(v int) Result { return Success{Value: v} },
+//	)
+//
+//	// Assert that the result is a Success
+//	isSuccess := assert.FromPrism(successPrism)
+//
+//	result1 := Success{Value: 42}
+//	isSuccess(result1)(t)  // Passes
+//
+//	result2 := Failure{Error: "something went wrong"}
+//	isSuccess(result2)(t)  // Fails
+//
+//go:inline
+func FromPrism[S, T any](p Prism[S, T]) reader.Reader[S, Reader] {
+	return fromOptionalGetter(p.GetOption, "Prism: %s", p)
+}

v2/assert/assert_test.go

@@ -16,94 +16,677 @@
 package assert
 
 import (
-	"fmt"
+	"errors"
 	"testing"
 
-	E "github.com/IBM/fp-go/v2/either"
-	EQ "github.com/IBM/fp-go/v2/eq"
-	"github.com/stretchr/testify/assert"
+	"github.com/IBM/fp-go/v2/optics/prism"
+	"github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/result"
+	S "github.com/IBM/fp-go/v2/string"
 )
 
-var (
-	errTest = fmt.Errorf("test failure")
-
-	// Eq is the equal predicate checking if objects are equal
-	Eq = EQ.FromEquals(assert.ObjectsAreEqual)
-)
-
-func wrap1[T any](wrapped func(t assert.TestingT, expected, actual any, msgAndArgs ...any) bool, t *testing.T, expected T) func(actual T) E.Either[error, T] {
-	return func(actual T) E.Either[error, T] {
-		ok := wrapped(t, expected, actual)
-		if ok {
-			return E.Of[error](actual)
+func TestEqual(t *testing.T) {
+	t.Run("should pass when values are equal", func(t *testing.T) {
+		result := Equal(42)(42)(t)
+		if !result {
+			t.Error("Expected Equal to pass for equal values")
 		}
-		return E.Left[T](errTest)
-	}
-}
+	})
 
-// NotEqual tests if the expected and the actual values are not equal
-func NotEqual[T any](t *testing.T, expected T) func(actual T) E.Either[error, T] {
-	return wrap1(assert.NotEqual, t, expected)
-}
-
-// Equal tests if the expected and the actual values are equal
-func Equal[T any](t *testing.T, expected T) func(actual T) E.Either[error, T] {
-	return wrap1(assert.Equal, t, expected)
-}
-
-// Length tests if an array has the expected length
-func Length[T any](t *testing.T, expected int) func(actual []T) E.Either[error, []T] {
-	return func(actual []T) E.Either[error, []T] {
-		ok := assert.Len(t, actual, expected)
-		if ok {
-			return E.Of[error](actual)
+	t.Run("should fail when values are not equal", func(t *testing.T) {
+		mockT := &testing.T{}
+		result := Equal(42)(43)(mockT)
+		if result {
+			t.Error("Expected Equal to fail for different values")
 		}
-		return E.Left[[]T](errTest)
-	}
+	})
+
+	t.Run("should work with strings", func(t *testing.T) {
+		result := Equal("hello")("hello")(t)
+		if !result {
+			t.Error("Expected Equal to pass for equal strings")
+		}
+	})
 }
 
-// NoError validates that there is no error
-func NoError[T any](t *testing.T) func(actual E.Either[error, T]) E.Either[error, T] {
-	return func(actual E.Either[error, T]) E.Either[error, T] {
-		return E.MonadFold(actual, func(e error) E.Either[error, T] {
-			assert.NoError(t, e)
-			return E.Left[T](e)
-		}, func(value T) E.Either[error, T] {
-			assert.NoError(t, nil)
-			return E.Right[error](value)
+func TestNotEqual(t *testing.T) {
+	t.Run("should pass when values are not equal", func(t *testing.T) {
+		result := NotEqual(42)(43)(t)
+		if !result {
+			t.Error("Expected NotEqual to pass for different values")
+		}
+	})
+
+	t.Run("should fail when values are equal", func(t *testing.T) {
+		mockT := &testing.T{}
+		result := NotEqual(42)(42)(mockT)
+		if result {
+			t.Error("Expected NotEqual to fail for equal values")
+		}
+	})
+}
+
+func TestArrayNotEmpty(t *testing.T) {
+	t.Run("should pass for non-empty array", func(t *testing.T) {
+		arr := []int{1, 2, 3}
+		result := ArrayNotEmpty(arr)(t)
+		if !result {
+			t.Error("Expected ArrayNotEmpty to pass for non-empty array")
+		}
+	})
+
+	t.Run("should fail for empty array", func(t *testing.T) {
+		mockT := &testing.T{}
+		arr := []int{}
+		result := ArrayNotEmpty(arr)(mockT)
+		if result {
+			t.Error("Expected ArrayNotEmpty to fail for empty array")
+		}
+	})
+}
+
+func TestRecordNotEmpty(t *testing.T) {
+	t.Run("should pass for non-empty map", func(t *testing.T) {
+		mp := map[string]int{"a": 1, "b": 2}
+		result := RecordNotEmpty(mp)(t)
+		if !result {
+			t.Error("Expected RecordNotEmpty to pass for non-empty map")
+		}
+	})
+
+	t.Run("should fail for empty map", func(t *testing.T) {
+		mockT := &testing.T{}
+		mp := map[string]int{}
+		result := RecordNotEmpty(mp)(mockT)
+		if result {
+			t.Error("Expected RecordNotEmpty to fail for empty map")
+		}
+	})
+}
+
+func TestArrayLength(t *testing.T) {
+	t.Run("should pass when length matches", func(t *testing.T) {
+		arr := []int{1, 2, 3}
+		result := ArrayLength[int](3)(arr)(t)
+		if !result {
+			t.Error("Expected ArrayLength to pass when length matches")
+		}
+	})
+
+	t.Run("should fail when length doesn't match", func(t *testing.T) {
+		mockT := &testing.T{}
+		arr := []int{1, 2, 3}
+		result := ArrayLength[int](5)(arr)(mockT)
+		if result {
+			t.Error("Expected ArrayLength to fail when length doesn't match")
+		}
+	})
+
+	t.Run("should work with empty array", func(t *testing.T) {
+		arr := []string{}
+		result := ArrayLength[string](0)(arr)(t)
+		if !result {
+			t.Error("Expected ArrayLength to pass for empty array with expected length 0")
+		}
+	})
+}
+
+func TestRecordLength(t *testing.T) {
+	t.Run("should pass when map length matches", func(t *testing.T) {
+		mp := map[string]int{"a": 1, "b": 2}
+		result := RecordLength[string, int](2)(mp)(t)
+		if !result {
+			t.Error("Expected RecordLength to pass when length matches")
+		}
+	})
+
+	t.Run("should fail when map length doesn't match", func(t *testing.T) {
+		mockT := &testing.T{}
+		mp := map[string]int{"a": 1}
+		result := RecordLength[string, int](3)(mp)(mockT)
+		if result {
+			t.Error("Expected RecordLength to fail when length doesn't match")
+		}
+	})
+}
+
+func TestStringLength(t *testing.T) {
+	t.Run("should pass when string length matches", func(t *testing.T) {
+		str := "hello"
+		result := StringLength[string, int](5)(str)(t)
+		if !result {
+			t.Error("Expected StringLength to pass when length matches")
+		}
+	})
+
+	t.Run("should fail when string length doesn't match", func(t *testing.T) {
+		mockT := &testing.T{}
+		str := "hello"
+		result := StringLength[string, int](10)(str)(mockT)
+		if result {
+			t.Error("Expected StringLength to fail when length doesn't match")
+		}
+	})
+
+	t.Run("should work with empty string", func(t *testing.T) {
+		str := ""
+		result := StringLength[string, int](0)(str)(t)
+		if !result {
+			t.Error("Expected StringLength to pass for empty string with expected length 0")
+		}
+	})
+}
+
+func TestNoError(t *testing.T) {
+	t.Run("should pass when error is nil", func(t *testing.T) {
+		result := NoError(nil)(t)
+		if !result {
+			t.Error("Expected NoError to pass when error is nil")
+		}
+	})
+
+	t.Run("should fail when error is not nil", func(t *testing.T) {
+		mockT := &testing.T{}
+		err := errors.New("test error")
+		result := NoError(err)(mockT)
+		if result {
+			t.Error("Expected NoError to fail when error is not nil")
+		}
+	})
+}
+
+func TestError(t *testing.T) {
+	t.Run("should pass when error is not nil", func(t *testing.T) {
+		err := errors.New("test error")
+		result := Error(err)(t)
+		if !result {
+			t.Error("Expected Error to pass when error is not nil")
+		}
+	})
+
+	t.Run("should fail when error is nil", func(t *testing.T) {
+		mockT := &testing.T{}
+		result := Error(nil)(mockT)
+		if result {
+			t.Error("Expected Error to fail when error is nil")
+		}
+	})
+}
+
+func TestSuccess(t *testing.T) {
+	t.Run("should pass for successful result", func(t *testing.T) {
+		res := result.Of(42)
+		result := Success(res)(t)
+		if !result {
+			t.Error("Expected Success to pass for successful result")
+		}
+	})
+
+	t.Run("should fail for error result", func(t *testing.T) {
+		mockT := &testing.T{}
+		res := result.Left[int](errors.New("test error"))
+		result := Success(res)(mockT)
+		if result {
+			t.Error("Expected Success to fail for error result")
+		}
+	})
+}
+
+func TestFailure(t *testing.T) {
+	t.Run("should pass for error result", func(t *testing.T) {
+		res := result.Left[int](errors.New("test error"))
+		result := Failure(res)(t)
+		if !result {
+			t.Error("Expected Failure to pass for error result")
+		}
+	})
+
+	t.Run("should fail for successful result", func(t *testing.T) {
+		mockT := &testing.T{}
+		res := result.Of(42)
+		result := Failure(res)(mockT)
+		if result {
+			t.Error("Expected Failure to fail for successful result")
+		}
+	})
+}
+
+func TestArrayContains(t *testing.T) {
+	t.Run("should pass when element is in array", func(t *testing.T) {
+		arr := []int{1, 2, 3, 4, 5}
+		result := ArrayContains(3)(arr)(t)
+		if !result {
+			t.Error("Expected ArrayContains to pass when element is in array")
+		}
+	})
+
+	t.Run("should fail when element is not in array", func(t *testing.T) {
+		mockT := &testing.T{}
+		arr := []int{1, 2, 3}
+		result := ArrayContains(10)(arr)(mockT)
+		if result {
+			t.Error("Expected ArrayContains to fail when element is not in array")
+		}
+	})
+
+	t.Run("should work with strings", func(t *testing.T) {
+		arr := []string{"apple", "banana", "cherry"}
+		result := ArrayContains("banana")(arr)(t)
+		if !result {
+			t.Error("Expected ArrayContains to pass for string element")
+		}
+	})
+}
+
+func TestContainsKey(t *testing.T) {
+	t.Run("should pass when key exists in map", func(t *testing.T) {
+		mp := map[string]int{"a": 1, "b": 2, "c": 3}
+		result := ContainsKey[int]("b")(mp)(t)
+		if !result {
+			t.Error("Expected ContainsKey to pass when key exists")
+		}
+	})
+
+	t.Run("should fail when key doesn't exist in map", func(t *testing.T) {
+		mockT := &testing.T{}
+		mp := map[string]int{"a": 1, "b": 2}
+		result := ContainsKey[int]("z")(mp)(mockT)
+		if result {
+			t.Error("Expected ContainsKey to fail when key doesn't exist")
+		}
+	})
+}
+
+func TestNotContainsKey(t *testing.T) {
+	t.Run("should pass when key doesn't exist in map", func(t *testing.T) {
+		mp := map[string]int{"a": 1, "b": 2}
+		result := NotContainsKey[int]("z")(mp)(t)
+		if !result {
+			t.Error("Expected NotContainsKey to pass when key doesn't exist")
+		}
+	})
+
+	t.Run("should fail when key exists in map", func(t *testing.T) {
+		mockT := &testing.T{}
+		mp := map[string]int{"a": 1, "b": 2}
+		result := NotContainsKey[int]("a")(mp)(mockT)
+		if result {
+			t.Error("Expected NotContainsKey to fail when key exists")
+		}
+	})
+}
+
+func TestThat(t *testing.T) {
+	t.Run("should pass when predicate is true", func(t *testing.T) {
+		isEven := func(n int) bool { return n%2 == 0 }
+		result := That(isEven)(42)(t)
+		if !result {
+			t.Error("Expected That to pass when predicate is true")
+		}
+	})
+
+	t.Run("should fail when predicate is false", func(t *testing.T) {
+		mockT := &testing.T{}
+		isEven := func(n int) bool { return n%2 == 0 }
+		result := That(isEven)(43)(mockT)
+		if result {
+			t.Error("Expected That to fail when predicate is false")
+		}
+	})
+
+	t.Run("should work with string predicates", func(t *testing.T) {
+		startsWithH := func(s string) bool { return S.IsNonEmpty(s) && s[0] == 'h' }
+		result := That(startsWithH)("hello")(t)
+		if !result {
+			t.Error("Expected That to pass for string predicate")
+		}
+	})
+}
+
+func TestAllOf(t *testing.T) {
+	t.Run("should pass when all assertions pass", func(t *testing.T) {
+		assertions := AllOf([]Reader{
+			Equal(42)(42),
+			Equal("hello")("hello"),
+			ArrayNotEmpty([]int{1, 2, 3}),
 		})
-	}
+		result := assertions(t)
+		if !result {
+			t.Error("Expected AllOf to pass when all assertions pass")
+		}
+	})
+
+	t.Run("should fail when any assertion fails", func(t *testing.T) {
+		mockT := &testing.T{}
+		assertions := AllOf([]Reader{
+			Equal(42)(42),
+			Equal("hello")("goodbye"),
+			ArrayNotEmpty([]int{1, 2, 3}),
+		})
+		result := assertions(mockT)
+		if result {
+			t.Error("Expected AllOf to fail when any assertion fails")
+		}
+	})
+
+	t.Run("should work with empty array", func(t *testing.T) {
+		assertions := AllOf([]Reader{})
+		result := assertions(t)
+		if !result {
+			t.Error("Expected AllOf to pass for empty array")
+		}
+	})
+
+	t.Run("should combine multiple array assertions", func(t *testing.T) {
+		arr := []int{1, 2, 3, 4, 5}
+		assertions := AllOf([]Reader{
+			ArrayNotEmpty(arr),
+			ArrayLength[int](5)(arr),
+			ArrayContains(3)(arr),
+		})
+		result := assertions(t)
+		if !result {
+			t.Error("Expected AllOf to pass for multiple array assertions")
+		}
+	})
 }
 
-// ArrayContains tests if a value is contained in an array
-func ArrayContains[T any](t *testing.T, expected T) func(actual []T) E.Either[error, []T] {
-	return func(actual []T) E.Either[error, []T] {
-		ok := assert.Contains(t, actual, expected)
-		if ok {
-			return E.Of[error](actual)
+func TestRunAll(t *testing.T) {
+	t.Run("should run all named test cases", func(t *testing.T) {
+		testcases := map[string]Reader{
+			"equality":     Equal(42)(42),
+			"string_check": Equal("test")("test"),
+			"array_check":  ArrayNotEmpty([]int{1, 2, 3}),
 		}
-		return E.Left[[]T](errTest)
-	}
+		result := RunAll(testcases)(t)
+		if !result {
+			t.Error("Expected RunAll to pass when all test cases pass")
+		}
+	})
+
+	// Note: Testing failure behavior of RunAll is tricky because subtests
+	// will actually fail in the test framework. The function works correctly
+	// as demonstrated by the passing test above.
+
+	t.Run("should work with empty test cases", func(t *testing.T) {
+		testcases := map[string]Reader{}
+		result := RunAll(testcases)(t)
+		if !result {
+			t.Error("Expected RunAll to pass for empty test cases")
+		}
+	})
 }
 
-// ContainsKey tests if a key is contained in a map
-func ContainsKey[T any, K comparable](t *testing.T, expected K) func(actual map[K]T) E.Either[error, map[K]T] {
-	return func(actual map[K]T) E.Either[error, map[K]T] {
-		ok := assert.Contains(t, actual, expected)
-		if ok {
-			return E.Of[error](actual)
+func TestEq(t *testing.T) {
+	t.Run("should return true for equal values", func(t *testing.T) {
+		if !Eq.Equals(42, 42) {
+			t.Error("Expected Eq to return true for equal integers")
 		}
-		return E.Left[map[K]T](errTest)
-	}
+	})
+
+	t.Run("should return false for different values", func(t *testing.T) {
+		if Eq.Equals(42, 43) {
+			t.Error("Expected Eq to return false for different integers")
+		}
+	})
+
+	t.Run("should work with strings", func(t *testing.T) {
+		if !Eq.Equals("hello", "hello") {
+			t.Error("Expected Eq to return true for equal strings")
+		}
+		if Eq.Equals("hello", "world") {
+			t.Error("Expected Eq to return false for different strings")
+		}
+	})
+
+	t.Run("should work with slices", func(t *testing.T) {
+		arr1 := []int{1, 2, 3}
+		arr2 := []int{1, 2, 3}
+		if !Eq.Equals(arr1, arr2) {
+			t.Error("Expected Eq to return true for equal slices")
+		}
+	})
 }
 
-// NotContainsKey tests if a key is not contained in a map
-func NotContainsKey[T any, K comparable](t *testing.T, expected K) func(actual map[K]T) E.Either[error, map[K]T] {
-	return func(actual map[K]T) E.Either[error, map[K]T] {
-		ok := assert.NotContains(t, actual, expected)
-		if ok {
-			return E.Of[error](actual)
-		}
-		return E.Left[map[K]T](errTest)
+func TestLocal(t *testing.T) {
+	type User struct {
+		Name string
+		Age  int
 	}
+
+	t.Run("should focus assertion on a property", func(t *testing.T) {
+		// Create an assertion that checks if age is positive
+		ageIsPositive := That(func(age int) bool { return age > 0 })
+
+		// Focus this assertion on the Age field of User
+		userAgeIsPositive := Local(func(u User) int { return u.Age })(ageIsPositive)
+
+		// Test with a user who has a positive age
+		user := User{Name: "Alice", Age: 30}
+		result := userAgeIsPositive(user)(t)
+		if !result {
+			t.Error("Expected focused assertion to pass for positive age")
+		}
+	})
+
+	t.Run("should fail when focused property doesn't match", func(t *testing.T) {
+		mockT := &testing.T{}
+		ageIsPositive := That(func(age int) bool { return age > 0 })
+		userAgeIsPositive := Local(func(u User) int { return u.Age })(ageIsPositive)
+
+		// Test with a user who has zero age
+		user := User{Name: "Bob", Age: 0}
+		result := userAgeIsPositive(user)(mockT)
+		if result {
+			t.Error("Expected focused assertion to fail for zero age")
+		}
+	})
+
+	t.Run("should compose with other assertions", func(t *testing.T) {
+		// Create multiple focused assertions
+		nameNotEmpty := Local(func(u User) string { return u.Name })(
+			That(S.IsNonEmpty),
+		)
+		ageInRange := Local(func(u User) int { return u.Age })(
+			That(func(age int) bool { return age >= 18 && age <= 100 }),
+		)
+
+		user := User{Name: "Charlie", Age: 25}
+		assertions := AllOf([]Reader{
+			nameNotEmpty(user),
+			ageInRange(user),
+		})
+
+		result := assertions(t)
+		if !result {
+			t.Error("Expected composed focused assertions to pass")
+		}
+	})
+
+	t.Run("should work with Equal assertion", func(t *testing.T) {
+		// Focus Equal assertion on Name field
+		nameIsAlice := Local(func(u User) string { return u.Name })(Equal("Alice"))
+
+		user := User{Name: "Alice", Age: 30}
+		result := nameIsAlice(user)(t)
+		if !result {
+			t.Error("Expected focused Equal assertion to pass")
+		}
+	})
+}
+
+func TestLocalL(t *testing.T) {
+	// Note: LocalL requires lens package which provides lens operations.
+	// This test demonstrates the concept, but actual usage would require
+	// proper lens definitions.
+
+	t.Run("conceptual test for LocalL", func(t *testing.T) {
+		// LocalL is similar to Local but uses lenses for focusing.
+		// It would be used like:
+		// validEmail := That(func(email string) bool { return strings.Contains(email, "@") })
+		// validPersonEmail := LocalL(emailLens)(validEmail)
+		//
+		// The actual implementation would require lens definitions from the lens package.
+		// This test serves as documentation for the intended usage.
+	})
+}
+
+func TestFromOptional(t *testing.T) {
+	type DatabaseConfig struct {
+		Host string
+		Port int
+	}
+
+	type Config struct {
+		Database *DatabaseConfig
+	}
+
+	// Create an Optional that focuses on the Database field
+	dbOptional := Optional[Config, *DatabaseConfig]{
+		GetOption: func(c Config) option.Option[*DatabaseConfig] {
+			if c.Database != nil {
+				return option.Of(c.Database)
+			}
+			return option.None[*DatabaseConfig]()
+		},
+		Set: func(db *DatabaseConfig) func(Config) Config {
+			return func(c Config) Config {
+				c.Database = db
+				return c
+			}
+		},
+	}
+
+	t.Run("should pass when optional value is present", func(t *testing.T) {
+		config := Config{Database: &DatabaseConfig{Host: "localhost", Port: 5432}}
+		hasDatabaseConfig := FromOptional(dbOptional)
+		result := hasDatabaseConfig(config)(t)
+		if !result {
+			t.Error("Expected FromOptional to pass when optional value is present")
+		}
+	})
+
+	t.Run("should fail when optional value is absent", func(t *testing.T) {
+		mockT := &testing.T{}
+		emptyConfig := Config{Database: nil}
+		hasDatabaseConfig := FromOptional(dbOptional)
+		result := hasDatabaseConfig(emptyConfig)(mockT)
+		if result {
+			t.Error("Expected FromOptional to fail when optional value is absent")
+		}
+	})
+
+	t.Run("should work with nested optionals", func(t *testing.T) {
+		type AdvancedSettings struct {
+			Cache bool
+		}
+
+		type Settings struct {
+			Advanced *AdvancedSettings
+		}
+
+		advancedOptional := Optional[Settings, *AdvancedSettings]{
+			GetOption: func(s Settings) option.Option[*AdvancedSettings] {
+				if s.Advanced != nil {
+					return option.Of(s.Advanced)
+				}
+				return option.None[*AdvancedSettings]()
+			},
+			Set: func(adv *AdvancedSettings) func(Settings) Settings {
+				return func(s Settings) Settings {
+					s.Advanced = adv
+					return s
+				}
+			},
+		}
+
+		settings := Settings{Advanced: &AdvancedSettings{Cache: true}}
+		hasAdvanced := FromOptional(advancedOptional)
+		result := hasAdvanced(settings)(t)
+		if !result {
+			t.Error("Expected FromOptional to pass for nested optional")
+		}
+	})
+}
+
+// Helper types for Prism testing
+type PrismTestResult interface {
+	isPrismTestResult()
+}
+
+type PrismTestSuccess struct {
+	Value int
+}
+
+type PrismTestFailure struct {
+	Error string
+}
+
+func (PrismTestSuccess) isPrismTestResult() {}
+func (PrismTestFailure) isPrismTestResult() {}
+
+func TestFromPrism(t *testing.T) {
+	// Create a Prism that focuses on Success variant using prism.MakePrism
+	successPrism := prism.MakePrism(
+		func(r PrismTestResult) option.Option[int] {
+			if s, ok := r.(PrismTestSuccess); ok {
+				return option.Of(s.Value)
+			}
+			return option.None[int]()
+		},
+		func(v int) PrismTestResult {
+			return PrismTestSuccess{Value: v}
+		},
+	)
+
+	// Create a Prism that focuses on Failure variant
+	failurePrism := prism.MakePrism(
+		func(r PrismTestResult) option.Option[string] {
+			if f, ok := r.(PrismTestFailure); ok {
+				return option.Of(f.Error)
+			}
+			return option.None[string]()
+		},
+		func(err string) PrismTestResult {
+			return PrismTestFailure{Error: err}
+		},
+	)
+
+	t.Run("should pass when prism successfully extracts", func(t *testing.T) {
+		result := PrismTestSuccess{Value: 42}
+		isSuccess := FromPrism(successPrism)
+		testResult := isSuccess(result)(t)
+		if !testResult {
+			t.Error("Expected FromPrism to pass when prism extracts successfully")
+		}
+	})
+
+	t.Run("should fail when prism cannot extract", func(t *testing.T) {
+		mockT := &testing.T{}
+		result := PrismTestFailure{Error: "something went wrong"}
+		isSuccess := FromPrism(successPrism)
+		testResult := isSuccess(result)(mockT)
+		if testResult {
+			t.Error("Expected FromPrism to fail when prism cannot extract")
+		}
+	})
+
+	t.Run("should work with failure prism", func(t *testing.T) {
+		result := PrismTestFailure{Error: "test error"}
+		isFailure := FromPrism(failurePrism)
+		testResult := isFailure(result)(t)
+		if !testResult {
+			t.Error("Expected FromPrism to pass for failure prism on failure result")
+		}
+	})
+
+	t.Run("should fail with failure prism on success result", func(t *testing.T) {
+		mockT := &testing.T{}
+		result := PrismTestSuccess{Value: 100}
+		isFailure := FromPrism(failurePrism)
+		testResult := isFailure(result)(mockT)
+		if testResult {
+			t.Error("Expected FromPrism to fail for failure prism on success result")
+		}
+	})
 }

v2/assert/example_test.go

@@ -0,0 +1,236 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package assert_test
+
+import (
+	"errors"
+	"strings"
+	"testing"
+
+	"github.com/IBM/fp-go/v2/assert"
+	N "github.com/IBM/fp-go/v2/number"
+	"github.com/IBM/fp-go/v2/result"
+)
+
+// Example_basicAssertions demonstrates basic equality and inequality assertions
+func Example_basicAssertions() {
+	// This would be in a real test function
+	var t *testing.T // placeholder for example
+
+	// Basic equality
+	value := 42
+	assert.Equal(42)(value)(t)
+
+	// String equality
+	name := "Alice"
+	assert.Equal("Alice")(name)(t)
+
+	// Inequality
+	assert.NotEqual(10)(value)(t)
+}
+
+// Example_arrayAssertions demonstrates array-related assertions
+func Example_arrayAssertions() {
+	var t *testing.T // placeholder for example
+
+	numbers := []int{1, 2, 3, 4, 5}
+
+	// Check array is not empty
+	assert.ArrayNotEmpty(numbers)(t)
+
+	// Check array length
+	assert.ArrayLength[int](5)(numbers)(t)
+
+	// Check array contains a value
+	assert.ArrayContains(3)(numbers)(t)
+}
+
+// Example_mapAssertions demonstrates map-related assertions
+func Example_mapAssertions() {
+	var t *testing.T // placeholder for example
+
+	config := map[string]int{
+		"timeout": 30,
+		"retries": 3,
+		"maxSize": 1000,
+	}
+
+	// Check map is not empty
+	assert.RecordNotEmpty(config)(t)
+
+	// Check map length
+	assert.RecordLength[string, int](3)(config)(t)
+
+	// Check map contains key
+	assert.ContainsKey[int]("timeout")(config)(t)
+
+	// Check map does not contain key
+	assert.NotContainsKey[int]("unknown")(config)(t)
+}
+
+// Example_errorAssertions demonstrates error-related assertions
+func Example_errorAssertions() {
+	var t *testing.T // placeholder for example
+
+	// Assert no error
+	err := doSomethingSuccessful()
+	assert.NoError(err)(t)
+
+	// Assert error exists
+	err2 := doSomethingThatFails()
+	assert.Error(err2)(t)
+}
+
+// Example_resultAssertions demonstrates Result type assertions
+func Example_resultAssertions() {
+	var t *testing.T // placeholder for example
+
+	// Assert success
+	successResult := result.Of(42)
+	assert.Success(successResult)(t)
+
+	// Assert failure
+	failureResult := result.Left[int](errors.New("something went wrong"))
+	assert.Failure(failureResult)(t)
+}
+
+// Example_predicateAssertions demonstrates custom predicate assertions
+func Example_predicateAssertions() {
+	var t *testing.T // placeholder for example
+
+	// Test if a number is positive
+	isPositive := N.MoreThan(0)
+	assert.That(isPositive)(42)(t)
+
+	// Test if a string is uppercase
+	isUppercase := func(s string) bool { return s == strings.ToUpper(s) }
+	assert.That(isUppercase)("HELLO")(t)
+
+	// Test if a number is even
+	isEven := func(n int) bool { return n%2 == 0 }
+	assert.That(isEven)(10)(t)
+}
+
+// Example_allOf demonstrates combining multiple assertions
+func Example_allOf() {
+	var t *testing.T // placeholder for example
+
+	type User struct {
+		Name   string
+		Age    int
+		Active bool
+	}
+
+	user := User{Name: "Alice", Age: 30, Active: true}
+
+	// Combine multiple assertions
+	assertions := assert.AllOf([]assert.Reader{
+		assert.Equal("Alice")(user.Name),
+		assert.Equal(30)(user.Age),
+		assert.Equal(true)(user.Active),
+	})
+
+	assertions(t)
+}
+
+// Example_runAll demonstrates running named test cases
+func Example_runAll() {
+	var t *testing.T // placeholder for example
+
+	testcases := map[string]assert.Reader{
+		"addition":       assert.Equal(4)(2 + 2),
+		"multiplication": assert.Equal(6)(2 * 3),
+		"subtraction":    assert.Equal(1)(3 - 2),
+		"division":       assert.Equal(2)(10 / 5),
+	}
+
+	assert.RunAll(testcases)(t)
+}
+
+// Example_local demonstrates focusing assertions on specific properties
+func Example_local() {
+	var t *testing.T // placeholder for example
+
+	type User struct {
+		Name string
+		Age  int
+	}
+
+	// Create an assertion that checks if age is positive
+	ageIsPositive := assert.That(func(age int) bool { return age > 0 })
+
+	// Focus this assertion on the Age field of User
+	userAgeIsPositive := assert.Local(func(u User) int { return u.Age })(ageIsPositive)
+
+	// Now we can test the whole User object
+	user := User{Name: "Alice", Age: 30}
+	userAgeIsPositive(user)(t)
+}
+
+// Example_composableAssertions demonstrates building complex assertions
+func Example_composableAssertions() {
+	var t *testing.T // placeholder for example
+
+	type Config struct {
+		Host    string
+		Port    int
+		Timeout int
+		Retries int
+	}
+
+	config := Config{
+		Host:    "localhost",
+		Port:    8080,
+		Timeout: 30,
+		Retries: 3,
+	}
+
+	// Create focused assertions for each field
+	validHost := assert.Local(func(c Config) string { return c.Host })(
+		assert.StringNotEmpty,
+	)
+
+	validPort := assert.Local(func(c Config) int { return c.Port })(
+		assert.That(func(p int) bool { return p > 0 && p < 65536 }),
+	)
+
+	validTimeout := assert.Local(func(c Config) int { return c.Timeout })(
+		assert.That(func(t int) bool { return t > 0 }),
+	)
+
+	validRetries := assert.Local(func(c Config) int { return c.Retries })(
+		assert.That(func(r int) bool { return r >= 0 }),
+	)
+
+	// Combine all assertions
+	validConfig := assert.AllOf([]assert.Reader{
+		validHost(config),
+		validPort(config),
+		validTimeout(config),
+		validRetries(config),
+	})
+
+	validConfig(t)
+}
+
+// Helper functions for examples
+func doSomethingSuccessful() error {
+	return nil
+}
+
+func doSomethingThatFails() error {
+	return errors.New("operation failed")
+}

v2/assert/types.go

@@ -0,0 +1,35 @@
+package assert
+
+import (
+	"testing"
+
+	"github.com/IBM/fp-go/v2/optics/lens"
+	"github.com/IBM/fp-go/v2/optics/optional"
+	"github.com/IBM/fp-go/v2/optics/prism"
+	"github.com/IBM/fp-go/v2/predicate"
+	"github.com/IBM/fp-go/v2/reader"
+	"github.com/IBM/fp-go/v2/result"
+)
+
+type (
+	// Result represents a computation that may fail with an error.
+	Result[T any] = result.Result[T]
+
+	// Reader represents a test assertion that depends on a testing.T context and returns a boolean.
+	Reader = reader.Reader[*testing.T, bool]
+
+	// Kleisli represents a function that produces a test assertion Reader from a value of type T.
+	Kleisli[T any] = reader.Reader[T, Reader]
+
+	// Predicate represents a function that tests a value of type T and returns a boolean.
+	Predicate[T any] = predicate.Predicate[T]
+
+	// Lens is a functional reference to a subpart of a data structure.
+	Lens[S, T any] = lens.Lens[S, T]
+
+	// Optional is an optic that focuses on a value that may or may not be present.
+	Optional[S, T any] = optional.Optional[S, T]
+
+	// Prism is an optic that focuses on a case of a sum type.
+	Prism[S, T any] = prism.Prism[S, T]
+)

v2/boolean/types.go

@@ -18,5 +18,7 @@ package boolean
 import "github.com/IBM/fp-go/v2/monoid"
 
 type (
+	// Monoid represents a monoid structure for boolean values.
+	// A monoid provides an associative binary operation and an identity element.
 	Monoid = monoid.Monoid[bool]
 )

v2/bounded/bounded.go

@@ -53,7 +53,7 @@ func MakeBounded[T any](o ord.Ord[T], t, b T) Bounded[T] {
 
 // Clamp returns a function that clamps against the bounds defined in the bounded type
 func Clamp[T any](b Bounded[T]) func(T) T {
-	return ord.Clamp[T](b)(b.Bottom(), b.Top())
+	return ord.Clamp(b)(b.Bottom(), b.Top())
 }
 
 // Reverse reverses the ordering and swaps the bounds

v2/builder/builder.go

@@ -0,0 +1,7 @@
+package builder
+
+type (
+	Builder[T any] interface {
+		Build() Result[T]
+	}
+)

v2/builder/prism.go

@@ -0,0 +1,12 @@
+package builder
+
+import (
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/optics/prism"
+	"github.com/IBM/fp-go/v2/result"
+)
+
+// BuilderPrism createa a [Prism] that converts between a builder and its type
+func BuilderPrism[T any, B Builder[T]](creator func(T) B) Prism[B, T] {
+	return prism.MakePrismWithName(F.Flow2(B.Build, result.ToOption[T]), creator, "BuilderPrism")
+}

v2/builder/types.go

@@ -0,0 +1,20 @@
+package builder
+
+import (
+	"github.com/IBM/fp-go/v2/optics/prism"
+	"github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/result"
+)
+
+type (
+	// Result represents a computation that may fail with an error.
+	// It's an alias for Either[error, T].
+	Result[T any] = result.Result[T]
+
+	// Prism is an optic that focuses on a case of a sum type.
+	// It provides a way to extract and construct values of a specific variant.
+	Prism[S, A any] = prism.Prism[S, A]
+
+	// Option represents an optional value that may or may not be present.
+	Option[T any] = option.Option[T]
+)

v2/bytes/bytes.go

@@ -15,14 +15,163 @@
 
 package bytes
 
+// Empty returns an empty byte slice.
+//
+// This function returns the identity element for the byte slice Monoid,
+// which is an empty byte slice. It's useful as a starting point for
+// building byte slices or as a default value.
+//
+// Returns:
+//   - An empty byte slice ([]byte{})
+//
+// Properties:
+//   - Empty() is the identity element for Monoid.Concat
+//   - Monoid.Concat(Empty(), x) == x
+//   - Monoid.Concat(x, Empty()) == x
+//
+// Example - Basic usage:
+//
+//	empty := Empty()
+//	fmt.Println(len(empty)) // 0
+//
+// Example - As identity element:
+//
+//	data := []byte("hello")
+//	result1 := Monoid.Concat(Empty(), data) // []byte("hello")
+//	result2 := Monoid.Concat(data, Empty()) // []byte("hello")
+//
+// Example - Building byte slices:
+//
+//	// Start with empty and build up
+//	buffer := Empty()
+//	buffer = Monoid.Concat(buffer, []byte("Hello"))
+//	buffer = Monoid.Concat(buffer, []byte(" "))
+//	buffer = Monoid.Concat(buffer, []byte("World"))
+//	// buffer: []byte("Hello World")
+//
+// See also:
+//   - Monoid.Empty(): Alternative way to get empty byte slice
+//   - ConcatAll(): For concatenating multiple byte slices
 func Empty() []byte {
 	return Monoid.Empty()
 }
 
+// ToString converts a byte slice to a string.
+//
+// This function performs a direct conversion from []byte to string.
+// The conversion creates a new string with a copy of the byte data.
+//
+// Parameters:
+//   - a: The byte slice to convert
+//
+// Returns:
+//   - A string containing the same data as the byte slice
+//
+// Performance Note:
+//
+// This conversion allocates a new string. For performance-critical code
+// that needs to avoid allocations, consider using unsafe.String (Go 1.20+)
+// or working directly with byte slices.
+//
+// Example - Basic conversion:
+//
+//	bytes := []byte("hello")
+//	str := ToString(bytes)
+//	fmt.Println(str) // "hello"
+//
+// Example - Converting binary data:
+//
+//	// ASCII codes for "Hello"
+//	data := []byte{0x48, 0x65, 0x6c, 0x6c, 0x6f}
+//	str := ToString(data)
+//	fmt.Println(str) // "Hello"
+//
+// Example - Empty byte slice:
+//
+//	empty := Empty()
+//	str := ToString(empty)
+//	fmt.Println(str == "") // true
+//
+// Example - UTF-8 encoded text:
+//
+//	utf8Bytes := []byte("Hello, 世界")
+//	str := ToString(utf8Bytes)
+//	fmt.Println(str) // "Hello, 世界"
+//
+// Example - Round-trip conversion:
+//
+//	original := "test string"
+//	bytes := []byte(original)
+//	result := ToString(bytes)
+//	fmt.Println(original == result) // true
+//
+// See also:
+//   - []byte(string): For converting string to byte slice
+//   - Size(): For getting the length of a byte slice
 func ToString(a []byte) string {
 	return string(a)
 }
 
