fix: better doc and NonEmptyString

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

.github/workflows/build.yml

@@ -39,9 +39,10 @@ jobs:
       - name: Run tests
         run: |
           go mod tidy
-          go test -v -race -coverprofile=coverage.txt -covermode=atomic ./...
+          go test -race -coverprofile=coverage.txt -covermode=atomic -coverpkg=./... ./...
       
       - name: Upload coverage to Coveralls
+        continue-on-error: true
         uses: coverallsapp/github-action@v2
         with:
           github-token: ${{ secrets.GITHUB_TOKEN }}
@@ -78,9 +79,10 @@ jobs:
         run: |
           cd v2
           go mod tidy
-          go test -v -race -coverprofile=coverage.txt -covermode=atomic ./...
+          go test -race -coverprofile=coverage.txt -covermode=atomic -coverpkg=./... ./...
       
       - name: Upload coverage to Coveralls
+        continue-on-error: true
         uses: coverallsapp/github-action@v2
         with:
           github-token: ${{ secrets.GITHUB_TOKEN }}
@@ -106,6 +108,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Coveralls Finished
+        continue-on-error: true
         uses: coverallsapp/github-action@v2
         with:
           github-token: ${{ secrets.GITHUB_TOKEN }}
@@ -131,7 +134,7 @@ jobs:
           fetch-depth: 0
 
       - name: Set up Node.js ${{ env.NODE_VERSION }}
-        uses: actions/setup-node@6044e13b5dc448c55e2357c09f80417699197238 # v6.2.0
+        uses: actions/setup-node@53b83947a5a98c8d113130e565377fae1a50d02f # v6.3.0
         with:
           node-version: ${{ env.NODE_VERSION }}
 

context7.json

@@ -0,0 +1,4 @@
+{
+  "url": "https://context7.com/ibm/fp-go",
+  "public_key": "pk_7wJdJRn8zGHxvIYu7eh9h"
+}

skills/fp-go-http/SKILL.md

@@ -0,0 +1,318 @@
+# fp-go HTTP Requests
+
+## Overview
+
+fp-go wraps `net/http` in the `ReaderIOResult` monad, giving you composable, context-aware HTTP operations with automatic error propagation. The core package is:
+
+```
+github.com/IBM/fp-go/v2/context/readerioresult/http
+```
+
+All HTTP operations are lazy — they describe what to do but do not execute until you call the resulting function with a `context.Context`.
+
+## Core Types
+
+```go
+// Requester builds an *http.Request given a context.
+type Requester = ReaderIOResult[*http.Request]  // func(context.Context) func() result.Result[*http.Request]
+
+// Client executes a Requester and returns the response wrapped in ReaderIOResult.
+type Client interface {
+    Do(Requester) ReaderIOResult[*http.Response]
+}
+```
+
+## Basic Usage
+
+### 1. Create a Client
+
+```go
+import (
+    HTTP "net/http"
+    H    "github.com/IBM/fp-go/v2/context/readerioresult/http"
+)
+
+client := H.MakeClient(HTTP.DefaultClient)
+
+// Or with a custom client:
+custom := &HTTP.Client{Timeout: 10 * time.Second}
+client := H.MakeClient(custom)
+```
+
+### 2. Build a Request
+
+```go
+// GET request (most common)
+req := H.MakeGetRequest("https://api.example.com/users/1")
+
+// Arbitrary method + body
+req := H.MakeRequest("POST", "https://api.example.com/users", bodyReader)
+```
+
+### 3. Execute and Parse
+
+```go
+import (
+    "context"
+    H "github.com/IBM/fp-go/v2/context/readerioresult/http"
+)
+
+type User struct {
+    ID   int    `json:"id"`
+    Name string `json:"name"`
+}
+
+client := H.MakeClient(HTTP.DefaultClient)
+
+// ReadJSON validates status, Content-Type, then unmarshals JSON
+result := H.ReadJSON[User](client)(H.MakeGetRequest("https://api.example.com/users/1"))
+
+// Execute — provide context once
+user, err := result(context.Background())()
+```
+
+## Response Readers
+
+All accept a `Client` and return a function `Requester → ReaderIOResult[A]`:
+
+| Function | Returns | Notes |
+|----------|---------|-------|
+| `ReadJSON[A](client)` | `ReaderIOResult[A]` | Validates status + Content-Type, unmarshals JSON |
+| `ReadText(client)` | `ReaderIOResult[string]` | Validates status, reads body as UTF-8 string |
+| `ReadAll(client)` | `ReaderIOResult[[]byte]` | Validates status, returns raw body bytes |
+| `ReadFullResponse(client)` | `ReaderIOResult[FullResponse]` | Returns `Pair[*http.Response, []byte]` |
+
+`FullResponse = Pair[*http.Response, []byte]` — use `pair.First` / `pair.Second` to access components.
+
+## Composing Requests in Pipelines
+
+```go
+import (
+    F   "github.com/IBM/fp-go/v2/function"
+    H   "github.com/IBM/fp-go/v2/context/readerioresult/http"
+    RIO "github.com/IBM/fp-go/v2/context/readerioresult"
+    IO  "github.com/IBM/fp-go/v2/io"
+)
+
+client     := H.MakeClient(HTTP.DefaultClient)
+readPost   := H.ReadJSON[Post](client)
+
+pipeline := F.Pipe2(
+    H.MakeGetRequest("https://jsonplaceholder.typicode.com/posts/1"),
+    readPost,
+    RIO.ChainFirstIOK(IO.Logf[Post]("Got post: %v")),
+)
+
+post, err := pipeline(context.Background())()
+```
+
+## Parallel Requests — Homogeneous Types
+
+Use `RIO.TraverseArray` when all requests return the same type:
+
+```go
+import (
+    A   "github.com/IBM/fp-go/v2/array"
+    F   "github.com/IBM/fp-go/v2/function"
+    H   "github.com/IBM/fp-go/v2/context/readerioresult/http"
+    RIO "github.com/IBM/fp-go/v2/context/readerioresult"
+    IO  "github.com/IBM/fp-go/v2/io"
+)
+
+type PostItem struct {
+    UserID uint   `json:"userId"`
+    ID     uint   `json:"id"`
+    Title  string `json:"title"`
+}
+
+client     := H.MakeClient(HTTP.DefaultClient)
+readPost   := H.ReadJSON[PostItem](client)
+
+// Fetch 10 posts in parallel
+data := F.Pipe3(
+    A.MakeBy(10, func(i int) string {
+        return fmt.Sprintf("https://jsonplaceholder.typicode.com/posts/%d", i+1)
+    }),
+    RIO.TraverseArray(F.Flow3(
+        H.MakeGetRequest,
+        readPost,
+        RIO.ChainFirstIOK(IO.Logf[PostItem]("Post: %v")),
+    )),
+    RIO.ChainFirstIOK(IO.Logf[[]PostItem]("All posts: %v")),
+    RIO.Map(A.Size[PostItem]),
+)
+
+count, err := data(context.Background())()
+```
+
+## Parallel Requests — Heterogeneous Types
+
+Use `RIO.TraverseTuple2` (or `Tuple3`, etc.) when requests return different types:
+
+```go
+import (
+    T   "github.com/IBM/fp-go/v2/tuple"
+    RIO "github.com/IBM/fp-go/v2/context/readerioresult"
+    H   "github.com/IBM/fp-go/v2/context/readerioresult/http"
+    F   "github.com/IBM/fp-go/v2/function"
+)
+
+type CatFact struct {
+    Fact string `json:"fact"`
+}
+
+client         := H.MakeClient(HTTP.DefaultClient)
+readPost       := H.ReadJSON[PostItem](client)
+readCatFact    := H.ReadJSON[CatFact](client)
+
+// Execute both requests in parallel with different response types
+data := F.Pipe3(
+    T.MakeTuple2(
+        "https://jsonplaceholder.typicode.com/posts/1",
+        "https://catfact.ninja/fact",
+    ),
+    T.Map2(H.MakeGetRequest, H.MakeGetRequest), // build both requesters
+    RIO.TraverseTuple2(readPost, readCatFact),  // run in parallel, typed
+    RIO.ChainFirstIOK(IO.Logf[T.Tuple2[PostItem, CatFact]]("Result: %v")),
+)
+
+both, err := data(context.Background())()
+// both.F1 is PostItem, both.F2 is CatFact
+```
+
+## Building Requests with the Builder API
+
+For complex requests (custom headers, query params, JSON body), use the builder:
+
+```go
+import (
+    B  "github.com/IBM/fp-go/v2/http/builder"
+    RB "github.com/IBM/fp-go/v2/context/readerioresult/http/builder"
+    F  "github.com/IBM/fp-go/v2/function"
+)
+
+// GET with query parameters
+req := F.Pipe2(
+    B.Default,
+    B.WithURL("https://api.example.com/items?page=1"),
+    B.WithQueryArg("limit")("50"),
+)
+requester := RB.Requester(req)
+
+// POST with JSON body
+req := F.Pipe3(
+    B.Default,
+    B.WithURL("https://api.example.com/users"),
+    B.WithMethod("POST"),
+    B.WithJSON(map[string]string{"name": "Alice"}),
+    // sets Content-Type: application/json automatically
+)
+requester := RB.Requester(req)
+
+// With authentication and custom headers
+req := F.Pipe3(
+    B.Default,
+    B.WithURL("https://api.example.com/protected"),
+    B.WithBearer("my-token"),           // sets Authorization: Bearer my-token
+    B.WithHeader("X-Request-ID")("123"),
+)
+requester := RB.Requester(req)
+
+// Execute
+result := H.ReadJSON[Response](client)(requester)
+data, err := result(ctx)()
+```
+
+### Builder Functions
+
+| Function | Effect |
+|----------|--------|
+| `B.WithURL(url)` | Set the target URL |
+| `B.WithMethod(method)` | Set HTTP method (GET, POST, PUT, DELETE, …) |
+| `B.WithJSON(v)` | Marshal `v` as JSON body, set `Content-Type: application/json` |
+| `B.WithBytes(data)` | Set raw bytes body, set `Content-Length` automatically |
+| `B.WithHeader(key)(value)` | Add a request header |