+// Size returns the number of bytes in a byte slice.
+//
+// This function returns the length of the byte slice, which is the number
+// of bytes it contains. This is equivalent to len(as) but provided as a
+// named function for use in functional composition.
+//
+// Parameters:
+//   - as: The byte slice to measure
+//
+// Returns:
+//   - The number of bytes in the slice
+//
+// Example - Basic usage:
+//
+//	data := []byte("hello")
+//	size := Size(data)
+//	fmt.Println(size) // 5
+//
+// Example - Empty slice:
+//
+//	empty := Empty()
+//	size := Size(empty)
+//	fmt.Println(size) // 0
+//
+// Example - Binary data:
+//
+//	binary := []byte{0x01, 0x02, 0x03, 0x04}
+//	size := Size(binary)
+//	fmt.Println(size) // 4
+//
+// Example - UTF-8 encoded text:
+//
+//	// Note: Size returns byte count, not character count
+//	utf8 := []byte("Hello, 世界")
+//	byteCount := Size(utf8)
+//	fmt.Println(byteCount) // 13 (not 9 characters)
+//
+// Example - Using in functional composition:
+//
+//	import "github.com/IBM/fp-go/v2/array"
+//
+//	slices := [][]byte{
+//	    []byte("a"),
+//	    []byte("bb"),
+//	    []byte("ccc"),
+//	}
+//
+//	// Map to get sizes
+//	sizes := array.Map(Size)(slices)
+//	// sizes: []int{1, 2, 3}
+//
+// Example - Checking if slice is empty:
+//
+//	data := []byte("test")
+//	isEmpty := Size(data) == 0
+//	fmt.Println(isEmpty) // false
+//
+// See also:
+//   - len(): Built-in function for getting slice length
+//   - ToString(): For converting byte slice to string
 func Size(as []byte) int {
 	return len(as)
 }

v2/bytes/bytes_test.go

@@ -187,6 +187,299 @@ func TestOrd(t *testing.T) {
 	})
 }
 
+// TestOrdProperties tests mathematical properties of Ord
+func TestOrdProperties(t *testing.T) {
+	t.Run("reflexivity: x == x", func(t *testing.T) {
+		testCases := [][]byte{
+			[]byte{},
+			[]byte("a"),
+			[]byte("test"),
+			[]byte{0x01, 0x02, 0x03},
+		}
+
+		for _, tc := range testCases {
+			assert.Equal(t, 0, Ord.Compare(tc, tc),
+				"Compare(%v, %v) should be 0", tc, tc)
+			assert.True(t, Ord.Equals(tc, tc),
+				"Equals(%v, %v) should be true", tc, tc)
+		}
+	})
+
+	t.Run("antisymmetry: if x <= y and y <= x then x == y", func(t *testing.T) {
+		testCases := []struct {
+			a, b []byte
+		}{
+			{[]byte("abc"), []byte("abc")},
+			{[]byte{}, []byte{}},
+			{[]byte{0x01}, []byte{0x01}},
+		}
+
+		for _, tc := range testCases {
+			cmp1 := Ord.Compare(tc.a, tc.b)
+			cmp2 := Ord.Compare(tc.b, tc.a)
+
+			if cmp1 <= 0 && cmp2 <= 0 {
+				assert.True(t, Ord.Equals(tc.a, tc.b),
+					"If %v <= %v and %v <= %v, they should be equal", tc.a, tc.b, tc.b, tc.a)
+			}
+		}
+	})
+
+	t.Run("transitivity: if x <= y and y <= z then x <= z", func(t *testing.T) {
+		x := []byte("a")
+		y := []byte("b")
+		z := []byte("c")
+
+		cmpXY := Ord.Compare(x, y)
+		cmpYZ := Ord.Compare(y, z)
+		cmpXZ := Ord.Compare(x, z)
+
+		if cmpXY <= 0 && cmpYZ <= 0 {
+			assert.True(t, cmpXZ <= 0,
+				"If %v <= %v and %v <= %v, then %v <= %v", x, y, y, z, x, z)
+		}
+	})
+
+	t.Run("totality: either x <= y or y <= x", func(t *testing.T) {
+		testCases := []struct {
+			a, b []byte
+		}{
+			{[]byte("abc"), []byte("abd")},
+			{[]byte("xyz"), []byte("abc")},
+			{[]byte{}, []byte("a")},
+			{[]byte{0x01}, []byte{0x02}},
+		}
+
+		for _, tc := range testCases {
+			cmp1 := Ord.Compare(tc.a, tc.b)
+			cmp2 := Ord.Compare(tc.b, tc.a)
+
+			assert.True(t, cmp1 <= 0 || cmp2 <= 0,
+				"Either %v <= %v or %v <= %v must be true", tc.a, tc.b, tc.b, tc.a)
+		}
+	})
+}
+
+// TestEdgeCases tests edge cases and boundary conditions
+func TestEdgeCases(t *testing.T) {
+	t.Run("very large byte slices", func(t *testing.T) {
+		large := make([]byte, 1000000)
+		for i := range large {
+			large[i] = byte(i % 256)
+		}
+
+		size := Size(large)
+		assert.Equal(t, 1000000, size)
+
+		str := ToString(large)
+		assert.Equal(t, 1000000, len(str))
+	})
+
+	t.Run("concatenating many slices", func(t *testing.T) {
+		slices := make([][]byte, 100)
+		for i := range slices {
+			slices[i] = []byte{byte(i)}
+		}
+
+		result := ConcatAll(slices...)
+		assert.Equal(t, 100, Size(result))
+	})
+
+	t.Run("null bytes in slice", func(t *testing.T) {
+		data := []byte{0x00, 0x01, 0x00, 0x02}
+		size := Size(data)
+		assert.Equal(t, 4, size)
+
+		str := ToString(data)
+		assert.Equal(t, 4, len(str))
+	})
+
+	t.Run("comparing slices with null bytes", func(t *testing.T) {
+		a := []byte{0x00, 0x01}
+		b := []byte{0x00, 0x02}
+		assert.Equal(t, -1, Ord.Compare(a, b))
+	})
+}
+
+// TestMonoidConcatPerformance tests concatenation performance characteristics
+func TestMonoidConcatPerformance(t *testing.T) {
+	t.Run("ConcatAll vs repeated Concat", func(t *testing.T) {
+		slices := [][]byte{
+			[]byte("a"),
+			[]byte("b"),
+			[]byte("c"),
+			[]byte("d"),
+			[]byte("e"),
+		}
+
+		// Using ConcatAll
+		result1 := ConcatAll(slices...)
+
+		// Using repeated Concat
+		result2 := Monoid.Empty()
+		for _, s := range slices {
+			result2 = Monoid.Concat(result2, s)
+		}
+
+		assert.Equal(t, result1, result2)
+		assert.Equal(t, []byte("abcde"), result1)
+	})
+}
+
+// TestRoundTrip tests round-trip conversions
+func TestRoundTrip(t *testing.T) {
+	t.Run("string to bytes to string", func(t *testing.T) {
+		original := "Hello, World! 世界"
+		bytes := []byte(original)
+		result := ToString(bytes)
+		assert.Equal(t, original, result)
+	})
+
+	t.Run("bytes to string to bytes", func(t *testing.T) {
+		original := []byte{0x48, 0x65, 0x6c, 0x6c, 0x6f}
+		str := ToString(original)
+		result := []byte(str)
+		assert.Equal(t, original, result)
+	})
+}
+
+// TestConcatAllVariadic tests ConcatAll with various argument counts
+func TestConcatAllVariadic(t *testing.T) {
+	t.Run("zero arguments", func(t *testing.T) {
+		result := ConcatAll()
+		assert.Equal(t, []byte{}, result)
+	})
+
+	t.Run("one argument", func(t *testing.T) {
+		result := ConcatAll([]byte("test"))
+		assert.Equal(t, []byte("test"), result)
+	})
+
+	t.Run("two arguments", func(t *testing.T) {
+		result := ConcatAll([]byte("hello"), []byte("world"))
+		assert.Equal(t, []byte("helloworld"), result)
+	})
+
+	t.Run("many arguments", func(t *testing.T) {
+		result := ConcatAll(
+			[]byte("a"),
+			[]byte("b"),
+			[]byte("c"),
+			[]byte("d"),
+			[]byte("e"),
+			[]byte("f"),
+			[]byte("g"),
+			[]byte("h"),
+			[]byte("i"),
+			[]byte("j"),
+		)
+		assert.Equal(t, []byte("abcdefghij"), result)
+	})
+}
+
+// Benchmark tests
+func BenchmarkToString(b *testing.B) {
+	data := []byte("Hello, World!")
+
+	b.Run("small", func(b *testing.B) {
+		for b.Loop() {
+			_ = ToString(data)
+		}
+	})
+
+	b.Run("large", func(b *testing.B) {
+		large := make([]byte, 10000)
+		for i := range large {
+			large[i] = byte(i % 256)
+		}
+		b.ResetTimer()
+		for b.Loop() {
+			_ = ToString(large)
+		}
+	})
+}
+
+func BenchmarkSize(b *testing.B) {
+	data := []byte("Hello, World!")
+
+	for b.Loop() {
+		_ = Size(data)
+	}
+}
+
+func BenchmarkMonoidConcat(b *testing.B) {
+	a := []byte("Hello")
+	c := []byte(" World")
+
+	b.Run("small slices", func(b *testing.B) {
+		for b.Loop() {
+			_ = Monoid.Concat(a, c)
+		}
+	})
+
+	b.Run("large slices", func(b *testing.B) {
+		large1 := make([]byte, 10000)
+		large2 := make([]byte, 10000)
+		b.ResetTimer()
+		for b.Loop() {
+			_ = Monoid.Concat(large1, large2)
+		}
+	})
+}
+
+func BenchmarkConcatAll(b *testing.B) {
+	slices := [][]byte{
+		[]byte("Hello"),
+		[]byte(" "),
+		[]byte("World"),
+		[]byte("!"),
+	}
+
+	b.Run("few slices", func(b *testing.B) {
+		for b.Loop() {
+			_ = ConcatAll(slices...)
+		}
+	})
+
+	b.Run("many slices", func(b *testing.B) {
+		many := make([][]byte, 100)
+		for i := range many {
+			many[i] = []byte{byte(i)}
+		}
+		b.ResetTimer()
+		for b.Loop() {
+			_ = ConcatAll(many...)
+		}
+	})
+}
+
+func BenchmarkOrdCompare(b *testing.B) {
+	a := []byte("abc")
+	c := []byte("abd")
+
+	b.Run("equal", func(b *testing.B) {
+		for b.Loop() {
+			_ = Ord.Compare(a, a)
+		}
+	})
+
+	b.Run("different", func(b *testing.B) {
+		for b.Loop() {
+			_ = Ord.Compare(a, c)
+		}
+	})
+
+	b.Run("large slices", func(b *testing.B) {
+		large1 := make([]byte, 10000)
+		large2 := make([]byte, 10000)
+		large2[9999] = 1
+		b.ResetTimer()
+		for b.Loop() {
+			_ = Ord.Compare(large1, large2)
+		}
+	})
+}
+
 // Example tests
 func ExampleEmpty() {
 	empty := Empty()
@@ -219,3 +512,17 @@ func ExampleConcatAll() {
 
 	// Output:
 }
+
+func ExampleMonoid_concat() {
+	result := Monoid.Concat([]byte("Hello"), []byte(" World"))
+	println(string(result)) // Hello World
+
+	// Output:
+}
+
+func ExampleOrd_compare() {
+	cmp := Ord.Compare([]byte("abc"), []byte("abd"))
+	println(cmp) // -1 (abc < abd)
+
+	// Output:
+}

v2/bytes/coverage.out

@@ -0,0 +1,4 @@
+mode: set
+github.com/IBM/fp-go/v2/bytes/bytes.go:55.21,57.2 1 1
+github.com/IBM/fp-go/v2/bytes/bytes.go:111.32,113.2 1 1
+github.com/IBM/fp-go/v2/bytes/bytes.go:175.26,177.2 1 1

v2/bytes/monoid.go

@@ -23,12 +23,219 @@ import (
 )
 
 var (
-	// monoid for byte arrays
+	// Monoid is the Monoid instance for byte slices.
+	//
+	// This Monoid combines byte slices through concatenation, with an empty
+	// byte slice as the identity element. It satisfies the monoid laws:
+	//
+	// Identity laws:
+	//   - Monoid.Concat(Monoid.Empty(), x) == x (left identity)
+	//   - Monoid.Concat(x, Monoid.Empty()) == x (right identity)
+	//
+	// Associativity law:
+	//   - Monoid.Concat(Monoid.Concat(a, b), c) == Monoid.Concat(a, Monoid.Concat(b, c))
+	//
+	// Operations:
+	//   - Empty(): Returns an empty byte slice []byte{}
+	//   - Concat(a, b []byte): Concatenates two byte slices
+	//
+	// Example - Basic concatenation:
+	//
+	//	result := Monoid.Concat([]byte("Hello"), []byte(" World"))
+	//	// result: []byte("Hello World")
+	//
+	// Example - Identity element:
+	//
+	//	empty := Monoid.Empty()
+	//	data := []byte("test")
+	//	result1 := Monoid.Concat(empty, data) // []byte("test")
+	//	result2 := Monoid.Concat(data, empty) // []byte("test")
+	//
+	// Example - Building byte buffers:
+	//
+	//	buffer := Monoid.Empty()
+	//	buffer = Monoid.Concat(buffer, []byte("Line 1\n"))
+	//	buffer = Monoid.Concat(buffer, []byte("Line 2\n"))
+	//	buffer = Monoid.Concat(buffer, []byte("Line 3\n"))
+	//
+	// Example - Associativity:
+	//
+	//	a := []byte("a")
+	//	b := []byte("b")
+	//	c := []byte("c")
+	//	left := Monoid.Concat(Monoid.Concat(a, b), c)   // []byte("abc")
+	//	right := Monoid.Concat(a, Monoid.Concat(b, c))  // []byte("abc")
+	//	// left == right
+	//
+	// See also:
+	//   - ConcatAll: For concatenating multiple byte slices at once
+	//   - Empty(): Convenience function for getting empty byte slice
 	Monoid = A.Monoid[byte]()
 
-	// ConcatAll concatenates all bytes
+	// ConcatAll efficiently concatenates multiple byte slices into a single slice.
+	//
+	// This function takes a variadic number of byte slices and combines them
+	// into a single byte slice. It pre-allocates the exact amount of memory
+	// needed, making it more efficient than repeated concatenation.
+	//
+	// Parameters:
+	//   - slices: Zero or more byte slices to concatenate
+	//
+	// Returns:
+	//   - A new byte slice containing all input slices concatenated in order
+	//
+	// Performance:
+	//
+	// ConcatAll is more efficient than using Monoid.Concat repeatedly because
+	// it calculates the total size upfront and allocates memory once, avoiding
+	// multiple allocations and copies.
+	//
+	// Example - Basic usage:
+	//
+	//	result := ConcatAll(
+	//	    []byte("Hello"),
+	//	    []byte(" "),
+	//	    []byte("World"),
+	//	)
+	//	// result: []byte("Hello World")
+	//
+	// Example - Empty input:
+	//
+	//	result := ConcatAll()
+	//	// result: []byte{}
+	//
+	// Example - Single slice:
+	//
+	//	result := ConcatAll([]byte("test"))
+	//	// result: []byte("test")
+	//
+	// Example - Building protocol messages:
+	//
+	//	import "encoding/binary"
+	//
+	//	header := []byte{0x01, 0x02}
+	//	length := make([]byte, 4)
+	//	binary.BigEndian.PutUint32(length, 100)
+	//	payload := []byte("data")
+	//	footer := []byte{0xFF}
+	//
+	//	message := ConcatAll(header, length, payload, footer)
+	//
+	// Example - With empty slices:
+	//
+	//	result := ConcatAll(
+	//	    []byte("a"),
+	//	    []byte{},
+	//	    []byte("b"),
+	//	    []byte{},
+	//	    []byte("c"),
+	//	)
+	//	// result: []byte("abc")
+	//
+	// Example - Building CSV line:
+	//
+	//	fields := [][]byte{
+	//	    []byte("John"),
+	//	    []byte("Doe"),
+	//	    []byte("30"),
+	//	}
+	//	separator := []byte(",")
+	//
+	//	// Interleave fields with separators
+	//	parts := [][]byte{
+	//	    fields[0], separator,
+	//	    fields[1], separator,
+	//	    fields[2],
+	//	}
+	//	line := ConcatAll(parts...)
+	//	// line: []byte("John,Doe,30")
+	//
+	// See also:
+	//   - Monoid.Concat: For concatenating exactly two byte slices
+	//   - bytes.Join: Standard library function for joining with separator
 	ConcatAll = A.ArrayConcatAll[byte]
 
-	// Ord implements the default ordering on bytes
+	// Ord is the Ord instance for byte slices providing lexicographic ordering.
+	//
+	// This Ord instance compares byte slices lexicographically (dictionary order),
+	// comparing bytes from left to right until a difference is found or one slice
+	// ends. It uses the standard library's bytes.Compare and bytes.Equal functions.
+	//
+	// Comparison rules:
+	//   - Compares byte-by-byte from left to right
+	//   - First differing byte determines the order
+	//   - Shorter slice is less than longer slice if all bytes match
+	//   - Empty slice is less than any non-empty slice
+	//
+	// Operations:
+	//   - Compare(a, b []byte) int: Returns -1 if a < b, 0 if a == b, 1 if a > b
+	//   - Equals(a, b []byte) bool: Returns true if slices are equal
+	//
+	// Example - Basic comparison:
+	//
+	//	cmp := Ord.Compare([]byte("abc"), []byte("abd"))
+	//	// cmp: -1 (abc < abd)
+	//
+	//	cmp = Ord.Compare([]byte("xyz"), []byte("abc"))
+	//	// cmp: 1 (xyz > abc)
+	//
+	//	cmp = Ord.Compare([]byte("test"), []byte("test"))
+	//	// cmp: 0 (equal)
+	//
+	// Example - Length differences:
+	//
+	//	cmp := Ord.Compare([]byte("ab"), []byte("abc"))
+	//	// cmp: -1 (shorter is less)
+	//
+	//	cmp = Ord.Compare([]byte("abc"), []byte("ab"))
+	//	// cmp: 1 (longer is greater)
+	//
+	// Example - Empty slices:
+	//
+	//	cmp := Ord.Compare([]byte{}, []byte("a"))
+	//	// cmp: -1 (empty is less)
+	//
+	//	cmp = Ord.Compare([]byte{}, []byte{})
+	//	// cmp: 0 (both empty)
+	//
+	// Example - Equality check:
+	//
+	//	equal := Ord.Equals([]byte("test"), []byte("test"))
+	//	// equal: true
+	//
+	//	equal = Ord.Equals([]byte("test"), []byte("Test"))
+	//	// equal: false (case-sensitive)
+	//
+	// Example - Sorting byte slices:
+	//
+	//	import "github.com/IBM/fp-go/v2/array"
+	//
+	//	data := [][]byte{
+	//	    []byte("zebra"),
+	//	    []byte("apple"),
+	//	    []byte("mango"),
+	//	}
+	//
+	//	sorted := array.Sort(Ord)(data)
+	//	// sorted: [[]byte("apple"), []byte("mango"), []byte("zebra")]
+	//
+	// Example - Binary data comparison:
+	//
+	//	cmp := Ord.Compare([]byte{0x01, 0x02}, []byte{0x01, 0x03})
+	//	// cmp: -1 (0x02 < 0x03)
+	//
+	// Example - Finding minimum:
+	//
+	//	import O "github.com/IBM/fp-go/v2/ord"
+	//
+	//	a := []byte("xyz")
+	//	b := []byte("abc")
+	//	min := O.Min(Ord)(a, b)
+	//	// min: []byte("abc")
+	//
+	// See also:
+	//   - bytes.Compare: Standard library comparison function
+	//   - bytes.Equal: Standard library equality function
+	//   - array.Sort: For sorting slices using an Ord instance
 	Ord = O.MakeOrd(bytes.Compare, bytes.Equal)
 )

v2/circuitbreaker/circuitbreaker.go

@@ -0,0 +1,623 @@
+package circuitbreaker
+
+import (
+	"time"
+
+	"github.com/IBM/fp-go/v2/either"
+	"github.com/IBM/fp-go/v2/function"
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/identity"
+	"github.com/IBM/fp-go/v2/io"
+	"github.com/IBM/fp-go/v2/ioref"
+	"github.com/IBM/fp-go/v2/lazy"
+	"github.com/IBM/fp-go/v2/optics/lens"
+	"github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/pair"
+	"github.com/IBM/fp-go/v2/reader"
+	"github.com/IBM/fp-go/v2/retry"
+)
+
+var (
+	canaryRequestLens = lens.MakeLensWithName(
+		func(os openState) bool { return os.canaryRequest },
+		func(os openState, flag bool) openState {
+			os.canaryRequest = flag
+			return os
+		},
+		"openState.CanaryRequest",
+	)
+
+	retryStatusLens = lens.MakeLensWithName(
+		func(os openState) retry.RetryStatus { return os.retryStatus },
+		func(os openState, status retry.RetryStatus) openState {
+			os.retryStatus = status
+			return os
+		},
+		"openState.RetryStatus",
+	)
+
+	resetAtLens = lens.MakeLensWithName(
+		func(os openState) time.Time { return os.resetAt },
+		func(os openState, tm time.Time) openState {
+			os.resetAt = tm
+			return os
+		},
+		"openState.ResetAt",
+	)
+
+	openedAtLens = lens.MakeLensWithName(
+		func(os openState) time.Time { return os.openedAt },
+		func(os openState, tm time.Time) openState {
+			os.openedAt = tm
+			return os
+		},
+		"openState.OpenedAt",
+	)
+
+	createClosedCircuit = either.Right[openState, ClosedState]
+	createOpenCircuit   = either.Left[ClosedState, openState]
+
+	// MakeClosedIORef creates an IORef containing a closed circuit breaker state.
+	// It wraps the provided ClosedState in a Right (closed) BreakerState and creates
+	// a mutable reference to it.
+	//
+	// Parameters:
+	//   - closedState: The initial closed state configuration
+	//
+	// Returns:
+	//   - An IO operation that creates an IORef[BreakerState] initialized to closed state
+	//
+	// Thread Safety: The returned IORef[BreakerState] is thread-safe. It uses atomic
+	// operations for all read/write/modify operations. The BreakerState itself is immutable.
+	MakeClosedIORef = F.Flow2(
+		createClosedCircuit,
+		ioref.MakeIORef,
+	)
+
+	// IsOpen checks if a BreakerState is in the open state.
+	// Returns true if the circuit breaker is open (blocking requests), false otherwise.
+	IsOpen = either.IsLeft[openState, ClosedState]
+
+	// IsClosed checks if a BreakerState is in the closed state.
+	// Returns true if the circuit breaker is closed (allowing requests), false otherwise.
+	IsClosed = either.IsRight[openState, ClosedState]
+
+	// modifyV creates a Reader that sequences an IORef modification operation.
+	// It takes an IORef[BreakerState] and returns a Reader that, when given an endomorphism
+	// (a function from BreakerState to BreakerState), produces an IO operation that modifies
+	// the IORef and returns the new state.
+	//
+	// This is used internally to create state modification operations that can be composed
+	// with other Reader-based operations in the circuit breaker logic.
+	//
+	// Thread Safety: The IORef modification is atomic. Multiple concurrent calls will be
+	// serialized by the IORef's atomic operations.
+	//
+	// Type signature: Reader[IORef[BreakerState], IO[Endomorphism[BreakerState]]]
+	modifyV = reader.Sequence(ioref.Modify[BreakerState])
+
+	initialRetry = retry.DefaultRetryStatus
+
+	// testCircuit sets the canaryRequest flag to true in an openState.
+	// This is used to mark that the circuit breaker is in half-open state,
+	// allowing a single test request (canary) to check if the service has recovered.
+	//
+	// When canaryRequest is true:
+	//   - One request is allowed through to test the service
+	//   - If the canary succeeds, the circuit closes
+	//   - If the canary fails, the circuit remains open with an extended reset time
+	//
+	// Thread Safety: This is a pure function that returns a new openState; it does not
+	// modify its input. Safe for concurrent use.
+	//
+	// Type signature: Endomorphism[openState]
+	testCircuit = canaryRequestLens.Set(true)
+)
+
+// makeOpenCircuitFromPolicy creates a function that constructs an openState from a retry policy.
+// This is a curried function that takes a retry policy and returns a function that takes a retry status
+// and current time to produce an openState with calculated reset time.
+//
+// The function applies the retry policy to determine the next retry delay and calculates
+// the resetAt time by adding the delay to the current time. If no previous delay exists
+// (first failure), the resetAt is set to the current time.
+//
+// Parameters:
+//   - policy: The retry policy that determines backoff strategy (e.g., exponential backoff)
+//
+// Returns:
+//   - A curried function that takes:
+//     1. rs (retry.RetryStatus): The current retry status containing retry count and previous delay
+//     2. ct (time.Time): The current time when the circuit is opening
+//     And returns an openState with:
+//   - openedAt: Set to the current time (ct)
+//   - resetAt: Current time plus the delay from the retry policy
+//   - retryStatus: The updated retry status from applying the policy
+//   - canaryRequest: false (will be set to true when reset time is reached)
+//
+// Thread Safety: This is a pure function that creates new openState instances.
+// Safe for concurrent use.
+//
+// Example:
+//
+//	policy := retry.ExponentialBackoff(1*time.Second, 2.0, 10)
+//	makeOpen := makeOpenCircuitFromPolicy(policy)
+//	openState := makeOpen(retry.DefaultRetryStatus)(time.Now())
+//	// openState.resetAt will be approximately 1 second from now
+func makeOpenCircuitFromPolicy(policy retry.RetryPolicy) func(rs retry.RetryStatus) func(ct time.Time) openState {
+
+	return func(rs retry.RetryStatus) func(ct time.Time) openState {
+
+		retryStatus := retry.ApplyPolicy(policy, rs)
+
+		return func(ct time.Time) openState {
+
+			resetTime := F.Pipe2(
+				retryStatus,
+				retry.PreviousDelayLens.Get,
+				option.Fold(
+					F.Pipe1(
+						ct,
+						lazy.Of,
+					),
+					ct.Add,
+				),
+			)
+
+			return openState{openedAt: ct, resetAt: resetTime, retryStatus: retryStatus}
+		}
+	}
+}
+
+// extendOpenCircuitFromMakeCircuit creates a function that extends the open state of a circuit breaker
+// when a canary request fails. It takes a circuit maker function and returns a function that,
+// given the current time, produces an endomorphism that updates an openState.
+//
+// This function is used when a canary request (test request in half-open state) fails.
+// It extends the circuit breaker's open period by:
+//  1. Extracting the current retry status from the open state
+//  2. Using the makeCircuit function to calculate a new open state with updated retry status
+//  3. Applying the current time to get the new state
+//  4. Setting the canaryRequest flag to true to allow another test request later
+//
+// Parameters:
+//   - makeCircuit: A function that creates an openState from a retry status and current time.
+//     This is typically created by makeOpenCircuitFromPolicy.
+//
+// Returns:
+//   - A curried function that takes:
+//     1. ct (time.Time): The current time when extending the circuit
+//     And returns an Endomorphism[openState] that:
+//   - Increments the retry count
+//   - Calculates a new resetAt time based on the retry policy (typically with exponential backoff)
+//   - Sets canaryRequest to true for the next test attempt
+//
+// Thread Safety: This is a pure function that returns new openState instances.
+// Safe for concurrent use.
+//
+// Usage Context:
+//   - Called when a canary request fails in the half-open state
+//   - Extends the open period with increased backoff delay
+//   - Prepares the circuit for another canary attempt at the new resetAt time
+func extendOpenCircuitFromMakeCircuit(
+	makeCircuit func(rs retry.RetryStatus) func(ct time.Time) openState,
+) func(time.Time) Endomorphism[openState] {
+	return func(ct time.Time) Endomorphism[openState] {
+		return F.Flow4(
+			retryStatusLens.Get,
+			makeCircuit,
+			identity.Flap[openState](ct),
+			testCircuit,
+		)
+	}
+}
+
+// isResetTimeExceeded checks if the reset time for an open circuit has been exceeded.
+// This is used to determine if the circuit breaker should transition from open to half-open state
+// by allowing a canary request.
+//
+// The function returns an option.Kleisli that succeeds (returns Some) only when:
+//  1. The circuit is not already in canary mode (canaryRequest is false)
+//  2. The current time is after the resetAt time
+//
+// Parameters:
+//   - ct: The current time to compare against the reset time
+//
+// Returns:
+//   - An option.Kleisli[openState, openState] that:
+//   - Returns Some(openState) if the reset time has been exceeded and no canary is active
+//   - Returns None if the reset time has not been exceeded or a canary request is already active
+//
+// Thread Safety: This is a pure function that does not modify its input.
+// Safe for concurrent use.
+//
+// Usage Context:
+//   - Called when the circuit is open to check if it's time to attempt a canary request
+//   - If this returns Some, the circuit transitions to half-open state (canary mode)
+//   - If this returns None, the circuit remains fully open and requests are blocked
+func isResetTimeExceeded(ct time.Time) option.Kleisli[openState, openState] {
+	return option.FromPredicate(func(open openState) bool {
+		return !open.canaryRequest && ct.After(resetAtLens.Get(open))
+	})
+}
+
+// handleSuccessOnClosed handles a successful request when the circuit breaker is in closed state.
+// It updates the closed state by recording the success and returns an IO operation that
+// modifies the breaker state.
+//
+// This function is part of the circuit breaker's state management for the closed state.
+// When a request succeeds in closed state:
+//  1. The current time is obtained
+//  2. The addSuccess function is called with the current time to update the ClosedState
+//  3. The updated ClosedState is wrapped in a Right (closed) BreakerState
+//  4. The breaker state is modified with the new state
+//
+// Parameters:
+//   - currentTime: An IO operation that provides the current time
+//   - addSuccess: A Reader that takes a time and returns an endomorphism for ClosedState,
+//     typically resetting failure counters or history
+//
+// Returns:
+//   - An io.Kleisli that takes another io.Kleisli and chains them together.
+//     The outer Kleisli takes an Endomorphism[BreakerState] and returns BreakerState.
+//     This allows composing the success handling with other state modifications.
+//
+// Thread Safety: This function creates IO operations that will atomically modify the
+// IORef[BreakerState] when executed. The state modifications are thread-safe.
+//
+// Type signature:
+//
+//	io.Kleisli[io.Kleisli[Endomorphism[BreakerState], BreakerState], BreakerState]
+//
+// Usage Context:
+//   - Called when a request succeeds while the circuit is closed
+//   - Resets failure tracking (counter or history) in the ClosedState
+//   - Keeps the circuit in closed state
+func handleSuccessOnClosed(
+	currentTime IO[time.Time],
+	addSuccess Reader[time.Time, Endomorphism[ClosedState]],
+) io.Kleisli[io.Kleisli[Endomorphism[BreakerState], BreakerState], BreakerState] {
+	return F.Flow2(
+		io.Chain,
+		identity.Flap[IO[BreakerState]](F.Pipe1(
+			currentTime,
+			io.Map(F.Flow2(
+				addSuccess,
+				either.Map[openState],
+			)))),
+	)
+}
+
+// handleFailureOnClosed handles a failed request when the circuit breaker is in closed state.
+// It updates the closed state by recording the failure and checks if the circuit should open.
+//
+// This function is part of the circuit breaker's state management for the closed state.
+// When a request fails in closed state:
+//  1. The current time is obtained
+//  2. The addError function is called to record the failure in the ClosedState
+//  3. The checkClosedState function is called to determine if the failure threshold is exceeded
+//  4. If the threshold is exceeded (Check returns None):
+//     - The circuit transitions to open state using openCircuit
+//     - A new openState is created with resetAt time calculated from the retry policy
+//  5. If the threshold is not exceeded (Check returns Some):
+//     - The circuit remains closed with the updated failure tracking
+//
+// Parameters:
+//   - currentTime: An IO operation that provides the current time
+//   - addError: A Reader that takes a time and returns an endomorphism for ClosedState,
+//     recording a failure (incrementing counter or adding to history)
+//   - checkClosedState: A Reader that takes a time and returns an option.Kleisli that checks
+//     if the ClosedState should remain closed. Returns Some if circuit stays closed, None if it should open.
+//   - openCircuit: A Reader that takes a time and returns an openState with calculated resetAt time
+//
+// Returns:
+//   - An io.Kleisli that takes another io.Kleisli and chains them together.
+//     The outer Kleisli takes an Endomorphism[BreakerState] and returns BreakerState.
+//     This allows composing the failure handling with other state modifications.
+//
+// Thread Safety: This function creates IO operations that will atomically modify the
+// IORef[BreakerState] when executed. The state modifications are thread-safe.
+//
+// Type signature:
+//
+//	io.Kleisli[io.Kleisli[Endomorphism[BreakerState], BreakerState], BreakerState]
+//
+// State Transitions:
+//   - Closed -> Closed: When failure threshold is not exceeded (Some from checkClosedState)
+//   - Closed -> Open: When failure threshold is exceeded (None from checkClosedState)
+//
+// Usage Context:
+//   - Called when a request fails while the circuit is closed
+//   - Records the failure in the ClosedState (counter or history)
+//   - May trigger transition to open state if threshold is exceeded
+func handleFailureOnClosed(
+	currentTime IO[time.Time],
+	addError Reader[time.Time, Endomorphism[ClosedState]],
+	checkClosedState Reader[time.Time, option.Kleisli[ClosedState, ClosedState]],
+	openCircuit Reader[time.Time, openState],
+) io.Kleisli[io.Kleisli[Endomorphism[BreakerState], BreakerState], BreakerState] {
+
+	return F.Flow2(
+		io.Chain,
+		identity.Flap[IO[BreakerState]](F.Pipe1(
+			currentTime,
+			io.Map(func(ct time.Time) either.Operator[openState, ClosedState, ClosedState] {
+				return either.Chain(F.Flow3(
+					addError(ct),
+					checkClosedState(ct),
+					option.Fold(
+						F.Pipe2(
+							ct,
+							lazy.Of,
+							lazy.Map(F.Flow2(
+								openCircuit,
+								createOpenCircuit,
+							)),
+						),
+						createClosedCircuit,
+					),
+				))
+			}))),
+	)
+
+}
+
+// MakeCircuitBreaker creates a circuit breaker implementation for a higher-kinded type.
+//
+// This is a generic circuit breaker factory that works with any monad-like type (HKTT).
+// It implements the circuit breaker pattern by wrapping operations and managing state transitions
+// between closed, open, and half-open states based on failure rates and retry policies.
+//
+// Type Parameters:
+//   - E: The error type
+//   - T: The success value type
+//   - HKTT: The higher-kinded type representing the computation (e.g., IO[T], ReaderIO[R, T])
+//   - HKTOP: The higher-kinded type for operators (e.g., IO[func(HKTT) HKTT])
+//   - HKTHKTT: The nested higher-kinded type (e.g., IO[IO[T]])
+//
+// Parameters:
+//   - left: Constructs an error result in HKTT from an error value
+//   - chainFirstIOK: Chains an IO operation that runs after success, preserving the original value
+//   - chainFirstLeftIOK: Chains an IO operation that runs after error, preserving the original error
+//   - fromIO: Lifts an IO operation into HKTOP
+//   - flap: Applies a value to a function wrapped in a higher-kinded type
+//   - flatten: Flattens nested higher-kinded types (join operation)
+//   - currentTime: IO operation that provides the current time
+//   - closedState: The initial closed state configuration
+//   - makeError: Creates an error from a reset time when the circuit is open
+//   - checkError: Predicate to determine if an error should trigger circuit breaker logic
+//   - policy: Retry policy for determining reset times when circuit opens
+//   - logger: Logging function for circuit breaker events
+//
+// Thread Safety: The returned State monad creates operations that are thread-safe when
+// executed. The IORef[BreakerState] uses atomic operations for all state modifications.
+// Multiple concurrent requests will be properly serialized at the IORef level.
+//
+// Returns:
+//   - A State monad that transforms a pair of (IORef[BreakerState], HKTT) into HKTT,
+//     applying circuit breaker logic to the computation
+func MakeCircuitBreaker[E, T, HKTT, HKTOP, HKTHKTT any](
+
+	left func(E) HKTT,
+	chainFirstIOK func(io.Kleisli[T, BreakerState]) func(HKTT) HKTT,
+	chainFirstLeftIOK func(io.Kleisli[E, BreakerState]) func(HKTT) HKTT,
+
+	fromIO func(IO[func(HKTT) HKTT]) HKTOP,
+	flap func(HKTT) func(HKTOP) HKTHKTT,
+	flatten func(HKTHKTT) HKTT,
+
+	currentTime IO[time.Time],
+	closedState ClosedState,
+	makeError Reader[time.Time, E],
+	checkError option.Kleisli[E, E],
+	policy retry.RetryPolicy,
+	metrics Metrics,
+) State[Pair[IORef[BreakerState], HKTT], HKTT] {
+
+	type Operator = func(HKTT) HKTT
+
+	addSuccess := reader.From1(ClosedState.AddSuccess)
+	addError := reader.From1(ClosedState.AddError)
+	checkClosedState := reader.From1(ClosedState.Check)
+
+	closedCircuit := createClosedCircuit(closedState.Empty())
+	makeOpenCircuit := makeOpenCircuitFromPolicy(policy)
+
+	openCircuit := F.Pipe1(
+		initialRetry,
+		makeOpenCircuit,
+	)
+
+	extendOpenCircuit := extendOpenCircuitFromMakeCircuit(makeOpenCircuit)
+
+	failWithError := F.Flow4(
+		resetAtLens.Get,
+		makeError,
+		left,
+		reader.Of[HKTT],
+	)
+
+	handleSuccess := handleSuccessOnClosed(currentTime, addSuccess)
+	handleFailure := handleFailureOnClosed(currentTime, addError, checkClosedState, openCircuit)
+
+	onClosed := func(modify io.Kleisli[Endomorphism[BreakerState], BreakerState]) Operator {
+
+		return F.Flow2(
+			// error case
+			chainFirstLeftIOK(F.Flow3(
+				checkError,
+				option.Fold(
+					// the error is not applicable, handle as success
+					F.Pipe2(
+						modify,
+						handleSuccess,
+						lazy.Of,
+					),
+					// the error is relevant, record it
+					F.Pipe2(
+						modify,
+						handleFailure,
+						reader.Of[E],
+					),
+				),
+				// metering
+				io.ChainFirst(either.Fold(
+					F.Flow2(
+						openedAtLens.Get,
+						metrics.Open,
+					),
+					func(c ClosedState) IO[Void] {
+						return io.Of(function.VOID)
+					},
+				)),
+			)),
+			// good case
+			chainFirstIOK(F.Pipe2(
+				modify,
+				handleSuccess,
+				reader.Of[T],
+			)),
+		)
+	}
+
+	onCanary := func(modify io.Kleisli[Endomorphism[BreakerState], BreakerState]) Operator {
+
+		handleSuccess := F.Pipe2(
+			closedCircuit,
+			reader.Of[BreakerState],
+			modify,
+		)
+
+		return F.Flow2(
+			// the canary request fails
+			chainFirstLeftIOK(F.Flow2(
+				checkError,
+				option.Fold(
+					// the canary request succeeds, we close the circuit
+					F.Pipe1(
+						handleSuccess,
+						lazy.Of,
+					),
+					// the canary request fails, we extend the circuit
+					F.Pipe1(
+						F.Pipe1(
+							currentTime,
+							io.Chain(func(ct time.Time) IO[BreakerState] {
+								return F.Pipe1(
+									F.Flow2(
+										either.Fold(
+											extendOpenCircuit(ct),
+											F.Pipe1(
+												openCircuit(ct),
+												reader.Of[ClosedState],
+											),
+										),
+										createOpenCircuit,
+									),
+									modify,
+								)
+							}),
+						),
+						reader.Of[E],
+					),
+				),
+			)),
+			// the canary request succeeds, we'll close the circuit
+			chainFirstIOK(F.Pipe1(
+				handleSuccess,
+				reader.Of[T],
+			)),
+		)
+	}
+
+	onOpen := func(ref IORef[BreakerState]) Operator {
+
+		modify := modifyV(ref)
+
+		return F.Pipe3(
+			currentTime,
+			io.Chain(func(ct time.Time) IO[Operator] {
+				return F.Pipe1(
+					ref,
+					ioref.ModifyWithResult(either.Fold(
+						func(open openState) Pair[BreakerState, Operator] {
+							return option.Fold(
+								func() Pair[BreakerState, Operator] {
+									return pair.MakePair(createOpenCircuit(open), failWithError(open))
+								},
+								func(open openState) Pair[BreakerState, Operator] {
+									return pair.MakePair(createOpenCircuit(testCircuit(open)), onCanary(modify))
+								},
+							)(isResetTimeExceeded(ct)(open))
+						},
+						func(closed ClosedState) Pair[BreakerState, Operator] {
+							return pair.MakePair(createClosedCircuit(closed), onClosed(modify))
+						},
+					)),
+				)
+			}),
+			fromIO,
+			func(src HKTOP) Operator {
+				return func(rdr HKTT) HKTT {
+					return F.Pipe2(
+						src,
+						flap(rdr),
+						flatten,
+					)
+				}
+			},
+		)
+	}
+
+	return func(e Pair[IORef[BreakerState], HKTT]) Pair[Pair[IORef[BreakerState], HKTT], HKTT] {
+		return pair.MakePair(e, onOpen(pair.Head(e))(pair.Tail(e)))
+	}
+}
+
+// MakeSingletonBreaker creates a singleton circuit breaker operator for a higher-kinded type.
+//
+// This function creates a circuit breaker that maintains its own internal state reference.
+// It's called "singleton" because it creates a single, self-contained circuit breaker instance
+// with its own IORef for state management. The returned function can be used to wrap
+// computations with circuit breaker protection.
+//
+// Type Parameters:
+//   - HKTT: The higher-kinded type representing the computation (e.g., IO[T], ReaderIO[R, T])
+//
+// Parameters:
+//   - cb: The circuit breaker State monad created by MakeCircuitBreaker
+//   - closedState: The initial closed state configuration for the circuit breaker
+//
+// Returns:
+//   - A function that wraps a computation (HKTT) with circuit breaker logic.
+//     The circuit breaker state is managed internally and persists across invocations.
+//
+// Thread Safety: The returned function is thread-safe. The internal IORef[BreakerState]
+// uses atomic operations to manage state. Multiple concurrent calls to the returned function
+// will be properly serialized at the state modification level.
+//
+// Example Usage:
+//
+//	// Create a circuit breaker for IO operations
+//	breaker := MakeSingletonBreaker(
+//	    MakeCircuitBreaker(...),
+//	    MakeClosedStateCounter(3),
+//	)
+//
+//	// Use it to wrap operations
+//	protectedOp := breaker(myIOOperation)
+func MakeSingletonBreaker[HKTT any](
+	cb State[Pair[IORef[BreakerState], HKTT], HKTT],
+	closedState ClosedState,
+) func(HKTT) HKTT {
+	return F.Flow3(
+		F.Pipe3(
+			closedState,
+			MakeClosedIORef,
+			io.Run,
+			pair.FromHead[HKTT],
+		),
+		cb,
+		pair.Tail,
+	)
+}