+| `B.WithBearer(token)` | Set `Authorization: Bearer <token>` |
+| `B.WithQueryArg(key)(value)` | Append a query parameter |
+
+## Error Handling
+
+Errors from request creation, HTTP status codes, Content-Type validation, and JSON parsing all propagate automatically through the `Result` monad. You only handle errors at the call site:
+
+```go
+// Pattern 1: direct extraction
+value, err := pipeline(ctx)()
+if err != nil { /* handle */ }
+
+// Pattern 2: Fold for clean HTTP handler
+RIO.Fold(
+    func(err error) { http.Error(w, err.Error(), http.StatusInternalServerError) },
+    func(data MyType) { json.NewEncoder(w).Encode(data) },
+)(pipeline)(ctx)()
+```
+
+## Full HTTP Handler Example
+
+```go
+package main
+
+import (
+    "context"
+    "encoding/json"
+    "net/http"
+    HTTP "net/http"
+    "fmt"
+
+    F   "github.com/IBM/fp-go/v2/function"
+    H   "github.com/IBM/fp-go/v2/context/readerioresult/http"
+    RIO "github.com/IBM/fp-go/v2/context/readerioresult"
+    IO  "github.com/IBM/fp-go/v2/io"
+)
+
+type Post struct {
+    ID    int    `json:"id"`
+    Title string `json:"title"`
+}
+
+var client = H.MakeClient(HTTP.DefaultClient)
+
+func fetchPost(id int) RIO.ReaderIOResult[Post] {
+    url := fmt.Sprintf("https://jsonplaceholder.typicode.com/posts/%d", id)
+    return F.Pipe2(
+        H.MakeGetRequest(url),
+        H.ReadJSON[Post](client),
+        RIO.ChainFirstIOK(IO.Logf[Post]("fetched: %v")),
+    )
+}
+
+func handler(w http.ResponseWriter, r *http.Request) {
+    RIO.Fold(
+        func(err error) {
+            http.Error(w, err.Error(), http.StatusBadGateway)
+        },
+        func(post Post) {
+            w.Header().Set("Content-Type", "application/json")
+            json.NewEncoder(w).Encode(post)
+        },
+    )(fetchPost(1))(r.Context())()
+}
+```
+
+## Import Reference
+
+```go
+import (
+    HTTP "net/http"
+
+    H   "github.com/IBM/fp-go/v2/context/readerioresult/http"
+    RB  "github.com/IBM/fp-go/v2/context/readerioresult/http/builder"
+    B   "github.com/IBM/fp-go/v2/http/builder"
+    RIO "github.com/IBM/fp-go/v2/context/readerioresult"
+    F   "github.com/IBM/fp-go/v2/function"
+    A   "github.com/IBM/fp-go/v2/array"
+    T   "github.com/IBM/fp-go/v2/tuple"
+    IO  "github.com/IBM/fp-go/v2/io"
+)
+```
+
+Requires Go 1.24+.

skills/fp-go-logging/SKILL.md

@@ -0,0 +1,410 @@
+# fp-go Logging
+
+## Overview
+
+fp-go provides logging utilities that integrate naturally with functional pipelines. Logging is always a **side effect** — it should not change the value being processed. The library achieves this through `ChainFirst`-style combinators that thread the original value through unchanged while performing the log.
+
+## Packages
+
+| Package | Purpose |
+|---------|---------|
+| `github.com/IBM/fp-go/v2/logging` | Global logger, context-embedded logger, `LoggingCallbacks` |
+| `github.com/IBM/fp-go/v2/io` | `Logf`, `Logger`, `LogGo`, `Printf`, `PrintGo` — IO-level logging helpers |
+| `github.com/IBM/fp-go/v2/readerio` | `SLog`, `SLogWithCallback` — structured logging for ReaderIO |
+| `github.com/IBM/fp-go/v2/context/readerio` | `SLog`, `SLogWithCallback` — structured logging for context ReaderIO |
+| `github.com/IBM/fp-go/v2/context/readerresult` | `SLog`, `TapSLog`, `SLogWithCallback` — structured logging for ReaderResult |
+| `github.com/IBM/fp-go/v2/context/readerioresult` | `SLog`, `TapSLog`, `SLogWithCallback`, `LogEntryExit`, `LogEntryExitWithCallback` — full suite for ReaderIOResult |
+
+## Logging Inside Pipelines
+
+The idiomatic way to log inside a monadic pipeline is `ChainFirstIOK` (or `ChainFirst` where the monad is already IO). These combinators execute a side-effecting function and pass the **original value** downstream unchanged.
+
+### With `IOResult` / `ReaderIOResult` — printf-style
+
+```go
+import (
+    RIO "github.com/IBM/fp-go/v2/context/readerioresult"
+    IO  "github.com/IBM/fp-go/v2/io"
+    F   "github.com/IBM/fp-go/v2/function"
+)
+
+pipeline := F.Pipe3(
+    fetchUser(42),
+    RIO.ChainEitherK(validateUser),
+    // Log after validation — value flows through unchanged
+    RIO.ChainFirstIOK(IO.Logf[User]("Validated user: %v")),
+    RIO.Map(enrichUser),
+)
+```
+
+`IO.Logf[A](format string) func(A) IO[A]` logs using `log.Printf` and returns the value unchanged. It's a Kleisli arrow suitable for `ChainFirst` and `ChainFirstIOK`.
+
+### With `IOEither` / plain `IO`
+
+```go
+import (
+    IOE "github.com/IBM/fp-go/v2/ioeither"
+    IO  "github.com/IBM/fp-go/v2/io"
+    F   "github.com/IBM/fp-go/v2/function"
+)
+
+pipeline := F.Pipe3(
+    file.ReadFile("config.json"),
+    IOE.ChainEitherK(J.Unmarshal[Config]),
+    IOE.ChainFirstIOK(IO.Logf[Config]("Loaded config: %v")),
+    IOE.Map[error](processConfig),
+)
+```
+
+### Logging Arrays in TraverseArray
+
+```go
+import (
+    A   "github.com/IBM/fp-go/v2/array"
+    RIO "github.com/IBM/fp-go/v2/context/readerioresult"
+    IO  "github.com/IBM/fp-go/v2/io"
+    F   "github.com/IBM/fp-go/v2/function"
+)
+
+// Log each item individually, then log the final slice
+pipeline := F.Pipe2(
+    A.MakeBy(3, idxToFilename),
+    RIO.TraverseArray(F.Flow3(
+        file.ReadFile,
+        RIO.ChainEitherK(J.Unmarshal[Record]),
+        RIO.ChainFirstIOK(IO.Logf[Record]("Parsed record: %v")),
+    )),
+    RIO.ChainFirstIOK(IO.Logf[[]Record]("All records: %v")),
+)
+```
+
+## IO Logging Functions
+
+All live in `github.com/IBM/fp-go/v2/io`:
+
+### `Logf` — printf-style
+
+```go
+IO.Logf[A any](format string) func(A) IO[A]
+```
+
+Uses `log.Printf`. The format string works like `fmt.Sprintf`.
+
+```go
+IO.Logf[User]("Processing user: %+v")
+IO.Logf[int]("Count: %d")
+```
+
+### `Logger` — with custom `*log.Logger`
+
+```go
+IO.Logger[A any](loggers ...*log.Logger) func(prefix string) func(A) IO[A]
+```
+
+Uses `logger.Printf(prefix+": %v", value)`. Pass your own `*log.Logger` instance.
+
+```go
+customLog := log.New(os.Stderr, "APP ", log.LstdFlags)
+logUser := IO.Logger[User](customLog)("user")
+// logs: "APP user: {ID:42 Name:Alice}"
+```
+
+### `LogGo` — Go template syntax
+
+```go
+IO.LogGo[A any](tmpl string) func(A) IO[A]
+```
+
+Uses Go's `text/template`. The template receives the value as `.`.
+
+```go
+type User struct{ Name string; Age int }
+IO.LogGo[User]("User {{.Name}} is {{.Age}} years old")
+```
+
+### `Printf` / `PrintGo` — stdout instead of log
+
+Same signatures as `Logf` / `LogGo` but use `fmt.Printf`/`fmt.Println` (no log prefix, no timestamp).
+
+```go
+IO.Printf[Result]("Result: %v\n")
+IO.PrintGo[User]("Name: {{.Name}}")
+```
+
+## Structured Logging in the `context` Package
+
+The `context/readerioresult`, `context/readerresult`, and `context/readerio` packages provide structured `slog`-based logging functions that are context-aware: they retrieve the logger from the context (via `logging.GetLoggerFromContext`) rather than using a fixed logger instance.
+
+### `TapSLog` — inline structured logging in a ReaderIOResult pipeline
+
+`TapSLog` is an **Operator** (`func(ReaderIOResult[A]) ReaderIOResult[A]`). It sits directly in a `F.Pipe` call on a `ReaderIOResult`, logs the current value or error using `slog`, and passes the result through unchanged.
+
+```go
+import (
+    RIO "github.com/IBM/fp-go/v2/context/readerioresult"
+    F   "github.com/IBM/fp-go/v2/function"
+)
+
+pipeline := F.Pipe4(
+    fetchOrder(orderID),
+    RIO.TapSLog[Order]("Order fetched"),        // logs value=<Order> or error=<err>
+    RIO.Chain(validateOrder),
+    RIO.TapSLog[Order]("Order validated"),
+    RIO.Chain(processPayment),
+)
+
+result, err := pipeline(ctx)()
+```
+
+- Logs **both** success values (`value=<A>`) and errors (`error=<err>`) using `slog` structured attributes.
+- Respects the logger level — if the logger is configured to discard Info-level logs, nothing is written.
+- Available in both `context/readerioresult` and `context/readerresult`.
+
+### `SLog` — Kleisli-style structured logging
+
+`SLog` is a **Kleisli arrow** (`func(Result[A]) ReaderResult[A]` / `func(Result[A]) ReaderIOResult[A]`). It is used with `Chain` when you want to intercept the raw `Result` directly.
+
+```go
+import (
+    RIO "github.com/IBM/fp-go/v2/context/readerioresult"
+    F   "github.com/IBM/fp-go/v2/function"
+)
+
+pipeline := F.Pipe3(
+    fetchData(id),
+    RIO.Chain(RIO.SLog[Data]("Data fetched")),    // log raw Result, pass it through
+    RIO.Chain(validateData),
+    RIO.Chain(RIO.SLog[Data]("Data validated")),
+    RIO.Chain(processData),
+)
+```
+
+**Difference from `TapSLog`:**
+- `TapSLog[A](msg)` is an `Operator[A, A]` — used directly in `F.Pipe` on a `ReaderIOResult[A]`.
+- `SLog[A](msg)` is a `Kleisli[Result[A], A]` — used with `Chain`, giving access to the raw `Result[A]`.
+
+Both log in the same format. `TapSLog` is more ergonomic in most pipelines.