v2/circuitbreaker/circuitbreaker_test.go

@@ -0,0 +1,579 @@
+package circuitbreaker
+
+import (
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/IBM/fp-go/v2/function"
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/io"
+	"github.com/IBM/fp-go/v2/ioref"
+	"github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/reader"
+	"github.com/IBM/fp-go/v2/retry"
+	"github.com/stretchr/testify/assert"
+)
+
+type testMetrics struct {
+	accepts int
+	rejects int
+	opens   int
+	closes  int
+	canary  int
+
+	mu sync.Mutex
+}
+
+func (m *testMetrics) Accept(_ time.Time) IO[Void] {
+	return func() Void {
+		m.mu.Lock()
+		defer m.mu.Unlock()
+		m.accepts++
+		return function.VOID
+	}
+}
+
+func (m *testMetrics) Open(_ time.Time) IO[Void] {
+	return func() Void {
+		m.mu.Lock()
+		defer m.mu.Unlock()
+		m.opens++
+		return function.VOID
+	}
+}
+
+func (m *testMetrics) Close(_ time.Time) IO[Void] {
+	return func() Void {
+		m.mu.Lock()
+		defer m.mu.Unlock()
+		m.closes++
+		return function.VOID
+	}
+}
+
+func (m *testMetrics) Reject(_ time.Time) IO[Void] {
+	return func() Void {
+		m.mu.Lock()
+		defer m.mu.Unlock()
+		m.rejects++
+		return function.VOID
+	}
+}
+
+func (m *testMetrics) Canary(_ time.Time) IO[Void] {
+	return func() Void {
+		m.mu.Lock()
+		defer m.mu.Unlock()
+		m.canary++
+		return function.VOID
+	}
+}
+
+// VirtualTimer provides a controllable time source for testing
+type VirtualTimer struct {
+	mu      sync.Mutex
+	current time.Time
+}
+
+func NewMockMetrics() Metrics {
+	return &testMetrics{}
+}
+
+// NewVirtualTimer creates a new virtual timer starting at the given time
+func NewVirtualTimer(start time.Time) *VirtualTimer {
+	return &VirtualTimer{current: start}
+}
+
+// Now returns the current virtual time
+func (vt *VirtualTimer) Now() time.Time {
+	vt.mu.Lock()
+	defer vt.mu.Unlock()
+	return vt.current
+}
+
+// Advance moves the virtual time forward by the given duration
+func (vt *VirtualTimer) Advance(d time.Duration) {
+	vt.mu.Lock()
+	defer vt.mu.Unlock()
+	vt.current = vt.current.Add(d)
+}
+
+// Set sets the virtual time to a specific value
+func (vt *VirtualTimer) Set(t time.Time) {
+	vt.mu.Lock()
+	defer vt.mu.Unlock()
+	vt.current = t
+}
+
+// TestModifyV tests the modifyV variable
+func TestModifyV(t *testing.T) {
+	t.Run("modifyV creates a Reader that modifies IORef", func(t *testing.T) {
+		vt := NewVirtualTimer(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
+
+		// Create initial state
+		initialState := createClosedCircuit(MakeClosedStateCounter(3))
+		ref := io.Run(ioref.MakeIORef(initialState))
+
+		// Create an endomorphism that opens the circuit
+		now := vt.Now()
+		openState := openState{
+			openedAt:      now,
+			resetAt:       now.Add(1 * time.Minute),
+			retryStatus:   retry.DefaultRetryStatus,
+			canaryRequest: false,
+		}
+		endomorphism := func(bs BreakerState) BreakerState {
+			return createOpenCircuit(openState)
+		}
+
+		// Apply modifyV
+		modifyOp := modifyV(ref)
+		result := io.Run(modifyOp(endomorphism))
+
+		// Verify the state was modified
+		assert.True(t, IsOpen(result), "state should be open after modification")
+	})
+
+	t.Run("modifyV returns the new state", func(t *testing.T) {
+		initialState := createClosedCircuit(MakeClosedStateCounter(3))
+		ref := io.Run(ioref.MakeIORef(initialState))
+
+		// Create a simple endomorphism
+		endomorphism := F.Identity[BreakerState]
+
+		modifyOp := modifyV(ref)
+		result := io.Run(modifyOp(endomorphism))
+
+		assert.True(t, IsClosed(result), "state should remain closed")
+	})
+}
+
+// TestTestCircuit tests the testCircuit variable
+func TestTestCircuit(t *testing.T) {
+	t.Run("testCircuit sets canaryRequest to true", func(t *testing.T) {
+		vt := NewVirtualTimer(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
+		now := vt.Now()
+
+		openState := openState{
+			openedAt:      now,
+			resetAt:       now.Add(1 * time.Minute),
+			retryStatus:   retry.DefaultRetryStatus,
+			canaryRequest: false,
+		}
+
+		result := testCircuit(openState)
+
+		assert.True(t, result.canaryRequest, "canaryRequest should be set to true")
+		assert.Equal(t, openState.openedAt, result.openedAt, "openedAt should be unchanged")
+		assert.Equal(t, openState.resetAt, result.resetAt, "resetAt should be unchanged")
+	})
+
+	t.Run("testCircuit is idempotent", func(t *testing.T) {
+		vt := NewVirtualTimer(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
+		now := vt.Now()
+
+		openState := openState{
+			openedAt:      now,
+			resetAt:       now.Add(1 * time.Minute),
+			retryStatus:   retry.DefaultRetryStatus,
+			canaryRequest: true, // already true
+		}
+
+		result := testCircuit(openState)
+
+		assert.True(t, result.canaryRequest, "canaryRequest should remain true")
+	})
+
+	t.Run("testCircuit preserves other fields", func(t *testing.T) {
+		vt := NewVirtualTimer(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
+		now := vt.Now()
+		resetTime := now.Add(2 * time.Minute)
+		retryStatus := retry.RetryStatus{
+			IterNumber:    5,
+			PreviousDelay: option.Some(30 * time.Second),
+		}
+
+		openState := openState{
+			openedAt:      now,
+			resetAt:       resetTime,
+			retryStatus:   retryStatus,
+			canaryRequest: false,
+		}
+
+		result := testCircuit(openState)
+
+		assert.Equal(t, now, result.openedAt, "openedAt should be preserved")
+		assert.Equal(t, resetTime, result.resetAt, "resetAt should be preserved")
+		assert.Equal(t, retryStatus.IterNumber, result.retryStatus.IterNumber, "retryStatus should be preserved")
+		assert.True(t, result.canaryRequest, "canaryRequest should be set to true")
+	})
+}
+
+// TestMakeOpenCircuitFromPolicy tests the makeOpenCircuitFromPolicy function
+func TestMakeOpenCircuitFromPolicy(t *testing.T) {
+	t.Run("creates openState with calculated reset time", func(t *testing.T) {
+		policy := retry.LimitRetries(5)
+		makeOpen := makeOpenCircuitFromPolicy(policy)
+
+		currentTime := time.Date(2026, 1, 9, 12, 0, 0, 0, time.UTC)
+		result := makeOpen(retry.DefaultRetryStatus)(currentTime)
+
+		assert.Equal(t, currentTime, result.openedAt, "openedAt should be current time")
+		assert.False(t, result.canaryRequest, "canaryRequest should be false initially")
+		assert.NotNil(t, result.retryStatus, "retryStatus should be set")
+	})
+
+	t.Run("applies retry policy to calculate delay", func(t *testing.T) {
+		// Use exponential backoff policy with limit and cap
+		policy := retry.Monoid.Concat(
+			retry.LimitRetries(10),
+			retry.CapDelay(10*time.Second, retry.ExponentialBackoff(1*time.Second)),
+		)
+		makeOpen := makeOpenCircuitFromPolicy(policy)
+
+		currentTime := time.Date(2026, 1, 9, 12, 0, 0, 0, time.UTC)
+
+		// First retry (iter 0)
+		result1 := makeOpen(retry.DefaultRetryStatus)(currentTime)
+
+		// The first delay should be approximately 1 second
+		expectedResetTime1 := currentTime.Add(1 * time.Second)
+		assert.WithinDuration(t, expectedResetTime1, result1.resetAt, 100*time.Millisecond,
+			"first reset time should be ~1 second from now")
+
+		// Second retry (iter 1) - should double
+		result2 := makeOpen(result1.retryStatus)(currentTime)
+		expectedResetTime2 := currentTime.Add(2 * time.Second)
+		assert.WithinDuration(t, expectedResetTime2, result2.resetAt, 100*time.Millisecond,
+			"second reset time should be ~2 seconds from now")
+	})
+
+	t.Run("handles first failure with no previous delay", func(t *testing.T) {
+		policy := retry.LimitRetries(3)
+		makeOpen := makeOpenCircuitFromPolicy(policy)
+
+		currentTime := time.Date(2026, 1, 9, 12, 0, 0, 0, time.UTC)
+		result := makeOpen(retry.DefaultRetryStatus)(currentTime)
+
+		// With no previous delay, resetAt should be current time
+		assert.Equal(t, currentTime, result.resetAt, "resetAt should be current time when no previous delay")
+	})
+
+	t.Run("increments retry iteration number", func(t *testing.T) {
+		vt := NewVirtualTimer(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
+		policy := retry.LimitRetries(10)
+		makeOpen := makeOpenCircuitFromPolicy(policy)
+
+		currentTime := vt.Now()
+		initialStatus := retry.DefaultRetryStatus
+
+		result := makeOpen(initialStatus)(currentTime)
+
+		assert.Greater(t, result.retryStatus.IterNumber, initialStatus.IterNumber,
+			"retry iteration should be incremented")
+	})
+
+	t.Run("curried function can be partially applied", func(t *testing.T) {
+		vt := NewVirtualTimer(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
+		policy := retry.LimitRetries(5)
+		makeOpen := makeOpenCircuitFromPolicy(policy)
+
+		// Partially apply with retry status
+		makeOpenWithStatus := makeOpen(retry.DefaultRetryStatus)
+
+		currentTime := vt.Now()
+		result := makeOpenWithStatus(currentTime)
+
+		assert.NotNil(t, result, "partially applied function should work")
+		assert.Equal(t, currentTime, result.openedAt)
+	})
+}
+
+// TestExtendOpenCircuitFromMakeCircuit tests the extendOpenCircuitFromMakeCircuit function
+func TestExtendOpenCircuitFromMakeCircuit(t *testing.T) {
+	t.Run("extends open circuit with new retry status", func(t *testing.T) {
+		policy := retry.Monoid.Concat(
+			retry.LimitRetries(10),
+			retry.ExponentialBackoff(1*time.Second),
+		)
+		makeCircuit := makeOpenCircuitFromPolicy(policy)
+		extendCircuit := extendOpenCircuitFromMakeCircuit(makeCircuit)
+
+		currentTime := time.Date(2026, 1, 9, 12, 0, 0, 0, time.UTC)
+
+		// Create initial open state
+		initialOpen := openState{
+			openedAt:      currentTime.Add(-1 * time.Minute),
+			resetAt:       currentTime,
+			retryStatus:   retry.DefaultRetryStatus,
+			canaryRequest: false,
+		}
+
+		// Extend the circuit
+		extendOp := extendCircuit(currentTime)
+		result := extendOp(initialOpen)
+
+		assert.True(t, result.canaryRequest, "canaryRequest should be set to true")
+		assert.Greater(t, result.retryStatus.IterNumber, initialOpen.retryStatus.IterNumber,
+			"retry iteration should be incremented")
+		assert.True(t, result.resetAt.After(currentTime), "resetAt should be in the future")
+	})
+
+	t.Run("sets canaryRequest to true for next test", func(t *testing.T) {
+		vt := NewVirtualTimer(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
+		policy := retry.LimitRetries(5)
+		makeCircuit := makeOpenCircuitFromPolicy(policy)
+		extendCircuit := extendOpenCircuitFromMakeCircuit(makeCircuit)
+
+		currentTime := vt.Now()
+		initialOpen := openState{
+			openedAt:      currentTime.Add(-30 * time.Second),
+			resetAt:       currentTime,
+			retryStatus:   retry.DefaultRetryStatus,
+			canaryRequest: false,
+		}
+
+		result := extendCircuit(currentTime)(initialOpen)
+
+		assert.True(t, result.canaryRequest, "canaryRequest must be true after extension")
+	})
+
+	t.Run("applies exponential backoff on successive extensions", func(t *testing.T) {
+		policy := retry.Monoid.Concat(
+			retry.LimitRetries(10),
+			retry.ExponentialBackoff(1*time.Second),
+		)
+		makeCircuit := makeOpenCircuitFromPolicy(policy)
+		extendCircuit := extendOpenCircuitFromMakeCircuit(makeCircuit)
+
+		currentTime := time.Date(2026, 1, 9, 12, 0, 0, 0, time.UTC)
+
+		// First extension
+		state1 := openState{
+			openedAt:      currentTime,
+			resetAt:       currentTime,
+			retryStatus:   retry.DefaultRetryStatus,
+			canaryRequest: false,
+		}
+		result1 := extendCircuit(currentTime)(state1)
+		delay1 := result1.resetAt.Sub(currentTime)
+
+		// Second extension (should have longer delay)
+		result2 := extendCircuit(currentTime)(result1)
+		delay2 := result2.resetAt.Sub(currentTime)
+
+		assert.Greater(t, delay2, delay1, "second extension should have longer delay due to exponential backoff")
+	})
+}
+
+// TestIsResetTimeExceeded tests the isResetTimeExceeded function
+func TestIsResetTimeExceeded(t *testing.T) {
+	t.Run("returns Some when reset time is exceeded and no canary active", func(t *testing.T) {
+		currentTime := time.Date(2026, 1, 9, 12, 0, 0, 0, time.UTC)
+		resetTime := currentTime.Add(-1 * time.Second) // in the past
+
+		openState := openState{
+			openedAt:      currentTime.Add(-1 * time.Minute),
+			resetAt:       resetTime,
+			retryStatus:   retry.DefaultRetryStatus,
+			canaryRequest: false,
+		}
+
+		result := isResetTimeExceeded(currentTime)(openState)
+
+		assert.True(t, option.IsSome(result), "should return Some when reset time exceeded")
+	})
+
+	t.Run("returns None when reset time not yet exceeded", func(t *testing.T) {
+		currentTime := time.Date(2026, 1, 9, 12, 0, 0, 0, time.UTC)
+		resetTime := currentTime.Add(1 * time.Minute) // in the future
+
+		openState := openState{
+			openedAt:      currentTime.Add(-30 * time.Second),
+			resetAt:       resetTime,
+			retryStatus:   retry.DefaultRetryStatus,
+			canaryRequest: false,
+		}
+
+		result := isResetTimeExceeded(currentTime)(openState)
+
+		assert.True(t, option.IsNone(result), "should return None when reset time not exceeded")
+	})
+
+	t.Run("returns None when canary request is already active", func(t *testing.T) {
+		currentTime := time.Date(2026, 1, 9, 12, 0, 0, 0, time.UTC)
+		resetTime := currentTime.Add(-1 * time.Second) // in the past
+
+		openState := openState{
+			openedAt:      currentTime.Add(-1 * time.Minute),
+			resetAt:       resetTime,
+			retryStatus:   retry.DefaultRetryStatus,
+			canaryRequest: true, // canary already active
+		}
+
+		result := isResetTimeExceeded(currentTime)(openState)
+
+		assert.True(t, option.IsNone(result), "should return None when canary is already active")
+	})
+
+	t.Run("returns Some at exact reset time boundary", func(t *testing.T) {
+		currentTime := time.Date(2026, 1, 9, 12, 0, 0, 0, time.UTC)
+		resetTime := currentTime.Add(-1 * time.Nanosecond) // just passed
+
+		openState := openState{
+			openedAt:      currentTime.Add(-1 * time.Minute),
+			resetAt:       resetTime,
+			retryStatus:   retry.DefaultRetryStatus,
+			canaryRequest: false,
+		}
+
+		result := isResetTimeExceeded(currentTime)(openState)
+
+		assert.True(t, option.IsSome(result), "should return Some when current time is after reset time")
+	})
+
+	t.Run("returns None when current time equals reset time", func(t *testing.T) {
+		currentTime := time.Date(2026, 1, 9, 12, 0, 0, 0, time.UTC)
+		resetTime := currentTime // exactly equal
+
+		openState := openState{
+			openedAt:      currentTime.Add(-1 * time.Minute),
+			resetAt:       resetTime,
+			retryStatus:   retry.DefaultRetryStatus,
+			canaryRequest: false,
+		}
+
+		result := isResetTimeExceeded(currentTime)(openState)
+
+		assert.True(t, option.IsNone(result), "should return None when times are equal (not After)")
+	})
+}
+
+// TestHandleSuccessOnClosed tests the handleSuccessOnClosed function
+func TestHandleSuccessOnClosed(t *testing.T) {
+	t.Run("resets failure count on success", func(t *testing.T) {
+		vt := NewVirtualTimer(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
+		currentTime := vt.Now
+		addSuccess := reader.From1(ClosedState.AddSuccess)
+
+		// Create initial state with some failures
+		now := vt.Now()
+		initialClosed := MakeClosedStateCounter(3)
+		initialClosed = initialClosed.AddError(now)
+		initialClosed = initialClosed.AddError(now)
+		initialState := createClosedCircuit(initialClosed)
+
+		ref := io.Run(ioref.MakeIORef(initialState))
+		modify := modifyV(ref)
+
+		handler := handleSuccessOnClosed(currentTime, addSuccess)
+
+		// Apply the handler
+		result := io.Run(handler(modify))
+
+		// Verify state is still closed and failures are reset
+		assert.True(t, IsClosed(result), "circuit should remain closed after success")
+	})
+
+	t.Run("keeps circuit closed", func(t *testing.T) {
+		vt := NewVirtualTimer(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
+		currentTime := vt.Now
+		addSuccess := reader.From1(ClosedState.AddSuccess)
+
+		initialState := createClosedCircuit(MakeClosedStateCounter(3))
+		ref := io.Run(ioref.MakeIORef(initialState))
+		modify := modifyV(ref)
+
+		handler := handleSuccessOnClosed(currentTime, addSuccess)
+		result := io.Run(handler(modify))
+
+		assert.True(t, IsClosed(result), "circuit should remain closed")
+	})
+}
+
+// TestHandleFailureOnClosed tests the handleFailureOnClosed function
+func TestHandleFailureOnClosed(t *testing.T) {
+	t.Run("keeps circuit closed when threshold not exceeded", func(t *testing.T) {
+		vt := NewVirtualTimer(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
+		currentTime := vt.Now
+		addError := reader.From1(ClosedState.AddError)
+		checkClosedState := reader.From1(ClosedState.Check)
+		openCircuit := func(ct time.Time) openState {
+			return openState{
+				openedAt:      ct,
+				resetAt:       ct.Add(1 * time.Minute),
+				retryStatus:   retry.DefaultRetryStatus,
+				canaryRequest: false,
+			}
+		}
+
+		// Create initial state with room for more failures
+		now := vt.Now()
+		initialClosed := MakeClosedStateCounter(5) // threshold is 5
+		initialClosed = initialClosed.AddError(now)
+		initialState := createClosedCircuit(initialClosed)
+
+		ref := io.Run(ioref.MakeIORef(initialState))
+		modify := modifyV(ref)
+
+		handler := handleFailureOnClosed(currentTime, addError, checkClosedState, openCircuit)
+		result := io.Run(handler(modify))
+
+		assert.True(t, IsClosed(result), "circuit should remain closed when threshold not exceeded")
+	})
+
+	t.Run("opens circuit when threshold exceeded", func(t *testing.T) {
+		vt := NewVirtualTimer(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
+		currentTime := vt.Now
+		addError := reader.From1(ClosedState.AddError)
+		checkClosedState := reader.From1(ClosedState.Check)
+		openCircuit := func(ct time.Time) openState {
+			return openState{
+				openedAt:      ct,
+				resetAt:       ct.Add(1 * time.Minute),
+				retryStatus:   retry.DefaultRetryStatus,
+				canaryRequest: false,
+			}
+		}
+
+		// Create initial state at threshold
+		now := vt.Now()
+		initialClosed := MakeClosedStateCounter(2) // threshold is 2
+		initialClosed = initialClosed.AddError(now)
+		initialState := createClosedCircuit(initialClosed)
+
+		ref := io.Run(ioref.MakeIORef(initialState))
+		modify := modifyV(ref)
+
+		handler := handleFailureOnClosed(currentTime, addError, checkClosedState, openCircuit)
+		result := io.Run(handler(modify))
+
+		assert.True(t, IsOpen(result), "circuit should open when threshold exceeded")
+	})
+
+	t.Run("records failure in closed state", func(t *testing.T) {
+		vt := NewVirtualTimer(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
+		currentTime := vt.Now
+		addError := reader.From1(ClosedState.AddError)
+		checkClosedState := reader.From1(ClosedState.Check)
+		openCircuit := func(ct time.Time) openState {
+			return openState{
+				openedAt:      ct,
+				resetAt:       ct.Add(1 * time.Minute),
+				retryStatus:   retry.DefaultRetryStatus,
+				canaryRequest: false,
+			}
+		}
+
+		initialState := createClosedCircuit(MakeClosedStateCounter(10))
+		ref := io.Run(ioref.MakeIORef(initialState))
+		modify := modifyV(ref)
+
+		handler := handleFailureOnClosed(currentTime, addError, checkClosedState, openCircuit)
+		result := io.Run(handler(modify))
+
+		// Should still be closed but with failure recorded
+		assert.True(t, IsClosed(result), "circuit should remain closed")
+	})
+}

v2/circuitbreaker/closed.go

@@ -0,0 +1,329 @@
+package circuitbreaker
+
+import (
+	"slices"
+	"time"
+
+	A "github.com/IBM/fp-go/v2/array"
+	F "github.com/IBM/fp-go/v2/function"
+	N "github.com/IBM/fp-go/v2/number"
+	"github.com/IBM/fp-go/v2/optics/lens"
+	"github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/ord"
+)
+
+type (
+	// ClosedState represents the closed state of a circuit breaker.
+	// In the closed state, requests are allowed to pass through, but failures are tracked.
+	// If a failure condition is met, the circuit breaker transitions to an open state.
+	//
+	// # Thread Safety
+	//
+	// All ClosedState implementations MUST be thread-safe. The recommended approach is to
+	// make all methods return new copies rather than modifying the receiver, which provides
+	// automatic thread safety through immutability.
+	//
+	// Implementations should ensure that:
+	//   - Empty() returns a new instance with cleared state
+	//   - AddError() returns a new instance with the error recorded
+	//   - AddSuccess() returns a new instance with success recorded
+	//   - Check() does not modify the receiver
+	//
+	// Both provided implementations (closedStateWithErrorCount and closedStateWithHistory)
+	// follow this pattern and are safe for concurrent use.
+	ClosedState interface {
+		// Empty returns a new ClosedState with all tracked failures cleared.
+		// This is used when transitioning back to a closed state from an open state.
+		//
+		// Thread Safety: Returns a new instance; safe for concurrent use.
+		Empty() ClosedState
+
+		// AddError records a failure at the given time.
+		// Returns an updated ClosedState reflecting the recorded failure.
+		//
+		// Thread Safety: Returns a new instance; safe for concurrent use.
+		// The original ClosedState is not modified.
+		AddError(time.Time) ClosedState
+
+		// AddSuccess records a successful request at the given time.
+		// Returns an updated ClosedState reflecting the successful request.
+		//
+		// Thread Safety: Returns a new instance; safe for concurrent use.
+		// The original ClosedState is not modified.
+		AddSuccess(time.Time) ClosedState
+
+		// Check verifies if the circuit breaker should remain closed at the given time.
+		// Returns Some(ClosedState) if the circuit should stay closed,
+		// or None if the circuit should open due to exceeding the failure threshold.
+		//
+		// Thread Safety: Does not modify the receiver; safe for concurrent use.
+		Check(time.Time) Option[ClosedState]
+	}
+
+	// closedStateWithErrorCount is a counter-based implementation of ClosedState.
+	// It tracks the number of consecutive failures and opens the circuit when
+	// the failure count exceeds a configured threshold.
+	//
+	// Thread Safety: This implementation is immutable. All methods return new instances
+	// rather than modifying the receiver, making it safe for concurrent use without locks.
+	closedStateWithErrorCount struct {
+		// checkFailures is a Kleisli arrow that checks if the failure count exceeds the threshold.
+		// Returns Some(count) if threshold is exceeded, None otherwise.
+		checkFailures option.Kleisli[uint, uint]
+		// failureCount tracks the current number of consecutive failures.
+		failureCount uint
+	}
+
+	// closedStateWithHistory is a time-window-based implementation of ClosedState.
+	// It tracks failures within a sliding time window and opens the circuit when
+	// the failure count within the window exceeds a configured threshold.
+	//
+	// Thread Safety: This implementation is immutable. All methods return new instances
+	// with new slices rather than modifying the receiver, making it safe for concurrent
+	// use without locks. The history slice is never modified in place; addToSlice always
+	// creates a new slice.
+	closedStateWithHistory struct {
+		ordTime Ord[time.Time]
+		// maxFailures is the maximum number of failures allowed within the time window.
+		checkFailures option.Kleisli[int, int]
+		timeWindow    time.Duration
+		history       []time.Time
+	}
+)
+
+var (
+	failureCountLens = lens.MakeLensStrictWithName(
+		func(s *closedStateWithErrorCount) uint { return s.failureCount },
+		func(s *closedStateWithErrorCount, c uint) *closedStateWithErrorCount {
+			s.failureCount = c
+			return s
+		},
+		"closeStateWithErrorCount.failureCount",
+	)
+
+	historyLens = lens.MakeLensRefWithName(
+		func(s *closedStateWithHistory) []time.Time { return s.history },
+		func(s *closedStateWithHistory, c []time.Time) *closedStateWithHistory {
+			s.history = c
+			return s
+		},
+		"closedStateWithHistory.history",
+	)
+
+	resetHistory      = historyLens.Set(A.Empty[time.Time]())
+	resetFailureCount = failureCountLens.Set(0)
+	incFailureCount   = lens.Modify[*closedStateWithErrorCount](N.Add(uint(1)))(failureCountLens)
+)
+
+// Empty returns a new closedStateWithErrorCount with the failure count reset to zero.
+//
+// Thread Safety: Returns a new instance; the original is not modified.
+// Safe for concurrent use.
+func (s *closedStateWithErrorCount) Empty() ClosedState {
+	return resetFailureCount(s)
+}
+
+// AddError increments the failure count and returns a new closedStateWithErrorCount.
+// The time parameter is ignored in this counter-based implementation.
+//
+// Thread Safety: Returns a new instance; the original is not modified.
+// Safe for concurrent use.
+func (s *closedStateWithErrorCount) AddError(_ time.Time) ClosedState {
+	return incFailureCount(s)
+}
+
+// AddSuccess resets the failure count to zero and returns a new closedStateWithErrorCount.
+// The time parameter is ignored in this counter-based implementation.
+//
+// Thread Safety: Returns a new instance; the original is not modified.
+// Safe for concurrent use.
+func (s *closedStateWithErrorCount) AddSuccess(_ time.Time) ClosedState {
+	return resetFailureCount(s)
+}
+
+// Check verifies if the failure count is below the threshold.
+// Returns Some(ClosedState) if below threshold, None if at or above threshold.
+// The time parameter is ignored in this counter-based implementation.
+//
+// Thread Safety: Does not modify the receiver; safe for concurrent use.
+func (s *closedStateWithErrorCount) Check(_ time.Time) Option[ClosedState] {
+	return F.Pipe3(
+		s,
+		failureCountLens.Get,
+		s.checkFailures,
+		option.MapTo[uint](ClosedState(s)),
+	)
+}
+
+// MakeClosedStateCounter creates a counter-based ClosedState implementation.
+// The circuit breaker will open when the number of consecutive failures reaches maxFailures.
+//
+// Parameters:
+//   - maxFailures: The threshold for consecutive failures. The circuit opens when
+//     failureCount >= maxFailures (greater than or equal to).
+//
+// Returns:
+//   - A ClosedState that tracks failures using a simple counter.
+//
+// Example:
+//   - If maxFailures is 3, the circuit will open on the 3rd consecutive failure.
+//   - Each AddError call increments the counter.
+//   - Each AddSuccess call resets the counter to 0 (only consecutive failures count).
+//   - Empty resets the counter to 0.
+//
+// Behavior:
+//   - Check returns Some(ClosedState) when failureCount < maxFailures (circuit stays closed)
+//   - Check returns None when failureCount >= maxFailures (circuit should open)
+//   - AddSuccess resets the failure count, so only consecutive failures trigger circuit opening
+//
+// Thread Safety: The returned ClosedState is safe for concurrent use. All methods
+// return new instances rather than modifying the receiver.
+func MakeClosedStateCounter(maxFailures uint) ClosedState {
+	return &closedStateWithErrorCount{
+		checkFailures: option.FromPredicate(N.LessThan(maxFailures)),
+	}
+}
+
+// Empty returns a new closedStateWithHistory with an empty failure history.
+//
+// Thread Safety: Returns a new instance with a new empty slice; the original is not modified.
+// Safe for concurrent use.
+func (s *closedStateWithHistory) Empty() ClosedState {
+	return resetHistory(s)
+}
+
+// addToSlice creates a new sorted slice by adding an item to an existing slice.
+// This function does not modify the input slice; it creates a new slice with the item added
+// and returns it in sorted order.
+//
+// Parameters:
+//   - o: An Ord instance for comparing time.Time values to determine sort order
+//   - ar: The existing slice of time.Time values (assumed to be sorted)
+//   - item: The new time.Time value to add to the slice
+//
+// Returns:
+//   - A new slice containing all elements from ar plus the new item, sorted in ascending order
+//
+// Implementation Details:
+//   - Creates a new slice with capacity len(ar)+1
+//   - Copies all elements from ar to the new slice
+//   - Appends the new item
+//   - Sorts the entire slice using the provided Ord comparator
+//
+// Thread Safety: This function is pure and does not modify its inputs. It always returns
+// a new slice, making it safe for concurrent use. This is a key component of the immutable
+// design of closedStateWithHistory.
+//
+// Note: This function is used internally by closedStateWithHistory.AddError to maintain
+// a sorted history of failure timestamps for efficient binary search operations.
+func addToSlice(o ord.Ord[time.Time], ar []time.Time, item time.Time) []time.Time {
+	cpy := make([]time.Time, len(ar)+1)
+	cpy[copy(cpy, ar)] = item
+	slices.SortFunc(cpy, o.Compare)
+	return cpy
+}
+
+// AddError records a failure at the given time and returns a new closedStateWithHistory.
+// The new instance contains the failure in its history, with old failures outside the
+// time window automatically pruned.
+//
+// Thread Safety: Returns a new instance with a new history slice; the original is not modified.
+// Safe for concurrent use. The addToSlice function creates a new slice, ensuring immutability.
+func (s *closedStateWithHistory) AddError(currentTime time.Time) ClosedState {
+
+	addFailureToHistory := F.Pipe1(
+		historyLens,
+		lens.Modify[*closedStateWithHistory](func(old []time.Time) []time.Time {
+			// oldest valid entry
+			idx, _ := slices.BinarySearchFunc(old, currentTime.Add(-s.timeWindow), s.ordTime.Compare)
+			return addToSlice(s.ordTime, old[idx:], currentTime)
+		}),
+	)
+
+	return addFailureToHistory(s)
+}
+
+// AddSuccess purges the entire failure history and returns a new closedStateWithHistory.
+// The time parameter is ignored; any success clears all tracked failures.
+//
+// Thread Safety: Returns a new instance with a new empty slice; the original is not modified.
+// Safe for concurrent use.
+func (s *closedStateWithHistory) AddSuccess(_ time.Time) ClosedState {
+	return resetHistory(s)
+}
+
+// Check verifies if the number of failures in the history is below the threshold.
+// Returns Some(ClosedState) if below threshold, None if at or above threshold.
+// The time parameter is ignored; the check is based on the current history size.
+//
+// Thread Safety: Does not modify the receiver; safe for concurrent use.
+func (s *closedStateWithHistory) Check(_ time.Time) Option[ClosedState] {
+
+	return F.Pipe4(
+		s,
+		historyLens.Get,
+		A.Size,
+		s.checkFailures,
+		option.MapTo[int](ClosedState(s)),
+	)
+}
+
+// MakeClosedStateHistory creates a time-window-based ClosedState implementation.
+// The circuit breaker will open when the number of failures within a sliding time window reaches maxFailures.
+//
+// Unlike MakeClosedStateCounter which tracks consecutive failures, this implementation tracks
+// all failures within a time window. However, any successful request will purge the entire history,
+// effectively resetting the failure tracking.
+//
+// Parameters:
+//   - timeWindow: The duration of the sliding time window. Failures older than this are automatically
+//     discarded from the history when new failures are added.
+//   - maxFailures: The threshold for failures within the time window. The circuit opens when
+//     the number of failures in the window reaches this value (failureCount >= maxFailures).
+//
+// Returns:
+//   - A ClosedState that tracks failures using a time-based sliding window.
+//
+// Example:
+//   - If timeWindow is 1 minute and maxFailures is 5, the circuit will open when 5 failures
+//     occur within any 1-minute period.
+//   - Failures older than 1 minute are automatically removed from the history when AddError is called.
+//   - Any successful request immediately purges all tracked failures from the history.
+//
+// Behavior:
+//   - AddError records the failure timestamp and removes failures outside the time window
+//     (older than currentTime - timeWindow).
+//   - AddSuccess purges the entire failure history (all tracked failures are removed).
+//   - Check returns Some(ClosedState) when failureCount < maxFailures (circuit stays closed).
+//   - Check returns None when failureCount >= maxFailures (circuit should open).
+//   - Empty purges the entire failure history.
+//
+// Time Window Management:
+//   - The history is automatically pruned on each AddError call to remove failures older than
+//     currentTime - timeWindow.
+//   - The history is kept sorted by time for efficient binary search and pruning.
+//
+// Important Note:
+//   - A successful request resets everything by purging the entire history. This means that
+//     unlike a pure sliding window, a single success will clear all tracked failures, even
+//     those within the time window. This behavior is similar to MakeClosedStateCounter but
+//     with time-based tracking for failures.
+//
+// Thread Safety: The returned ClosedState is safe for concurrent use. All methods return
+// new instances with new slices rather than modifying the receiver. The history slice is
+// never modified in place.
+//
+// Use Cases:
+//   - Systems where a successful request indicates recovery and past failures should be forgotten.
+//   - Rate limiting with success-based reset: Allow bursts of failures but reset on success.
+//   - Hybrid approach: Time-based failure tracking with success-based recovery.
+func MakeClosedStateHistory(
+	timeWindow time.Duration,
+	maxFailures uint) ClosedState {
+	return &closedStateWithHistory{
+		checkFailures: option.FromPredicate(N.LessThan(int(maxFailures))),
+		ordTime:       ord.OrdTime(),
+		history:       A.Empty[time.Time](),
+		timeWindow:    timeWindow,
+	}
+}

v2/circuitbreaker/closed_test.go

@@ -0,0 +1,934 @@
+package circuitbreaker
+
+import (
+	"testing"
+	"time"
+
+	"github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/ord"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestMakeClosedStateCounter(t *testing.T) {
+	t.Run("creates a valid ClosedState", func(t *testing.T) {
+		maxFailures := uint(3)
+		state := MakeClosedStateCounter(maxFailures)
+
+		assert.NotNil(t, state, "MakeClosedStateCounter should return a non-nil ClosedState")
+	})
+
+	t.Run("initial state passes Check", func(t *testing.T) {
+		vt := NewVirtualTimer(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
+		maxFailures := uint(3)
+		state := MakeClosedStateCounter(maxFailures)
+		now := vt.Now()
+
+		result := state.Check(now)
+
+		assert.True(t, option.IsSome(result), "initial state should pass Check (return Some, circuit stays closed)")
+	})
+
+	t.Run("Empty resets failure count", func(t *testing.T) {
+		vt := NewVirtualTimer(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
+		maxFailures := uint(2)
+		state := MakeClosedStateCounter(maxFailures)
+		now := vt.Now()
+
+		// Add some errors
+		state = state.AddError(now)
+		state = state.AddError(now)
+
+		// Reset the state
+		state = state.Empty()
+
+		// Should pass check after reset
+		result := state.Check(now)
+		assert.True(t, option.IsSome(result), "state should pass Check after Empty")
+	})
+
+	t.Run("AddSuccess resets failure count", func(t *testing.T) {
+		vt := NewVirtualTimer(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
+		maxFailures := uint(3)
+		state := MakeClosedStateCounter(maxFailures)
+
+		// Add errors
+		state = state.AddError(vt.Now())
+		vt.Advance(1 * time.Second)
+		state = state.AddError(vt.Now())
+		vt.Advance(1 * time.Second)
+
+		// Add success (should reset counter)
+		state = state.AddSuccess(vt.Now())
+		vt.Advance(1 * time.Second)
+
+		// Add another error (this is now the first consecutive error)
+		state = state.AddError(vt.Now())
+
+		// Should still pass check (only 1 consecutive error, threshold is 3)
+		result := state.Check(vt.Now())
+		assert.True(t, option.IsSome(result), "AddSuccess should reset failure count")
+	})
+
+	t.Run("circuit opens when failures reach threshold", func(t *testing.T) {
+		vt := NewVirtualTimer(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
+		maxFailures := uint(3)
+		state := MakeClosedStateCounter(maxFailures)
+		now := vt.Now()
+
+		// Add errors up to but not including threshold
+		state = state.AddError(now)
+		state = state.AddError(now)
+
+		// Should still pass before threshold
+		result := state.Check(now)
+		assert.True(t, option.IsSome(result), "should pass Check before threshold")
+
+		// Add one more error to reach threshold
+		state = state.AddError(now)
+
+		// Should fail check at threshold
+		result = state.Check(now)
+		assert.True(t, option.IsNone(result), "should fail Check when reaching threshold")
+	})
+
+	t.Run("circuit opens exactly at maxFailures", func(t *testing.T) {
+		vt := NewVirtualTimer(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
+		maxFailures := uint(5)
+		state := MakeClosedStateCounter(maxFailures)
+		now := vt.Now()
+
+		// Add exactly maxFailures - 1 errors
+		for i := uint(0); i < maxFailures-1; i++ {
+			state = state.AddError(now)
+		}
+
+		// Should still pass
+		result := state.Check(now)
+		assert.True(t, option.IsSome(result), "should pass Check before maxFailures")
+
+		// Add one more to reach maxFailures
+		state = state.AddError(now)
+
+		// Should fail now
+		result = state.Check(now)
+		assert.True(t, option.IsNone(result), "should fail Check at maxFailures")
+	})
+
+	t.Run("zero maxFailures means circuit is always open", func(t *testing.T) {
+		vt := NewVirtualTimer(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
+		maxFailures := uint(0)
+		state := MakeClosedStateCounter(maxFailures)
+		now := vt.Now()
+
+		// Initial state should already fail (0 >= 0)
+		result := state.Check(now)
+		assert.True(t, option.IsNone(result), "initial state should fail Check with maxFailures=0")
+
+		// Add one error
+		state = state.AddError(now)
+
+		// Should still fail
+		result = state.Check(now)
+		assert.True(t, option.IsNone(result), "should fail Check after error with maxFailures=0")
+	})
+
+	t.Run("AddSuccess resets counter between errors", func(t *testing.T) {
+		vt := NewVirtualTimer(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
+		maxFailures := uint(3)
+		state := MakeClosedStateCounter(maxFailures)
+
+		// Add errors
+		state = state.AddError(vt.Now())
+		vt.Advance(1 * time.Second)
+		state = state.AddError(vt.Now())
+		vt.Advance(1 * time.Second)
+
+		// Add success (resets counter)
+		state = state.AddSuccess(vt.Now())
+		vt.Advance(1 * time.Second)
+
+		// Add more errors
+		state = state.AddError(vt.Now())
+		vt.Advance(1 * time.Second)
+		state = state.AddError(vt.Now())
+
+		// Should still pass (only 2 consecutive errors after reset)
+		result := state.Check(vt.Now())
+		assert.True(t, option.IsSome(result), "should pass with 2 consecutive errors after reset")
+
+		// Add one more to reach threshold
+		vt.Advance(1 * time.Second)
+		state = state.AddError(vt.Now())
+
+		// Should fail at threshold
+		result = state.Check(vt.Now())
+		assert.True(t, option.IsNone(result), "should fail after reaching threshold")
+	})
+
+	t.Run("Empty can be called multiple times", func(t *testing.T) {
+		vt := NewVirtualTimer(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
+		maxFailures := uint(2)
+		state := MakeClosedStateCounter(maxFailures)
+		now := vt.Now()
+
+		// Add errors
+		state = state.AddError(now)
+		state = state.AddError(now)
+		state = state.AddError(now)
+
+		// Reset multiple times
+		state = state.Empty()
+		state = state.Empty()
+		state = state.Empty()
+
+		// Should still pass
+		result := state.Check(now)
+		assert.True(t, option.IsSome(result), "state should pass Check after multiple Empty calls")
+	})
+
+	t.Run("time parameter is ignored in counter implementation", func(t *testing.T) {
+		vt := NewVirtualTimer(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
+		maxFailures := uint(3)
+		state := MakeClosedStateCounter(maxFailures)
+
+		// Use different times for each operation
+		time1 := vt.Now()
+		time2 := time1.Add(1 * time.Hour)
+
+		state = state.AddError(time1)
+		state = state.AddError(time2)
+
+		// Check with yet another time
+		time3 := time1.Add(2 * time.Hour)
+		result := state.Check(time3)
+
+		// Should still pass (2 errors, threshold is 3, not reached yet)
+		assert.True(t, option.IsSome(result), "time parameter should not affect counter behavior")
+
+		// Add one more to reach threshold
+		state = state.AddError(time1)
+		result = state.Check(time1)
+		assert.True(t, option.IsNone(result), "should fail after reaching threshold regardless of time")
+	})
+
+	t.Run("large maxFailures value", func(t *testing.T) {
+		vt := NewVirtualTimer(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
+		maxFailures := uint(1000)
+		state := MakeClosedStateCounter(maxFailures)
+		now := vt.Now()
+
+		// Add many errors but not reaching threshold
+		for i := uint(0); i < maxFailures-1; i++ {
+			state = state.AddError(now)
+		}
+
+		// Should still pass
+		result := state.Check(now)
+		assert.True(t, option.IsSome(result), "should pass Check with large maxFailures before threshold")
+
+		// Add one more to reach threshold
+		state = state.AddError(now)
+
+		// Should fail
+		result = state.Check(now)
+		assert.True(t, option.IsNone(result), "should fail Check with large maxFailures at threshold")
+	})
+
+	t.Run("state is immutable - original unchanged after AddError", func(t *testing.T) {
+		vt := NewVirtualTimer(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
+		maxFailures := uint(2)
+		originalState := MakeClosedStateCounter(maxFailures)
+		now := vt.Now()
+
+		// Create new state by adding error
+		newState := originalState.AddError(now)
+
+		// Original should still pass check
+		result := originalState.Check(now)
+		assert.True(t, option.IsSome(result), "original state should be unchanged")
+
+		// New state should reach threshold (2 errors total, threshold is 2)
+		newState = newState.AddError(now)
+
+		result = newState.Check(now)
+		assert.True(t, option.IsNone(result), "new state should fail after reaching threshold")
+
+		// Original should still pass
+		result = originalState.Check(now)
+		assert.True(t, option.IsSome(result), "original state should still be unchanged")
+	})
+
+	t.Run("state is immutable - original unchanged after Empty", func(t *testing.T) {
+		vt := NewVirtualTimer(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
+		maxFailures := uint(2)
+		state := MakeClosedStateCounter(maxFailures)
+		now := vt.Now()
+
+		// Add errors to original
+		state = state.AddError(now)
+		state = state.AddError(now)
+		stateWithErrors := state
+
+		// Create new state by calling Empty
+		emptyState := stateWithErrors.Empty()
+
+		// Original with errors should reach threshold (2 errors total, threshold is 2)
+		result := stateWithErrors.Check(now)
+		assert.True(t, option.IsNone(result), "state with errors should fail after reaching threshold")
+
+		// Empty state should pass
+		result = emptyState.Check(now)
+		assert.True(t, option.IsSome(result), "empty state should pass Check")
+	})
+
+	t.Run("AddSuccess prevents circuit from opening", func(t *testing.T) {
+		vt := NewVirtualTimer(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
+		maxFailures := uint(3)
+		state := MakeClosedStateCounter(maxFailures)
+
+		// Add errors close to threshold
+		state = state.AddError(vt.Now())
+		vt.Advance(1 * time.Second)
+		state = state.AddError(vt.Now())
+		vt.Advance(1 * time.Second)
+
+		// Add success before reaching threshold
+		state = state.AddSuccess(vt.Now())
+		vt.Advance(1 * time.Second)
+
+		// Add more errors
+		state = state.AddError(vt.Now())
+		vt.Advance(1 * time.Second)
+		state = state.AddError(vt.Now())
+
+		// Should still pass (only 2 consecutive errors)
+		result := state.Check(vt.Now())
+		assert.True(t, option.IsSome(result), "circuit should stay closed after success reset")
+	})
+
+	t.Run("multiple AddSuccess calls keep counter at zero", func(t *testing.T) {
+		vt := NewVirtualTimer(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
+		maxFailures := uint(2)
+		state := MakeClosedStateCounter(maxFailures)
+
+		// Add error
+		state = state.AddError(vt.Now())
+		vt.Advance(1 * time.Second)
+
+		// Multiple successes
+		state = state.AddSuccess(vt.Now())
+		vt.Advance(1 * time.Second)
+		state = state.AddSuccess(vt.Now())
+		vt.Advance(1 * time.Second)
+		state = state.AddSuccess(vt.Now())
+		vt.Advance(1 * time.Second)
+
+		// Should still pass
+		result := state.Check(vt.Now())
+		assert.True(t, option.IsSome(result), "multiple AddSuccess should keep counter at zero")
+
+		// Add errors to reach threshold
+		state = state.AddError(vt.Now())
+		vt.Advance(1 * time.Second)
+		state = state.AddError(vt.Now())
+
+		// Should fail
+		result = state.Check(vt.Now())
+		assert.True(t, option.IsNone(result), "should fail after reaching threshold")
+	})
+
+	t.Run("alternating errors and successes never opens circuit", func(t *testing.T) {
+		vt := NewVirtualTimer(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
+		maxFailures := uint(3)
+		state := MakeClosedStateCounter(maxFailures)
+
+		// Alternate errors and successes
+		for i := 0; i < 10; i++ {
+			state = state.AddError(vt.Now())
+			vt.Advance(500 * time.Millisecond)
+			state = state.AddSuccess(vt.Now())
+			vt.Advance(500 * time.Millisecond)
+		}
+
+		// Should still pass (never had consecutive failures)
+		result := state.Check(vt.Now())
+		assert.True(t, option.IsSome(result), "alternating errors and successes should never open circuit")
+	})
+}
+
+func TestAddToSlice(t *testing.T) {
+	ordTime := ord.OrdTime()
+
+	t.Run("adds item to empty slice and returns sorted result", func(t *testing.T) {
+		input := []time.Time{}
+		item := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+
+		result := addToSlice(ordTime, input, item)
+
+		assert.Len(t, result, 1, "result should have 1 element")
+		assert.Equal(t, item, result[0], "result should contain the added item")
+	})
+
+	t.Run("adds item and maintains sorted order", func(t *testing.T) {
+		baseTime := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+		input := []time.Time{
+			baseTime,
+			baseTime.Add(20 * time.Second),
+			baseTime.Add(40 * time.Second),
+		}
+		item := baseTime.Add(30 * time.Second)
+
+		result := addToSlice(ordTime, input, item)
+
+		assert.Len(t, result, 4, "result should have 4 elements")
+		// Verify sorted order
+		assert.Equal(t, baseTime, result[0])
+		assert.Equal(t, baseTime.Add(20*time.Second), result[1])
+		assert.Equal(t, baseTime.Add(30*time.Second), result[2])
+		assert.Equal(t, baseTime.Add(40*time.Second), result[3])
+	})
+
+	t.Run("adds item at beginning when it's earliest", func(t *testing.T) {
+		baseTime := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+		input := []time.Time{
+			baseTime.Add(20 * time.Second),
+			baseTime.Add(40 * time.Second),
+		}
+		item := baseTime
+
+		result := addToSlice(ordTime, input, item)
+
+		assert.Len(t, result, 3, "result should have 3 elements")
+		assert.Equal(t, baseTime, result[0], "earliest item should be first")
+		assert.Equal(t, baseTime.Add(20*time.Second), result[1])
+		assert.Equal(t, baseTime.Add(40*time.Second), result[2])
+	})
+
+	t.Run("adds item at end when it's latest", func(t *testing.T) {
+		baseTime := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+		input := []time.Time{
+			baseTime,
+			baseTime.Add(20 * time.Second),
+		}
+		item := baseTime.Add(40 * time.Second)
+
+		result := addToSlice(ordTime, input, item)
+
+		assert.Len(t, result, 3, "result should have 3 elements")
+		assert.Equal(t, baseTime, result[0])
+		assert.Equal(t, baseTime.Add(20*time.Second), result[1])
+		assert.Equal(t, baseTime.Add(40*time.Second), result[2], "latest item should be last")
+	})
+
+	t.Run("does not modify original slice", func(t *testing.T) {
+		baseTime := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+		input := []time.Time{
+			baseTime,
+			baseTime.Add(20 * time.Second),
+		}
+		originalLen := len(input)
+		originalFirst := input[0]
+		originalLast := input[1]
+		item := baseTime.Add(10 * time.Second)
+
+		result := addToSlice(ordTime, input, item)
+
+		// Verify original slice is unchanged
+		assert.Len(t, input, originalLen, "original slice length should be unchanged")
+		assert.Equal(t, originalFirst, input[0], "original slice first element should be unchanged")
+		assert.Equal(t, originalLast, input[1], "original slice last element should be unchanged")
+
+		// Verify result is different and has correct length
+		assert.Len(t, result, 3, "result should have new length")
+		// Verify the result contains the new item in sorted order
+		assert.Equal(t, baseTime, result[0])
+		assert.Equal(t, baseTime.Add(10*time.Second), result[1])
+		assert.Equal(t, baseTime.Add(20*time.Second), result[2])
+	})
+
+	t.Run("handles duplicate timestamps", func(t *testing.T) {
+		baseTime := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+		input := []time.Time{
+			baseTime,
+			baseTime.Add(20 * time.Second),
+		}
+		item := baseTime // duplicate of first element
+
+		result := addToSlice(ordTime, input, item)
+
+		assert.Len(t, result, 3, "result should have 3 elements including duplicate")
+		// Both instances of baseTime should be present
+		count := 0
+		for _, t := range result {
+			if t.Equal(baseTime) {
+				count++
+			}
+		}
+		assert.Equal(t, 2, count, "should have 2 instances of the duplicate timestamp")
+	})
+
+	t.Run("maintains sort order with unsorted input", func(t *testing.T) {
+		baseTime := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+		// Input is intentionally unsorted
+		input := []time.Time{
+			baseTime.Add(40 * time.Second),
+			baseTime,
+			baseTime.Add(20 * time.Second),
+		}
+		item := baseTime.Add(30 * time.Second)
+
+		result := addToSlice(ordTime, input, item)
+
+		assert.Len(t, result, 4, "result should have 4 elements")
+		// Verify result is sorted regardless of input order
+		for i := 0; i < len(result)-1; i++ {
+			assert.True(t, result[i].Before(result[i+1]) || result[i].Equal(result[i+1]),
+				"result should be sorted: element %d (%v) should be <= element %d (%v)",
+				i, result[i], i+1, result[i+1])
+		}
+	})
+
+	t.Run("works with nanosecond precision", func(t *testing.T) {
+		baseTime := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+		input := []time.Time{
+			baseTime,
+			baseTime.Add(2 * time.Nanosecond),
+		}
+		item := baseTime.Add(1 * time.Nanosecond)
+
+		result := addToSlice(ordTime, input, item)
+
+		assert.Len(t, result, 3, "result should have 3 elements")
+		assert.Equal(t, baseTime, result[0])
+		assert.Equal(t, baseTime.Add(1*time.Nanosecond), result[1])
+		assert.Equal(t, baseTime.Add(2*time.Nanosecond), result[2])
+	})
+}
+
+func TestMakeClosedStateHistory(t *testing.T) {
+	t.Run("creates a valid ClosedState", func(t *testing.T) {
+		timeWindow := 1 * time.Minute
+		maxFailures := uint(3)
+		state := MakeClosedStateHistory(timeWindow, maxFailures)
+
+		assert.NotNil(t, state, "MakeClosedStateHistory should return a non-nil ClosedState")
+	})
+
+	t.Run("initial state passes Check", func(t *testing.T) {
+		timeWindow := 1 * time.Minute
+		maxFailures := uint(3)
+		state := MakeClosedStateHistory(timeWindow, maxFailures)
+		now := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+
+		result := state.Check(now)
+
+		assert.True(t, option.IsSome(result), "initial state should pass Check (return Some, circuit stays closed)")
+	})
+
+	t.Run("Empty purges failure history", func(t *testing.T) {
+		timeWindow := 1 * time.Minute
+		maxFailures := uint(2)
+		state := MakeClosedStateHistory(timeWindow, maxFailures)
+		baseTime := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+
+		// Add some errors
+		state = state.AddError(baseTime)
+		state = state.AddError(baseTime.Add(10 * time.Second))
+
+		// Reset the state
+		state = state.Empty()
+
+		// Should pass check after reset
+		result := state.Check(baseTime.Add(20 * time.Second))
+		assert.True(t, option.IsSome(result), "state should pass Check after Empty")
+	})
+
+	t.Run("AddSuccess purges entire failure history", func(t *testing.T) {
+		timeWindow := 1 * time.Minute
+		maxFailures := uint(3)
+		state := MakeClosedStateHistory(timeWindow, maxFailures)
+		baseTime := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+
+		// Add errors
+		state = state.AddError(baseTime)
+		state = state.AddError(baseTime.Add(10 * time.Second))
+
+		// Add success (should purge all history)
+		state = state.AddSuccess(baseTime.Add(20 * time.Second))
+
+		// Add another error (this is now the first error in history)
+		state = state.AddError(baseTime.Add(30 * time.Second))
+
+		// Should still pass check (only 1 error in history, threshold is 3)
+		result := state.Check(baseTime.Add(30 * time.Second))
+		assert.True(t, option.IsSome(result), "AddSuccess should purge entire failure history")
+	})
+
+	t.Run("circuit opens when failures reach threshold within time window", func(t *testing.T) {
+		timeWindow := 1 * time.Minute
+		maxFailures := uint(3)
+		state := MakeClosedStateHistory(timeWindow, maxFailures)
+		baseTime := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+
+		// Add errors within time window but not reaching threshold
+		state = state.AddError(baseTime)
+		state = state.AddError(baseTime.Add(10 * time.Second))
+
+		// Should still pass before threshold
+		result := state.Check(baseTime.Add(20 * time.Second))
+		assert.True(t, option.IsSome(result), "should pass Check before threshold")
+
+		// Add one more error to reach threshold
+		state = state.AddError(baseTime.Add(30 * time.Second))
+
+		// Should fail check at threshold
+		result = state.Check(baseTime.Add(30 * time.Second))
+		assert.True(t, option.IsNone(result), "should fail Check when reaching threshold")
+	})
+
+	t.Run("old failures outside time window are automatically removed", func(t *testing.T) {
+		timeWindow := 1 * time.Minute
+		maxFailures := uint(3)
+		state := MakeClosedStateHistory(timeWindow, maxFailures)
+		baseTime := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+
+		// Add errors that will be outside the time window
+		state = state.AddError(baseTime)
+		state = state.AddError(baseTime.Add(10 * time.Second))
+
+		// Add error after time window has passed (this should remove old errors)
+		state = state.AddError(baseTime.Add(2 * time.Minute))
+
+		// Should pass check (only 1 error in window, old ones removed)
+		result := state.Check(baseTime.Add(2 * time.Minute))
+		assert.True(t, option.IsSome(result), "old failures should be removed from history")
+	})
+
+	t.Run("failures within time window are retained", func(t *testing.T) {
+		timeWindow := 1 * time.Minute
+		maxFailures := uint(3)
+		state := MakeClosedStateHistory(timeWindow, maxFailures)
+		baseTime := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+
+		// Add errors within time window
+		state = state.AddError(baseTime)
+		state = state.AddError(baseTime.Add(30 * time.Second))
+		state = state.AddError(baseTime.Add(50 * time.Second))
+
+		// All errors are within 1 minute window, should fail check
+		result := state.Check(baseTime.Add(50 * time.Second))
+		assert.True(t, option.IsNone(result), "failures within time window should be retained")
+	})
+
+	t.Run("sliding window behavior - errors slide out over time", func(t *testing.T) {
+		timeWindow := 1 * time.Minute
+		maxFailures := uint(3)
+		state := MakeClosedStateHistory(timeWindow, maxFailures)
+		baseTime := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+
+		// Add 3 errors to reach threshold
+		state = state.AddError(baseTime)
+		state = state.AddError(baseTime.Add(10 * time.Second))
+		state = state.AddError(baseTime.Add(20 * time.Second))
+
+		// Circuit should be open
+		result := state.Check(baseTime.Add(20 * time.Second))
+		assert.True(t, option.IsNone(result), "circuit should be open with 3 failures")
+
+		// Add error after first failure has expired (> 1 minute from first error)
+		// This should remove the first error, leaving only 3 in window
+		state = state.AddError(baseTime.Add(70 * time.Second))
+
+		// Should still fail check (3 errors in window after pruning)
+		result = state.Check(baseTime.Add(70 * time.Second))
+		assert.True(t, option.IsNone(result), "circuit should remain open with 3 failures in window")
+	})
+
+	t.Run("zero maxFailures means circuit is always open", func(t *testing.T) {
+		timeWindow := 1 * time.Minute
+		maxFailures := uint(0)
+		state := MakeClosedStateHistory(timeWindow, maxFailures)
+		baseTime := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+
+		// Initial state should already fail (0 >= 0)
+		result := state.Check(baseTime)
+		assert.True(t, option.IsNone(result), "initial state should fail Check with maxFailures=0")
+
+		// Add one error
+		state = state.AddError(baseTime)
+
+		// Should still fail
+		result = state.Check(baseTime)
+		assert.True(t, option.IsNone(result), "should fail Check after error with maxFailures=0")
+	})
+
+	t.Run("success purges history even with failures in time window", func(t *testing.T) {
+		timeWindow := 1 * time.Minute
+		maxFailures := uint(3)
+		state := MakeClosedStateHistory(timeWindow, maxFailures)
+		baseTime := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+
+		// Add errors within time window
+		state = state.AddError(baseTime)
+		state = state.AddError(baseTime.Add(10 * time.Second))
+
+		// Add success (purges all history)
+		state = state.AddSuccess(baseTime.Add(20 * time.Second))
+
+		// Add more errors
+		state = state.AddError(baseTime.Add(30 * time.Second))
+		state = state.AddError(baseTime.Add(40 * time.Second))
+
+		// Should still pass (only 2 errors after purge)
+		result := state.Check(baseTime.Add(40 * time.Second))
+		assert.True(t, option.IsSome(result), "success should purge all history")
+
+		// Add one more to reach threshold
+		state = state.AddError(baseTime.Add(50 * time.Second))
+
+		// Should fail at threshold
+		result = state.Check(baseTime.Add(50 * time.Second))
+		assert.True(t, option.IsNone(result), "should fail after reaching threshold")
+	})
+
+	t.Run("multiple successes keep history empty", func(t *testing.T) {
+		timeWindow := 1 * time.Minute
+		maxFailures := uint(2)
+		state := MakeClosedStateHistory(timeWindow, maxFailures)
+		baseTime := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+
+		// Add error
+		state = state.AddError(baseTime)
+
+		// Multiple successes
+		state = state.AddSuccess(baseTime.Add(10 * time.Second))
+		state = state.AddSuccess(baseTime.Add(20 * time.Second))
+		state = state.AddSuccess(baseTime.Add(30 * time.Second))
+
+		// Should still pass
+		result := state.Check(baseTime.Add(30 * time.Second))
+		assert.True(t, option.IsSome(result), "multiple AddSuccess should keep history empty")
+
+		// Add errors to reach threshold
+		state = state.AddError(baseTime.Add(40 * time.Second))
+		state = state.AddError(baseTime.Add(50 * time.Second))
+
+		// Should fail
+		result = state.Check(baseTime.Add(50 * time.Second))
+		assert.True(t, option.IsNone(result), "should fail after reaching threshold")
+	})
+
+	t.Run("state is immutable - original unchanged after AddError", func(t *testing.T) {
+		timeWindow := 1 * time.Minute
+		maxFailures := uint(2)
+		originalState := MakeClosedStateHistory(timeWindow, maxFailures)
+		baseTime := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+
+		// Create new state by adding error
+		newState := originalState.AddError(baseTime)
+
+		// Original should still pass check
+		result := originalState.Check(baseTime)
+		assert.True(t, option.IsSome(result), "original state should be unchanged")
+
+		// New state should reach threshold after another error
+		newState = newState.AddError(baseTime.Add(10 * time.Second))
+
+		result = newState.Check(baseTime.Add(10 * time.Second))
+		assert.True(t, option.IsNone(result), "new state should fail after reaching threshold")
+
+		// Original should still pass
+		result = originalState.Check(baseTime)
+		assert.True(t, option.IsSome(result), "original state should still be unchanged")
+	})
+
+	t.Run("state is immutable - original unchanged after Empty", func(t *testing.T) {
+		timeWindow := 1 * time.Minute
+		maxFailures := uint(2)
+		state := MakeClosedStateHistory(timeWindow, maxFailures)
+		baseTime := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+
+		// Add errors to original
+		state = state.AddError(baseTime)
+		state = state.AddError(baseTime.Add(10 * time.Second))
+		stateWithErrors := state
+
+		// Create new state by calling Empty
+		emptyState := stateWithErrors.Empty()
+
+		// Original with errors should fail check
+		result := stateWithErrors.Check(baseTime.Add(10 * time.Second))
+		assert.True(t, option.IsNone(result), "state with errors should fail after reaching threshold")
+
+		// Empty state should pass
+		result = emptyState.Check(baseTime.Add(10 * time.Second))
+		assert.True(t, option.IsSome(result), "empty state should pass Check")
+	})
+
+	t.Run("exact time window boundary behavior", func(t *testing.T) {
+		timeWindow := 1 * time.Minute
+		maxFailures := uint(3)
+		state := MakeClosedStateHistory(timeWindow, maxFailures)
+		baseTime := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+
+		// Add error at baseTime
+		state = state.AddError(baseTime)
+
+		// Add error exactly at time window boundary
+		state = state.AddError(baseTime.Add(1 * time.Minute))
+
+		// The first error should be removed (it's now outside the window)
+		// Only 1 error should remain
+		result := state.Check(baseTime.Add(1 * time.Minute))
+		assert.True(t, option.IsSome(result), "error at exact window boundary should remove older errors")
+	})
+
+	t.Run("multiple errors at same timestamp", func(t *testing.T) {
+		timeWindow := 1 * time.Minute
+		maxFailures := uint(3)
+		state := MakeClosedStateHistory(timeWindow, maxFailures)
+		baseTime := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+
+		// Add multiple errors at same time
+		state = state.AddError(baseTime)
+		state = state.AddError(baseTime)
+		state = state.AddError(baseTime)
+
+		// Should fail check (3 errors at same time)
+		result := state.Check(baseTime)
+		assert.True(t, option.IsNone(result), "multiple errors at same timestamp should count separately")
+	})
+
+	t.Run("errors added out of chronological order are sorted", func(t *testing.T) {
+		timeWindow := 1 * time.Minute
+		maxFailures := uint(4)
+		state := MakeClosedStateHistory(timeWindow, maxFailures)
+		baseTime := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+
+		// Add errors out of order
+		state = state.AddError(baseTime.Add(30 * time.Second))
+		state = state.AddError(baseTime.Add(5 * time.Second))
+		state = state.AddError(baseTime.Add(50 * time.Second))
+
+		// Add error that should trigger pruning
+		state = state.AddError(baseTime.Add(70 * time.Second))
+
+		// The error at 5s should be removed (> 1 minute from 70s: 70-5=65 > 60)
+		// Should have 3 errors remaining (30s, 50s, 70s)
+		result := state.Check(baseTime.Add(70 * time.Second))
+		assert.True(t, option.IsSome(result), "errors should be sorted and pruned correctly")
+	})
+
+	t.Run("large time window with many failures", func(t *testing.T) {
+		timeWindow := 24 * time.Hour
+		maxFailures := uint(100)
+		state := MakeClosedStateHistory(timeWindow, maxFailures)
+		baseTime := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+
+		// Add many failures within the window
+		for i := 0; i < 99; i++ {
+			state = state.AddError(baseTime.Add(time.Duration(i) * time.Minute))
+		}
+
+		// Should still pass (99 < 100)
+		result := state.Check(baseTime.Add(99 * time.Minute))
+		assert.True(t, option.IsSome(result), "should pass with 99 failures when threshold is 100")
+
+		// Add one more to reach threshold
+		state = state.AddError(baseTime.Add(100 * time.Minute))
+
+		// Should fail
+		result = state.Check(baseTime.Add(100 * time.Minute))
+		assert.True(t, option.IsNone(result), "should fail at threshold with large window")
+	})
+
+	t.Run("very short time window", func(t *testing.T) {
+		timeWindow := 100 * time.Millisecond
+		maxFailures := uint(3)
+		state := MakeClosedStateHistory(timeWindow, maxFailures)
+		baseTime := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+
+		// Add errors within short window
+		state = state.AddError(baseTime)
+		state = state.AddError(baseTime.Add(50 * time.Millisecond))
+		state = state.AddError(baseTime.Add(90 * time.Millisecond))
+
+		// Should fail (3 errors within 100ms)
+		result := state.Check(baseTime.Add(90 * time.Millisecond))
+		assert.True(t, option.IsNone(result), "should fail with errors in short time window")
+
+		// Add error after window expires
+		state = state.AddError(baseTime.Add(200 * time.Millisecond))
+
+		// Should pass (old errors removed, only 1 in window)
+		result = state.Check(baseTime.Add(200 * time.Millisecond))
+		assert.True(t, option.IsSome(result), "should pass after short window expires")
+	})
+
+	t.Run("success prevents circuit from opening", func(t *testing.T) {
+		timeWindow := 1 * time.Minute
+		maxFailures := uint(3)
+		state := MakeClosedStateHistory(timeWindow, maxFailures)
+		baseTime := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+
+		// Add errors close to threshold
+		state = state.AddError(baseTime)
+		state = state.AddError(baseTime.Add(10 * time.Second))
+
+		// Add success before reaching threshold
+		state = state.AddSuccess(baseTime.Add(20 * time.Second))
+
+		// Add more errors
+		state = state.AddError(baseTime.Add(30 * time.Second))
+		state = state.AddError(baseTime.Add(40 * time.Second))
+
+		// Should still pass (only 2 errors after success purge)
+		result := state.Check(baseTime.Add(40 * time.Second))
+		assert.True(t, option.IsSome(result), "circuit should stay closed after success purge")
+	})
+
+	t.Run("Empty can be called multiple times", func(t *testing.T) {
+		timeWindow := 1 * time.Minute
+		maxFailures := uint(2)
+		state := MakeClosedStateHistory(timeWindow, maxFailures)
+		baseTime := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+
+		// Add errors
+		state = state.AddError(baseTime)
+		state = state.AddError(baseTime.Add(10 * time.Second))
+		state = state.AddError(baseTime.Add(20 * time.Second))
+
+		// Reset multiple times
+		state = state.Empty()
+		state = state.Empty()
+		state = state.Empty()
+
+		// Should still pass
+		result := state.Check(baseTime.Add(30 * time.Second))
+		assert.True(t, option.IsSome(result), "state should pass Check after multiple Empty calls")
+	})
+
+	t.Run("gradual failure accumulation within window", func(t *testing.T) {
+		timeWindow := 1 * time.Minute
+		maxFailures := uint(5)
+		state := MakeClosedStateHistory(timeWindow, maxFailures)
+		baseTime := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+
+		// Add failures gradually
+		state = state.AddError(baseTime)
+		state = state.AddError(baseTime.Add(15 * time.Second))
+		state = state.AddError(baseTime.Add(30 * time.Second))
+		state = state.AddError(baseTime.Add(45 * time.Second))
+
+		// Should still pass (4 < 5)
+		result := state.Check(baseTime.Add(45 * time.Second))
+		assert.True(t, option.IsSome(result), "should pass before threshold")
+
+		// Add one more within window
+		state = state.AddError(baseTime.Add(55 * time.Second))
+
+		// Should fail (5 >= 5)
+		result = state.Check(baseTime.Add(55 * time.Second))
+		assert.True(t, option.IsNone(result), "should fail at threshold")
+	})
+}

v2/circuitbreaker/error.go

@@ -0,0 +1,335 @@
+// Package circuitbreaker provides error types and utilities for circuit breaker implementations.
+package circuitbreaker
+
+import (
+	"crypto/x509"
+	"errors"
+	"fmt"
+	"net"
+	"net/http"
+	"net/url"
+	"syscall"
+	"time"
+
+	E "github.com/IBM/fp-go/v2/errors"
+	FH "github.com/IBM/fp-go/v2/http"
+	"github.com/IBM/fp-go/v2/option"
+)
+
+// CircuitBreakerError represents an error that occurs when a circuit breaker is in the open state.
+//
+// When a circuit breaker opens due to too many failures, it prevents further operations
+// from executing until a reset time is reached. This error type communicates that state
+// and provides information about when the circuit breaker will attempt to close again.
+//
+// Fields:
+//   - Name: The name identifying this circuit breaker instance
+//   - ResetAt: The time at which the circuit breaker will transition from open to half-open state
+//
+// Thread Safety: This type is immutable and safe for concurrent use.
+type CircuitBreakerError struct {
+	Name    string
+	ResetAt time.Time
+}
+
+// Error implements the error interface for CircuitBreakerError.
+//
+// Returns a formatted error message indicating that the circuit breaker is open
+// and when it will attempt to close.
+//
+// Returns:
+//   - A string describing the circuit breaker state and reset time
+//
+// Thread Safety: This method is safe for concurrent use as it only reads immutable fields.
+//
+// Example:
+//
+//	err := &CircuitBreakerError{Name: "API", ResetAt: time.Now().Add(30 * time.Second)}
+//	fmt.Println(err.Error())
+//	// Output: circuit breaker is open [API], will close at 2026-01-09 12:20:47.123 +0100 CET
+func (e *CircuitBreakerError) Error() string {
+	return fmt.Sprintf("circuit breaker is open [%s], will close at %s", e.Name, e.ResetAt)
+}
+
+// MakeCircuitBreakerErrorWithName creates a circuit breaker error constructor with a custom name.
+//
+// This function returns a constructor that creates CircuitBreakerError instances with a specific
+// circuit breaker name. This is useful when you have multiple circuit breakers in your system
+// and want to identify which one is open in error messages.
+//
+// Parameters:
+//   - name: The name to identify this circuit breaker in error messages
+//
+// Returns:
+//   - A function that takes a reset time and returns a CircuitBreakerError with the specified name
+//
+// Thread Safety: The returned function is safe for concurrent use as it creates new error
+// instances on each call.
+//
+// Example:
+//
+//	makeDBError := MakeCircuitBreakerErrorWithName("Database Circuit Breaker")
+//	err := makeDBError(time.Now().Add(30 * time.Second))
+//	fmt.Println(err.Error())
+//	// Output: circuit breaker is open [Database Circuit Breaker], will close at 2026-01-09 12:20:47.123 +0100 CET
+func MakeCircuitBreakerErrorWithName(name string) func(time.Time) error {
+	return func(resetTime time.Time) error {
+		return &CircuitBreakerError{Name: name, ResetAt: resetTime}
+	}
+}
+
+// MakeCircuitBreakerError creates a new CircuitBreakerError with the specified reset time.
+//
+// This constructor function creates a circuit breaker error that indicates when the
+// circuit breaker will transition from the open state to the half-open state, allowing
+// test requests to determine if the underlying service has recovered.
+//
+// Parameters:
+//   - resetTime: The time at which the circuit breaker will attempt to close
+//
+// Returns:
+//   - An error representing the circuit breaker open state
+//
+// Thread Safety: This function is safe for concurrent use as it creates new error
+// instances on each call.
+//
+// Example:
+//
+//	resetTime := time.Now().Add(30 * time.Second)
+//	err := MakeCircuitBreakerError(resetTime)
+//	if cbErr, ok := err.(*CircuitBreakerError); ok {
+//	    fmt.Printf("Circuit breaker will reset at: %s\n", cbErr.ResetAt)
+//	}
+var MakeCircuitBreakerError = MakeCircuitBreakerErrorWithName("Generic Circuit Breaker")
+
+// AnyError converts an error to an Option, wrapping non-nil errors in Some and nil errors in None.
+//
+// This variable provides a functional way to handle errors by converting them to Option types.
+// It's particularly useful in functional programming contexts where you want to treat errors
+// as optional values rather than using traditional error handling patterns.
+//
+// Behavior:
+//   - If the error is non-nil, returns Some(error)
+//   - If the error is nil, returns None
+//
+// Thread Safety: This function is pure and safe for concurrent use.
+//
+// Example:
+//
+//	err := errors.New("something went wrong")
+//	optErr := AnyError(err)  // Some(error)
+//
+//	var noErr error = nil
+//	optNoErr := AnyError(noErr)  // None
+//
+//	// Using in functional pipelines
+//	result := F.Pipe2(
+//	    someOperation(),
+//	    AnyError,
+//	    O.Map(func(e error) string { return e.Error() }),
+//	)
+var AnyError = option.FromPredicate(E.IsNonNil)
+
+// shouldOpenCircuit determines if an error should cause a circuit breaker to open.
+//
+// This function checks if an error represents an infrastructure or server problem
+// that indicates the service is unhealthy and should trigger circuit breaker protection.
+// It examines both the error type and, for HTTP errors, the status code.
+//
+// Errors that should open the circuit include:
+//   - HTTP 5xx server errors (500-599) indicating server-side problems
+//   - Network errors (connection refused, connection reset, timeouts)
+//   - DNS resolution errors
+//   - TLS/certificate errors
+//   - Other infrastructure-related errors
+//
+// Errors that should NOT open the circuit include:
+//   - HTTP 4xx client errors (bad request, unauthorized, not found, etc.)
+//   - Application-level validation errors
+//   - Business logic errors
+//
+// The function unwraps error chains to find the root cause, making it compatible
+// with wrapped errors created by fmt.Errorf with %w or errors.Join.
+//
+// Parameters:
+//   - err: The error to evaluate (may be nil)
+//
+// Returns:
+//   - true if the error should cause the circuit to open, false otherwise
+//
+// Thread Safety: This function is pure and safe for concurrent use. It does not
+// modify any state.
+//
+// Example:
+//
+//	// HTTP 500 error - should open circuit
+//	httpErr := &FH.HttpError{...} // status 500
+//	if shouldOpenCircuit(httpErr) {
+//	    // Open circuit breaker
+//	}
+//
+//	// HTTP 404 error - should NOT open circuit (client error)
+//	notFoundErr := &FH.HttpError{...} // status 404
+//	if !shouldOpenCircuit(notFoundErr) {
+//	    // Don't open circuit, this is a client error
+//	}
+//
+//	// Network timeout - should open circuit
+//	timeoutErr := &net.OpError{Op: "dial", Err: syscall.ETIMEDOUT}
+//	if shouldOpenCircuit(timeoutErr) {
+//	    // Open circuit breaker
+//	}
+func shouldOpenCircuit(err error) bool {
+	if err == nil {
+		return false
+	}
+
+	// Check for HTTP errors with server status codes (5xx)
+	var httpErr *FH.HttpError
+	if errors.As(err, &httpErr) {
+		statusCode := httpErr.StatusCode()
+		// Only 5xx errors should open the circuit
+		// 4xx errors are client errors and shouldn't affect circuit state
+		return statusCode >= http.StatusInternalServerError && statusCode < 600
+	}
+
+	// Check for network operation errors
+	var opErr *net.OpError
+	if errors.As(err, &opErr) {
+		// Network timeouts should open the circuit
+		if opErr.Timeout() {
+			return true
+		}
+		// Check the underlying error
+		if opErr.Err != nil {
+			return isInfrastructureError(opErr.Err)
+		}
+		return true
+	}
+
+	// Check for DNS errors
+	var dnsErr *net.DNSError
+	if errors.As(err, &dnsErr) {
+		return true
+	}
+
+	// Check for URL errors (often wrap network errors)
+	var urlErr *url.Error
+	if errors.As(err, &urlErr) {
+		if urlErr.Timeout() {
+			return true
+		}
+		// Recursively check the wrapped error
+		return shouldOpenCircuit(urlErr.Err)
+	}
+
+	// Check for specific syscall errors that indicate infrastructure problems
+	return isInfrastructureError(err) || isTLSError(err)
+}
+
+// isInfrastructureError checks if an error is a low-level infrastructure error
+// that should cause the circuit to open.
+//
+// This function examines syscall errors to identify network and system-level failures
+// that indicate the service is unavailable or unreachable.
+//
+// Infrastructure errors include:
+//   - ECONNREFUSED: Connection refused (service not listening)
+//   - ECONNRESET: Connection reset by peer (service crashed or network issue)
+//   - ECONNABORTED: Connection aborted (network issue)
+//   - ENETUNREACH: Network unreachable (routing problem)
+//   - EHOSTUNREACH: Host unreachable (host down or network issue)
+//   - EPIPE: Broken pipe (connection closed unexpectedly)
+//   - ETIMEDOUT: Operation timed out (service not responding)
+//
+// Parameters:
+//   - err: The error to check
+//
+// Returns:
+//   - true if the error is an infrastructure error, false otherwise
+//
+// Thread Safety: This function is pure and safe for concurrent use.
+func isInfrastructureError(err error) bool {
+
+	var syscallErr *syscall.Errno
+
+	if errors.As(err, &syscallErr) {
+		switch *syscallErr {
+		case syscall.ECONNREFUSED,
+			syscall.ECONNRESET,
+			syscall.ECONNABORTED,
+			syscall.ENETUNREACH,
+			syscall.EHOSTUNREACH,
+			syscall.EPIPE,
+			syscall.ETIMEDOUT:
+			return true
+		}
+
+	}
+	return false
+}
+
+// isTLSError checks if an error is a TLS/certificate error that should cause the circuit to open.
+//
+// TLS errors typically indicate infrastructure or configuration problems that prevent
+// secure communication with the service. These errors suggest the service is not properly
+// configured or accessible.
+//
+// TLS errors include:
+//   - Certificate verification failures (invalid, expired, or malformed certificates)
+//   - Unknown certificate authority errors (untrusted CA)
+//
+// Parameters:
+//   - err: The error to check
+//
+// Returns:
+//   - true if the error is a TLS/certificate error, false otherwise
+//
+// Thread Safety: This function is pure and safe for concurrent use.
+func isTLSError(err error) bool {
+	// Certificate verification failed
+	var certErr *x509.CertificateInvalidError
+	if errors.As(err, &certErr) {
+		return true
+	}
+
+	// Unknown authority
+	var unknownAuthErr *x509.UnknownAuthorityError
+	if errors.As(err, &unknownAuthErr) {
+		return true
+	}
+
+	return false
+}
+
+// InfrastructureError is a predicate that converts errors to Options based on whether
+// they should trigger circuit breaker opening.
+//
+// This variable provides a functional way to filter errors that represent infrastructure
+// failures (network issues, server errors, timeouts, etc.) from application-level errors
+// (validation errors, business logic errors, client errors).
+//
+// Behavior:
+//   - Returns Some(error) if the error should open the circuit (infrastructure failure)
+//   - Returns None if the error should not open the circuit (application error)
+//
+// Thread Safety: This function is pure and safe for concurrent use.
+//
+// Use this in circuit breaker configurations to determine which errors should count
+// toward the failure threshold.
+//
+// Example:
+//
+//	// In a circuit breaker configuration
+//	breaker := MakeCircuitBreaker(
+//	    ...,
+//	    checkError: InfrastructureError,  // Only infrastructure errors open the circuit
+//	    ...,
+//	)
+//
+//	// HTTP 500 error - returns Some(error)
+//	result := InfrastructureError(&FH.HttpError{...}) // Some(error)
+//
+//	// HTTP 404 error - returns None
+//	result := InfrastructureError(&FH.HttpError{...}) // None
+var InfrastructureError = option.FromPredicate(shouldOpenCircuit)

v2/circuitbreaker/error_test.go

@@ -0,0 +1,503 @@
+package circuitbreaker
+
+import (
+	"crypto/x509"
+	"errors"
+	"fmt"
+	"net"
+	"net/http"
+	"net/url"
+	"testing"
+	"time"
+
+	FH "github.com/IBM/fp-go/v2/http"
+	"github.com/IBM/fp-go/v2/option"
+	"github.com/stretchr/testify/assert"
+)
+
+// TestCircuitBreakerError tests the CircuitBreakerError type
+func TestCircuitBreakerError(t *testing.T) {
+	t.Run("Error returns formatted message with reset time", func(t *testing.T) {
+		resetTime := time.Date(2026, 1, 9, 12, 30, 0, 0, time.UTC)
+		err := &CircuitBreakerError{ResetAt: resetTime}
+
+		result := err.Error()
+
+		assert.Contains(t, result, "circuit breaker is open")
+		assert.Contains(t, result, "will close at")
+		assert.Contains(t, result, resetTime.String())
+	})
+
+	t.Run("Error message includes full timestamp", func(t *testing.T) {
+		resetTime := time.Now().Add(30 * time.Second)
+		err := &CircuitBreakerError{ResetAt: resetTime}
+
+		result := err.Error()
+
+		assert.NotEmpty(t, result)
+		assert.Contains(t, result, "circuit breaker is open")
+	})
+}
+
+// TestMakeCircuitBreakerError tests the constructor function
+func TestMakeCircuitBreakerError(t *testing.T) {
+	t.Run("creates CircuitBreakerError with correct reset time", func(t *testing.T) {
+		resetTime := time.Date(2026, 1, 9, 13, 0, 0, 0, time.UTC)
+
+		err := MakeCircuitBreakerError(resetTime)
+
+		assert.NotNil(t, err)
+		cbErr, ok := err.(*CircuitBreakerError)
+		assert.True(t, ok, "should return *CircuitBreakerError type")
+		assert.Equal(t, resetTime, cbErr.ResetAt)
+	})
+
+	t.Run("returns error interface", func(t *testing.T) {
+		resetTime := time.Now().Add(1 * time.Minute)
+
+		err := MakeCircuitBreakerError(resetTime)
+
+		// Should be assignable to error interface
+		var _ error = err
+		assert.NotNil(t, err)
+	})
+
+	t.Run("created error can be type asserted", func(t *testing.T) {
+		resetTime := time.Now().Add(45 * time.Second)
+
+		err := MakeCircuitBreakerError(resetTime)
+
+		cbErr, ok := err.(*CircuitBreakerError)
+		assert.True(t, ok)
+		assert.Equal(t, resetTime, cbErr.ResetAt)
+	})
+}
+
+// TestAnyError tests the AnyError function
+func TestAnyError(t *testing.T) {
+	t.Run("returns Some for non-nil error", func(t *testing.T) {
+		err := errors.New("test error")
+
+		result := AnyError(err)
+
+		assert.True(t, option.IsSome(result), "should return Some for non-nil error")
+		value := option.GetOrElse(func() error { return nil })(result)
+		assert.Equal(t, err, value)
+	})
+
+	t.Run("returns None for nil error", func(t *testing.T) {
+		var err error = nil
+
+		result := AnyError(err)
+
+		assert.True(t, option.IsNone(result), "should return None for nil error")
+	})
+
+	t.Run("works with different error types", func(t *testing.T) {
+		err1 := fmt.Errorf("wrapped: %w", errors.New("inner"))
+		err2 := &CircuitBreakerError{ResetAt: time.Now()}
+
+		result1 := AnyError(err1)
+		result2 := AnyError(err2)
+
+		assert.True(t, option.IsSome(result1))
+		assert.True(t, option.IsSome(result2))
+	})
+}
+
+// TestShouldOpenCircuit tests the shouldOpenCircuit function
+func TestShouldOpenCircuit(t *testing.T) {
+	t.Run("returns false for nil error", func(t *testing.T) {
+		result := shouldOpenCircuit(nil)
+		assert.False(t, result)
+	})
+
+	t.Run("HTTP 5xx errors should open circuit", func(t *testing.T) {
+		testCases := []struct {
+			name       string
+			statusCode int
+			expected   bool
+		}{
+			{"500 Internal Server Error", 500, true},
+			{"501 Not Implemented", 501, true},
+			{"502 Bad Gateway", 502, true},
+			{"503 Service Unavailable", 503, true},
+			{"504 Gateway Timeout", 504, true},
+			{"599 Custom Server Error", 599, true},
+		}
+
+		for _, tc := range testCases {
+			t.Run(tc.name, func(t *testing.T) {
+				testURL, _ := url.Parse("http://example.com")
+				resp := &http.Response{
+					StatusCode: tc.statusCode,
+					Request:    &http.Request{URL: testURL},
+					Body:       http.NoBody,
+				}
+				httpErr := FH.StatusCodeError(resp)
+
+				result := shouldOpenCircuit(httpErr)
+
+				assert.Equal(t, tc.expected, result)
+			})
+		}
+	})
+
+	t.Run("HTTP 4xx errors should NOT open circuit", func(t *testing.T) {
+		testCases := []struct {
+			name       string
+			statusCode int
+			expected   bool
+		}{
+			{"400 Bad Request", 400, false},
+			{"401 Unauthorized", 401, false},
+			{"403 Forbidden", 403, false},
+			{"404 Not Found", 404, false},
+			{"422 Unprocessable Entity", 422, false},
+			{"429 Too Many Requests", 429, false},
+			{"499 Custom Client Error", 499, false},
+		}
+
+		for _, tc := range testCases {
+			t.Run(tc.name, func(t *testing.T) {
+				testURL, _ := url.Parse("http://example.com")
+				resp := &http.Response{
+					StatusCode: tc.statusCode,
+					Request:    &http.Request{URL: testURL},
+					Body:       http.NoBody,
+				}
+				httpErr := FH.StatusCodeError(resp)
+
+				result := shouldOpenCircuit(httpErr)
+
+				assert.Equal(t, tc.expected, result)
+			})
+		}
+	})
+
+	t.Run("HTTP 2xx and 3xx should NOT open circuit", func(t *testing.T) {
+		testCases := []int{200, 201, 204, 301, 302, 304}
+
+		for _, statusCode := range testCases {
+			t.Run(fmt.Sprintf("Status %d", statusCode), func(t *testing.T) {
+				testURL, _ := url.Parse("http://example.com")
+				resp := &http.Response{
+					StatusCode: statusCode,
+					Request:    &http.Request{URL: testURL},
+					Body:       http.NoBody,
+				}
+				httpErr := FH.StatusCodeError(resp)
+
+				result := shouldOpenCircuit(httpErr)
+
+				assert.False(t, result)
+			})
+		}
+	})
+
+	t.Run("network timeout errors should open circuit", func(t *testing.T) {
+		opErr := &net.OpError{
+			Op:  "dial",
+			Err: &timeoutError{},
+		}
+
+		result := shouldOpenCircuit(opErr)
+
+		assert.True(t, result)
+	})
+
+	t.Run("DNS errors should open circuit", func(t *testing.T) {
+		dnsErr := &net.DNSError{
+			Err:  "no such host",
+			Name: "example.com",
+		}
+
+		result := shouldOpenCircuit(dnsErr)
+
+		assert.True(t, result)
+	})
+
+	t.Run("URL timeout errors should open circuit", func(t *testing.T) {
+		urlErr := &url.Error{
+			Op:  "Get",
+			URL: "http://example.com",
+			Err: &timeoutError{},
+		}
+
+		result := shouldOpenCircuit(urlErr)
+
+		assert.True(t, result)
+	})
+
+	t.Run("URL errors with nested network timeout should open circuit", func(t *testing.T) {
+		urlErr := &url.Error{
+			Op:  "Get",
+			URL: "http://example.com",
+			Err: &net.OpError{
+				Op:  "dial",
+				Err: &timeoutError{},
+			},
+		}
+
+		result := shouldOpenCircuit(urlErr)
+
+		assert.True(t, result)
+	})
+
+	t.Run("OpError with nil Err should open circuit", func(t *testing.T) {
+		opErr := &net.OpError{
+			Op:  "dial",
+			Err: nil,
+		}
+
+		result := shouldOpenCircuit(opErr)
+
+		assert.True(t, result)
+	})
+
+	t.Run("wrapped HTTP 5xx error should open circuit", func(t *testing.T) {
+		testURL, _ := url.Parse("http://example.com")
+		resp := &http.Response{
+			StatusCode: 503,
+			Request:    &http.Request{URL: testURL},
+			Body:       http.NoBody,
+		}
+		httpErr := FH.StatusCodeError(resp)
+		wrappedErr := fmt.Errorf("service error: %w", httpErr)
+
+		result := shouldOpenCircuit(wrappedErr)
+
+		assert.True(t, result)
+	})
+
+	t.Run("wrapped HTTP 4xx error should NOT open circuit", func(t *testing.T) {
+		testURL, _ := url.Parse("http://example.com")
+		resp := &http.Response{
+			StatusCode: 404,
+			Request:    &http.Request{URL: testURL},
+			Body:       http.NoBody,
+		}
+		httpErr := FH.StatusCodeError(resp)
+		wrappedErr := fmt.Errorf("not found: %w", httpErr)
+
+		result := shouldOpenCircuit(wrappedErr)
+
+		assert.False(t, result)
+	})
+
+	t.Run("generic application error should NOT open circuit", func(t *testing.T) {
+		err := errors.New("validation failed")
+
+		result := shouldOpenCircuit(err)
+
+		assert.False(t, result)
+	})
+}
+
+// TestIsInfrastructureError tests infrastructure error detection through shouldOpenCircuit
+func TestIsInfrastructureError(t *testing.T) {
+	t.Run("network timeout is infrastructure error", func(t *testing.T) {
+		opErr := &net.OpError{Op: "dial", Err: &timeoutError{}}
+		result := shouldOpenCircuit(opErr)
+		assert.True(t, result)
+	})
+
+	t.Run("OpError with nil Err is infrastructure error", func(t *testing.T) {
+		opErr := &net.OpError{Op: "dial", Err: nil}
+		result := shouldOpenCircuit(opErr)
+		assert.True(t, result)
+	})
+
+	t.Run("generic error returns false", func(t *testing.T) {
+		err := errors.New("generic error")
+		result := shouldOpenCircuit(err)
+		assert.False(t, result)
+	})
+
+	t.Run("wrapped network timeout is detected", func(t *testing.T) {
+		opErr := &net.OpError{Op: "dial", Err: &timeoutError{}}
+		wrappedErr := fmt.Errorf("connection failed: %w", opErr)
+		result := shouldOpenCircuit(wrappedErr)
+		assert.True(t, result)
+	})
+}
+
+// TestIsTLSError tests the isTLSError function
+func TestIsTLSError(t *testing.T) {
+	t.Run("certificate invalid error is TLS error", func(t *testing.T) {
+		certErr := &x509.CertificateInvalidError{
+			Reason: x509.Expired,
+		}
+
+		result := isTLSError(certErr)
+
+		assert.True(t, result)
+	})
+
+	t.Run("unknown authority error is TLS error", func(t *testing.T) {
+		authErr := &x509.UnknownAuthorityError{}
+
+		result := isTLSError(authErr)
+
+		assert.True(t, result)
+	})
+
+	t.Run("generic error is not TLS error", func(t *testing.T) {
+		err := errors.New("generic error")
+
+		result := isTLSError(err)
+
+		assert.False(t, result)
+	})
+
+	t.Run("wrapped certificate error is detected", func(t *testing.T) {
+		certErr := &x509.CertificateInvalidError{
+			Reason: x509.Expired,
+		}
+		wrappedErr := fmt.Errorf("TLS handshake failed: %w", certErr)
+
+		result := isTLSError(wrappedErr)
+
+		assert.True(t, result)
+	})
+
+	t.Run("wrapped unknown authority error is detected", func(t *testing.T) {
+		authErr := &x509.UnknownAuthorityError{}
+		wrappedErr := fmt.Errorf("certificate verification failed: %w", authErr)
+
+		result := isTLSError(wrappedErr)
+
+		assert.True(t, result)
+	})
+}
+
+// TestInfrastructureError tests the InfrastructureError variable
+func TestInfrastructureError(t *testing.T) {
+	t.Run("returns Some for infrastructure errors", func(t *testing.T) {
+		testURL, _ := url.Parse("http://example.com")
+		resp := &http.Response{
+			StatusCode: 503,
+			Request:    &http.Request{URL: testURL},
+			Body:       http.NoBody,
+		}
+		httpErr := FH.StatusCodeError(resp)
+
+		result := InfrastructureError(httpErr)
+
+		assert.True(t, option.IsSome(result))
+	})
+
+	t.Run("returns None for non-infrastructure errors", func(t *testing.T) {
+		testURL, _ := url.Parse("http://example.com")
+		resp := &http.Response{
+			StatusCode: 404,
+			Request:    &http.Request{URL: testURL},
+			Body:       http.NoBody,
+		}
+		httpErr := FH.StatusCodeError(resp)
+
+		result := InfrastructureError(httpErr)
+
+		assert.True(t, option.IsNone(result))
+	})
+
+	t.Run("returns None for nil error", func(t *testing.T) {
+		result := InfrastructureError(nil)
+
+		assert.True(t, option.IsNone(result))
+	})
+
+	t.Run("returns Some for network timeout", func(t *testing.T) {
+		opErr := &net.OpError{
+			Op:  "dial",
+			Err: &timeoutError{},
+		}
+
+		result := InfrastructureError(opErr)
+
+		assert.True(t, option.IsSome(result))
+	})
+}
+
+// TestComplexErrorScenarios tests complex real-world error scenarios
+func TestComplexErrorScenarios(t *testing.T) {
+	t.Run("deeply nested URL error with HTTP 5xx", func(t *testing.T) {
+		testURL, _ := url.Parse("http://api.example.com")
+		resp := &http.Response{
+			StatusCode: 502,
+			Request:    &http.Request{URL: testURL},
+			Body:       http.NoBody,
+		}
+		httpErr := FH.StatusCodeError(resp)
+		urlErr := &url.Error{
+			Op:  "Get",
+			URL: "http://api.example.com",
+			Err: httpErr,
+		}
+		wrappedErr := fmt.Errorf("API call failed: %w", urlErr)
+
+		result := shouldOpenCircuit(wrappedErr)
+
+		assert.True(t, result, "should detect HTTP 5xx through multiple layers")
+	})
+
+	t.Run("URL error with timeout nested in OpError", func(t *testing.T) {
+		opErr := &net.OpError{
+			Op:  "dial",
+			Err: &timeoutError{},
+		}
+		urlErr := &url.Error{
+			Op:  "Post",
+			URL: "http://api.example.com",
+			Err: opErr,
+		}
+
+		result := shouldOpenCircuit(urlErr)
+
+		assert.True(t, result, "should detect timeout through URL error")
+	})
+
+	t.Run("multiple wrapped errors with infrastructure error at core", func(t *testing.T) {
+		coreErr := &net.OpError{Op: "dial", Err: &timeoutError{}}
+		layer1 := fmt.Errorf("connection attempt failed: %w", coreErr)
+		layer2 := fmt.Errorf("retry exhausted: %w", layer1)
+		layer3 := fmt.Errorf("service unavailable: %w", layer2)
+
+		result := shouldOpenCircuit(layer3)
+
+		assert.True(t, result, "should unwrap to find infrastructure error")
+	})
+
+	t.Run("OpError with nil Err should open circuit", func(t *testing.T) {
+		opErr := &net.OpError{
+			Op:  "dial",
+			Err: nil,
+		}
+
+		result := shouldOpenCircuit(opErr)
+
+		assert.True(t, result, "OpError with nil Err should be treated as infrastructure error")
+	})
+
+	t.Run("mixed error types - HTTP 4xx with network error", func(t *testing.T) {
+		// This tests that we correctly identify the error type
+		testURL, _ := url.Parse("http://example.com")
+		resp := &http.Response{
+			StatusCode: 400,
+			Request:    &http.Request{URL: testURL},
+			Body:       http.NoBody,
+		}
+		httpErr := FH.StatusCodeError(resp)
+
+		result := shouldOpenCircuit(httpErr)
+
+		assert.False(t, result, "HTTP 4xx should not open circuit even if wrapped")
+	})
+}
+
+// Helper type for testing timeout errors
+type timeoutError struct{}
+
+func (e *timeoutError) Error() string   { return "timeout" }
+func (e *timeoutError) Timeout() bool   { return true }
+func (e *timeoutError) Temporary() bool { return true }