+
+### `SLogWithCallback` — custom log level and logger source
+
+```go
+import (
+    RIO  "github.com/IBM/fp-go/v2/context/readerioresult"
+    "log/slog"
+)
+
+// Log at DEBUG level with a custom logger extracted from context
+debugLog := RIO.SLogWithCallback[User](
+    slog.LevelDebug,
+    logging.GetLoggerFromContext, // or any func(context.Context) *slog.Logger
+    "Fetched user",
+)
+
+pipeline := F.Pipe2(
+    fetchUser(123),
+    RIO.Chain(debugLog),
+    RIO.Map(func(u User) string { return u.Name }),
+)
+```
+
+### `LogEntryExit` — automatic entry/exit timing with correlation IDs
+
+`LogEntryExit` wraps a `ReaderIOResult` computation with structured entry and exit log messages. It assigns a unique **correlation ID** (`ID=<n>`) to each invocation so concurrent or nested operations can be correlated in logs.
+
+```go
+import (
+    RIO "github.com/IBM/fp-go/v2/context/readerioresult"
+    F   "github.com/IBM/fp-go/v2/function"
+)
+
+pipeline := F.Pipe3(
+    fetchUser(123),
+    RIO.LogEntryExit[User]("fetchUser"),   // wraps the operation
+    RIO.Chain(func(user User) RIO.ReaderIOResult[[]Order] {
+        return F.Pipe1(
+            fetchOrders(user.ID),
+            RIO.LogEntryExit[[]Order]("fetchOrders"),
+        )
+    }),
+)
+
+result, err := pipeline(ctx)()
+// Logs:
+// level=INFO msg="[entering]" name=fetchUser ID=1
+// level=INFO msg="[exiting ]" name=fetchUser ID=1 duration=42ms
+// level=INFO msg="[entering]" name=fetchOrders ID=2
+// level=INFO msg="[exiting ]" name=fetchOrders ID=2 duration=18ms
+```
+
+On error, the exit log changes to `[throwing]` and includes the error:
+
+```
+level=INFO msg="[throwing]" name=fetchUser ID=3 duration=5ms error="user not found"
+```
+
+Key properties:
+- **Correlation ID** (`ID=`) is unique per operation, monotonically increasing, and stored in the context so nested operations can access the parent's ID.
+- **Duration** (`duration=`) is measured from entry to exit.
+- **Logger is taken from the context** — embed a request-scoped logger with `logging.WithLogger` before executing the pipeline and `LogEntryExit` picks it up automatically.
+- **Level-aware** — if the logger does not have the log level enabled, the entire entry/exit instrumentation is skipped (zero overhead).
+- The original `ReaderIOResult[A]` value flows through **unchanged**.
+
+```go
+// Use a context logger so all log messages carry request metadata
+cancelFn, ctxWithLogger := pair.Unpack(
+    logging.WithLogger(
+        slog.Default().With("requestID", r.Header.Get("X-Request-ID")),
+    )(r.Context()),
+)
+defer cancelFn()
+
+result, err := pipeline(ctxWithLogger)()
+```
+
+### `LogEntryExitWithCallback` — custom log level
+
+```go
+import (
+    RIO  "github.com/IBM/fp-go/v2/context/readerioresult"
+    "log/slog"
+)
+
+// Log at DEBUG level instead of INFO
+debugPipeline := F.Pipe1(
+    expensiveComputation(),
+    RIO.LogEntryExitWithCallback[Result](
+        slog.LevelDebug,
+        logging.GetLoggerFromContext,
+        "expensiveComputation",
+    ),
+)
+```
+
+### `SLog` / `SLogWithCallback` in `context/readerresult`
+
+The same `SLog` and `TapSLog` functions are also available in `context/readerresult` for use with the synchronous `ReaderResult[A] = func(context.Context) (A, error)` monad:
+
+```go
+import RR "github.com/IBM/fp-go/v2/context/readerresult"
+
+pipeline := F.Pipe3(
+    queryDB(id),
+    RR.TapSLog[Row]("Row fetched"),
+    RR.Chain(parseRow),
+    RR.TapSLog[Record]("Record parsed"),
+)
+```
+
+## Global Logger (`logging` package)
+
+The `logging` package manages a global `*slog.Logger` (structured logging, Go 1.21+).
+
+```go
+import "github.com/IBM/fp-go/v2/logging"
+
+// Get the current global logger (defaults to slog.Default())
+logger := logging.GetLogger()
+logger.Info("application started", "version", "1.0")
+
+// Replace the global logger; returns the old one for deferred restore
+old := logging.SetLogger(slog.New(slog.NewJSONHandler(os.Stdout, nil)))
+defer logging.SetLogger(old)
+```
+
+## Context-Embedded Logger
+
+Embed a `*slog.Logger` in a `context.Context` to carry request-scoped loggers across the call stack. All context-package logging functions (`TapSLog`, `SLog`, `LogEntryExit`) pick up this logger automatically.
+
+```go
+import (
+    "github.com/IBM/fp-go/v2/logging"
+    "github.com/IBM/fp-go/v2/pair"
+    "log/slog"
+)
+
+// Create a request-scoped logger
+reqLogger := slog.Default().With("requestID", "abc-123")
+
+// Embed it into a context using the Kleisli arrow WithLogger
+cancelFn, ctxWithLogger := pair.Unpack(logging.WithLogger(reqLogger)(ctx))
+defer cancelFn()
+
+// All downstream logging (TapSLog, LogEntryExit, etc.) uses reqLogger
+result, err := pipeline(ctxWithLogger)()
+```
+
+`WithLogger` returns a `ContextCancel = Pair[context.CancelFunc, context.Context]`. The cancel function is a no-op — the context is only enriched, not made cancellable.