v2/circuitbreaker/metrics.go

@@ -0,0 +1,208 @@
+// Package circuitbreaker provides metrics collection for circuit breaker state transitions and events.
+package circuitbreaker
+
+import (
+	"log"
+	"time"
+
+	"github.com/IBM/fp-go/v2/function"
+)
+
+type (
+	// Metrics defines the interface for collecting circuit breaker metrics and events.
+	// Implementations can use this interface to track circuit breaker behavior for
+	// monitoring, alerting, and debugging purposes.
+	//
+	// All methods accept a time.Time parameter representing when the event occurred,
+	// and return an IO[Void] operation that performs the metric recording when executed.
+	//
+	// Thread Safety: Implementations must be thread-safe as circuit breakers may be
+	// accessed concurrently from multiple goroutines.
+	//
+	// Example Usage:
+	//
+	//	logger := log.New(os.Stdout, "[CircuitBreaker] ", log.LstdFlags)
+	//	metrics := MakeMetricsFromLogger("API-Service", logger)
+	//
+	//	// In circuit breaker implementation
+	//	io.Run(metrics.Accept(time.Now()))  // Record accepted request
+	//	io.Run(metrics.Reject(time.Now()))  // Record rejected request
+	//	io.Run(metrics.Open(time.Now()))    // Record circuit opening
+	//	io.Run(metrics.Close(time.Now()))   // Record circuit closing
+	//	io.Run(metrics.Canary(time.Now()))  // Record canary request
+	Metrics interface {
+		// Accept records that a request was accepted and allowed through the circuit breaker.
+		// This is called when the circuit is closed or in half-open state (canary request).
+		//
+		// Parameters:
+		//   - time.Time: The timestamp when the request was accepted
+		//
+		// Returns:
+		//   - IO[Void]: An IO operation that records the acceptance when executed
+		//
+		// Thread Safety: Must be safe to call concurrently.
+		Accept(time.Time) IO[Void]
+
+		// Reject records that a request was rejected because the circuit breaker is open.
+		// This is called when a request is blocked due to the circuit being in open state
+		// and the reset time has not been reached.
+		//
+		// Parameters:
+		//   - time.Time: The timestamp when the request was rejected
+		//
+		// Returns:
+		//   - IO[Void]: An IO operation that records the rejection when executed
+		//
+		// Thread Safety: Must be safe to call concurrently.
+		Reject(time.Time) IO[Void]
+
+		// Open records that the circuit breaker transitioned to the open state.
+		// This is called when the failure threshold is exceeded and the circuit opens
+		// to prevent further requests from reaching the failing service.
+		//
+		// Parameters:
+		//   - time.Time: The timestamp when the circuit opened
+		//
+		// Returns:
+		//   - IO[Void]: An IO operation that records the state transition when executed
+		//
+		// Thread Safety: Must be safe to call concurrently.
+		Open(time.Time) IO[Void]
+
+		// Close records that the circuit breaker transitioned to the closed state.
+		// This is called when:
+		//   - A canary request succeeds in half-open state
+		//   - The circuit is manually reset
+		//   - The circuit breaker is initialized
+		//
+		// Parameters:
+		//   - time.Time: The timestamp when the circuit closed
+		//
+		// Returns:
+		//   - IO[Void]: An IO operation that records the state transition when executed
+		//
+		// Thread Safety: Must be safe to call concurrently.
+		Close(time.Time) IO[Void]
+
+		// Canary records that a canary (test) request is being attempted.
+		// This is called when the circuit is in half-open state and a single test request
+		// is allowed through to check if the service has recovered.
+		//
+		// Parameters:
+		//   - time.Time: The timestamp when the canary request was initiated
+		//
+		// Returns:
+		//   - IO[Void]: An IO operation that records the canary attempt when executed
+		//
+		// Thread Safety: Must be safe to call concurrently.
+		Canary(time.Time) IO[Void]
+	}
+
+	// loggingMetrics is a simple implementation of the Metrics interface that logs
+	// circuit breaker events using Go's standard log.Logger.
+	//
+	// This implementation is thread-safe as log.Logger is safe for concurrent use.
+	//
+	// Fields:
+	//   - name: A human-readable name identifying the circuit breaker instance
+	//   - logger: The log.Logger instance used for writing log messages
+	loggingMetrics struct {
+		name   string
+		logger *log.Logger
+	}
+)
+
+// doLog is a helper method that creates an IO operation for logging a circuit breaker event.
+// It formats the log message with the event prefix, circuit breaker name, and timestamp.
+//
+// Parameters:
+//   - prefix: The event type (e.g., "Accept", "Reject", "Open", "Close", "Canary")
+//   - ct: The timestamp when the event occurred
+//
+// Returns:
+//   - IO[Void]: An IO operation that logs the event when executed
+//
+// Thread Safety: Safe for concurrent use as log.Logger is thread-safe.
+//
+// Log Format: "<prefix>: <name>, <timestamp>"
+// Example: "Open: API-Service, 2026-01-09 15:30:45.123 +0100 CET"
+func (m *loggingMetrics) doLog(prefix string, ct time.Time) IO[Void] {
+	return func() Void {
+		m.logger.Printf("%s: %s, %s\n", prefix, m.name, ct)
+		return function.VOID
+	}
+}
+
+// Accept implements the Metrics interface for loggingMetrics.
+// Logs when a request is accepted through the circuit breaker.
+//
+// Thread Safety: Safe for concurrent use.
+func (m *loggingMetrics) Accept(ct time.Time) IO[Void] {
+	return m.doLog("Accept", ct)
+}
+
+// Open implements the Metrics interface for loggingMetrics.
+// Logs when the circuit breaker transitions to open state.
+//
+// Thread Safety: Safe for concurrent use.
+func (m *loggingMetrics) Open(ct time.Time) IO[Void] {
+	return m.doLog("Open", ct)
+}
+
+// Close implements the Metrics interface for loggingMetrics.
+// Logs when the circuit breaker transitions to closed state.
+//
+// Thread Safety: Safe for concurrent use.
+func (m *loggingMetrics) Close(ct time.Time) IO[Void] {
+	return m.doLog("Close", ct)
+}
+
+// Reject implements the Metrics interface for loggingMetrics.
+// Logs when a request is rejected because the circuit breaker is open.
+//
+// Thread Safety: Safe for concurrent use.
+func (m *loggingMetrics) Reject(ct time.Time) IO[Void] {
+	return m.doLog("Reject", ct)
+}
+
+// Canary implements the Metrics interface for loggingMetrics.
+// Logs when a canary (test) request is attempted in half-open state.
+//
+// Thread Safety: Safe for concurrent use.
+func (m *loggingMetrics) Canary(ct time.Time) IO[Void] {
+	return m.doLog("Canary", ct)
+}
+
+// MakeMetricsFromLogger creates a Metrics implementation that logs circuit breaker events
+// using the provided log.Logger.
+//
+// This is a simple metrics implementation suitable for development, debugging, and
+// basic production monitoring. For more sophisticated metrics collection (e.g., Prometheus,
+// StatsD), implement the Metrics interface with a custom type.
+//
+// Parameters:
+//   - name: A human-readable name identifying the circuit breaker instance.
+//     This name appears in all log messages to distinguish between multiple circuit breakers.
+//   - logger: The log.Logger instance to use for writing log messages.
+//     If nil, this will panic when metrics are recorded.
+//
+// Returns:
+//   - Metrics: A thread-safe Metrics implementation that logs events
+//
+// Thread Safety: The returned Metrics implementation is safe for concurrent use
+// as log.Logger is thread-safe.
+//
+// Example:
+//
+//	logger := log.New(os.Stdout, "[CB] ", log.LstdFlags)
+//	metrics := MakeMetricsFromLogger("UserService", logger)
+//
+//	// Use with circuit breaker
+//	io.Run(metrics.Open(time.Now()))
+//	// Output: [CB] 2026/01/09 15:30:45 Open: UserService, 2026-01-09 15:30:45.123 +0100 CET
+//
+//	io.Run(metrics.Reject(time.Now()))
+//	// Output: [CB] 2026/01/09 15:30:46 Reject: UserService, 2026-01-09 15:30:46.456 +0100 CET
+func MakeMetricsFromLogger(name string, logger *log.Logger) Metrics {
+	return &loggingMetrics{name: name, logger: logger}
+}

v2/circuitbreaker/metrics_test.go

@@ -0,0 +1,506 @@
+package circuitbreaker
+
+import (
+	"bytes"
+	"log"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/IBM/fp-go/v2/io"
+	"github.com/stretchr/testify/assert"
+)
+
+// TestMakeMetricsFromLogger tests the MakeMetricsFromLogger constructor
+func TestMakeMetricsFromLogger(t *testing.T) {
+	t.Run("creates valid Metrics implementation", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+
+		metrics := MakeMetricsFromLogger("TestCircuit", logger)
+
+		assert.NotNil(t, metrics, "MakeMetricsFromLogger should return non-nil Metrics")
+	})
+
+	t.Run("returns loggingMetrics type", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+
+		metrics := MakeMetricsFromLogger("TestCircuit", logger)
+
+		_, ok := metrics.(*loggingMetrics)
+		assert.True(t, ok, "should return *loggingMetrics type")
+	})
+
+	t.Run("stores name correctly", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+		name := "MyCircuitBreaker"
+
+		metrics := MakeMetricsFromLogger(name, logger).(*loggingMetrics)
+
+		assert.Equal(t, name, metrics.name, "name should be stored correctly")
+	})
+
+	t.Run("stores logger correctly", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+
+		metrics := MakeMetricsFromLogger("TestCircuit", logger).(*loggingMetrics)
+
+		assert.Equal(t, logger, metrics.logger, "logger should be stored correctly")
+	})
+}
+
+// TestLoggingMetricsAccept tests the Accept method
+func TestLoggingMetricsAccept(t *testing.T) {
+	t.Run("logs accept event with correct format", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+		metrics := MakeMetricsFromLogger("TestCircuit", logger)
+		timestamp := time.Date(2026, 1, 9, 15, 30, 45, 0, time.UTC)
+
+		io.Run(metrics.Accept(timestamp))
+
+		output := buf.String()
+		assert.Contains(t, output, "Accept:", "should contain Accept prefix")
+		assert.Contains(t, output, "TestCircuit", "should contain circuit name")
+		assert.Contains(t, output, timestamp.String(), "should contain timestamp")
+	})
+
+	t.Run("returns IO[Void] that can be executed", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+		metrics := MakeMetricsFromLogger("TestCircuit", logger)
+		timestamp := time.Now()
+
+		ioOp := metrics.Accept(timestamp)
+
+		assert.NotNil(t, ioOp, "should return non-nil IO operation")
+		result := io.Run(ioOp)
+		assert.NotNil(t, result, "IO operation should execute successfully")
+	})
+
+	t.Run("logs multiple accept events", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+		metrics := MakeMetricsFromLogger("TestCircuit", logger)
+		time1 := time.Date(2026, 1, 9, 15, 30, 0, 0, time.UTC)
+		time2 := time.Date(2026, 1, 9, 15, 31, 0, 0, time.UTC)
+
+		io.Run(metrics.Accept(time1))
+		io.Run(metrics.Accept(time2))
+
+		output := buf.String()
+		lines := strings.Split(strings.TrimSpace(output), "\n")
+		assert.Len(t, lines, 2, "should have 2 log lines")
+		assert.Contains(t, lines[0], time1.String())
+		assert.Contains(t, lines[1], time2.String())
+	})
+}
+
+// TestLoggingMetricsReject tests the Reject method
+func TestLoggingMetricsReject(t *testing.T) {
+	t.Run("logs reject event with correct format", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+		metrics := MakeMetricsFromLogger("TestCircuit", logger)
+		timestamp := time.Date(2026, 1, 9, 15, 30, 45, 0, time.UTC)
+
+		io.Run(metrics.Reject(timestamp))
+
+		output := buf.String()
+		assert.Contains(t, output, "Reject:", "should contain Reject prefix")
+		assert.Contains(t, output, "TestCircuit", "should contain circuit name")
+		assert.Contains(t, output, timestamp.String(), "should contain timestamp")
+	})
+
+	t.Run("returns IO[Void] that can be executed", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+		metrics := MakeMetricsFromLogger("TestCircuit", logger)
+		timestamp := time.Now()
+
+		ioOp := metrics.Reject(timestamp)
+
+		assert.NotNil(t, ioOp, "should return non-nil IO operation")
+		result := io.Run(ioOp)
+		assert.NotNil(t, result, "IO operation should execute successfully")
+	})
+}
+
+// TestLoggingMetricsOpen tests the Open method
+func TestLoggingMetricsOpen(t *testing.T) {
+	t.Run("logs open event with correct format", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+		metrics := MakeMetricsFromLogger("TestCircuit", logger)
+		timestamp := time.Date(2026, 1, 9, 15, 30, 45, 0, time.UTC)
+
+		io.Run(metrics.Open(timestamp))
+
+		output := buf.String()
+		assert.Contains(t, output, "Open:", "should contain Open prefix")
+		assert.Contains(t, output, "TestCircuit", "should contain circuit name")
+		assert.Contains(t, output, timestamp.String(), "should contain timestamp")
+	})
+
+	t.Run("returns IO[Void] that can be executed", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+		metrics := MakeMetricsFromLogger("TestCircuit", logger)
+		timestamp := time.Now()
+
+		ioOp := metrics.Open(timestamp)
+
+		assert.NotNil(t, ioOp, "should return non-nil IO operation")
+		result := io.Run(ioOp)
+		assert.NotNil(t, result, "IO operation should execute successfully")
+	})
+}
+
+// TestLoggingMetricsClose tests the Close method
+func TestLoggingMetricsClose(t *testing.T) {
+	t.Run("logs close event with correct format", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+		metrics := MakeMetricsFromLogger("TestCircuit", logger)
+		timestamp := time.Date(2026, 1, 9, 15, 30, 45, 0, time.UTC)
+
+		io.Run(metrics.Close(timestamp))
+
+		output := buf.String()
+		assert.Contains(t, output, "Close:", "should contain Close prefix")
+		assert.Contains(t, output, "TestCircuit", "should contain circuit name")
+		assert.Contains(t, output, timestamp.String(), "should contain timestamp")
+	})
+
+	t.Run("returns IO[Void] that can be executed", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+		metrics := MakeMetricsFromLogger("TestCircuit", logger)
+		timestamp := time.Now()
+
+		ioOp := metrics.Close(timestamp)
+
+		assert.NotNil(t, ioOp, "should return non-nil IO operation")
+		result := io.Run(ioOp)
+		assert.NotNil(t, result, "IO operation should execute successfully")
+	})
+}
+
+// TestLoggingMetricsCanary tests the Canary method
+func TestLoggingMetricsCanary(t *testing.T) {
+	t.Run("logs canary event with correct format", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+		metrics := MakeMetricsFromLogger("TestCircuit", logger)
+		timestamp := time.Date(2026, 1, 9, 15, 30, 45, 0, time.UTC)
+
+		io.Run(metrics.Canary(timestamp))
+
+		output := buf.String()
+		assert.Contains(t, output, "Canary:", "should contain Canary prefix")
+		assert.Contains(t, output, "TestCircuit", "should contain circuit name")
+		assert.Contains(t, output, timestamp.String(), "should contain timestamp")
+	})
+
+	t.Run("returns IO[Void] that can be executed", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+		metrics := MakeMetricsFromLogger("TestCircuit", logger)
+		timestamp := time.Now()
+
+		ioOp := metrics.Canary(timestamp)
+
+		assert.NotNil(t, ioOp, "should return non-nil IO operation")
+		result := io.Run(ioOp)
+		assert.NotNil(t, result, "IO operation should execute successfully")
+	})
+}
+
+// TestLoggingMetricsDoLog tests the doLog helper method
+func TestLoggingMetricsDoLog(t *testing.T) {
+	t.Run("formats log message correctly", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+		metrics := &loggingMetrics{name: "TestCircuit", logger: logger}
+		timestamp := time.Date(2026, 1, 9, 15, 30, 45, 0, time.UTC)
+
+		io.Run(metrics.doLog("CustomEvent", timestamp))
+
+		output := buf.String()
+		assert.Contains(t, output, "CustomEvent:", "should contain custom prefix")
+		assert.Contains(t, output, "TestCircuit", "should contain circuit name")
+		assert.Contains(t, output, timestamp.String(), "should contain timestamp")
+	})
+
+	t.Run("handles different prefixes", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+		metrics := &loggingMetrics{name: "TestCircuit", logger: logger}
+		timestamp := time.Now()
+
+		prefixes := []string{"Accept", "Reject", "Open", "Close", "Canary", "Custom"}
+		for _, prefix := range prefixes {
+			buf.Reset()
+			io.Run(metrics.doLog(prefix, timestamp))
+			output := buf.String()
+			assert.Contains(t, output, prefix+":", "should contain prefix: "+prefix)
+		}
+	})
+}
+
+// TestMetricsIntegration tests integration scenarios
+func TestMetricsIntegration(t *testing.T) {
+	t.Run("logs complete circuit breaker lifecycle", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+		metrics := MakeMetricsFromLogger("APICircuit", logger)
+		baseTime := time.Date(2026, 1, 9, 15, 30, 0, 0, time.UTC)
+
+		// Simulate circuit breaker lifecycle
+		io.Run(metrics.Accept(baseTime))                       // Request accepted
+		io.Run(metrics.Accept(baseTime.Add(1 * time.Second)))  // Another request
+		io.Run(metrics.Open(baseTime.Add(2 * time.Second)))    // Circuit opens
+		io.Run(metrics.Reject(baseTime.Add(3 * time.Second)))  // Request rejected
+		io.Run(metrics.Canary(baseTime.Add(30 * time.Second))) // Canary attempt
+		io.Run(metrics.Close(baseTime.Add(31 * time.Second)))  // Circuit closes
+
+		output := buf.String()
+		lines := strings.Split(strings.TrimSpace(output), "\n")
+		assert.Len(t, lines, 6, "should have 6 log lines")
+
+		assert.Contains(t, lines[0], "Accept:")
+		assert.Contains(t, lines[1], "Accept:")
+		assert.Contains(t, lines[2], "Open:")
+		assert.Contains(t, lines[3], "Reject:")
+		assert.Contains(t, lines[4], "Canary:")
+		assert.Contains(t, lines[5], "Close:")
+	})
+
+	t.Run("distinguishes between multiple circuit breakers", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+		metrics1 := MakeMetricsFromLogger("Circuit1", logger)
+		metrics2 := MakeMetricsFromLogger("Circuit2", logger)
+		timestamp := time.Now()
+
+		io.Run(metrics1.Accept(timestamp))
+		io.Run(metrics2.Accept(timestamp))
+
+		output := buf.String()
+		assert.Contains(t, output, "Circuit1", "should contain first circuit name")
+		assert.Contains(t, output, "Circuit2", "should contain second circuit name")
+	})
+}
+
+// TestMetricsThreadSafety tests concurrent access to metrics
+func TestMetricsThreadSafety(t *testing.T) {
+	t.Run("handles concurrent metric recording", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+		metrics := MakeMetricsFromLogger("ConcurrentCircuit", logger)
+
+		var wg sync.WaitGroup
+		numGoroutines := 100
+		wg.Add(numGoroutines)
+
+		// Launch multiple goroutines recording metrics concurrently
+		for i := 0; i < numGoroutines; i++ {
+			go func(id int) {
+				defer wg.Done()
+				timestamp := time.Now()
+				io.Run(metrics.Accept(timestamp))
+			}(i)
+		}
+
+		wg.Wait()
+
+		output := buf.String()
+		lines := strings.Split(strings.TrimSpace(output), "\n")
+		assert.Len(t, lines, numGoroutines, "should have logged all events")
+	})
+
+	t.Run("handles concurrent different event types", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+		metrics := MakeMetricsFromLogger("ConcurrentCircuit", logger)
+
+		var wg sync.WaitGroup
+		numIterations := 20
+		wg.Add(numIterations * 5) // 5 event types
+
+		timestamp := time.Now()
+
+		for i := 0; i < numIterations; i++ {
+			go func() {
+				defer wg.Done()
+				io.Run(metrics.Accept(timestamp))
+			}()
+			go func() {
+				defer wg.Done()
+				io.Run(metrics.Reject(timestamp))
+			}()
+			go func() {
+				defer wg.Done()
+				io.Run(metrics.Open(timestamp))
+			}()
+			go func() {
+				defer wg.Done()
+				io.Run(metrics.Close(timestamp))
+			}()
+			go func() {
+				defer wg.Done()
+				io.Run(metrics.Canary(timestamp))
+			}()
+		}
+
+		wg.Wait()
+
+		output := buf.String()
+		lines := strings.Split(strings.TrimSpace(output), "\n")
+		assert.Len(t, lines, numIterations*5, "should have logged all events")
+	})
+}
+
+// TestMetricsEdgeCases tests edge cases and special scenarios
+func TestMetricsEdgeCases(t *testing.T) {
+	t.Run("handles empty circuit breaker name", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+		metrics := MakeMetricsFromLogger("", logger)
+		timestamp := time.Now()
+
+		io.Run(metrics.Accept(timestamp))
+
+		output := buf.String()
+		assert.NotEmpty(t, output, "should still log even with empty name")
+	})
+
+	t.Run("handles very long circuit breaker name", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+		longName := strings.Repeat("VeryLongCircuitBreakerName", 100)
+		metrics := MakeMetricsFromLogger(longName, logger)
+		timestamp := time.Now()
+
+		io.Run(metrics.Accept(timestamp))
+
+		output := buf.String()
+		assert.Contains(t, output, longName, "should handle long names")
+	})
+
+	t.Run("handles special characters in name", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+		specialName := "Circuit-Breaker_123!@#$%^&*()"
+		metrics := MakeMetricsFromLogger(specialName, logger)
+		timestamp := time.Now()
+
+		io.Run(metrics.Accept(timestamp))
+
+		output := buf.String()
+		assert.Contains(t, output, specialName, "should handle special characters")
+	})
+
+	t.Run("handles zero time", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+		metrics := MakeMetricsFromLogger("TestCircuit", logger)
+		zeroTime := time.Time{}
+
+		io.Run(metrics.Accept(zeroTime))
+
+		output := buf.String()
+		assert.NotEmpty(t, output, "should handle zero time")
+		assert.Contains(t, output, "Accept:")
+	})
+
+	t.Run("handles far future time", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+		metrics := MakeMetricsFromLogger("TestCircuit", logger)
+		futureTime := time.Date(9999, 12, 31, 23, 59, 59, 0, time.UTC)
+
+		io.Run(metrics.Accept(futureTime))
+
+		output := buf.String()
+		assert.NotEmpty(t, output, "should handle far future time")
+		assert.Contains(t, output, "9999")
+	})
+}
+
+// TestMetricsWithCustomLogger tests metrics with different logger configurations
+func TestMetricsWithCustomLogger(t *testing.T) {
+	t.Run("works with logger with custom prefix", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "[CB] ", 0)
+		metrics := MakeMetricsFromLogger("TestCircuit", logger)
+		timestamp := time.Now()
+
+		io.Run(metrics.Accept(timestamp))
+
+		output := buf.String()
+		assert.Contains(t, output, "[CB]", "should include custom prefix")
+		assert.Contains(t, output, "Accept:")
+	})
+
+	t.Run("works with logger with flags", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", log.Ldate|log.Ltime)
+		metrics := MakeMetricsFromLogger("TestCircuit", logger)
+		timestamp := time.Now()
+
+		io.Run(metrics.Accept(timestamp))
+
+		output := buf.String()
+		assert.NotEmpty(t, output, "should log with flags")
+		assert.Contains(t, output, "Accept:")
+	})
+}
+
+// TestMetricsIOOperations tests IO operation behavior
+func TestMetricsIOOperations(t *testing.T) {
+	t.Run("IO operations are lazy", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+		metrics := MakeMetricsFromLogger("TestCircuit", logger)
+		timestamp := time.Now()
+
+		// Create IO operation but don't execute it
+		_ = metrics.Accept(timestamp)
+
+		// Buffer should be empty because IO wasn't executed
+		assert.Empty(t, buf.String(), "IO operation should be lazy")
+	})
+
+	t.Run("IO operations execute when run", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+		metrics := MakeMetricsFromLogger("TestCircuit", logger)
+		timestamp := time.Now()
+
+		ioOp := metrics.Accept(timestamp)
+		io.Run(ioOp)
+
+		assert.NotEmpty(t, buf.String(), "IO operation should execute when run")
+	})
+
+	t.Run("same IO operation can be executed multiple times", func(t *testing.T) {
+		var buf bytes.Buffer
+		logger := log.New(&buf, "", 0)
+		metrics := MakeMetricsFromLogger("TestCircuit", logger)
+		timestamp := time.Now()
+
+		ioOp := metrics.Accept(timestamp)
+		io.Run(ioOp)
+		io.Run(ioOp)
+		io.Run(ioOp)
+
+		output := buf.String()
+		lines := strings.Split(strings.TrimSpace(output), "\n")
+		assert.Len(t, lines, 3, "should execute multiple times")
+	})
+}

v2/circuitbreaker/types.go

@@ -0,0 +1,118 @@
+// Package circuitbreaker provides a functional implementation of the circuit breaker pattern.
+// A circuit breaker prevents cascading failures by temporarily blocking requests to a failing service,
+// allowing it time to recover before retrying.
+//
+// # Thread Safety
+//
+// All data structures in this package are immutable except for IORef[BreakerState].
+// The IORef provides thread-safe mutable state through atomic operations.
+//
+// Immutable types (safe for concurrent use):
+//   - BreakerState (Either[openState, ClosedState])
+//   - openState
+//   - ClosedState implementations (closedStateWithErrorCount, closedStateWithHistory)
+//   - All function types and readers
+//
+// Mutable types (thread-safe through atomic operations):
+//   - IORef[BreakerState] - provides atomic read/write/modify operations
+//
+// ClosedState implementations must be thread-safe. The recommended approach is to
+// return new copies for all operations (Empty, AddError, AddSuccess, Check), which
+// provides automatic thread safety through immutability.
+package circuitbreaker
+
+import (
+	"time"
+
+	"github.com/IBM/fp-go/v2/either"
+	"github.com/IBM/fp-go/v2/endomorphism"
+	"github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/io"
+	"github.com/IBM/fp-go/v2/ioref"
+	"github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/ord"
+	"github.com/IBM/fp-go/v2/pair"
+	"github.com/IBM/fp-go/v2/predicate"
+	"github.com/IBM/fp-go/v2/reader"
+	"github.com/IBM/fp-go/v2/retry"
+	"github.com/IBM/fp-go/v2/state"
+)
+
+type (
+	// Ord is a type alias for ord.Ord, representing a total ordering on type A.
+	// Used for comparing values in a consistent way.
+	Ord[A any] = ord.Ord[A]
+
+	// Option is a type alias for option.Option, representing an optional value.
+	// It can be either Some(value) or None, used for safe handling of nullable values.
+	Option[A any] = option.Option[A]
+
+	// Endomorphism is a type alias for endomorphism.Endomorphism, representing a function from A to A.
+	// Used for transformations that preserve the type.
+	Endomorphism[A any] = endomorphism.Endomorphism[A]
+
+	// IO is a type alias for io.IO, representing a lazy computation that produces a value of type T.
+	// Used for side-effectful operations that are deferred until execution.
+	IO[T any] = io.IO[T]
+
+	// Pair is a type alias for pair.Pair, representing a tuple of two values.
+	// Used for grouping related values together.
+	Pair[L, R any] = pair.Pair[L, R]
+
+	// IORef is a type alias for ioref.IORef, representing a mutable reference to a value of type T.
+	// Used for managing mutable state in a functional way with IO operations.
+	IORef[T any] = ioref.IORef[T]
+
+	// State is a type alias for state.State, representing a stateful computation.
+	// It transforms a state of type T and produces a result of type R.
+	State[T, R any] = state.State[T, R]
+
+	// Either is a type alias for either.Either, representing a value that can be one of two types.
+	// Left[E] represents an error or alternative path, Right[A] represents the success path.
+	Either[E, A any] = either.Either[E, A]
+
+	// Predicate is a type alias for predicate.Predicate, representing a function that tests a value.
+	// Returns true if the value satisfies the predicate condition, false otherwise.
+	Predicate[A any] = predicate.Predicate[A]
+
+	// Reader is a type alias for reader.Reader, representing a computation that depends on an environment R
+	// and produces a value of type A. Used for dependency injection and configuration.
+	Reader[R, A any] = reader.Reader[R, A]
+
+	// openState represents the internal state when the circuit breaker is open.
+	// In the open state, requests are blocked to give the failing service time to recover.
+	// The circuit breaker will transition to a half-open state (canary request) after resetAt.
+	openState struct {
+		openedAt time.Time
+
+		// resetAt is the time when the circuit breaker should attempt a canary request
+		// to test if the service has recovered. Calculated based on the retry policy.
+		resetAt time.Time
+
+		// retryStatus tracks the current retry attempt information, including the number
+		// of retries and the delay between attempts. Used by the retry policy to calculate
+		// exponential backoff or other retry strategies.
+		retryStatus retry.RetryStatus
+
+		// canaryRequest indicates whether the circuit is in half-open state, allowing
+		// a single test request (canary) to check if the service has recovered.
+		// If true, one request is allowed through to test the service.
+		// If the canary succeeds, the circuit closes; if it fails, the circuit remains open
+		// with an extended reset time.
+		canaryRequest bool
+	}
+
+	// BreakerState represents the current state of the circuit breaker.
+	// It is an Either type where:
+	//   - Left[openState] represents an open circuit (requests are blocked)
+	//   - Right[ClosedState] represents a closed circuit (requests are allowed through)
+	//
+	// State Transitions:
+	//   - Closed -> Open: When failure threshold is exceeded in ClosedState
+	//   - Open -> Half-Open: When resetAt is reached (canaryRequest = true)
+	//   - Half-Open -> Closed: When canary request succeeds
+	//   - Half-Open -> Open: When canary request fails (with extended resetAt)
+	BreakerState = Either[openState, ClosedState]
+
+	Void = function.Void
+)

v2/cli/commands.go

@@ -35,5 +35,6 @@ func Commands() []*C.Command {
 		IOCommand(),
 		IOOptionCommand(),
 		DICommand(),
+		LensCommand(),
 	}
 }

v2/cli/lens.go

@@ -0,0 +1,946 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package cli
+
+import (
+	"bytes"
+	"go/ast"
+	"go/parser"
+	"go/token"
+	"log"
+	"os"
+	"path/filepath"
+	"reflect"
+	"strings"
+	"text/template"
+
+	S "github.com/IBM/fp-go/v2/string"
+	C "github.com/urfave/cli/v2"
+)
+
+const (
+	keyLensDir         = "dir"
+	keyVerbose         = "verbose"
+	keyIncludeTestFile = "include-test-files"
+	lensAnnotation     = "fp-go:Lens"
+)
+
+var (
+	flagLensDir = &C.StringFlag{
+		Name:  keyLensDir,
+		Value: ".",
+		Usage: "Directory to scan for Go files",
+	}
+
+	flagVerbose = &C.BoolFlag{
+		Name:    keyVerbose,
+		Aliases: []string{"v"},
+		Value:   false,
+		Usage:   "Enable verbose output",
+	}
+
+	flagIncludeTestFiles = &C.BoolFlag{
+		Name:    keyIncludeTestFile,
+		Aliases: []string{"t"},
+		Value:   false,
+		Usage:   "Include test files (*_test.go) when scanning for annotated types",
+	}
+)
+
+// structInfo holds information about a struct that needs lens generation
+type structInfo struct {
+	Name           string
+	TypeParams     string // e.g., "[T any]" or "[K comparable, V any]" - for type declarations
+	TypeParamNames string // e.g., "[T]" or "[K, V]" - for type usage in function signatures
+	Fields         []fieldInfo
+	Imports        map[string]string // package path -> alias
+}
+
+// fieldInfo holds information about a struct field
+type fieldInfo struct {
+	Name         string
+	TypeName     string
+	BaseType     string // TypeName without leading * for pointer types
+	IsOptional   bool   // true if field is a pointer or has json omitempty tag
+	IsComparable bool   // true if the type is comparable (can use ==)
+	IsEmbedded   bool   // true if this field comes from an embedded struct
+}
+
+// templateData holds data for template rendering
+type templateData struct {
+	PackageName string
+	Structs     []structInfo
+}
+
+const lensStructTemplate = `
+// {{.Name}}Lenses provides lenses for accessing fields of {{.Name}}
+type {{.Name}}Lenses{{.TypeParams}} struct {
+	// mandatory fields
+{{- range .Fields}}
+	{{.Name}} __lens.Lens[{{$.Name}}{{$.TypeParamNames}}, {{.TypeName}}]
+{{- end}}
+	// optional fields
+{{- range .Fields}}
+{{- if .IsComparable}}
+	{{.Name}}O __lens_option.LensO[{{$.Name}}{{$.TypeParamNames}}, {{.TypeName}}]
+{{- end}}
+{{- end}}
+}
+
+// {{.Name}}RefLenses provides lenses for accessing fields of {{.Name}} via a reference to {{.Name}}
+type {{.Name}}RefLenses{{.TypeParams}} struct {
+	// mandatory fields
+{{- range .Fields}}
+	{{.Name}} __lens.Lens[*{{$.Name}}{{$.TypeParamNames}}, {{.TypeName}}]
+{{- end}}
+	// optional fields
+{{- range .Fields}}
+{{- if .IsComparable}}
+	{{.Name}}O __lens_option.LensO[*{{$.Name}}{{$.TypeParamNames}}, {{.TypeName}}]
+{{- end}}
+{{- end}}
+	// prisms
+{{- range .Fields}}
+	{{.Name}}P __prism.Prism[*{{$.Name}}{{$.TypeParamNames}}, {{.TypeName}}]
+{{- end}}
+}
+
+// {{.Name}}Prisms provides prisms for accessing fields of {{.Name}}
+type {{.Name}}Prisms{{.TypeParams}} struct {
+{{- range .Fields}}
+	{{.Name}} __prism.Prism[{{$.Name}}{{$.TypeParamNames}}, {{.TypeName}}]
+{{- end}}
+}
+`
+
+const lensConstructorTemplate = `
+// Make{{.Name}}Lenses creates a new {{.Name}}Lenses with lenses for all fields
+func Make{{.Name}}Lenses{{.TypeParams}}() {{.Name}}Lenses{{.TypeParamNames}} {
+	// mandatory lenses
+{{- range .Fields}}
+	lens{{.Name}} := __lens.MakeLensWithName(
+		func(s {{$.Name}}{{$.TypeParamNames}}) {{.TypeName}} { return s.{{.Name}} },
+		func(s {{$.Name}}{{$.TypeParamNames}}, v {{.TypeName}}) {{$.Name}}{{$.TypeParamNames}} { s.{{.Name}} = v; return s },
+		"{{$.Name}}{{$.TypeParamNames}}.{{.Name}}",
+	)
+{{- end}}
+	// optional lenses
+{{- range .Fields}}
+{{- if .IsComparable}}
+	lens{{.Name}}O := __lens_option.FromIso[{{$.Name}}{{$.TypeParamNames}}](__iso_option.FromZero[{{.TypeName}}]())(lens{{.Name}})
+{{- end}}
+{{- end}}
+	return {{.Name}}Lenses{{.TypeParamNames}}{
+		// mandatory lenses
+{{- range .Fields}}
+		{{.Name}}: lens{{.Name}},
+{{- end}}
+		// optional lenses
+{{- range .Fields}}
+{{- if .IsComparable}}
+		{{.Name}}O: lens{{.Name}}O,
+{{- end}}
+{{- end}}
+	}
+}
+
+// Make{{.Name}}RefLenses creates a new {{.Name}}RefLenses with lenses for all fields
+func Make{{.Name}}RefLenses{{.TypeParams}}() {{.Name}}RefLenses{{.TypeParamNames}} {
+	// mandatory lenses
+{{- range .Fields}}
+{{- if .IsComparable}}
+	lens{{.Name}} := __lens.MakeLensStrictWithName(
+		func(s *{{$.Name}}{{$.TypeParamNames}}) {{.TypeName}} { return s.{{.Name}} },
+		func(s *{{$.Name}}{{$.TypeParamNames}}, v {{.TypeName}}) *{{$.Name}}{{$.TypeParamNames}} { s.{{.Name}} = v; return s },
+		"(*{{$.Name}}{{$.TypeParamNames}}).{{.Name}}",
+	)
+{{- else}}
+	lens{{.Name}} := __lens.MakeLensRefWithName(
+		func(s *{{$.Name}}{{$.TypeParamNames}}) {{.TypeName}} { return s.{{.Name}} },
+		func(s *{{$.Name}}{{$.TypeParamNames}}, v {{.TypeName}}) *{{$.Name}}{{$.TypeParamNames}} { s.{{.Name}} = v; return s },
+		"(*{{$.Name}}{{$.TypeParamNames}}).{{.Name}}",
+	)
+{{- end}}
+{{- end}}
+	// optional lenses
+{{- range .Fields}}
+{{- if .IsComparable}}
+	lens{{.Name}}O := __lens_option.FromIso[*{{$.Name}}{{$.TypeParamNames}}](__iso_option.FromZero[{{.TypeName}}]())(lens{{.Name}})
+{{- end}}
+{{- end}}
+	return {{.Name}}RefLenses{{.TypeParamNames}}{
+		// mandatory lenses
+{{- range .Fields}}
+		{{.Name}}: lens{{.Name}},
+{{- end}}
+		// optional lenses
+{{- range .Fields}}
+{{- if .IsComparable}}
+		{{.Name}}O: lens{{.Name}}O,
+{{- end}}
+{{- end}}
+	}
+}
+
+// Make{{.Name}}Prisms creates a new {{.Name}}Prisms with prisms for all fields
+func Make{{.Name}}Prisms{{.TypeParams}}() {{.Name}}Prisms{{.TypeParamNames}} {
+{{- range .Fields}}
+{{- if .IsComparable}}
+	_fromNonZero{{.Name}} := __option.FromNonZero[{{.TypeName}}]()
+	_prism{{.Name}} := __prism.MakePrismWithName(
+		func(s {{$.Name}}{{$.TypeParamNames}}) __option.Option[{{.TypeName}}] { return _fromNonZero{{.Name}}(s.{{.Name}}) },
+		func(v {{.TypeName}}) {{$.Name}}{{$.TypeParamNames}} {
+			{{- if .IsEmbedded}}
+			var result {{$.Name}}{{$.TypeParamNames}}
+			result.{{.Name}} = v
+			return result
+			{{- else}}
+			return {{$.Name}}{{$.TypeParamNames}}{ {{.Name}}: v }
+			{{- end}}
+		},
+		"{{$.Name}}{{$.TypeParamNames}}.{{.Name}}",
+	)
+{{- else}}
+	_prism{{.Name}} := __prism.MakePrismWithName(
+		func(s {{$.Name}}{{$.TypeParamNames}}) __option.Option[{{.TypeName}}] { return __option.Some(s.{{.Name}}) },
+		func(v {{.TypeName}}) {{$.Name}}{{$.TypeParamNames}} {
+			{{- if .IsEmbedded}}
+			var result {{$.Name}}{{$.TypeParamNames}}
+			result.{{.Name}} = v
+			return result
+			{{- else}}
+			return {{$.Name}}{{$.TypeParamNames}}{ {{.Name}}: v }
+			{{- end}}
+		},
+		"{{$.Name}}{{$.TypeParamNames}}.{{.Name}}",
+	)
+{{- end}}
+{{- end}}
+	return {{.Name}}Prisms{{.TypeParamNames}} {
+{{- range .Fields}}
+		{{.Name}}: _prism{{.Name}},
+{{- end}}
+	}
+}
+`
+
+var (
+	structTmpl      *template.Template
+	constructorTmpl *template.Template
+)
+
+func init() {
+	var err error
+	structTmpl, err = template.New("struct").Parse(lensStructTemplate)
+	if err != nil {
+		panic(err)
+	}
+	constructorTmpl, err = template.New("constructor").Parse(lensConstructorTemplate)
+	if err != nil {
+		panic(err)
+	}
+}
+
+// hasLensAnnotation checks if a comment group contains the lens annotation
+func hasLensAnnotation(doc *ast.CommentGroup) bool {
+	if doc == nil {
+		return false
+	}
+	for _, comment := range doc.List {
+		if strings.Contains(comment.Text, lensAnnotation) {
+			return true
+		}
+	}
+	return false
+}
+
+// getTypeName extracts the type name from a field type expression
+func getTypeName(expr ast.Expr) string {
+	switch t := expr.(type) {
+	case *ast.Ident:
+		return t.Name
+	case *ast.StarExpr:
+		return "*" + getTypeName(t.X)
+	case *ast.ArrayType:
+		return "[]" + getTypeName(t.Elt)
+	case *ast.MapType:
+		return "map[" + getTypeName(t.Key) + "]" + getTypeName(t.Value)
+	case *ast.SelectorExpr:
+		return getTypeName(t.X) + "." + t.Sel.Name
+	case *ast.InterfaceType:
+		return "interface{}"
+	case *ast.IndexExpr:
+		// Generic type with single type parameter (Go 1.18+)
+		// e.g., Option[string]
+		return getTypeName(t.X) + "[" + getTypeName(t.Index) + "]"
+	case *ast.IndexListExpr:
+		// Generic type with multiple type parameters (Go 1.18+)
+		// e.g., Map[string, int]
+		var params []string
+		for _, index := range t.Indices {
+			params = append(params, getTypeName(index))
+		}
+		return getTypeName(t.X) + "[" + strings.Join(params, ", ") + "]"
+	default:
+		return "any"
+	}
+}
+
+// extractImports extracts package imports from a type expression
+// Returns a map of package path -> package name
+func extractImports(expr ast.Expr, imports map[string]string) {
+	switch t := expr.(type) {
+	case *ast.StarExpr:
+		extractImports(t.X, imports)
+	case *ast.ArrayType:
+		extractImports(t.Elt, imports)
+	case *ast.MapType:
+		extractImports(t.Key, imports)
+		extractImports(t.Value, imports)
+	case *ast.SelectorExpr:
+		// This is a qualified identifier like "option.Option"
+		if ident, ok := t.X.(*ast.Ident); ok {
+			// ident.Name is the package name (e.g., "option")
+			// We need to track this for import resolution
+			imports[ident.Name] = ident.Name
+		}
+	case *ast.IndexExpr:
+		// Generic type with single type parameter
+		extractImports(t.X, imports)
+		extractImports(t.Index, imports)
+	case *ast.IndexListExpr:
+		// Generic type with multiple type parameters
+		extractImports(t.X, imports)
+		for _, index := range t.Indices {
+			extractImports(index, imports)
+		}
+	}
+}
+
+// hasOmitEmpty checks if a struct tag contains json omitempty
+func hasOmitEmpty(tag *ast.BasicLit) bool {
+	if tag == nil {
+		return false
+	}
+	// Parse the struct tag
+	tagValue := strings.Trim(tag.Value, "`")
+	structTag := reflect.StructTag(tagValue)
+	jsonTag := structTag.Get("json")
+
+	// Check if omitempty is present
+	parts := strings.Split(jsonTag, ",")
+	for _, part := range parts {
+		if strings.TrimSpace(part) == "omitempty" {
+			return true
+		}
+	}
+	return false
+}
+
+// isPointerType checks if a type expression is a pointer
+func isPointerType(expr ast.Expr) bool {
+	_, ok := expr.(*ast.StarExpr)
+	return ok
+}
+
+// isComparableType checks if a type expression represents a comparable type.
+// Comparable types in Go include:
+// - Basic types (bool, numeric types, string)
+// - Pointer types
+// - Channel types
+// - Interface types
+// - Structs where all fields are comparable
+// - Arrays where the element type is comparable
+//
+// Non-comparable types include:
+// - Slices
+// - Maps
+// - Functions
+//
+// typeParams is a map of type parameter names to their constraints (e.g., "T" -> "any", "K" -> "comparable")
+func isComparableType(expr ast.Expr, typeParams map[string]string) bool {
+	switch t := expr.(type) {
+	case *ast.Ident:
+		// Check if this is a type parameter
+		if constraint, isTypeParam := typeParams[t.Name]; isTypeParam {
+			// Type parameter - check its constraint
+			return constraint == "comparable"
+		}
+
+		// Basic types and named types
+		// We assume named types are comparable unless they're known non-comparable types
+		name := t.Name
+		// Known non-comparable built-in types
+		if name == "error" {
+			// error is an interface, which is comparable
+			return true
+		}
+		// Most basic types and named types are comparable
+		// We can't determine if a custom type is comparable without type checking,
+		// so we assume it is (conservative approach)
+		return true
+	case *ast.StarExpr:
+		// Pointer types are always comparable
+		return true
+	case *ast.ArrayType:
+		// Arrays are comparable if their element type is comparable
+		if t.Len == nil {
+			// This is a slice (no length), slices are not comparable
+			return false
+		}
+		// Fixed-size array, check element type
+		return isComparableType(t.Elt, typeParams)
+	case *ast.MapType:
+		// Maps are not comparable
+		return false
+	case *ast.FuncType:
+		// Functions are not comparable
+		return false
+	case *ast.InterfaceType:
+		// Interface types are comparable
+		return true
+	case *ast.StructType:
+		// Structs are comparable if all fields are comparable
+		// We can't easily determine this without full type information,
+		// so we conservatively return false for struct literals
+		return false
+	case *ast.SelectorExpr:
+		// Qualified identifier (e.g., pkg.Type)
+		// We can't determine comparability without type information
+		// Check for known non-comparable types from standard library
+		if ident, ok := t.X.(*ast.Ident); ok {
+			pkgName := ident.Name
+			typeName := t.Sel.Name
+			// Check for known non-comparable types
+			if pkgName == "context" && typeName == "Context" {
+				// context.Context is an interface, which is comparable
+				return true
+			}
+			// For other qualified types, we assume they're comparable
+			// This is a conservative approach
+		}
+		return true
+	case *ast.IndexExpr, *ast.IndexListExpr:
+		// Generic types - we can't determine comparability without type information
+		// For common generic types, we can make educated guesses
+		var baseExpr ast.Expr
+		if idx, ok := t.(*ast.IndexExpr); ok {
+			baseExpr = idx.X
+		} else if idxList, ok := t.(*ast.IndexListExpr); ok {
+			baseExpr = idxList.X
+		}
+
+		if sel, ok := baseExpr.(*ast.SelectorExpr); ok {
+			if ident, ok := sel.X.(*ast.Ident); ok {
+				pkgName := ident.Name
+				typeName := sel.Sel.Name
+				// Check for known non-comparable generic types
+				if pkgName == "option" && typeName == "Option" {
+					// Option types are not comparable (they contain a slice internally)
+					return false
+				}
+				if pkgName == "either" && typeName == "Either" {
+					// Either types are not comparable
+					return false
+				}
+			}
+		}
+		// For other generic types, conservatively assume not comparable
+		log.Printf("Not comparable type: %v\n", t)
+		return false
+	case *ast.ChanType:
+		// Channel types are comparable
+		return true
+	default:
+		// Unknown type, conservatively assume not comparable
+		return false
+	}
+}
+
+// embeddedFieldResult holds both the field info and its AST type for import extraction
+type embeddedFieldResult struct {
+	fieldInfo fieldInfo
+	fieldType ast.Expr
+}
+
+// extractEmbeddedFields extracts fields from an embedded struct type
+// It returns a slice of embeddedFieldResult for all exported fields in the embedded struct
+// typeParamsMap contains the type parameters of the parent struct (for checking comparability)
+func extractEmbeddedFields(embedType ast.Expr, fileImports map[string]string, file *ast.File, typeParamsMap map[string]string) []embeddedFieldResult {
+	var results []embeddedFieldResult
+
+	// Get the type name of the embedded field
+	var typeName string
+	var typeIdent *ast.Ident
+
+	switch t := embedType.(type) {
+	case *ast.Ident:
+		// Direct embedded type: type MyStruct struct { EmbeddedType }
+		typeName = t.Name
+		typeIdent = t
+	case *ast.StarExpr:
+		// Pointer embedded type: type MyStruct struct { *EmbeddedType }
+		if ident, ok := t.X.(*ast.Ident); ok {
+			typeName = ident.Name
+			typeIdent = ident
+		}
+	case *ast.SelectorExpr:
+		// Qualified embedded type: type MyStruct struct { pkg.EmbeddedType }
+		// We can't easily resolve this without full type information
+		// For now, skip these
+		return results
+	}
+
+	if S.IsEmpty(typeName) || typeIdent == nil {
+		return results
+	}
+
+	// Find the struct definition in the same file
+	var embeddedStructType *ast.StructType
+	ast.Inspect(file, func(n ast.Node) bool {
+		if ts, ok := n.(*ast.TypeSpec); ok {
+			if ts.Name.Name == typeName {
+				if st, ok := ts.Type.(*ast.StructType); ok {
+					embeddedStructType = st
+					return false
+				}
+			}
+		}
+		return true
+	})
+
+	if embeddedStructType == nil {
+		// Struct not found in this file, might be from another package
+		return results
+	}
+
+	// Extract fields from the embedded struct
+	for _, field := range embeddedStructType.Fields.List {
+		// Skip embedded fields within embedded structs (for now, to avoid infinite recursion)
+		if len(field.Names) == 0 {
+			continue
+		}
+
+		for _, name := range field.Names {
+			// Only export lenses for exported fields
+			if name.IsExported() {
+				fieldTypeName := getTypeName(field.Type)
+				isOptional := false
+				baseType := fieldTypeName
+
+				// Check if field is optional
+				if isPointerType(field.Type) {
+					isOptional = true
+					baseType = strings.TrimPrefix(fieldTypeName, "*")
+				} else if hasOmitEmpty(field.Tag) {
+					isOptional = true
+				}
+
+				// Check if the type is comparable
+				isComparable := isComparableType(field.Type, typeParamsMap)
+
+				results = append(results, embeddedFieldResult{
+					fieldInfo: fieldInfo{
+						Name:         name.Name,
+						TypeName:     fieldTypeName,
+						BaseType:     baseType,
+						IsOptional:   isOptional,
+						IsComparable: isComparable,
+						IsEmbedded:   true,
+					},
+					fieldType: field.Type,
+				})
+			}
+		}
+	}
+
+	return results
+}
+
+// extractTypeParams extracts type parameters from a type spec
+// Returns two strings: full params like "[T any]" and names only like "[T]"
+func extractTypeParams(typeSpec *ast.TypeSpec) (string, string) {
+	if typeSpec.TypeParams == nil || len(typeSpec.TypeParams.List) == 0 {
+		return "", ""
+	}
+
+	var params []string
+	var names []string
+	for _, field := range typeSpec.TypeParams.List {
+		for _, name := range field.Names {
+			constraint := getTypeName(field.Type)
+			params = append(params, name.Name+" "+constraint)
+			names = append(names, name.Name)
+		}
+	}
+
+	fullParams := "[" + strings.Join(params, ", ") + "]"
+	nameParams := "[" + strings.Join(names, ", ") + "]"
+	return fullParams, nameParams
+}
+
+// buildTypeParamsMap creates a map of type parameter names to their constraints
+// e.g., for "type Box[T any, K comparable]", returns {"T": "any", "K": "comparable"}
+func buildTypeParamsMap(typeSpec *ast.TypeSpec) map[string]string {
+	typeParamsMap := make(map[string]string)
+	if typeSpec.TypeParams == nil || len(typeSpec.TypeParams.List) == 0 {
+		return typeParamsMap
+	}
+
+	for _, field := range typeSpec.TypeParams.List {
+		constraint := getTypeName(field.Type)
+		for _, name := range field.Names {
+			typeParamsMap[name.Name] = constraint
+		}
+	}
+
+	return typeParamsMap
+}
+
+// parseFile parses a Go file and extracts structs with lens annotations
+func parseFile(filename string) ([]structInfo, string, error) {
+	fset := token.NewFileSet()
+	node, err := parser.ParseFile(fset, filename, nil, parser.ParseComments)
+	if err != nil {
+		return nil, "", err
+	}
+
+	var structs []structInfo
+	packageName := node.Name.Name
+
+	// Build import map: package name -> import path
+	fileImports := make(map[string]string)
+	for _, imp := range node.Imports {
+		path := strings.Trim(imp.Path.Value, `"`)
+		var name string
+		if imp.Name != nil {
+			name = imp.Name.Name
+		} else {
+			// Extract package name from path (last component)
+			parts := strings.Split(path, "/")
+			name = parts[len(parts)-1]
+		}
+		fileImports[name] = path
+	}
+
+	// First pass: collect all GenDecls with their doc comments
+	declMap := make(map[*ast.TypeSpec]*ast.CommentGroup)
+	ast.Inspect(node, func(n ast.Node) bool {
+		if gd, ok := n.(*ast.GenDecl); ok {
+			for _, spec := range gd.Specs {
+				if ts, ok := spec.(*ast.TypeSpec); ok {
+					declMap[ts] = gd.Doc
+				}
+			}
+		}
+		return true
+	})
+
+	// Second pass: process type specs
+	ast.Inspect(node, func(n ast.Node) bool {
+		// Look for type declarations
+		typeSpec, ok := n.(*ast.TypeSpec)
+		if !ok {
+			return true
+		}
+
+		// Check if it's a struct type
+		structType, ok := typeSpec.Type.(*ast.StructType)
+		if !ok {
+			return true
+		}
+
+		// Get the doc comment from our map
+		doc := declMap[typeSpec]
+		if !hasLensAnnotation(doc) {
+			return true
+		}
+
+		// Extract field information and collect imports
+		var fields []fieldInfo
+		structImports := make(map[string]string)
+
+		// Build type parameters map for this struct
+		typeParamsMap := buildTypeParamsMap(typeSpec)
+
+		for _, field := range structType.Fields.List {
+			if len(field.Names) == 0 {
+				// Embedded field - promote its fields
+				embeddedResults := extractEmbeddedFields(field.Type, fileImports, node, typeParamsMap)
+				for _, embResult := range embeddedResults {
+					// Extract imports from embedded field's type
+					fieldImports := make(map[string]string)
+					extractImports(embResult.fieldType, fieldImports)
+
+					// Resolve package names to full import paths
+					for pkgName := range fieldImports {
+						if importPath, ok := fileImports[pkgName]; ok {
+							structImports[importPath] = pkgName
+						}
+					}
+
+					fields = append(fields, embResult.fieldInfo)
+				}
+				continue
+			}
+			for _, name := range field.Names {
+				// Only export lenses for exported fields
+				if name.IsExported() {
+					typeName := getTypeName(field.Type)
+					isOptional := false
+					baseType := typeName
+					isComparable := false
+
+					// Check if field is optional:
+					// 1. Pointer types are always optional
+					// 2. Non-pointer types with json omitempty tag are optional
+					if isPointerType(field.Type) {
+						isOptional = true
+						// Strip leading * for base type
+						baseType = strings.TrimPrefix(typeName, "*")
+					} else if hasOmitEmpty(field.Tag) {
+						// Non-pointer type with omitempty is also optional
+						isOptional = true
+					}
+
+					// Check if the type is comparable (for non-optional fields)
+					// For optional fields, we don't need to check since they use LensO
+					isComparable = isComparableType(field.Type, typeParamsMap)
+					// log.Printf("field %s, type: %v, isComparable: %b\n", name, field.Type, isComparable)
+
+					// Extract imports from this field's type
+					fieldImports := make(map[string]string)
+					extractImports(field.Type, fieldImports)
+
+					// Resolve package names to full import paths
+					for pkgName := range fieldImports {
+						if importPath, ok := fileImports[pkgName]; ok {
+							structImports[importPath] = pkgName
+						}
+					}
+
+					fields = append(fields, fieldInfo{
+						Name:         name.Name,
+						TypeName:     typeName,
+						BaseType:     baseType,
+						IsOptional:   isOptional,
+						IsComparable: isComparable,
+					})
+				}
+			}
+		}
+
+		if len(fields) > 0 {
+			typeParams, typeParamNames := extractTypeParams(typeSpec)
+			structs = append(structs, structInfo{
+				Name:           typeSpec.Name.Name,
+				TypeParams:     typeParams,
+				TypeParamNames: typeParamNames,
+				Fields:         fields,
+				Imports:        structImports,
+			})
+		}
+
+		return true
+	})
+
+	return structs, packageName, nil
+}
+
+// generateLensHelpers scans a directory for Go files and generates lens code
+func generateLensHelpers(dir, filename string, verbose, includeTestFiles bool) error {
+	// Get absolute path
+	absDir, err := filepath.Abs(dir)
+	if err != nil {
+		return err
+	}
+
+	if verbose {
+		log.Printf("Scanning directory: %s", absDir)
+	}
+
+	// Find all Go files in the directory
+	files, err := filepath.Glob(filepath.Join(absDir, "*.go"))
+	if err != nil {
+		return err
+	}
+
+	if verbose {
+		log.Printf("Found %d Go files", len(files))
+	}
+
+	// Parse all files and collect structs, separating test and non-test files
+	var regularStructs []structInfo
+	var testStructs []structInfo
+	var packageName string
+
+	for _, file := range files {
+		baseName := filepath.Base(file)
+
+		// Skip generated lens files (both regular and test)
+		if strings.HasPrefix(baseName, "gen_lens") && strings.HasSuffix(baseName, ".go") {
+			if verbose {
+				log.Printf("Skipping generated lens file: %s", baseName)
+			}
+			continue
+		}
+
+		isTestFile := strings.HasSuffix(file, "_test.go")
+
+		// Skip test files unless includeTestFiles is true
+		if isTestFile && !includeTestFiles {
+			if verbose {
+				log.Printf("Skipping test file: %s", baseName)
+			}
+			continue
+		}
+
+		if verbose {
+			log.Printf("Parsing file: %s", baseName)
+		}
+
+		structs, pkg, err := parseFile(file)
+		if err != nil {
+			log.Printf("Warning: failed to parse %s: %v", file, err)
+			continue
+		}
+
+		if verbose && len(structs) > 0 {
+			log.Printf("Found %d annotated struct(s) in %s", len(structs), baseName)
+			for _, s := range structs {
+				log.Printf("  - %s (%d fields)", s.Name, len(s.Fields))
+			}
+		}
+
+		if S.IsEmpty(packageName) {
+			packageName = pkg
+		}
+
+		// Separate structs based on source file type
+		if isTestFile {
+			testStructs = append(testStructs, structs...)
+		} else {
+			regularStructs = append(regularStructs, structs...)
+		}
+	}
+
+	if len(regularStructs) == 0 && len(testStructs) == 0 {
+		log.Printf("No structs with %s annotation found in %s", lensAnnotation, absDir)
+		return nil
+	}
+
+	// Generate regular lens file if there are regular structs
+	if len(regularStructs) > 0 {
+		if err := generateLensFile(absDir, filename, packageName, regularStructs, verbose); err != nil {
+			return err
+		}
+	}
+
+	// Generate test lens file if there are test structs
+	if len(testStructs) > 0 {
+		testFilename := strings.TrimSuffix(filename, ".go") + "_test.go"
+		if err := generateLensFile(absDir, testFilename, packageName, testStructs, verbose); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+// generateLensFile generates a lens file for the given structs
+func generateLensFile(absDir, filename, packageName string, structs []structInfo, verbose bool) error {
+	// Collect all unique imports from all structs
+	allImports := make(map[string]string) // import path -> alias
+	for _, s := range structs {
+		for importPath, alias := range s.Imports {
+			allImports[importPath] = alias
+		}
+	}
+
+	// Create output file
+	outPath := filepath.Join(absDir, filename)
+	f, err := os.Create(filepath.Clean(outPath))
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	log.Printf("Generating lens code in [%s] for package [%s] with [%d] structs ...", outPath, packageName, len(structs))
+
+	// Write header
+	writePackage(f, packageName)
+
+	// Write imports
+	f.WriteString("import (\n")
+	// Standard fp-go imports always needed
+	f.WriteString("\t__lens \"github.com/IBM/fp-go/v2/optics/lens\"\n")
+	f.WriteString("\t__option \"github.com/IBM/fp-go/v2/option\"\n")
+	f.WriteString("\t__prism \"github.com/IBM/fp-go/v2/optics/prism\"\n")
+	f.WriteString("\t__lens_option \"github.com/IBM/fp-go/v2/optics/lens/option\"\n")
+	f.WriteString("\t__iso_option \"github.com/IBM/fp-go/v2/optics/iso/option\"\n")
+
+	// Add additional imports collected from field types
+	for importPath, alias := range allImports {
+		f.WriteString("\t" + alias + " \"" + importPath + "\"\n")
+	}
+
+	f.WriteString(")\n")
+
+	// Generate lens code for each struct using templates
+	for _, s := range structs {
+		var buf bytes.Buffer
+
+		// Generate struct type
+		if err := structTmpl.Execute(&buf, s); err != nil {
+			return err
+		}
+
+		// Generate constructor
+		if err := constructorTmpl.Execute(&buf, s); err != nil {
+			return err
+		}
+
+		// Write to file
+		if _, err := f.Write(buf.Bytes()); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+// LensCommand creates the CLI command for lens generation
+func LensCommand() *C.Command {
+	return &C.Command{
+		Name:        "lens",
+		Usage:       "generate lens code for annotated structs",
+		Description: "Scans Go files for structs annotated with 'fp-go:Lens' and generates lens types. Pointer types and non-pointer types with json omitempty tag generate LensO (optional lens).",
+		Flags: []C.Flag{
+			flagLensDir,
+			flagFilename,
+			flagVerbose,
+			flagIncludeTestFiles,
+		},
+		Action: func(ctx *C.Context) error {
+			return generateLensHelpers(
+				ctx.String(keyLensDir),
+				ctx.String(keyFilename),
+				ctx.Bool(keyVerbose),
+				ctx.Bool(keyIncludeTestFile),
+			)
+		},
+	}
+}

v2/cli/monad.go

@@ -19,6 +19,8 @@ import (
 	"fmt"
 	"os"
 	"strings"
+
+	S "github.com/IBM/fp-go/v2/string"
 )
 
 // Deprecated:
@@ -176,7 +178,7 @@ func generateTraverseTuple1(
 			}
 			fmt.Fprintf(f, "F%d ~func(A%d) %s", j+1, j+1, hkt(fmt.Sprintf("T%d", j+1)))
 		}
-		if infix != "" {
+		if S.IsNonEmpty(infix) {
 			fmt.Fprintf(f, ", %s", infix)
 		}
 		// types
@@ -209,7 +211,7 @@ func generateTraverseTuple1(
 		fmt.Fprintf(f, "    return A.TraverseTuple%d(\n", i)
 		// map
 		fmt.Fprintf(f, "      Map[")
-		if infix != "" {
+		if S.IsNonEmpty(infix) {
 			fmt.Fprintf(f, "%s, T1,", infix)
 		} else {
 			fmt.Fprintf(f, "T1,")
@@ -231,7 +233,7 @@ func generateTraverseTuple1(
 				fmt.Fprintf(f, " ")
 			}
 			fmt.Fprintf(f, "%s", tuple)
-			if infix != "" {
+			if S.IsNonEmpty(infix) {
 				fmt.Fprintf(f, ", %s", infix)
 			}
 			fmt.Fprintf(f, ", T%d],\n", j+1)
@@ -256,11 +258,11 @@ func generateSequenceTuple1(
 
 		fmt.Fprintf(f, "\n// SequenceTuple%d converts a [Tuple%d] of [%s] into an [%s].\n", i, i, hkt("T"), hkt(fmt.Sprintf("Tuple%d", i)))
 		fmt.Fprintf(f, "func SequenceTuple%d[", i)
-		if infix != "" {
+		if S.IsNonEmpty(infix) {
 			fmt.Fprintf(f, "%s", infix)
 		}
 		for j := 0; j < i; j++ {
-			if infix != "" || j > 0 {
+			if S.IsNonEmpty(infix) || j > 0 {
 				fmt.Fprintf(f, ", ")
 			}
 			fmt.Fprintf(f, "T%d", j+1)
@@ -276,7 +278,7 @@ func generateSequenceTuple1(
 		fmt.Fprintf(f, "  return A.SequenceTuple%d(\n", i)
 		// map
 		fmt.Fprintf(f, "    Map[")
-		if infix != "" {
+		if S.IsNonEmpty(infix) {
 			fmt.Fprintf(f, "%s, T1,", infix)
 		} else {
 			fmt.Fprintf(f, "T1,")
@@ -298,7 +300,7 @@ func generateSequenceTuple1(
 				fmt.Fprintf(f, " ")
 			}
 			fmt.Fprintf(f, "%s", tuple)
-			if infix != "" {
+			if S.IsNonEmpty(infix) {
 				fmt.Fprintf(f, ", %s", infix)
 			}
 			fmt.Fprintf(f, ", T%d],\n", j+1)
@@ -319,11 +321,11 @@ func generateSequenceT1(
 
 		fmt.Fprintf(f, "\n// SequenceT%d converts %d parameters of [%s] into a [%s].\n", i, i, hkt("T"), hkt(fmt.Sprintf("Tuple%d", i)))
 		fmt.Fprintf(f, "func SequenceT%d[", i)
-		if infix != "" {
+		if S.IsNonEmpty(infix) {
 			fmt.Fprintf(f, "%s", infix)
 		}
 		for j := 0; j < i; j++ {
-			if infix != "" || j > 0 {
+			if S.IsNonEmpty(infix) || j > 0 {
 				fmt.Fprintf(f, ", ")
 			}
 			fmt.Fprintf(f, "T%d", j+1)
@@ -339,7 +341,7 @@ func generateSequenceT1(
 		fmt.Fprintf(f, "  return A.SequenceT%d(\n", i)
 		// map
 		fmt.Fprintf(f, "    Map[")
-		if infix != "" {
+		if S.IsNonEmpty(infix) {
 			fmt.Fprintf(f, "%s, T1,", infix)
 		} else {
 			fmt.Fprintf(f, "T1,")
@@ -361,7 +363,7 @@ func generateSequenceT1(
 				fmt.Fprintf(f, " ")
 			}
 			fmt.Fprintf(f, "%s", tuple)
-			if infix != "" {
+			if S.IsNonEmpty(infix) {
 				fmt.Fprintf(f, ", %s", infix)
 			}
 			fmt.Fprintf(f, ", T%d],\n", j+1)

v2/constant/const_test.go

@@ -27,7 +27,7 @@ import (
 
 func TestMap(t *testing.T) {
 	fa := Make[string, int]("foo")
-	assert.Equal(t, fa, F.Pipe1(fa, Map[string, int](utils.Double)))
+	assert.Equal(t, fa, F.Pipe1(fa, Map[string](utils.Double)))
 }
 
 func TestOf(t *testing.T) {

v2/constant/monoid.go

@@ -0,0 +1,11 @@
+package constant
+
+import (
+	"github.com/IBM/fp-go/v2/function"
+	M "github.com/IBM/fp-go/v2/monoid"
+)
+
+// Monoid returns a [M.Monoid] that returns a constant value in all operations
+func Monoid[A any](a A) M.Monoid[A] {
+	return M.MakeMonoid(function.Constant2[A, A](a), a)
+}

v2/consumer/consumer.go

@@ -0,0 +1,177 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package consumer
+
+// Local transforms a Consumer by preprocessing its input through a function.
+// This is the contravariant map operation for Consumers, analogous to reader.Local
+// but operating on the input side rather than the output side.
+//
+// Given a Consumer[R1] that consumes values of type R1, and a function f that
+// converts R2 to R1, Local creates a new Consumer[R2] that:
+//   1. Takes a value of type R2
+//   2. Applies f to convert it to R1
+//   3. Passes the result to the original Consumer[R1]
+//
+// This is particularly useful for adapting consumers to work with different input types,
+// similar to how reader.Local adapts readers to work with different environment types.
+//
+// Comparison with reader.Local:
+//   - reader.Local: Transforms the environment BEFORE passing it to a Reader (preprocessing input)
+//   - consumer.Local: Transforms the value BEFORE passing it to a Consumer (preprocessing input)
+//   - Both are contravariant operations on the input type
+//   - Reader produces output, Consumer performs side effects
+//
+// Type Parameters:
+//   - R2: The input type of the new Consumer (what you have)
+//   - R1: The input type of the original Consumer (what it expects)
+//
+// Parameters:
+//   - f: A function that converts R2 to R1 (preprocessing function)
+//
+// Returns:
+//   - An Operator that transforms Consumer[R1] into Consumer[R2]
+//
+// Example - Basic type adaptation:
+//
+//	// Consumer that logs integers
+//	logInt := func(x int) {
+//	    fmt.Printf("Value: %d\n", x)
+//	}
+//
+//	// Adapt it to consume strings by parsing them first
+//	parseToInt := func(s string) int {
+//	    n, _ := strconv.Atoi(s)
+//	    return n
+//	}
+//
+//	logString := consumer.Local(parseToInt)(logInt)
+//	logString("42") // Logs: "Value: 42"
+//
+// Example - Extracting fields from structs:
+//
+//	type User struct {
+//	    Name string
+//	    Age  int
+//	}
+//
+//	// Consumer that logs names
+//	logName := func(name string) {
+//	    fmt.Printf("Name: %s\n", name)
+//	}
+//
+//	// Adapt it to consume User structs
+//	extractName := func(u User) string {
+//	    return u.Name
+//	}
+//
+//	logUser := consumer.Local(extractName)(logName)
+//	logUser(User{Name: "Alice", Age: 30}) // Logs: "Name: Alice"
+//
+// Example - Simplifying complex types:
+//
+//	type DetailedConfig struct {
+//	    Host     string
+//	    Port     int
+//	    Timeout  time.Duration
+//	    MaxRetry int
+//	}
+//
+//	type SimpleConfig struct {
+//	    Host string
+//	    Port int
+//	}
+//
+//	// Consumer that logs simple configs
+//	logSimple := func(c SimpleConfig) {
+//	    fmt.Printf("Server: %s:%d\n", c.Host, c.Port)
+//	}
+//
+//	// Adapt it to consume detailed configs
+//	simplify := func(d DetailedConfig) SimpleConfig {
+//	    return SimpleConfig{Host: d.Host, Port: d.Port}
+//	}
+//
+//	logDetailed := consumer.Local(simplify)(logSimple)
+//	logDetailed(DetailedConfig{
+//	    Host:     "localhost",
+//	    Port:     8080,
+//	    Timeout:  time.Second,
+//	    MaxRetry: 3,
+//	}) // Logs: "Server: localhost:8080"
+//
+// Example - Composing multiple transformations:
+//
+//	type Response struct {
+//	    StatusCode int
+//	    Body       string
+//	}
+//
+//	// Consumer that logs status codes
+//	logStatus := func(code int) {
+//	    fmt.Printf("Status: %d\n", code)
+//	}
+//
+//	// Extract status code from response
+//	getStatus := func(r Response) int {
+//	    return r.StatusCode
+//	}
+//
+//	// Adapt to consume responses
+//	logResponse := consumer.Local(getStatus)(logStatus)
+//	logResponse(Response{StatusCode: 200, Body: "OK"}) // Logs: "Status: 200"
+//
+// Example - Using with multiple consumers:
+//
+//	type Event struct {
+//	    Type      string
+//	    Timestamp time.Time
+//	    Data      map[string]any
+//	}
+//
+//	// Consumers for different aspects
+//	logType := func(t string) { fmt.Printf("Type: %s\n", t) }
+//	logTime := func(t time.Time) { fmt.Printf("Time: %v\n", t) }
+//
+//	// Adapt them to consume events
+//	logEventType := consumer.Local(func(e Event) string { return e.Type })(logType)
+//	logEventTime := consumer.Local(func(e Event) time.Time { return e.Timestamp })(logTime)
+//
+//	event := Event{Type: "UserLogin", Timestamp: time.Now(), Data: nil}
+//	logEventType(event) // Logs: "Type: UserLogin"
+//	logEventTime(event) // Logs: "Time: ..."
+//
+// Use Cases:
+//   - Type adaptation: Convert between different input types
+//   - Field extraction: Extract specific fields from complex structures
+//   - Data transformation: Preprocess data before consumption
+//   - Interface adaptation: Adapt consumers to work with different interfaces
+//   - Logging pipelines: Transform data before logging
+//   - Event handling: Extract relevant data from events before processing
+//
+// Relationship to Reader:
+// Consumer is the dual of Reader in category theory:
+//   - Reader[R, A] = R -> A (produces output from environment)
+//   - Consumer[A] = A -> () (consumes input, produces side effects)
+//   - reader.Local transforms the environment before reading
+//   - consumer.Local transforms the input before consuming
+//   - Both are contravariant functors on their input type
+func Local[R2, R1 any](f func(R2) R1) Operator[R1, R2] {
+	return func(c Consumer[R1]) Consumer[R2] {
+		return func(r2 R2) {
+			c(f(r2))
+		}
+	}
+}

v2/consumer/consumer_test.go

@@ -0,0 +1,383 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package consumer
+
+import (
+	"strconv"
+	"testing"
+	"time"
+
+	"github.com/IBM/fp-go/v2/function"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestLocal(t *testing.T) {
+	t.Run("basic type transformation", func(t *testing.T) {
+		var captured int
+		consumeInt := func(x int) {
+			captured = x
+		}
+
+		// Transform string to int before consuming
+		stringToInt := func(s string) int {
+			n, _ := strconv.Atoi(s)
+			return n
+		}
+
+		consumeString := Local(stringToInt)(consumeInt)
+		consumeString("42")
+
+		assert.Equal(t, 42, captured)
+	})
+
+	t.Run("field extraction from struct", func(t *testing.T) {
+		type User struct {
+			Name string
+			Age  int
+		}
+
+		var capturedName string
+		consumeName := func(name string) {
+			capturedName = name
+		}
+
+		extractName := func(u User) string {
+			return u.Name
+		}
+
+		consumeUser := Local(extractName)(consumeName)
+		consumeUser(User{Name: "Alice", Age: 30})
+
+		assert.Equal(t, "Alice", capturedName)
+	})
+
+	t.Run("simplifying complex types", func(t *testing.T) {
+		type DetailedConfig struct {
+			Host     string
+			Port     int
+			Timeout  time.Duration
+			MaxRetry int
+		}
+
+		type SimpleConfig struct {
+			Host string
+			Port int
+		}
+
+		var captured SimpleConfig
+		consumeSimple := func(c SimpleConfig) {
+			captured = c
+		}
+
+		simplify := func(d DetailedConfig) SimpleConfig {
+			return SimpleConfig{Host: d.Host, Port: d.Port}
+		}
+
+		consumeDetailed := Local(simplify)(consumeSimple)
+		consumeDetailed(DetailedConfig{
+			Host:     "localhost",
+			Port:     8080,
+			Timeout:  time.Second,
+			MaxRetry: 3,
+		})
+
+		assert.Equal(t, SimpleConfig{Host: "localhost", Port: 8080}, captured)
+	})
+
+	t.Run("multiple transformations", func(t *testing.T) {
+		type Response struct {
+			StatusCode int
+			Body       string
+		}
+
+		var capturedStatus int
+		consumeStatus := func(code int) {
+			capturedStatus = code
+		}
+
+		getStatus := func(r Response) int {
+			return r.StatusCode
+		}
+
+		consumeResponse := Local(getStatus)(consumeStatus)
+		consumeResponse(Response{StatusCode: 200, Body: "OK"})
+
+		assert.Equal(t, 200, capturedStatus)
+	})
+
+	t.Run("chaining Local transformations", func(t *testing.T) {
+		type Level3 struct{ Value int }
+		type Level2 struct{ L3 Level3 }
+		type Level1 struct{ L2 Level2 }
+
+		var captured int
+		consumeInt := func(x int) {
+			captured = x
+		}
+
+		// Chain multiple Local transformations
+		extract3 := func(l3 Level3) int { return l3.Value }
+		extract2 := func(l2 Level2) Level3 { return l2.L3 }
+		extract1 := func(l1 Level1) Level2 { return l1.L2 }
+
+		// Compose the transformations
+		consumeLevel3 := Local(extract3)(consumeInt)
+		consumeLevel2 := Local(extract2)(consumeLevel3)
+		consumeLevel1 := Local(extract1)(consumeLevel2)
+
+		consumeLevel1(Level1{L2: Level2{L3: Level3{Value: 42}}})
+
+		assert.Equal(t, 42, captured)
+	})
+
+	t.Run("identity transformation", func(t *testing.T) {
+		var captured string
+		consumeString := func(s string) {
+			captured = s
+		}
+
+		identity := function.Identity[string]
+
+		consumeIdentity := Local(identity)(consumeString)
+		consumeIdentity("test")
+
+		assert.Equal(t, "test", captured)
+	})
+
+	t.Run("transformation with calculation", func(t *testing.T) {
+		type Rectangle struct {
+			Width  int
+			Height int
+		}
+
+		var capturedArea int
+		consumeArea := func(area int) {
+			capturedArea = area
+		}
+
+		calculateArea := func(r Rectangle) int {
+			return r.Width * r.Height
+		}
+
+		consumeRectangle := Local(calculateArea)(consumeArea)
+		consumeRectangle(Rectangle{Width: 5, Height: 10})
+
+		assert.Equal(t, 50, capturedArea)
+	})
+
+	t.Run("multiple consumers with same transformation", func(t *testing.T) {
+		type Event struct {
+			Type      string
+			Timestamp time.Time
+		}
+
+		var capturedType string
+		var capturedTime time.Time
+
+		consumeType := func(t string) {
+			capturedType = t
+		}
+
+		consumeTime := func(t time.Time) {
+			capturedTime = t
+		}
+
+		extractType := func(e Event) string { return e.Type }
+		extractTime := func(e Event) time.Time { return e.Timestamp }
+
+		consumeEventType := Local(extractType)(consumeType)
+		consumeEventTime := Local(extractTime)(consumeTime)
+
+		now := time.Now()
+		event := Event{Type: "UserLogin", Timestamp: now}
+
+		consumeEventType(event)
+		consumeEventTime(event)
+
+		assert.Equal(t, "UserLogin", capturedType)
+		assert.Equal(t, now, capturedTime)
+	})
+
+	t.Run("transformation with slice", func(t *testing.T) {
+		var captured int
+		consumeLength := func(n int) {
+			captured = n
+		}
+
+		getLength := func(s []string) int {
+			return len(s)
+		}
+
+		consumeSlice := Local(getLength)(consumeLength)
+		consumeSlice([]string{"a", "b", "c"})
+
+		assert.Equal(t, 3, captured)
+	})
+
+	t.Run("transformation with map", func(t *testing.T) {
+		var captured int
+		consumeCount := func(n int) {
+			captured = n
+		}
+
+		getCount := func(m map[string]int) int {
+			return len(m)
+		}
+
+		consumeMap := Local(getCount)(consumeCount)
+		consumeMap(map[string]int{"a": 1, "b": 2, "c": 3})
+
+		assert.Equal(t, 3, captured)
+	})
+
+	t.Run("transformation with pointer", func(t *testing.T) {
+		var captured int
+		consumeInt := func(x int) {
+			captured = x
+		}
+
+		dereference := func(p *int) int {
+			if p == nil {
+				return 0
+			}
+			return *p
+		}
+
+		consumePointer := Local(dereference)(consumeInt)
+
+		value := 42
+		consumePointer(&value)
+		assert.Equal(t, 42, captured)
+
+		consumePointer(nil)
+		assert.Equal(t, 0, captured)
+	})
+
+	t.Run("transformation with custom type", func(t *testing.T) {
+		type MyType struct {
+			Value string
+		}
+
+		var captured string
+		consumeString := func(s string) {
+			captured = s
+		}
+
+		extractValue := func(m MyType) string {
+			return m.Value
+		}
+
+		consumeMyType := Local(extractValue)(consumeString)
+		consumeMyType(MyType{Value: "test"})
+
+		assert.Equal(t, "test", captured)
+	})
+
+	t.Run("accumulation through multiple calls", func(t *testing.T) {
+		var sum int
+		accumulate := func(x int) {
+			sum += x
+		}
+
+		double := func(x int) int {
+			return x * 2
+		}
+
+		accumulateDoubled := Local(double)(accumulate)
+
+		accumulateDoubled(1)
+		accumulateDoubled(2)
+		accumulateDoubled(3)
+
+		assert.Equal(t, 12, sum) // (1*2) + (2*2) + (3*2) = 2 + 4 + 6 = 12
+	})
+
+	t.Run("transformation with error handling", func(t *testing.T) {
+		type Result struct {
+			Value int
+			Error error
+		}
+
+		var captured int
+		consumeInt := func(x int) {
+			captured = x
+		}
+
+		extractValue := func(r Result) int {
+			if r.Error != nil {
+				return -1
+			}
+			return r.Value
+		}
+
+		consumeResult := Local(extractValue)(consumeInt)
+
+		consumeResult(Result{Value: 42, Error: nil})
+		assert.Equal(t, 42, captured)
+
+		consumeResult(Result{Value: 100, Error: assert.AnError})
+		assert.Equal(t, -1, captured)
+	})
+
+	t.Run("transformation preserves consumer behavior", func(t *testing.T) {
+		callCount := 0
+		consumer := func(x int) {
+			callCount++
+		}
+
+		transform := func(s string) int {
+			n, _ := strconv.Atoi(s)
+			return n
+		}
+
+		transformedConsumer := Local(transform)(consumer)
+
+		transformedConsumer("1")
+		transformedConsumer("2")
+		transformedConsumer("3")
+
+		assert.Equal(t, 3, callCount)
+	})
+
+	t.Run("comparison with reader.Local behavior", func(t *testing.T) {
+		// This test demonstrates the dual nature of Consumer and Reader
+		// Consumer: transforms input before consumption (contravariant)
+		// Reader: transforms environment before reading (also contravariant on input)
+
+		type DetailedEnv struct {
+			Value int
+			Extra string
+		}
+
+		type SimpleEnv struct {
+			Value int
+		}
+
+		var captured int
+		consumeSimple := func(e SimpleEnv) {
+			captured = e.Value
+		}
+
+		simplify := func(d DetailedEnv) SimpleEnv {
+			return SimpleEnv{Value: d.Value}
+		}
+
+		consumeDetailed := Local(simplify)(consumeSimple)
+		consumeDetailed(DetailedEnv{Value: 42, Extra: "ignored"})
+
+		assert.Equal(t, 42, captured)
+	})
+}