+
+`GetLoggerFromContext` falls back to the global logger if no logger is found in the context.
+
+## `LoggingCallbacks` — Dual-Logger Pattern
+
+```go
+import "github.com/IBM/fp-go/v2/logging"
+
+// Returns (infoCallback, errorCallback) — both are func(string, ...any)
+infoLog, errLog := logging.LoggingCallbacks()                    // use log.Default() for both
+infoLog, errLog := logging.LoggingCallbacks(myLogger)            // same logger for both
+infoLog, errLog := logging.LoggingCallbacks(infoLog, errorLog)   // separate loggers
+```
+
+Used internally by `io.Logger` and by packages that need separate info/error sinks.
+
+## Choosing the Right Logging Function
+
+| Situation | Use |
+|-----------|-----|
+| Quick printf logging mid-pipeline | `IO.Logf[A]("fmt")` with `ChainFirstIOK` |
+| Go template formatting mid-pipeline | `IO.LogGo[A]("tmpl")` with `ChainFirstIOK` |
+| Print to stdout (no log prefix) | `IO.Printf[A]("fmt")` with `ChainFirstIOK` |
+| Structured slog — log value or error inline | `RIO.TapSLog[A]("msg")` (Operator, used in Pipe) |
+| Structured slog — intercept raw Result | `RIO.Chain(RIO.SLog[A]("msg"))` (Kleisli) |
+| Structured slog — custom log level | `RIO.SLogWithCallback[A](level, cb, "msg")` |
+| Entry/exit timing + correlation IDs | `RIO.LogEntryExit[A]("name")` |
+| Entry/exit at custom log level | `RIO.LogEntryExitWithCallback[A](level, cb, "name")` |
+| Structured logging globally | `logging.GetLogger()` / `logging.SetLogger()` |
+| Request-scoped logger in context | `logging.WithLogger(logger)` + `logging.GetLoggerFromContext(ctx)` |
+| Custom `*log.Logger` in pipeline | `IO.Logger[A](logger)("prefix")` with `ChainFirstIOK` |
+
+## Complete Example
+
+```go
+package main
+
+import (
+    "context"
+    "log/slog"
+    "os"
+
+    F   "github.com/IBM/fp-go/v2/function"
+    IO  "github.com/IBM/fp-go/v2/io"
+    L   "github.com/IBM/fp-go/v2/logging"
+    P   "github.com/IBM/fp-go/v2/pair"
+    RIO "github.com/IBM/fp-go/v2/context/readerioresult"
+)
+
+func main() {
+    // Configure JSON structured logging globally
+    L.SetLogger(slog.New(slog.NewJSONHandler(os.Stdout, nil)))
+
+    // Embed a request-scoped logger into the context
+    _, ctx := P.Unpack(L.WithLogger(
+        L.GetLogger().With("requestID", "req-001"),
+    )(context.Background()))
+
+    pipeline := F.Pipe5(
+        fetchData(42),
+        RIO.LogEntryExit[Data]("fetchData"),              // entry/exit with timing + ID
+        RIO.TapSLog[Data]("raw data"),                    // inline structured value log
+        RIO.ChainEitherK(transformData),
+        RIO.LogEntryExit[Result]("transformData"),
+        RIO.ChainFirstIOK(IO.LogGo[Result]("result: {{.Value}}")), // template log
+    )
+
+    value, err := pipeline(ctx)()
+    if err != nil {
+        L.GetLogger().Error("pipeline failed", "error", err)
+    }
+    _ = value
+}
+```

skills/fp-go-monadic-operations/SKILL.md

@@ -0,0 +1,520 @@
+# fp-go Monadic Operations
+
+## Overview
+
+`fp-go` (import path `github.com/IBM/fp-go/v2`) brings type-safe functional programming to Go using generics. Every monad follows a **consistent interface**: once you know the pattern in one monad, it transfers to all others.
+
+All functions use the **data-last** principle: the data being transformed is always the last argument, enabling partial application and pipeline composition.
+
+## Core Types
+
+| Type | Package | Represents |
+|------|---------|------------|
+| `Option[A]` | `option` | A value that may or may not be present (replaces nil) |
+| `Either[E, A]` | `either` | A value that is either a left error `E` or a right success `A` |
+| `Result[A]` | `result` | `Either[error, A]` — shorthand for the common case |
+| `IO[A]` | `io` | A lazy computation that produces `A` (possibly with side effects) |
+| `IOResult[A]` | `ioresult` | `IO[Result[A]]` — lazy computation that can fail |
+| `ReaderIOResult[A]` | `context/readerioresult` | `func(context.Context) IOResult[A]` — context-aware IO with errors |
+| `Effect[C, A]` | `effect` | `func(C) ReaderIOResult[A]` — typed dependency injection + IO + errors |
+
+Idiomatic (high-performance, tuple-based) equivalents live in `idiomatic/`:
+- `idiomatic/option` — `(A, bool)` tuples
+- `idiomatic/result` — `(A, error)` tuples
+- `idiomatic/ioresult` — `func() (A, error)`
+- `idiomatic/context/readerresult` — `func(context.Context) (A, error)`
+
+## Standard Operations
+
+Every monad exports these operations (PascalCase for exported Go names):
+
+| fp-go | fp-ts / Haskell | Description |
+|-------|----------------|-------------|
+| `Of` | `of` / `pure` | Lift a pure value into the monad |
+| `Map` | `map` / `fmap` | Transform the value inside without changing the context |
+| `Chain` | `chain` / `>>=` | Sequence a computation that itself returns a monadic value |
+| `Ap` | `ap` / `<*>` | Apply a wrapped function to a wrapped value |
+| `Fold` | `fold` / `either` | Eliminate the context — handle every case and extract a plain value |
+| `GetOrElse` | `getOrElse` / `fromMaybe` | Extract the value or use a default (Option/Result) |
+| `Filter` | `filter` / `mfilter` | Keep only values satisfying a predicate |
+| `Flatten` | `flatten` / `join` | Remove one level of nesting (`M[M[A]]` → `M[A]`) |
+| `ChainFirst` | `chainFirst` / `>>` | Sequence for side effects; keeps the original value |
+| `Alt` | `alt` / `<\|>` | Provide an alternative when the first computation fails |
+| `FromPredicate` | `fromPredicate` / `guard` | Build a monadic value from a predicate |
+| `Sequence` | `sequence` | Turn `[]M[A]` into `M[[]A]` |
+| `Traverse` | `traverse` | Map and sequence in one step |
+
+Curried (composable) vs. monadic (direct) form:
+
+```go
+// Curried — data last, returns a transformer function
+option.Map(strings.ToUpper)              // func(Option[string]) Option[string]
+
+// Monadic — data first, immediate execution
+option.MonadMap(option.Some("hello"), strings.ToUpper)
+```
+
+Use curried form for pipelines; use `Monad*` form when you already have all arguments.