v2/consumer/types.go

@@ -0,0 +1,58 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package consumer provides types and utilities for functions that consume values without returning results.
+//
+// A Consumer represents a side-effecting operation that accepts a value but produces no output.
+// This is useful for operations like logging, printing, updating state, or any action where
+// the return value is not needed.
+package consumer
+
+type (
+	// Consumer represents a function that accepts a value of type A and performs a side effect.
+	// It does not return any value, making it useful for operations where only the side effect matters,
+	// such as logging, printing, or updating external state.
+	//
+	// This is a fundamental concept in functional programming for handling side effects in a
+	// controlled manner. Consumers can be composed, chained, or used in higher-order functions
+	// to build complex side-effecting behaviors.
+	//
+	// Type Parameters:
+	//   - A: The type of value consumed by the function
+	//
+	// Example:
+	//
+	//	// A simple consumer that prints values
+	//	var printInt Consumer[int] = func(x int) {
+	//	    fmt.Println(x)
+	//	}
+	//	printInt(42) // Prints: 42
+	//
+	//	// A consumer that logs messages
+	//	var logger Consumer[string] = func(msg string) {
+	//	    log.Println(msg)
+	//	}
+	//	logger("Hello, World!") // Logs: Hello, World!
+	//
+	//	// Consumers can be used in functional pipelines
+	//	var saveToDatabase Consumer[User] = func(user User) {
+	//	    db.Save(user)
+	//	}
+	Consumer[A any] = func(A)
+
+	// Operator represents a function that transforms a Consumer[A] into a Consumer[B].
+	// This is useful for composing and adapting consumers to work with different types.
+	Operator[A, B any] = func(Consumer[A]) Consumer[B]
+)

v2/context/ioresult/ioeither.go

@@ -0,0 +1,59 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ioresult
+
+import (
+	"context"
+
+	"github.com/IBM/fp-go/v2/result"
+)
+
+// WithContext wraps an IOResult and performs a context check for cancellation before executing.
+// This ensures that if the context is already cancelled, the computation short-circuits immediately
+// without executing the wrapped computation.
+//
+// This is useful for adding cancellation awareness to computations that might not check the context themselves.
+//
+// Type Parameters:
+//   - A: The type of the success value
+//
+// Parameters:
+//   - ctx: The context to check for cancellation
+//   - ma: The IOResult to wrap with context checking
+//
+// Returns:
+//   - An IOResult that checks for cancellation before executing
+//
+// Example:
+//
+//	computation := func() Result[string] {
+//	    // Long-running operation
+//	    return result.Of("done")
+//	}
+//
+//	ctx, cancel := context.WithCancel(context.Background())
+//	cancel() // Cancel immediately
+//
+//	wrapped := WithContext(ctx, computation)
+//	result := wrapped() // Returns Left with context.Canceled error
+func WithContext[A any](ctx context.Context, ma IOResult[A]) IOResult[A] {
+	return func() Result[A] {
+		if ctx.Err() != nil {
+			return result.Left[A](context.Cause(ctx))
+		}
+		return ma()
+	}
+}

v2/context/ioresult/types.go

@@ -0,0 +1,16 @@
+package ioresult
+
+import (
+	"github.com/IBM/fp-go/v2/ioresult"
+	"github.com/IBM/fp-go/v2/result"
+)
+
+type (
+	// IOResult represents a synchronous computation that may fail with an error.
+	// It's an alias for ioresult.IOResult[T].
+	IOResult[T any] = ioresult.IOResult[T]
+
+	// Result represents a computation that may fail with an error.
+	// It's an alias for result.Result[T].
+	Result[T any] = result.Result[T]
+)

v2/context/readereither/bind.go

@@ -1,68 +0,0 @@
-// Copyright (c) 2023 - 2025 IBM Corp.
-// All rights reserved.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-// http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-package readereither
-
-import (
-	"context"
-
-	G "github.com/IBM/fp-go/v2/readereither/generic"
-)
-
-// Bind creates an empty context of type [S] to be used with the [Bind] operation
-func Do[S any](
-	empty S,
-) ReaderEither[S] {
-	return G.Do[ReaderEither[S], context.Context, error, S](empty)
-}
-
-// Bind attaches the result of a computation to a context [S1] to produce a context [S2]
-func Bind[S1, S2, T any](
-	setter func(T) func(S1) S2,
-	f func(S1) ReaderEither[T],
-) func(ReaderEither[S1]) ReaderEither[S2] {
-	return G.Bind[ReaderEither[S1], ReaderEither[S2], ReaderEither[T], context.Context, error, S1, S2, T](setter, f)
-}
-
-// Let attaches the result of a computation to a context [S1] to produce a context [S2]
-func Let[S1, S2, T any](
-	setter func(T) func(S1) S2,
-	f func(S1) T,
-) func(ReaderEither[S1]) ReaderEither[S2] {
-	return G.Let[ReaderEither[S1], ReaderEither[S2], context.Context, error, S1, S2, T](setter, f)
-}
-
-// LetTo attaches the a value to a context [S1] to produce a context [S2]
-func LetTo[S1, S2, T any](
-	setter func(T) func(S1) S2,
-	b T,
-) func(ReaderEither[S1]) ReaderEither[S2] {
-	return G.LetTo[ReaderEither[S1], ReaderEither[S2], context.Context, error, S1, S2, T](setter, b)
-}
-
-// BindTo initializes a new state [S1] from a value [T]
-func BindTo[S1, T any](
-	setter func(T) S1,
-) func(ReaderEither[T]) ReaderEither[S1] {
-	return G.BindTo[ReaderEither[S1], ReaderEither[T], context.Context, error, S1, T](setter)
-}
-
-// ApS attaches a value to a context [S1] to produce a context [S2] by considering the context and the value concurrently
-func ApS[S1, S2, T any](
-	setter func(T) func(S1) S2,
-	fa ReaderEither[T],
-) func(ReaderEither[S1]) ReaderEither[S2] {
-	return G.ApS[ReaderEither[S1], ReaderEither[S2], ReaderEither[T], context.Context, error, S1, S2, T](setter, fa)
-}

v2/context/readereither/reader.go

@@ -1,94 +0,0 @@
-// Copyright (c) 2023 - 2025 IBM Corp.
-// All rights reserved.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-// http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-package readereither
-
-import (
-	"context"
-
-	"github.com/IBM/fp-go/v2/readereither"
-)
-
-func FromEither[A any](e Either[A]) ReaderEither[A] {
-	return readereither.FromEither[context.Context](e)
-}
-
-func Left[A any](l error) ReaderEither[A] {
-	return readereither.Left[context.Context, A](l)
-}
-
-func Right[A any](r A) ReaderEither[A] {
-	return readereither.Right[context.Context, error](r)
-}
-
-func MonadMap[A, B any](fa ReaderEither[A], f func(A) B) ReaderEither[B] {
-	return readereither.MonadMap(fa, f)
-}
-
-func Map[A, B any](f func(A) B) Operator[A, B] {
-	return readereither.Map[context.Context, error](f)
-}
-
-func MonadChain[A, B any](ma ReaderEither[A], f func(A) ReaderEither[B]) ReaderEither[B] {
-	return readereither.MonadChain(ma, f)
-}
-
-func Chain[A, B any](f func(A) ReaderEither[B]) Operator[A, B] {
-	return readereither.Chain(f)
-}
-
-func Of[A any](a A) ReaderEither[A] {
-	return readereither.Of[context.Context, error](a)
-}
-
-func MonadAp[A, B any](fab ReaderEither[func(A) B], fa ReaderEither[A]) ReaderEither[B] {
-	return readereither.MonadAp(fab, fa)
-}
-
-func Ap[A, B any](fa ReaderEither[A]) func(ReaderEither[func(A) B]) ReaderEither[B] {
-	return readereither.Ap[B](fa)
-}
-
-func FromPredicate[A any](pred func(A) bool, onFalse func(A) error) func(A) ReaderEither[A] {
-	return readereither.FromPredicate[context.Context](pred, onFalse)
-}
-
-func OrElse[A any](onLeft func(error) ReaderEither[A]) func(ReaderEither[A]) ReaderEither[A] {
-	return readereither.OrElse(onLeft)
-}
-
-func Ask() ReaderEither[context.Context] {
-	return readereither.Ask[context.Context, error]()
-}
-
-func MonadChainEitherK[A, B any](ma ReaderEither[A], f func(A) Either[B]) ReaderEither[B] {
-	return readereither.MonadChainEitherK(ma, f)
-}
-
-func ChainEitherK[A, B any](f func(A) Either[B]) func(ma ReaderEither[A]) ReaderEither[B] {
-	return readereither.ChainEitherK[context.Context](f)
-}
-
-func ChainOptionK[A, B any](onNone func() error) func(func(A) Option[B]) Operator[A, B] {
-	return readereither.ChainOptionK[context.Context, A, B](onNone)
-}
-
-func MonadFlap[B, A any](fab ReaderEither[func(A) B], a A) ReaderEither[B] {
-	return readereither.MonadFlap(fab, a)
-}
-
-func Flap[B, A any](a A) Operator[func(A) B, B] {
-	return readereither.Flap[context.Context, error, B](a)
-}

v2/context/readerio/bracket.go

@@ -0,0 +1,75 @@
+package readerio
+
+import (
+	RIO "github.com/IBM/fp-go/v2/readerio"
+)
+
+// Bracket ensures that a resource is properly acquired, used, and released, even if an error occurs.
+// This implements the bracket pattern for safe resource management with [ReaderIO].
+//
+// The bracket pattern guarantees that:
+//   - The acquire action is executed first to obtain the resource
+//   - The use function is called with the acquired resource
+//   - The release function is always called with the resource and result, regardless of success or failure
+//   - The final result from the use function is returned
+//
+// This is particularly useful for managing resources like file handles, database connections,
+// or locks that must be cleaned up properly.
+//
+// Type Parameters:
+//   - A: The type of the acquired resource
+//   - B: The type of the result produced by the use function
+//   - ANY: The type returned by the release function (typically ignored)
+//
+// Parameters:
+//   - acquire: A ReaderIO that acquires the resource
+//   - use: A Kleisli arrow that uses the resource and produces a result
+//   - release: A function that releases the resource, receiving both the resource and the result
+//
+// Returns:
+//   - A ReaderIO[B] that safely manages the resource lifecycle
+//
+// Example:
+//
+//	// Acquire a file handle
+//	acquireFile := func(ctx context.Context) IO[*os.File] {
+//	    return func() *os.File {
+//	        f, _ := os.Open("data.txt")
+//	        return f
+//	    }
+//	}
+//
+//	// Use the file
+//	readFile := func(f *os.File) ReaderIO[string] {
+//	    return func(ctx context.Context) IO[string] {
+//	        return func() string {
+//	            data, _ := io.ReadAll(f)
+//	            return string(data)
+//	        }
+//	    }
+//	}
+//
+//	// Release the file
+//	closeFile := func(f *os.File, result string) ReaderIO[any] {
+//	    return func(ctx context.Context) IO[any] {
+//	        return func() any {
+//	            f.Close()
+//	            return nil
+//	        }
+//	    }
+//	}
+//
+//	// Safely read file with automatic cleanup
+//	safeRead := Bracket(acquireFile, readFile, closeFile)
+//	result := safeRead(context.Background())()
+//
+//go:inline
+func Bracket[
+	A, B, ANY any](
+
+	acquire ReaderIO[A],
+	use Kleisli[A, B],
+	release func(A, B) ReaderIO[ANY],
+) ReaderIO[B] {
+	return RIO.Bracket(acquire, use, release)
+}

v2/context/readerio/consumer.go

@@ -0,0 +1,65 @@
+package readerio
+
+import "github.com/IBM/fp-go/v2/io"
+
+// ChainConsumer chains a consumer function into a ReaderIO computation, discarding the original value.
+// This is useful for performing side effects (like logging or metrics) that consume a value
+// but don't produce a meaningful result.
+//
+// The consumer is executed for its side effects, and the computation returns an empty struct.
+//
+// Type Parameters:
+//   - A: The type of value to consume
+//
+// Parameters:
+//   - c: A consumer function that performs side effects on the value
+//
+// Returns:
+//   - An Operator that chains the consumer and returns struct{}
+//
+// Example:
+//
+//	logUser := func(u User) {
+//	    log.Printf("Processing user: %s", u.Name)
+//	}
+//
+//	pipeline := F.Pipe2(
+//	    fetchUser(123),
+//	    ChainConsumer(logUser),
+//	)
+//
+//go:inline
+func ChainConsumer[A any](c Consumer[A]) Operator[A, Void] {
+	return ChainIOK(io.FromConsumer(c))
+}
+
+// ChainFirstConsumer chains a consumer function into a ReaderIO computation, preserving the original value.
+// This is useful for performing side effects (like logging or metrics) while passing the value through unchanged.
+//
+// The consumer is executed for its side effects, but the original value is returned.
+//
+// Type Parameters:
+//   - A: The type of value to consume and return
+//
+// Parameters:
+//   - c: A consumer function that performs side effects on the value
+//
+// Returns:
+//   - An Operator that chains the consumer and returns the original value
+//
+// Example:
+//
+//	logUser := func(u User) {
+//	    log.Printf("User: %s", u.Name)
+//	}
+//
+//	pipeline := F.Pipe3(
+//	    fetchUser(123),
+//	    ChainFirstConsumer(logUser),  // Logs but passes user through
+//	    Map(func(u User) string { return u.Email }),
+//	)
+//
+//go:inline
+func ChainFirstConsumer[A any](c Consumer[A]) Operator[A, A] {
+	return ChainFirstIOK(io.FromConsumer(c))
+}

v2/context/readerio/flip.go

@@ -0,0 +1,117 @@
+package readerio
+
+import (
+	"context"
+
+	"github.com/IBM/fp-go/v2/reader"
+	RIO "github.com/IBM/fp-go/v2/readerio"
+)
+
+// SequenceReader transforms a ReaderIO containing a Reader into a Reader containing a ReaderIO.
+// This "flips" the nested structure, allowing you to provide the Reader's environment first,
+// then get a ReaderIO that can be executed with a context.
+//
+// Type transformation:
+//
+//	From: ReaderIO[Reader[R, A]]
+//	      = func(context.Context) func() func(R) A
+//
+//	To:   Reader[R, ReaderIO[A]]
+//	      = func(R) func(context.Context) func() A
+//
+// This is useful for point-free style programming where you want to partially apply
+// the Reader's environment before dealing with the context.
+//
+// Type Parameters:
+//   - R: The environment type that the Reader depends on
+//   - A: The value type
+//
+// Parameters:
+//   - ma: A ReaderIO containing a Reader
+//
+// Returns:
+//   - A Reader that produces a ReaderIO when given an environment
+//
+// Example:
+//
+//	type Config struct {
+//	    Timeout int
+//	}
+//
+//	// A computation that produces a Reader
+//	getMultiplier := func(ctx context.Context) IO[func(Config) int] {
+//	    return func() func(Config) int {
+//	        return func(cfg Config) int {
+//	            return cfg.Timeout * 2
+//	        }
+//	    }
+//	}
+//
+//	// Sequence it to apply Config first
+//	sequenced := SequenceReader[Config, int](getMultiplier)
+//	cfg := Config{Timeout: 30}
+//	result := sequenced(cfg)(context.Background())() // Returns 60
+//
+//go:inline
+func SequenceReader[R, A any](ma ReaderIO[Reader[R, A]]) Reader[R, ReaderIO[A]] {
+	return RIO.SequenceReader(ma)
+}
+
+// TraverseReader applies a Reader-based transformation to a ReaderIO, introducing a new environment dependency.
+//
+// This function takes a Reader-based Kleisli arrow and returns a function that can transform
+// a ReaderIO. The result allows you to provide the Reader's environment (R) first, which then
+// produces a ReaderIO that depends on the context.
+//
+// Type transformation:
+//
+//	From: ReaderIO[A]
+//	      = func(context.Context) func() A
+//
+//	With: reader.Kleisli[R, A, B]
+//	      = func(A) func(R) B
+//
+//	To:   func(ReaderIO[A]) func(R) ReaderIO[B]
+//	      = func(ReaderIO[A]) func(R) func(context.Context) func() B
+//
+// This enables transforming values within a ReaderIO using environment-dependent logic.
+//
+// Type Parameters:
+//   - R: The environment type that the Reader depends on
+//   - A: The input value type
+//   - B: The output value type
+//
+// Parameters:
+//   - f: A Reader-based Kleisli arrow that transforms A to B using environment R
+//
+// Returns:
+//   - A function that takes a ReaderIO[A] and returns a function from R to ReaderIO[B]
+//
+// Example:
+//
+//	type Config struct {
+//	    Multiplier int
+//	}
+//
+//	// A Reader-based transformation
+//	multiply := func(x int) func(Config) int {
+//	    return func(cfg Config) int {
+//	        return x * cfg.Multiplier
+//	    }
+//	}
+//
+//	// Apply TraverseReader
+//	traversed := TraverseReader[Config, int, int](multiply)
+//	computation := Of(10)
+//	result := traversed(computation)
+//
+//	// Provide Config to get final result
+//	cfg := Config{Multiplier: 5}
+//	finalResult := result(cfg)(context.Background())() // Returns 50
+//
+//go:inline
+func TraverseReader[R, A, B any](
+	f reader.Kleisli[R, A, B],
+) func(ReaderIO[A]) Kleisli[R, B] {
+	return RIO.TraverseReader[context.Context](f)
+}

v2/context/readerio/logging.go

@@ -0,0 +1,91 @@
+package readerio
+
+import (
+	"context"
+	"log/slog"
+
+	"github.com/IBM/fp-go/v2/logging"
+)
+
+// SLogWithCallback creates a Kleisli arrow that logs a value with a custom logger and log level.
+// The value is logged and then passed through unchanged, making this useful for debugging
+// and monitoring values as they flow through a ReaderIO computation.
+//
+// Type Parameters:
+//   - A: The type of value to log and pass through
+//
+// Parameters:
+//   - logLevel: The slog.Level to use for logging (e.g., slog.LevelInfo, slog.LevelDebug)
+//   - cb: Callback function to retrieve the *slog.Logger from the context
+//   - message: A descriptive message to include in the log entry
+//
+// Returns:
+//   - A Kleisli arrow that logs the value and returns it unchanged
+//
+// Example:
+//
+//	getMyLogger := func(ctx context.Context) *slog.Logger {
+//	    if logger := ctx.Value("logger"); logger != nil {
+//	        return logger.(*slog.Logger)
+//	    }
+//	    return slog.Default()
+//	}
+//
+//	debugLog := SLogWithCallback[User](
+//	    slog.LevelDebug,
+//	    getMyLogger,
+//	    "Processing user",
+//	)
+//
+//	pipeline := F.Pipe2(
+//	    fetchUser(123),
+//	    Chain(debugLog),
+//	)
+func SLogWithCallback[A any](
+	logLevel slog.Level,
+	cb func(context.Context) *slog.Logger,
+	message string) Kleisli[A, A] {
+	return func(a A) ReaderIO[A] {
+		return func(ctx context.Context) IO[A] {
+			// logger
+			logger := cb(ctx)
+			return func() A {
+				logger.LogAttrs(ctx, logLevel, message, slog.Any("value", a))
+				return a
+			}
+		}
+	}
+}
+
+// SLog creates a Kleisli arrow that logs a value at Info level and passes it through unchanged.
+// This is a convenience wrapper around SLogWithCallback with standard settings.
+//
+// The value is logged with the provided message and then returned unchanged, making this
+// useful for debugging and monitoring values in a ReaderIO computation pipeline.
+//
+// Type Parameters:
+//   - A: The type of value to log and pass through
+//
+// Parameters:
+//   - message: A descriptive message to include in the log entry
+//
+// Returns:
+//   - A Kleisli arrow that logs the value at Info level and returns it unchanged
+//
+// Example:
+//
+//	pipeline := F.Pipe3(
+//	    fetchUser(123),
+//	    Chain(SLog[User]("Fetched user")),
+//	    Map(func(u User) string { return u.Name }),
+//	    Chain(SLog[string]("Extracted name")),
+//	)
+//
+//	result := pipeline(context.Background())()
+//	// Logs: "Fetched user" value={ID:123 Name:"Alice"}
+//	// Logs: "Extracted name" value="Alice"
+//
+//go:inline
+func SLog[A any](message string) Kleisli[A, A] {
+	return SLogWithCallback[A](slog.LevelInfo, logging.GetLoggerFromContext, message)
+}

v2/context/readerio/reader.go

@@ -0,0 +1,769 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package readerio
+
+import (
+	"context"
+	"time"
+
+	"github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/reader"
+	RIO "github.com/IBM/fp-go/v2/readerio"
+)
+
+const (
+	// useParallel is the feature flag to control if we use the parallel or the sequential implementation of ap
+	useParallel = true
+)
+
+// MonadMap transforms the success value of a [ReaderIO] using the provided function.
+// If the computation fails, the error is propagated unchanged.
+//
+// Parameters:
+//   - fa: The ReaderIO to transform
+//   - f: The transformation function
+//
+// Returns a new ReaderIO with the transformed value.
+//
+//go:inline
+func MonadMap[A, B any](fa ReaderIO[A], f func(A) B) ReaderIO[B] {
+	return RIO.MonadMap(fa, f)
+}
+
+// Map transforms the success value of a [ReaderIO] using the provided function.
+// This is the curried version of [MonadMap], useful for composition.
+//
+// Parameters:
+//   - f: The transformation function
+//
+// Returns a function that transforms a ReaderIO.
+//
+//go:inline
+func Map[A, B any](f func(A) B) Operator[A, B] {
+	return RIO.Map[context.Context](f)
+}
+
+// MonadMapTo replaces the success value of a [ReaderIO] with a constant value.
+// If the computation fails, the error is propagated unchanged.
+//
+// Parameters:
+//   - fa: The ReaderIO to transform
+//   - b: The constant value to use
+//
+// Returns a new ReaderIO with the constant value.
+//
+//go:inline
+func MonadMapTo[A, B any](fa ReaderIO[A], b B) ReaderIO[B] {
+	return RIO.MonadMapTo(fa, b)
+}
+
+// MapTo replaces the success value of a [ReaderIO] with a constant value.
+// This is the curried version of [MonadMapTo].
+//
+// Parameters:
+//   - b: The constant value to use
+//
+// Returns a function that transforms a ReaderIO.
+//
+//go:inline
+func MapTo[A, B any](b B) Operator[A, B] {
+	return RIO.MapTo[context.Context, A](b)
+}
+
+// MonadChain sequences two [ReaderIO] computations, where the second depends on the result of the first.
+// If the first computation fails, the second is not executed.
+//
+// Parameters:
+//   - ma: The first ReaderIO
+//   - f: Function that produces the second ReaderIO based on the first's result
+//
+// Returns a new ReaderIO representing the sequenced computation.
+//
+//go:inline
+func MonadChain[A, B any](ma ReaderIO[A], f Kleisli[A, B]) ReaderIO[B] {
+	return RIO.MonadChain(ma, f)
+}
+
+// Chain sequences two [ReaderIO] computations, where the second depends on the result of the first.
+// This is the curried version of [MonadChain], useful for composition.
+//
+// Parameters:
+//   - f: Function that produces the second ReaderIO based on the first's result
+//
+// Returns a function that sequences ReaderIO computations.
+//
+//go:inline
+func Chain[A, B any](f Kleisli[A, B]) Operator[A, B] {
+	return RIO.Chain(f)
+}
+
+// MonadChainFirst sequences two [ReaderIO] computations but returns the result of the first.
+// The second computation is executed for its side effects only.
+//
+// Parameters:
+//   - ma: The first ReaderIO
+//   - f: Function that produces the second ReaderIO
+//
+// Returns a ReaderIO with the result of the first computation.
+//
+//go:inline
+func MonadChainFirst[A, B any](ma ReaderIO[A], f Kleisli[A, B]) ReaderIO[A] {
+	return RIO.MonadChainFirst(ma, f)
+}
+
+// MonadTap executes a side-effect computation but returns the original value.
+// This is an alias for [MonadChainFirst] and is useful for operations like logging
+// or validation that should not affect the main computation flow.
+//
+// Parameters:
+//   - ma: The ReaderIO to tap
+//   - f: Function that produces a side-effect ReaderIO
+//
+// Returns a ReaderIO with the original value after executing the side effect.
+//
+//go:inline
+func MonadTap[A, B any](ma ReaderIO[A], f Kleisli[A, B]) ReaderIO[A] {
+	return RIO.MonadTap(ma, f)
+}
+
+// ChainFirst sequences two [ReaderIO] computations but returns the result of the first.
+// This is the curried version of [MonadChainFirst].
+//
+// Parameters:
+//   - f: Function that produces the second ReaderIO
+//
+// Returns a function that sequences ReaderIO computations.
+//
+//go:inline
+func ChainFirst[A, B any](f Kleisli[A, B]) Operator[A, A] {
+	return RIO.ChainFirst(f)
+}
+
+// Tap executes a side-effect computation but returns the original value.
+// This is the curried version of [MonadTap], an alias for [ChainFirst].
+//
+// Parameters:
+//   - f: Function that produces a side-effect ReaderIO
+//
+// Returns a function that taps ReaderIO computations.
+//
+//go:inline
+func Tap[A, B any](f Kleisli[A, B]) Operator[A, A] {
+	return RIO.Tap(f)
+}
+
+// Of creates a [ReaderIO] that always succeeds with the given value.
+// This is the same as [Right] and represents the monadic return operation.
+//
+// Parameters:
+//   - a: The value to wrap
+//
+// Returns a ReaderIO that always succeeds with the given value.
+//
+//go:inline
+func Of[A any](a A) ReaderIO[A] {
+	return RIO.Of[context.Context](a)
+}
+
+// MonadApPar implements parallel applicative application for [ReaderIO].
+// It executes the function and value computations in parallel where possible,
+// potentially improving performance for independent operations.
+//
+// Parameters:
+//   - fab: ReaderIO containing a function
+//   - fa: ReaderIO containing a value
+//
+// Returns a ReaderIO with the function applied to the value.
+//
+//go:inline
+func MonadApPar[B, A any](fab ReaderIO[func(A) B], fa ReaderIO[A]) ReaderIO[B] {
+	return RIO.MonadApPar(fab, fa)
+}
+
+// MonadAp implements applicative application for [ReaderIO].
+// By default, it uses parallel execution ([MonadApPar]) but can be configured to use
+// sequential execution ([MonadApSeq]) via the useParallel constant.
+//
+// Parameters:
+//   - fab: ReaderIO containing a function
+//   - fa: ReaderIO containing a value
+//
+// Returns a ReaderIO with the function applied to the value.
+//
+//go:inline
+func MonadAp[B, A any](fab ReaderIO[func(A) B], fa ReaderIO[A]) ReaderIO[B] {
+	// dispatch to the configured version
+	if useParallel {
+		return MonadApPar(fab, fa)
+	}
+	return MonadApSeq(fab, fa)
+}
+
+// MonadApSeq implements sequential applicative application for [ReaderIO].
+// It executes the function computation first, then the value computation.
+//
+// Parameters:
+//   - fab: ReaderIO containing a function
+//   - fa: ReaderIO containing a value
+//
+// Returns a ReaderIO with the function applied to the value.
+//
+//go:inline
+func MonadApSeq[B, A any](fab ReaderIO[func(A) B], fa ReaderIO[A]) ReaderIO[B] {
+	return RIO.MonadApSeq(fab, fa)
+}
+
+// Ap applies a function wrapped in a [ReaderIO] to a value wrapped in a ReaderIO.
+// This is the curried version of [MonadAp], using the default execution mode.
+//
+// Parameters:
+//   - fa: ReaderIO containing a value
+//
+// Returns a function that applies a ReaderIO function to the value.
+//
+//go:inline
+func Ap[B, A any](fa ReaderIO[A]) Operator[func(A) B, B] {
+	return RIO.Ap[B](fa)
+}
+
+// ApSeq applies a function wrapped in a [ReaderIO] to a value sequentially.
+// This is the curried version of [MonadApSeq].
+//
+// Parameters:
+//   - fa: ReaderIO containing a value
+//
+// Returns a function that applies a ReaderIO function to the value sequentially.
+//
+//go:inline
+func ApSeq[B, A any](fa ReaderIO[A]) Operator[func(A) B, B] {
+	return function.Bind2nd(MonadApSeq[B, A], fa)
+}
+
+// ApPar applies a function wrapped in a [ReaderIO] to a value in parallel.
+// This is the curried version of [MonadApPar].
+//
+// Parameters:
+//   - fa: ReaderIO containing a value
+//
+// Returns a function that applies a ReaderIO function to the value in parallel.
+//
+//go:inline
+func ApPar[B, A any](fa ReaderIO[A]) Operator[func(A) B, B] {
+	return function.Bind2nd(MonadApPar[B, A], fa)
+}
+
+// Ask returns a [ReaderIO] that provides access to the context.
+// This is useful for accessing the [context.Context] within a computation.
+//
+// Returns a ReaderIO that produces the context.
+//
+//go:inline
+func Ask() ReaderIO[context.Context] {
+	return RIO.Ask[context.Context]()
+}
+
+// FromIO converts an [IO] into a [ReaderIO].
+// The IO computation always succeeds, so it's wrapped in Right.
+//
+// Parameters:
+//   - t: The IO to convert
+//
+// Returns a ReaderIO that executes the IO and wraps the result in Right.
+//
+//go:inline
+func FromIO[A any](t IO[A]) ReaderIO[A] {
+	return RIO.FromIO[context.Context](t)
+}
+
+// FromReader converts a [Reader] into a [ReaderIO].
+// The Reader computation is lifted into the IO context, allowing it to be
+// composed with other ReaderIO operations.
+//
+// Parameters:
+//   - t: The Reader to convert
+//
+// Returns a ReaderIO that executes the Reader and wraps the result in IO.
+//
+//go:inline
+func FromReader[A any](t Reader[context.Context, A]) ReaderIO[A] {
+	return RIO.FromReader(t)
+}
+
+// FromLazy converts a [Lazy] computation into a [ReaderIO].
+// The Lazy computation always succeeds, so it's wrapped in Right.
+// This is an alias for [FromIO] since Lazy and IO have the same structure.
+//
+// Parameters:
+//   - t: The Lazy computation to convert
+//
+// Returns a ReaderIO that executes the Lazy computation and wraps the result in Right.
+//
+//go:inline
+func FromLazy[A any](t Lazy[A]) ReaderIO[A] {
+	return RIO.FromIO[context.Context](t)
+}
+
+// MonadChainIOK chains a function that returns an [IO] into a [ReaderIO] computation.
+// The IO computation always succeeds, so it's wrapped in Right.
+//
+// Parameters:
+//   - ma: The ReaderIO to chain from
+//   - f: Function that produces an IO
+//
+// Returns a new ReaderIO with the chained IO computation.
+//
+//go:inline
+func MonadChainIOK[A, B any](ma ReaderIO[A], f func(A) IO[B]) ReaderIO[B] {
+	return RIO.MonadChainIOK(ma, f)
+}
+
+// ChainIOK chains a function that returns an [IO] into a [ReaderIO] computation.
+// This is the curried version of [MonadChainIOK].
+//
+// Parameters:
+//   - f: Function that produces an IO
+//
+// Returns a function that chains the IO-returning function.
+//
+//go:inline
+func ChainIOK[A, B any](f func(A) IO[B]) Operator[A, B] {
+	return RIO.ChainIOK[context.Context](f)
+}
+
+// MonadChainFirstIOK chains a function that returns an [IO] but keeps the original value.
+// The IO computation is executed for its side effects only.
+//
+// Parameters:
+//   - ma: The ReaderIO to chain from
+//   - f: Function that produces an IO
+//
+// Returns a ReaderIO with the original value after executing the IO.
+//
+//go:inline
+func MonadChainFirstIOK[A, B any](ma ReaderIO[A], f func(A) IO[B]) ReaderIO[A] {
+	return RIO.MonadChainFirstIOK(ma, f)
+}
+
+// MonadTapIOK chains a function that returns an [IO] but keeps the original value.
+// This is an alias for [MonadChainFirstIOK] and is useful for side effects like logging.
+//
+// Parameters:
+//   - ma: The ReaderIO to tap
+//   - f: Function that produces an IO for side effects
+//
+// Returns a ReaderIO with the original value after executing the IO.
+//
+//go:inline
+func MonadTapIOK[A, B any](ma ReaderIO[A], f func(A) IO[B]) ReaderIO[A] {
+	return RIO.MonadTapIOK(ma, f)
+}
+
+// ChainFirstIOK chains a function that returns an [IO] but keeps the original value.
+// This is the curried version of [MonadChainFirstIOK].
+//
+// Parameters:
+//   - f: Function that produces an IO
+//
+// Returns a function that chains the IO-returning function.
+//
+//go:inline
+func ChainFirstIOK[A, B any](f func(A) IO[B]) Operator[A, A] {
+	return RIO.ChainFirstIOK[context.Context](f)
+}
+
+// TapIOK chains a function that returns an [IO] but keeps the original value.
+// This is the curried version of [MonadTapIOK], an alias for [ChainFirstIOK].
+//
+// Parameters:
+//   - f: Function that produces an IO for side effects
+//
+// Returns a function that taps with IO-returning functions.
+//
+//go:inline
+func TapIOK[A, B any](f func(A) IO[B]) Operator[A, A] {
+	return RIO.TapIOK[context.Context](f)
+}
+
+// Defer creates a [ReaderIO] by lazily generating a new computation each time it's executed.
+// This is useful for creating computations that should be re-evaluated on each execution.
+//
+// Parameters:
+//   - gen: Lazy generator function that produces a ReaderIO
+//
+// Returns a ReaderIO that generates a fresh computation on each execution.
+//
+//go:inline
+func Defer[A any](gen Lazy[ReaderIO[A]]) ReaderIO[A] {
+	return RIO.Defer(gen)
+}
+
+// Memoize computes the value of the provided [ReaderIO] monad lazily but exactly once.
+// The context used to compute the value is the context of the first call, so do not use this
+// method if the value has a functional dependency on the content of the context.
+//
+// Parameters:
+//   - rdr: The ReaderIO to memoize
+//
+// Returns a ReaderIO that caches its result after the first execution.
+//
+//go:inline
+func Memoize[A any](rdr ReaderIO[A]) ReaderIO[A] {
+	return RIO.Memoize(rdr)
+}
+
+// Flatten converts a nested [ReaderIO] into a flat [ReaderIO].
+// This is equivalent to [MonadChain] with the identity function.
+//
+// Parameters:
+//   - rdr: The nested ReaderIO to flatten
+//
+// Returns a flattened ReaderIO.
+//
+//go:inline
+func Flatten[A any](rdr ReaderIO[ReaderIO[A]]) ReaderIO[A] {
+	return RIO.Flatten(rdr)
+}
+
+// MonadFlap applies a value to a function wrapped in a [ReaderIO].
+// This is the reverse of [MonadAp], useful in certain composition scenarios.
+//
+// Parameters:
+//   - fab: ReaderIO containing a function
+//   - a: The value to apply to the function
+//
+// Returns a ReaderIO with the function applied to the value.
+//
+//go:inline
+func MonadFlap[B, A any](fab ReaderIO[func(A) B], a A) ReaderIO[B] {
+	return RIO.MonadFlap(fab, a)
+}
+
+// Flap applies a value to a function wrapped in a [ReaderIO].
+// This is the curried version of [MonadFlap].
+//
+// Parameters:
+//   - a: The value to apply to the function
+//
+// Returns a function that applies the value to a ReaderIO function.
+//
+//go:inline
+func Flap[B, A any](a A) Operator[func(A) B, B] {
+	return RIO.Flap[context.Context, B](a)
+}
+
+// MonadChainReaderK chains a [ReaderIO] with a function that returns a [Reader].
+// The Reader is lifted into the ReaderIO context, allowing composition of
+// Reader and ReaderIO operations.
+//
+// Parameters:
+//   - ma: The ReaderIO to chain from
+//   - f: Function that produces a Reader
+//
+// Returns a new ReaderIO with the chained Reader computation.
+//
+//go:inline
+func MonadChainReaderK[A, B any](ma ReaderIO[A], f reader.Kleisli[context.Context, A, B]) ReaderIO[B] {
+	return RIO.MonadChainReaderK(ma, f)
+}
+
+// ChainReaderK chains a [ReaderIO] with a function that returns a [Reader].
+// This is the curried version of [MonadChainReaderK].
+//
+// Parameters:
+//   - f: Function that produces a Reader
+//
+// Returns a function that chains Reader-returning functions.
+//
+//go:inline
+func ChainReaderK[A, B any](f reader.Kleisli[context.Context, A, B]) Operator[A, B] {
+	return RIO.ChainReaderK(f)
+}
+
+// MonadChainFirstReaderK chains a function that returns a [Reader] but keeps the original value.
+// The Reader computation is executed for its side effects only.
+//
+// Parameters:
+//   - ma: The ReaderIO to chain from
+//   - f: Function that produces a Reader
+//
+// Returns a ReaderIO with the original value after executing the Reader.
+//
+//go:inline
+func MonadChainFirstReaderK[A, B any](ma ReaderIO[A], f reader.Kleisli[context.Context, A, B]) ReaderIO[A] {
+	return RIO.MonadChainFirstReaderK(ma, f)
+}
+
+// MonadTapReaderK chains a function that returns a [Reader] but keeps the original value.
+// This is an alias for [MonadChainFirstReaderK] and is useful for side effects.
+//
+// Parameters:
+//   - ma: The ReaderIO to tap
+//   - f: Function that produces a Reader for side effects
+//
+// Returns a ReaderIO with the original value after executing the Reader.
+//
+//go:inline
+func MonadTapReaderK[A, B any](ma ReaderIO[A], f reader.Kleisli[context.Context, A, B]) ReaderIO[A] {
+	return RIO.MonadTapReaderK(ma, f)
+}
+
+// ChainFirstReaderK chains a function that returns a [Reader] but keeps the original value.
+// This is the curried version of [MonadChainFirstReaderK].
+//
+// Parameters:
+//   - f: Function that produces a Reader
+//
+// Returns a function that chains Reader-returning functions while preserving the original value.
+//
+//go:inline
+func ChainFirstReaderK[A, B any](f reader.Kleisli[context.Context, A, B]) Operator[A, A] {
+	return RIO.ChainFirstReaderK(f)
+}
+
+// TapReaderK chains a function that returns a [Reader] but keeps the original value.
+// This is the curried version of [MonadTapReaderK], an alias for [ChainFirstReaderK].
+//
+// Parameters:
+//   - f: Function that produces a Reader for side effects
+//
+// Returns a function that taps with Reader-returning functions.
+//
+//go:inline
+func TapReaderK[A, B any](f reader.Kleisli[context.Context, A, B]) Operator[A, A] {
+	return RIO.TapReaderK(f)
+}
+
+// Read executes a [ReaderIO] with a given context, returning the resulting [IO].
+// This is useful for providing the context dependency and obtaining an IO action
+// that can be executed later.
+//
+// Parameters:
+//   - r: The context to provide to the ReaderIO
+//
+// Returns a function that converts a ReaderIO into an IO by applying the context.
+//
+//go:inline
+func Read[A any](r context.Context) func(ReaderIO[A]) IO[A] {
+	return RIO.Read[A](r)
+}
+
+// Local transforms the context.Context environment before passing it to a ReaderIO computation.
+//
+// This is the Reader's local operation, which allows you to modify the environment
+// for a specific computation without affecting the outer context. The transformation
+// function receives the current context and returns a new context along with a
+// cancel function. The cancel function is automatically called when the computation
+// completes (via defer), ensuring proper cleanup of resources.
+//
+// This is useful for:
+//   - Adding timeouts or deadlines to specific operations
+//   - Adding context values for nested computations
+//   - Creating isolated context scopes
+//   - Implementing context-based dependency injection
+//
+// Type Parameters:
+//   - A: The value type of the ReaderIO
+//
+// Parameters:
+//   - f: A function that transforms the context and returns a cancel function
+//
+// Returns:
+//   - An Operator that runs the computation with the transformed context
+//
+// Example:
+//
+//	import F "github.com/IBM/fp-go/v2/function"
+//
+//	// Add a custom value to the context
+//	type key int
+//	const userKey key = 0
+//
+//	addUser := readerio.Local[string](func(ctx context.Context) (context.Context, context.CancelFunc) {
+//	    newCtx := context.WithValue(ctx, userKey, "Alice")
+//	    return newCtx, func() {} // No-op cancel
+//	})
+//
+//	getUser := readerio.FromReader(func(ctx context.Context) string {
+//	    if user := ctx.Value(userKey); user != nil {
+//	        return user.(string)
+//	    }
+//	    return "unknown"
+//	})
+//
+//	result := F.Pipe1(
+//	    getUser,
+//	    addUser,
+//	)
+//	user := result(context.Background())()  // Returns "Alice"
+//
+// Timeout Example:
+//
+//	// Add a 5-second timeout to a specific operation
+//	withTimeout := readerio.Local[Data](func(ctx context.Context) (context.Context, context.CancelFunc) {
+//	    return context.WithTimeout(ctx, 5*time.Second)
+//	})
+//
+//	result := F.Pipe1(
+//	    fetchData,
+//	    withTimeout,
+//	)
+func Local[A any](f func(context.Context) (context.Context, context.CancelFunc)) Operator[A, A] {
+	return func(rr ReaderIO[A]) ReaderIO[A] {
+		return func(ctx context.Context) IO[A] {
+			return func() A {
+				otherCtx, otherCancel := f(ctx)
+				defer otherCancel()
+				return rr(otherCtx)()
+			}
+		}
+	}
+}
+
+// WithTimeout adds a timeout to the context for a ReaderIO computation.
+//
+// This is a convenience wrapper around Local that uses context.WithTimeout.
+// The computation must complete within the specified duration, or it will be
+// cancelled. This is useful for ensuring operations don't run indefinitely
+// and for implementing timeout-based error handling.
+//
+// The timeout is relative to when the ReaderIO is executed, not when
+// WithTimeout is called. The cancel function is automatically called when
+// the computation completes, ensuring proper cleanup.
+//
+// Type Parameters:
+//   - A: The value type of the ReaderIO
+//
+// Parameters:
+//   - timeout: The maximum duration for the computation
+//
+// Returns:
+//   - An Operator that runs the computation with a timeout
+//
+// Example:
+//
+//	import (
+//	    "time"
+//	    F "github.com/IBM/fp-go/v2/function"
+//	)
+//
+//	// Fetch data with a 5-second timeout
+//	fetchData := readerio.FromReader(func(ctx context.Context) Data {
+//	    // Simulate slow operation
+//	    select {
+//	    case <-time.After(10 * time.Second):
+//	        return Data{Value: "slow"}
+//	    case <-ctx.Done():
+//	        return Data{}
+//	    }
+//	})
+//
+//	result := F.Pipe1(
+//	    fetchData,
+//	    readerio.WithTimeout[Data](5*time.Second),
+//	)
+//	data := result(context.Background())()  // Returns Data{} after 5s timeout
+//
+// Successful Example:
+//
+//	quickFetch := readerio.Of(Data{Value: "quick"})
+//	result := F.Pipe1(
+//	    quickFetch,
+//	    readerio.WithTimeout[Data](5*time.Second),
+//	)
+//	data := result(context.Background())()  // Returns Data{Value: "quick"}
+func WithTimeout[A any](timeout time.Duration) Operator[A, A] {
+	return Local[A](func(ctx context.Context) (context.Context, context.CancelFunc) {
+		return context.WithTimeout(ctx, timeout)
+	})
+}
+
+// WithDeadline adds an absolute deadline to the context for a ReaderIO computation.
+//
+// This is a convenience wrapper around Local that uses context.WithDeadline.
+// The computation must complete before the specified time, or it will be
+// cancelled. This is useful for coordinating operations that must finish
+// by a specific time, such as request deadlines or scheduled tasks.
+//
+// The deadline is an absolute time, unlike WithTimeout which uses a relative
+// duration. The cancel function is automatically called when the computation
+// completes, ensuring proper cleanup.
+//
+// Type Parameters:
+//   - A: The value type of the ReaderIO
+//
+// Parameters:
+//   - deadline: The absolute time by which the computation must complete
+//
+// Returns:
+//   - An Operator that runs the computation with a deadline
+//
+// Example:
+//
+//	import (
+//	    "time"
+//	    F "github.com/IBM/fp-go/v2/function"
+//	)
+//
+//	// Operation must complete by 3 PM
+//	deadline := time.Date(2024, 1, 1, 15, 0, 0, 0, time.UTC)
+//
+//	fetchData := readerio.FromReader(func(ctx context.Context) Data {
+//	    // Simulate operation
+//	    select {
+//	    case <-time.After(1 * time.Hour):
+//	        return Data{Value: "done"}
+//	    case <-ctx.Done():
+//	        return Data{}
+//	    }
+//	})
+//
+//	result := F.Pipe1(
+//	    fetchData,
+//	    readerio.WithDeadline[Data](deadline),
+//	)
+//	data := result(context.Background())()  // Returns Data{} if past deadline
+//
+// Combining with Parent Context:
+//
+//	// If parent context already has a deadline, the earlier one takes precedence
+//	parentCtx, cancel := context.WithDeadline(context.Background(), time.Now().Add(1*time.Hour))
+//	defer cancel()
+//
+//	laterDeadline := time.Now().Add(2 * time.Hour)
+//	result := F.Pipe1(
+//	    fetchData,
+//	    readerio.WithDeadline[Data](laterDeadline),
+//	)
+//	data := result(parentCtx)()  // Will use parent's 1-hour deadline
+func WithDeadline[A any](deadline time.Time) Operator[A, A] {
+	return Local[A](func(ctx context.Context) (context.Context, context.CancelFunc) {
+		return context.WithDeadline(ctx, deadline)
+	})
+}
+
+// Delay creates an operation that passes in the value after some delay
+//
+//go:inline
+func Delay[A any](delay time.Duration) Operator[A, A] {
+	return RIO.Delay[context.Context, A](delay)
+}
+
+// After creates an operation that passes after the given [time.Time]
+//
+//go:inline
+func After[R, E, A any](timestamp time.Time) Operator[A, A] {
+	return RIO.After[context.Context, A](timestamp)
+}