+
+## Key Type Aliases (defined per monad)
+
+```go
+// A Kleisli arrow: a function from A to a monadic B
+type Kleisli[A, B any] = func(A) M[B]
+
+// An operator: transforms one monadic value into another
+type Operator[A, B any] = func(M[A]) M[B]
+```
+
+`Chain` takes a `Kleisli`, `Map` returns an `Operator`. The naming is consistent across all monads.
+
+## Examples
+
+### Option — nullable values without nil
+
+```go
+import (
+    O "github.com/IBM/fp-go/v2/option"
+    F "github.com/IBM/fp-go/v2/function"
+    "strconv"
+)
+
+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)
+    }),
+)
+
+parseAndDouble("21")  // Some(42)
+parseAndDouble("")    // None
+parseAndDouble("abc") // None
+```
+
+### Result — error handling without if-err boilerplate
+
+```go
+import (
+    R  "github.com/IBM/fp-go/v2/result"
+    F  "github.com/IBM/fp-go/v2/function"
+    "strconv"
+    "errors"
+)
+
+parse := R.Eitherize1(strconv.Atoi)  // lifts (int, error) → Result[int]
+
+validate := func(n int) R.Result[int] {
+    if n < 0 {
+        return R.Error[int](errors.New("must be non-negative"))
+    }
+    return R.Of(n)
+}
+
+pipeline := F.Flow2(parse, R.Chain(validate))
+
+pipeline("42")   // Ok(42)
+pipeline("-1")   // Error("must be non-negative")
+pipeline("abc")  // Error(strconv parse error)
+```
+
+### IOResult — lazy IO with error handling
+
+```go
+import (
+    IOE "github.com/IBM/fp-go/v2/ioresult"
+    F   "github.com/IBM/fp-go/v2/function"
+    J   "github.com/IBM/fp-go/v2/json"
+    "os"
+)
+
+readConfig := F.Flow2(
+    IOE.Eitherize1(os.ReadFile),           // func(string) IOResult[[]byte]
+    IOE.ChainEitherK(J.Unmarshal[Config]), // parse JSON, propagate errors
+)
+
+result := readConfig("config.json")() // execute lazily
+```
+
+### ReaderIOResult — context-aware pipelines (recommended for services)
+
+```go
+import (
+    RIO "github.com/IBM/fp-go/v2/context/readerioresult"
+    F   "github.com/IBM/fp-go/v2/function"
+    "context"
+)
+
+// type ReaderIOResult[A any] = func(context.Context) func() result.Result[A]
+
+fetchUser := func(id int) RIO.ReaderIOResult[User] {
+    return func(ctx context.Context) func() result.Result[User] {
+        return func() result.Result[User] {
+            // perform IO here
+        }
+    }
+}
+
+pipeline := F.Pipe3(
+    fetchUser(42),
+    RIO.ChainEitherK(validateUser),    // lift pure (User, error) function
+    RIO.Map(enrichUser),               // lift pure User → User function
+    RIO.ChainFirstIOK(IO.Logf[User]("Fetched: %v")), // side-effect logging
+)
+
+user, err := pipeline(ctx)() // provide context once, execute
+```
+
+### Traversal — process slices monadically
+
+```go
+import (
+    A   "github.com/IBM/fp-go/v2/array"
+    RIO "github.com/IBM/fp-go/v2/context/readerioresult"
+    F   "github.com/IBM/fp-go/v2/function"
+)
+
+// Fetch all users, stop on first error
+fetchAll := F.Pipe1(
+    A.MakeBy(10, userID),
+    RIO.TraverseArray(fetchUser),  // []ReaderIOResult[User] → ReaderIOResult[[]User]
+)
+```
+
+## Function Composition with Flow and Pipe
+
+```go
+import F "github.com/IBM/fp-go/v2/function"
+
+// Flow: compose functions left-to-right, returns a new function
+transform := F.Flow3(
+    option.Map(strings.TrimSpace),
+    option.Filter(func(s string) bool { return s != "" }),
+    option.GetOrElse(func() string { return "default" }),
+)
+result := transform(option.Some("  hello  ")) // "hello"
+
+// Pipe: apply a value through a pipeline immediately
+result := F.Pipe3(
+    option.Some("  hello  "),
+    option.Map(strings.TrimSpace),
+    option.Filter(func(s string) bool { return s != "" }),
+    option.GetOrElse(func() string { return "default" }),
+)
+```
+
+## Lifting Pure Functions into Monadic Context
+
+fp-go provides helpers to promote non-monadic functions:
+
+| Helper | Lifts |
+|--------|-------|
+| `ChainEitherK` | `func(A) (B, error)` → works inside the monad |
+| `ChainOptionK` | `func(A) Option[B]` → works inside the monad |
+| `ChainFirstIOK` | `func(A) IO[B]` for side effects, keeps original value |
+| `Eitherize1..N` | `func(A) (B, error)` → `func(A) Result[B]` |