v2/context/readerio/reader_test.go

@@ -0,0 +1,502 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package readerio
+
+import (
+	"context"
+	"testing"
+
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/internal/utils"
+	G "github.com/IBM/fp-go/v2/io"
+	N "github.com/IBM/fp-go/v2/number"
+	"github.com/IBM/fp-go/v2/reader"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestMonadMap(t *testing.T) {
+	rio := Of(5)
+	doubled := MonadMap(rio, N.Mul(2))
+
+	result := doubled(context.Background())()
+	assert.Equal(t, 10, result)
+}
+
+func TestMap(t *testing.T) {
+	g := F.Pipe1(
+		Of(1),
+		Map(utils.Double),
+	)
+
+	assert.Equal(t, 2, g(context.Background())())
+}
+
+func TestMonadMapTo(t *testing.T) {
+	rio := Of(42)
+	replaced := MonadMapTo(rio, "constant")
+
+	result := replaced(context.Background())()
+	assert.Equal(t, "constant", result)
+}
+
+func TestMapTo(t *testing.T) {
+	result := F.Pipe1(
+		Of(42),
+		MapTo[int]("constant"),
+	)
+
+	assert.Equal(t, "constant", result(context.Background())())
+}
+
+func TestMonadChain(t *testing.T) {
+	rio1 := Of(5)
+	result := MonadChain(rio1, func(n int) ReaderIO[int] {
+		return Of(n * 3)
+	})
+
+	assert.Equal(t, 15, result(context.Background())())
+}
+
+func TestChain(t *testing.T) {
+	result := F.Pipe1(
+		Of(5),
+		Chain(func(n int) ReaderIO[int] {
+			return Of(n * 3)
+		}),
+	)
+
+	assert.Equal(t, 15, result(context.Background())())
+}
+
+func TestMonadChainFirst(t *testing.T) {
+	sideEffect := 0
+	rio := Of(42)
+	result := MonadChainFirst(rio, func(n int) ReaderIO[string] {
+		sideEffect = n
+		return Of("side effect")
+	})
+
+	value := result(context.Background())()
+	assert.Equal(t, 42, value)
+	assert.Equal(t, 42, sideEffect)
+}
+
+func TestChainFirst(t *testing.T) {
+	sideEffect := 0
+	result := F.Pipe1(
+		Of(42),
+		ChainFirst(func(n int) ReaderIO[string] {
+			sideEffect = n
+			return Of("side effect")
+		}),
+	)
+
+	value := result(context.Background())()
+	assert.Equal(t, 42, value)
+	assert.Equal(t, 42, sideEffect)
+}
+
+func TestMonadTap(t *testing.T) {
+	sideEffect := 0
+	rio := Of(42)
+	result := MonadTap(rio, func(n int) ReaderIO[func()] {
+		sideEffect = n
+		return Of(func() {})
+	})
+
+	value := result(context.Background())()
+	assert.Equal(t, 42, value)
+	assert.Equal(t, 42, sideEffect)
+}
+
+func TestTap(t *testing.T) {
+	sideEffect := 0
+	result := F.Pipe1(
+		Of(42),
+		Tap(func(n int) ReaderIO[func()] {
+			sideEffect = n
+			return Of(func() {})
+		}),
+	)
+
+	value := result(context.Background())()
+	assert.Equal(t, 42, value)
+	assert.Equal(t, 42, sideEffect)
+}
+
+func TestOf(t *testing.T) {
+	rio := Of(100)
+	result := rio(context.Background())()
+
+	assert.Equal(t, 100, result)
+}
+
+func TestMonadAp(t *testing.T) {
+	fabIO := Of(N.Mul(2))
+	faIO := Of(5)
+	result := MonadAp(fabIO, faIO)
+
+	assert.Equal(t, 10, result(context.Background())())
+}
+
+func TestAp(t *testing.T) {
+	g := F.Pipe1(
+		Of(utils.Double),
+		Ap[int](Of(1)),
+	)
+
+	assert.Equal(t, 2, g(context.Background())())
+}
+
+func TestMonadApSeq(t *testing.T) {
+	fabIO := Of(N.Add(10))
+	faIO := Of(5)
+	result := MonadApSeq(fabIO, faIO)
+
+	assert.Equal(t, 15, result(context.Background())())
+}
+
+func TestApSeq(t *testing.T) {
+	g := F.Pipe1(
+		Of(N.Add(10)),
+		ApSeq[int](Of(5)),
+	)
+
+	assert.Equal(t, 15, g(context.Background())())
+}
+
+func TestMonadApPar(t *testing.T) {
+	fabIO := Of(N.Add(10))
+	faIO := Of(5)
+	result := MonadApPar(fabIO, faIO)
+
+	assert.Equal(t, 15, result(context.Background())())
+}
+
+func TestApPar(t *testing.T) {
+	g := F.Pipe1(
+		Of(N.Add(10)),
+		ApPar[int](Of(5)),
+	)
+
+	assert.Equal(t, 15, g(context.Background())())
+}
+
+func TestAsk(t *testing.T) {
+	rio := Ask()
+	ctx := context.WithValue(context.Background(), "key", "value")
+	result := rio(ctx)()
+
+	assert.Equal(t, ctx, result)
+}
+
+func TestFromIO(t *testing.T) {
+	ioAction := G.Of(42)
+	rio := FromIO(ioAction)
+
+	result := rio(context.Background())()
+	assert.Equal(t, 42, result)
+}
+
+func TestFromReader(t *testing.T) {
+	rdr := func(ctx context.Context) int {
+		return 42
+	}
+
+	rio := FromReader(rdr)
+	result := rio(context.Background())()
+
+	assert.Equal(t, 42, result)
+}
+
+func TestFromLazy(t *testing.T) {
+	lazy := func() int { return 42 }
+	rio := FromLazy(lazy)
+
+	result := rio(context.Background())()
+	assert.Equal(t, 42, result)
+}
+
+func TestMonadChainIOK(t *testing.T) {
+	rio := Of(5)
+	result := MonadChainIOK(rio, func(n int) G.IO[int] {
+		return G.Of(n * 4)
+	})
+
+	assert.Equal(t, 20, result(context.Background())())
+}
+
+func TestChainIOK(t *testing.T) {
+	result := F.Pipe1(
+		Of(5),
+		ChainIOK(func(n int) G.IO[int] {
+			return G.Of(n * 4)
+		}),
+	)
+
+	assert.Equal(t, 20, result(context.Background())())
+}
+
+func TestMonadChainFirstIOK(t *testing.T) {
+	sideEffect := 0
+	rio := Of(42)
+	result := MonadChainFirstIOK(rio, func(n int) G.IO[string] {
+		sideEffect = n
+		return G.Of("side effect")
+	})
+
+	value := result(context.Background())()
+	assert.Equal(t, 42, value)
+	assert.Equal(t, 42, sideEffect)
+}
+
+func TestChainFirstIOK(t *testing.T) {
+	sideEffect := 0
+	result := F.Pipe1(
+		Of(42),
+		ChainFirstIOK(func(n int) G.IO[string] {
+			sideEffect = n
+			return G.Of("side effect")
+		}),
+	)
+
+	value := result(context.Background())()
+	assert.Equal(t, 42, value)
+	assert.Equal(t, 42, sideEffect)
+}
+
+func TestMonadTapIOK(t *testing.T) {
+	sideEffect := 0
+	rio := Of(42)
+	result := MonadTapIOK(rio, func(n int) G.IO[func()] {
+		sideEffect = n
+		return G.Of(func() {})
+	})
+
+	value := result(context.Background())()
+	assert.Equal(t, 42, value)
+	assert.Equal(t, 42, sideEffect)
+}
+
+func TestTapIOK(t *testing.T) {
+	sideEffect := 0
+	result := F.Pipe1(
+		Of(42),
+		TapIOK(func(n int) G.IO[func()] {
+			sideEffect = n
+			return G.Of(func() {})
+		}),
+	)
+
+	value := result(context.Background())()
+	assert.Equal(t, 42, value)
+	assert.Equal(t, 42, sideEffect)
+}
+
+func TestDefer(t *testing.T) {
+	counter := 0
+	rio := Defer(func() ReaderIO[int] {
+		counter++
+		return Of(counter)
+	})
+
+	result1 := rio(context.Background())()
+	result2 := rio(context.Background())()
+
+	assert.Equal(t, 1, result1)
+	assert.Equal(t, 2, result2)
+}
+
+func TestMemoize(t *testing.T) {
+	counter := 0
+	rio := Of(0)
+	memoized := Memoize(MonadMap(rio, func(int) int {
+		counter++
+		return counter
+	}))
+
+	result1 := memoized(context.Background())()
+	result2 := memoized(context.Background())()
+
+	assert.Equal(t, 1, result1)
+	assert.Equal(t, 1, result2) // Same value, memoized
+}
+
+func TestFlatten(t *testing.T) {
+	nested := Of(Of(42))
+	flattened := Flatten(nested)
+
+	result := flattened(context.Background())()
+	assert.Equal(t, 42, result)
+}
+
+func TestMonadFlap(t *testing.T) {
+	fabIO := Of(N.Mul(3))
+	result := MonadFlap(fabIO, 7)
+
+	assert.Equal(t, 21, result(context.Background())())
+}
+
+func TestFlap(t *testing.T) {
+	result := F.Pipe1(
+		Of(N.Mul(3)),
+		Flap[int](7),
+	)
+
+	assert.Equal(t, 21, result(context.Background())())
+}
+
+func TestMonadChainReaderK(t *testing.T) {
+	rio := Of(5)
+	result := MonadChainReaderK(rio, func(n int) reader.Reader[context.Context, int] {
+		return func(ctx context.Context) int { return n * 2 }
+	})
+
+	assert.Equal(t, 10, result(context.Background())())
+}
+
+func TestChainReaderK(t *testing.T) {
+	result := F.Pipe1(
+		Of(5),
+		ChainReaderK(func(n int) reader.Reader[context.Context, int] {
+			return func(ctx context.Context) int { return n * 2 }
+		}),
+	)
+
+	assert.Equal(t, 10, result(context.Background())())
+}
+
+func TestMonadChainFirstReaderK(t *testing.T) {
+	sideEffect := 0
+	rio := Of(42)
+	result := MonadChainFirstReaderK(rio, func(n int) reader.Reader[context.Context, string] {
+		return func(ctx context.Context) string {
+			sideEffect = n
+			return "side effect"
+		}
+	})
+
+	value := result(context.Background())()
+	assert.Equal(t, 42, value)
+	assert.Equal(t, 42, sideEffect)
+}
+
+func TestChainFirstReaderK(t *testing.T) {
+	sideEffect := 0
+	result := F.Pipe1(
+		Of(42),
+		ChainFirstReaderK(func(n int) reader.Reader[context.Context, string] {
+			return func(ctx context.Context) string {
+				sideEffect = n
+				return "side effect"
+			}
+		}),
+	)
+
+	value := result(context.Background())()
+	assert.Equal(t, 42, value)
+	assert.Equal(t, 42, sideEffect)
+}
+
+func TestMonadTapReaderK(t *testing.T) {
+	sideEffect := 0
+	rio := Of(42)
+	result := MonadTapReaderK(rio, func(n int) reader.Reader[context.Context, func()] {
+		return func(ctx context.Context) func() {
+			sideEffect = n
+			return func() {}
+		}
+	})
+
+	value := result(context.Background())()
+	assert.Equal(t, 42, value)
+	assert.Equal(t, 42, sideEffect)
+}
+
+func TestTapReaderK(t *testing.T) {
+	sideEffect := 0
+	result := F.Pipe1(
+		Of(42),
+		TapReaderK(func(n int) reader.Reader[context.Context, func()] {
+			return func(ctx context.Context) func() {
+				sideEffect = n
+				return func() {}
+			}
+		}),
+	)
+
+	value := result(context.Background())()
+	assert.Equal(t, 42, value)
+	assert.Equal(t, 42, sideEffect)
+}
+
+func TestRead(t *testing.T) {
+	rio := Of(42)
+	ctx := context.Background()
+	ioAction := Read[int](ctx)(rio)
+	result := ioAction()
+
+	assert.Equal(t, 42, result)
+}
+
+func TestComplexPipeline(t *testing.T) {
+	// Test a complex pipeline combining multiple operations
+	result := F.Pipe3(
+		Ask(),
+		Map(func(ctx context.Context) int { return 5 }),
+		Chain(func(n int) ReaderIO[int] {
+			return Of(n * 2)
+		}),
+		Map(N.Add(10)),
+	)
+
+	assert.Equal(t, 20, result(context.Background())()) // (5 * 2) + 10 = 20
+}
+
+func TestFromIOWithChain(t *testing.T) {
+	ioAction := G.Of(10)
+
+	result := F.Pipe1(
+		FromIO(ioAction),
+		Chain(func(n int) ReaderIO[int] {
+			return Of(n + 5)
+		}),
+	)
+
+	assert.Equal(t, 15, result(context.Background())())
+}
+
+func TestTapWithLogging(t *testing.T) {
+	// Simulate logging scenario
+	logged := []int{}
+
+	result := F.Pipe3(
+		Of(42),
+		Tap(func(n int) ReaderIO[func()] {
+			logged = append(logged, n)
+			return Of(func() {})
+		}),
+		Map(N.Mul(2)),
+		Tap(func(n int) ReaderIO[func()] {
+			logged = append(logged, n)
+			return Of(func() {})
+		}),
+	)
+
+	value := result(context.Background())()
+	assert.Equal(t, 84, value)
+	assert.Equal(t, []int{42, 84}, logged)
+}

v2/context/readerio/rec.go

@@ -0,0 +1,86 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package readerio
+
+import (
+	"github.com/IBM/fp-go/v2/readerio"
+)
+
+// TailRec implements stack-safe tail recursion for the ReaderIO monad.
+//
+// This function enables recursive computations that depend on a [context.Context] and
+// perform side effects, without risking stack overflow. It uses an iterative loop to
+// execute the recursion, making it safe for deep or unbounded recursion.
+//
+// The function takes a Kleisli arrow that returns Trampoline[A, B]:
+//   - Bounce(A): Continue recursion with the new state A
+//   - Land(B): Terminate recursion and return the final result B
+//
+// Type Parameters:
+//   - A: The state type that changes during recursion
+//   - B: The final result type when recursion terminates
+//
+// Parameters:
+//   - f: A Kleisli arrow (A => ReaderIO[Trampoline[A, B]]) that controls recursion flow
+//
+// Returns:
+//   - A Kleisli arrow (A => ReaderIO[B]) that executes the recursion safely
+//
+// Example - Countdown:
+//
+//	countdownStep := func(n int) ReaderIO[tailrec.Trampoline[int, string]] {
+//	    return func(ctx context.Context) IO[tailrec.Trampoline[int, string]] {
+//	        return func() tailrec.Trampoline[int, string] {
+//	            if n <= 0 {
+//	                return tailrec.Land[int]("Done!")
+//	            }
+//	            return tailrec.Bounce[string](n - 1)
+//	        }
+//	    }
+//	}
+//
+//	countdown := TailRec(countdownStep)
+//	result := countdown(10)(context.Background())() // Returns "Done!"
+//
+// Example - Sum with context:
+//
+//	type SumState struct {
+//	    numbers []int
+//	    total   int
+//	}
+//
+//	sumStep := func(state SumState) ReaderIO[tailrec.Trampoline[SumState, int]] {
+//	    return func(ctx context.Context) IO[tailrec.Trampoline[SumState, int]] {
+//	        return func() tailrec.Trampoline[SumState, int] {
+//	            if len(state.numbers) == 0 {
+//	                return tailrec.Land[SumState](state.total)
+//	            }
+//	            return tailrec.Bounce[int](SumState{
+//	                numbers: state.numbers[1:],
+//	                total:   state.total + state.numbers[0],
+//	            })
+//	        }
+//	    }
+//	}
+//
+//	sum := TailRec(sumStep)
+//	result := sum(SumState{numbers: []int{1, 2, 3, 4, 5}})(context.Background())()
+//	// Returns 15, safe even for very large slices
+//
+//go:inline
+func TailRec[A, B any](f Kleisli[A, Trampoline[A, B]]) Kleisli[A, B] {
+	return readerio.TailRec(f)
+}

v2/context/readerio/retry.go

@@ -0,0 +1,106 @@
+// Copyright (c) 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package readerio
+
+import (
+	"github.com/IBM/fp-go/v2/retry"
+	RG "github.com/IBM/fp-go/v2/retry/generic"
+)
+
+// Retrying retries a ReaderIO computation according to a retry policy.
+//
+// This function implements a retry mechanism for operations that depend on a [context.Context]
+// and perform side effects (IO). The retry loop continues until one of the following occurs:
+//   - The action succeeds and the check function returns false (no retry needed)
+//   - The retry policy returns None (retry limit reached)
+//   - The check function returns false (indicating success or a non-retryable condition)
+//
+// Type Parameters:
+//   - A: The type of the value produced by the action
+//
+// Parameters:
+//
+//   - policy: A RetryPolicy that determines when and how long to wait between retries.
+//     The policy receives a RetryStatus on each iteration and returns an optional delay.
+//     If it returns None, retrying stops. Common policies include LimitRetries,
+//     ExponentialBackoff, and CapDelay from the retry package.
+//
+//   - action: A Kleisli arrow that takes a RetryStatus and returns a ReaderIO[A].
+//     This function is called on each retry attempt and receives information about the
+//     current retry state (iteration number, cumulative delay, etc.).
+//
+//   - check: A predicate function that examines the result A and returns true if the
+//     operation should be retried, or false if it should stop. This allows you to
+//     distinguish between retryable conditions and successful/permanent results.
+//
+// Returns:
+//   - A ReaderIO[A] that, when executed with a context, will perform the retry logic
+//     and return the final result.
+//
+// Example:
+//
+//	// Create a retry policy: exponential backoff with a cap, limited to 5 retries
+//	policy := M.Concat(
+//	    retry.LimitRetries(5),
+//	    retry.CapDelay(10*time.Second, retry.ExponentialBackoff(100*time.Millisecond)),
+//	)(retry.Monoid)
+//
+//	// Action that fetches data, with retry status information
+//	fetchData := func(status retry.RetryStatus) ReaderIO[string] {
+//	    return func(ctx context.Context) IO[string] {
+//	        return func() string {
+//	            // Simulate an operation that might fail
+//	            if status.IterNumber < 3 {
+//	                return ""  // Empty result indicates failure
+//	            }
+//	            return "success"
+//	        }
+//	    }
+//	}
+//
+//	// Check function: retry if result is empty
+//	shouldRetry := func(s string) bool {
+//	    return s == ""
+//	}
+//
+//	// Create the retrying computation
+//	retryingFetch := Retrying(policy, fetchData, shouldRetry)
+//
+//	// Execute
+//	ctx := context.Background()
+//	result := retryingFetch(ctx)() // Returns "success" after 3 attempts
+//
+//go:inline
+func Retrying[A any](
+	policy retry.RetryPolicy,
+	action Kleisli[retry.RetryStatus, A],
+	check Predicate[A],
+) ReaderIO[A] {
+	// get an implementation for the types
+	return RG.Retrying(
+		Chain[A, Trampoline[retry.RetryStatus, A]],
+		Map[retry.RetryStatus, Trampoline[retry.RetryStatus, A]],
+		Of[Trampoline[retry.RetryStatus, A]],
+		Of[retry.RetryStatus],
+		Delay[retry.RetryStatus],
+
+		TailRec,
+
+		policy,
+		action,
+		check,
+	)
+}

v2/context/readerio/type.go

@@ -0,0 +1,84 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package readerio
+
+import (
+	"context"
+
+	"github.com/IBM/fp-go/v2/consumer"
+	"github.com/IBM/fp-go/v2/either"
+	"github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/io"
+	"github.com/IBM/fp-go/v2/lazy"
+	"github.com/IBM/fp-go/v2/predicate"
+	"github.com/IBM/fp-go/v2/reader"
+	"github.com/IBM/fp-go/v2/readerio"
+	"github.com/IBM/fp-go/v2/tailrec"
+)
+
+type (
+	// Lazy represents a deferred computation that produces a value of type A when executed.
+	// The computation is not executed until explicitly invoked.
+	Lazy[A any] = lazy.Lazy[A]
+
+	// IO represents a side-effectful computation that produces a value of type A.
+	// The computation is deferred and only executed when invoked.
+	//
+	// IO[A] is equivalent to func() A
+	IO[A any] = io.IO[A]
+
+	// Reader represents a computation that depends on a context of type R.
+	// This is used for dependency injection and accessing shared context.
+	//
+	// Reader[R, A] is equivalent to func(R) A
+	Reader[R, A any] = reader.Reader[R, A]
+
+	// ReaderIO represents a context-dependent computation that performs side effects.
+	// This is specialized to use [context.Context] as the context type.
+	//
+	// ReaderIO[A] is equivalent to func(context.Context) func() A
+	ReaderIO[A any] = readerio.ReaderIO[context.Context, A]
+
+	// Kleisli represents a Kleisli arrow for the ReaderIO monad.
+	// It is a function that takes a value of type A and returns a ReaderIO computation
+	// that produces a value of type B.
+	//
+	// Kleisli arrows are used for composing monadic computations and are fundamental
+	// to functional programming patterns involving effects and context.
+	//
+	// Kleisli[A, B] is equivalent to func(A) func(context.Context) func() B
+	Kleisli[A, B any] = reader.Reader[A, ReaderIO[B]]
+
+	// Operator represents a transformation from one ReaderIO computation to another.
+	// It takes a ReaderIO[A] and returns a ReaderIO[B], allowing for the composition
+	// of context-dependent, side-effectful computations.
+	//
+	// Operators are useful for building pipelines of ReaderIO computations where
+	// each step can depend on the previous computation's result.
+	//
+	// Operator[A, B] is equivalent to func(ReaderIO[A]) func(context.Context) func() B
+	Operator[A, B any] = Kleisli[ReaderIO[A], B]
+
+	Consumer[A any] = consumer.Consumer[A]
+
+	Either[E, A any] = either.Either[E, A]
+
+	Trampoline[B, L any] = tailrec.Trampoline[B, L]
+
+	Predicate[A any] = predicate.Predicate[A]
+
+	Void = function.Void
+)

v2/context/readerioeither/bind.go

@@ -1,89 +0,0 @@
-// Copyright (c) 2023 - 2025 IBM Corp.
-// All rights reserved.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-// http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-package readerioeither
-
-import (
-	"github.com/IBM/fp-go/v2/internal/apply"
-	"github.com/IBM/fp-go/v2/internal/chain"
-	"github.com/IBM/fp-go/v2/internal/functor"
-)
-
-// Bind creates an empty context of type [S] to be used with the [Bind] operation
-func Do[S any](
-	empty S,
-) ReaderIOEither[S] {
-	return Of(empty)
-}
-
-// Bind attaches the result of a computation to a context [S1] to produce a context [S2]
-func Bind[S1, S2, T any](
-	setter func(T) func(S1) S2,
-	f func(S1) ReaderIOEither[T],
-) func(ReaderIOEither[S1]) ReaderIOEither[S2] {
-	return chain.Bind(
-		Chain[S1, S2],
-		Map[T, S2],
-		setter,
-		f,
-	)
-}
-
-// Let attaches the result of a computation to a context [S1] to produce a context [S2]
-func Let[S1, S2, T any](
-	setter func(T) func(S1) S2,
-	f func(S1) T,
-) func(ReaderIOEither[S1]) ReaderIOEither[S2] {
-	return functor.Let(
-		Map[S1, S2],
-		setter,
-		f,
-	)
-}
-
-// LetTo attaches the a value to a context [S1] to produce a context [S2]
-func LetTo[S1, S2, T any](
-	setter func(T) func(S1) S2,
-	b T,
-) func(ReaderIOEither[S1]) ReaderIOEither[S2] {
-	return functor.LetTo(
-		Map[S1, S2],
-		setter,
-		b,
-	)
-}
-
-// BindTo initializes a new state [S1] from a value [T]
-func BindTo[S1, T any](
-	setter func(T) S1,
-) Operator[T, S1] {
-	return chain.BindTo(
-		Map[T, S1],
-		setter,
-	)
-}
-
-// ApS attaches a value to a context [S1] to produce a context [S2] by considering the context and the value concurrently
-func ApS[S1, S2, T any](
-	setter func(T) func(S1) S2,
-	fa ReaderIOEither[T],
-) func(ReaderIOEither[S1]) ReaderIOEither[S2] {
-	return apply.ApS(
-		Ap[S2, T],
-		Map[S1, func(T) S2],
-		setter,
-		fa,
-	)
-}

v2/context/readerioeither/cancel.go

@@ -1,42 +0,0 @@
-// Copyright (c) 2023 - 2025 IBM Corp.
-// All rights reserved.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-// http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-package readerioeither
-
-import (
-	"context"
-
-	CIOE "github.com/IBM/fp-go/v2/context/ioeither"
-	"github.com/IBM/fp-go/v2/ioeither"
-)
-
-// WithContext wraps an existing [ReaderIOEither] and performs a context check for cancellation before delegating.
-// This ensures that if the context is already canceled, the computation short-circuits immediately
-// without executing the wrapped computation.
-//
-// This is useful for adding cancellation awareness to computations that might not check the context themselves.
-//
-// Parameters:
-//   - ma: The ReaderIOEither to wrap with context checking
-//
-// Returns a ReaderIOEither that checks for cancellation before executing.
-func WithContext[A any](ma ReaderIOEither[A]) ReaderIOEither[A] {
-	return func(ctx context.Context) IOEither[A] {
-		if err := context.Cause(ctx); err != nil {
-			return ioeither.Left[A](err)
-		}
-		return CIOE.WithContext(ctx, ma(ctx))
-	}
-}

v2/context/readerioeither/coverage.out

@@ -1,251 +0,0 @@
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-github.com/IBM/fp-go/v2/context/readerioeither/monoid.go:68.64,75.2 1 0
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v2/context/readerioeither/http/builder/builder.go

@@ -1,72 +0,0 @@
-// Copyright (c) 2023 - 2025 IBM Corp.
-// All rights reserved.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-// http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-package builder
-
-import (
-	"bytes"
-	"context"
-	"net/http"
-	"strconv"
-
-	RIOE "github.com/IBM/fp-go/v2/context/readerioeither"
-	RIOEH "github.com/IBM/fp-go/v2/context/readerioeither/http"
-	E "github.com/IBM/fp-go/v2/either"
-	F "github.com/IBM/fp-go/v2/function"
-	R "github.com/IBM/fp-go/v2/http/builder"
-	H "github.com/IBM/fp-go/v2/http/headers"
-	LZ "github.com/IBM/fp-go/v2/lazy"
-	O "github.com/IBM/fp-go/v2/option"
-)
-
-func Requester(builder *R.Builder) RIOEH.Requester {
-
-	withBody := F.Curry3(func(data []byte, url string, method string) RIOE.ReaderIOEither[*http.Request] {
-		return RIOE.TryCatch(func(ctx context.Context) func() (*http.Request, error) {
-			return func() (*http.Request, error) {
-				req, err := http.NewRequestWithContext(ctx, method, url, bytes.NewReader(data))
-				if err == nil {
-					req.Header.Set(H.ContentLength, strconv.Itoa(len(data)))
-					H.Monoid.Concat(req.Header, builder.GetHeaders())
-				}
-				return req, err
-			}
-		})
-	})
-
-	withoutBody := F.Curry2(func(url string, method string) RIOE.ReaderIOEither[*http.Request] {
-		return RIOE.TryCatch(func(ctx context.Context) func() (*http.Request, error) {
-			return func() (*http.Request, error) {
-				req, err := http.NewRequestWithContext(ctx, method, url, nil)
-				if err == nil {
-					H.Monoid.Concat(req.Header, builder.GetHeaders())
-				}
-				return req, err
-			}
-		})
-	})
-
-	return F.Pipe5(
-		builder.GetBody(),
-		O.Fold(LZ.Of(E.Of[error](withoutBody)), E.Map[error](withBody)),
-		E.Ap[func(string) RIOE.ReaderIOEither[*http.Request]](builder.GetTargetURL()),
-		E.Flap[error, RIOE.ReaderIOEither[*http.Request]](builder.GetMethod()),
-		E.GetOrElse(RIOE.Left[*http.Request]),
-		RIOE.Map(func(req *http.Request) *http.Request {
-			req.Header = H.Monoid.Concat(req.Header, builder.GetHeaders())
-			return req
-		}),
-	)
-}

v2/context/readerioeither/http/request.go

@@ -1,129 +0,0 @@
-// Copyright (c) 2023 - 2025 IBM Corp.
-// All rights reserved.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-// http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-package http
-
-import (
-	"io"
-	"net/http"
-
-	B "github.com/IBM/fp-go/v2/bytes"
-	RIOE "github.com/IBM/fp-go/v2/context/readerioeither"
-	F "github.com/IBM/fp-go/v2/function"
-	H "github.com/IBM/fp-go/v2/http"
-	IOE "github.com/IBM/fp-go/v2/ioeither"
-	IOEF "github.com/IBM/fp-go/v2/ioeither/file"
-	J "github.com/IBM/fp-go/v2/json"
-	P "github.com/IBM/fp-go/v2/pair"
-)
-
-type (
-	// Requester is a reader that constructs a request
-	Requester = RIOE.ReaderIOEither[*http.Request]
-
-	Client interface {
-		// Do can send an HTTP request considering a context
-		Do(Requester) RIOE.ReaderIOEither[*http.Response]
-	}
-
-	client struct {
-		delegate *http.Client
-		doIOE    func(*http.Request) IOE.IOEither[error, *http.Response]
-	}
-)
-
-var (
-	// MakeRequest is an eitherized version of [http.NewRequestWithContext]
-	MakeRequest = RIOE.Eitherize3(http.NewRequestWithContext)
-	makeRequest = F.Bind13of3(MakeRequest)
-
-	// specialize
-	MakeGetRequest = makeRequest("GET", nil)
-)
-
-func (client client) Do(req Requester) RIOE.ReaderIOEither[*http.Response] {
-	return F.Pipe1(
-		req,
-		RIOE.ChainIOEitherK(client.doIOE),
-	)
-}
-
-// MakeClient creates an HTTP client proxy
-func MakeClient(httpClient *http.Client) Client {
-	return client{delegate: httpClient, doIOE: IOE.Eitherize1(httpClient.Do)}
-}
-
-// ReadFullResponse sends a request,  reads the response as a byte array and represents the result as a tuple
-func ReadFullResponse(client Client) func(Requester) RIOE.ReaderIOEither[H.FullResponse] {
-	return func(req Requester) RIOE.ReaderIOEither[H.FullResponse] {
-		return F.Flow3(
-			client.Do(req),
-			IOE.ChainEitherK(H.ValidateResponse),
-			IOE.Chain(func(resp *http.Response) IOE.IOEither[error, H.FullResponse] {
-				return F.Pipe1(
-					F.Pipe3(
-						resp,
-						H.GetBody,
-						IOE.Of[error, io.ReadCloser],
-						IOEF.ReadAll[io.ReadCloser],
-					),
-					IOE.Map[error](F.Bind1st(P.MakePair[*http.Response, []byte], resp)),
-				)
-			}),
-		)
-	}
-}
-
-// ReadAll sends a request and reads the response as bytes
-func ReadAll(client Client) func(Requester) RIOE.ReaderIOEither[[]byte] {
-	return F.Flow2(
-		ReadFullResponse(client),
-		RIOE.Map(H.Body),
-	)
-}
-
-// ReadText sends a request, reads the response and represents the response as a text string
-func ReadText(client Client) func(Requester) RIOE.ReaderIOEither[string] {
-	return F.Flow2(
-		ReadAll(client),
-		RIOE.Map(B.ToString),
-	)
-}
-
-// ReadJson sends a request, reads the response and parses the response as JSON
-//
-// Deprecated: use [ReadJSON] instead
-func ReadJson[A any](client Client) func(Requester) RIOE.ReaderIOEither[A] {
-	return ReadJSON[A](client)
-}
-
-func readJSON(client Client) func(Requester) RIOE.ReaderIOEither[[]byte] {
-	return F.Flow3(
-		ReadFullResponse(client),
-		RIOE.ChainFirstEitherK(F.Flow2(
-			H.Response,
-			H.ValidateJSONResponse,
-		)),
-		RIOE.Map(H.Body),
-	)
-}
-
-// ReadJSON sends a request, reads the response and parses the response as JSON
-func ReadJSON[A any](client Client) func(Requester) RIOE.ReaderIOEither[A] {
-	return F.Flow2(
-		readJSON(client),
-		RIOE.ChainEitherK(J.Unmarshal[A]),
-	)
-}

v2/context/readerioeither/http/request_test.go

@@ -1,157 +0,0 @@
-// Copyright (c) 2023 - 2025 IBM Corp.
-// All rights reserved.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-// http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-package http
-
-import (
-	"context"
-	"fmt"
-	"testing"
-
-	H "net/http"
-
-	R "github.com/IBM/fp-go/v2/context/readerioeither"
-	E "github.com/IBM/fp-go/v2/either"
-	"github.com/IBM/fp-go/v2/errors"
-	F "github.com/IBM/fp-go/v2/function"
-	IOE "github.com/IBM/fp-go/v2/ioeither"
-	"github.com/stretchr/testify/assert"
-)
-
-type PostItem struct {
-	UserID uint   `json:"userId"`
-	Id     uint   `json:"id"`
-	Title  string `json:"title"`
-	Body   string `json:"body"`
-}
-
-func getTitle(item PostItem) string {
-	return item.Title
-}
-
-type simpleRequestBuilder struct {
-	method  string
-	url     string
-	headers H.Header
-}
-
-func requestBuilder() simpleRequestBuilder {
-	return simpleRequestBuilder{method: "GET"}
-}
-
-func (b simpleRequestBuilder) WithURL(url string) simpleRequestBuilder {
-	b.url = url
-	return b
-}
-
-func (b simpleRequestBuilder) WithHeader(key, value string) simpleRequestBuilder {
-	if b.headers == nil {
-		b.headers = make(H.Header)
-	} else {
-		b.headers = b.headers.Clone()
-	}
-	b.headers.Set(key, value)
-	return b
-}
-
-func (b simpleRequestBuilder) Build() R.ReaderIOEither[*H.Request] {
-	return func(ctx context.Context) IOE.IOEither[error, *H.Request] {
-		return IOE.TryCatchError(func() (*H.Request, error) {
-			req, err := H.NewRequestWithContext(ctx, b.method, b.url, nil)
-			if err == nil {
-				req.Header = b.headers
-			}
-			return req, err
-		})
-	}
-}
-
-func TestSendSingleRequest(t *testing.T) {
-
-	client := MakeClient(H.DefaultClient)
-
-	req1 := MakeGetRequest("https://jsonplaceholder.typicode.com/posts/1")
-
-	readItem := ReadJSON[PostItem](client)
-
-	resp1 := readItem(req1)
-
-	resE := resp1(context.TODO())()
-
-	fmt.Println(resE)
-}
-
-// setHeaderUnsafe updates a header value in a request object by mutating the request object
-func setHeaderUnsafe(key, value string) func(*H.Request) *H.Request {
-	return func(req *H.Request) *H.Request {
-		req.Header.Set(key, value)
-		return req
-	}
-}
-
-func TestSendSingleRequestWithHeaderUnsafe(t *testing.T) {
-
-	client := MakeClient(H.DefaultClient)
-
-	// this is not safe from a puristic perspective, because the map call mutates the request object
-	req1 := F.Pipe2(
-		"https://jsonplaceholder.typicode.com/posts/1",
-		MakeGetRequest,
-		R.Map(setHeaderUnsafe("Content-Type", "text/html")),
-	)
-
-	readItem := ReadJSON[PostItem](client)
-
-	resp1 := F.Pipe2(
-		req1,
-		readItem,
-		R.Map(getTitle),
-	)
-
-	res := F.Pipe1(
-		resp1(context.TODO())(),
-		E.GetOrElse(errors.ToString),
-	)
-
-	assert.Equal(t, "sunt aut facere repellat provident occaecati excepturi optio reprehenderit", res)
-}
-
-func TestSendSingleRequestWithHeaderSafe(t *testing.T) {
-
-	client := MakeClient(H.DefaultClient)
-
-	// the request builder assembles config values to construct
-	// the final http request. Each `With` step creates a copy of the settings
-	// so the flow is pure
-	request := requestBuilder().
-		WithURL("https://jsonplaceholder.typicode.com/posts/1").
-		WithHeader("Content-Type", "text/html").
-		Build()
-
-	readItem := ReadJSON[PostItem](client)
-
-	response := F.Pipe2(
-		request,
-		readItem,
-		R.Map(getTitle),
-	)
-
-	res := F.Pipe1(
-		response(context.TODO())(),
-		E.GetOrElse(errors.ToString),
-	)
-
-	assert.Equal(t, "sunt aut facere repellat provident occaecati excepturi optio reprehenderit", res)
-}