+| `FromPredicate` | `func(A) bool` + error builder → `func(A) Result[A]` |
+
+## Type Parameter Ordering Rule (V2)
+
+Non-inferrable type parameters come **first**, so the compiler can infer the rest:
+
+```go
+// B cannot be inferred from the argument — it comes first
+result := either.Ap[string](value)(funcInEither)
+
+// All types inferrable — no explicit params needed
+result := either.Map(transform)(value)
+result := either.Chain(validator)(value)
+```
+
+## When to Use Which Monad
+
+| Situation | Use |
+|-----------|-----|
+| Value that might be absent | `Option[A]` |
+| Operation that can fail with custom error type | `Either[E, A]` |
+| Operation that can fail with `error` | `Result[A]` |
+| Lazy IO, side effects | `IO[A]` |
+| IO that can fail | `IOResult[A]` |
+| IO + context (cancellation, deadlines) | `ReaderIOResult[A]` from `context/readerioresult` |
+| IO + context + typed dependencies | `Effect[C, A]` |
+| High-performance services | Idiomatic packages in `idiomatic/` |
+
+## Do-Notation: Accumulating State with `Bind` and `ApS`
+
+When a pipeline needs to carry **multiple intermediate results** forward — not just a single value — the `Chain`/`Map` style becomes unwieldy because each step only threads one value and prior results are lost. Do-notation solves this by accumulating results into a growing struct (the "state") at each step.
+
+Every monad that supports do-notation exports the same family of functions. The examples below use `context/readerioresult` (`RIO`), but the identical API is available in `result`, `option`, `ioresult`, `readerioresult`, and others.
+
+### The Function Family
+
+| Function | Kind | What it does |
+|----------|------|-------------|
+| `Do(empty S)` | — | Lift an empty struct into the monad; starting point |
+| `BindTo(setter)` | monadic | Convert an existing `M[T]` into `M[S]`; alternative start |
+| `Bind(setter, f)` | monadic | Add a result; `f` receives the **current state** and returns `M[T]` |
+| `ApS(setter, fa)` | applicative | Add a result; `fa` is **independent** of the current state |
+| `Let(setter, f)` | pure | Add a value computed by a **pure function** of the state |
+| `LetTo(setter, value)` | pure | Add a **constant** value |
+
+Lens variants (`BindL`, `ApSL`, `LetL`, `LetToL`) accept a `Lens[S, T]` instead of a manual setter, integrating naturally with the optics system.
+
+### `Bind` — Sequential, Dependent Steps
+
+`Bind` sequences two monadic computations. The function `f` receives the **full accumulated state** so it can read anything gathered so far. Errors short-circuit automatically.
+
+```go
+import (
+    RIO "github.com/IBM/fp-go/v2/context/readerioresult"
+    F   "github.com/IBM/fp-go/v2/function"
+    L   "github.com/IBM/fp-go/v2/optics/lens"
+    "context"
+)
+
+type Pipeline struct {
+    User   User
+    Config Config
+    Posts  []Post
+}
+
+// Lenses — focus on individual fields; .Set is already func(T) func(S) S
+var (
+    userLens   = L.MakeLens(func(s Pipeline) User   { return s.User },   func(s Pipeline, u User)   Pipeline { s.User = u; return s })
+    configLens = L.MakeLens(func(s Pipeline) Config { return s.Config }, func(s Pipeline, c Config) Pipeline { s.Config = c; return s })
+    postsLens  = L.MakeLens(func(s Pipeline) []Post { return s.Posts },  func(s Pipeline, p []Post) Pipeline { s.Posts = p; return s })
+)
+
+result := F.Pipe3(
+    RIO.Do(Pipeline{}),                                                   // lift empty struct
+    RIO.Bind(userLens.Set,   func(_ Pipeline) RIO.ReaderIOResult[User] { return fetchUser(42) }),
+    RIO.Bind(configLens.Set, F.Flow2(userLens.Get, fetchConfigForUser)), // read s.User, pass to fetcher
+    RIO.Bind(postsLens.Set,  F.Flow2(userLens.Get, fetchPostsForUser)),  // read s.User, pass to fetcher
+)
+
+pipeline, err := result(context.Background())()
+// pipeline.User, pipeline.Config, pipeline.Posts are all populated
+```
+
+The setter signature is `func(T) func(S1) S2` — it takes the new value and returns a state transformer. `lens.Set` already has this shape, so no manual setter functions are needed. `F.Flow2(lens.Get, f)` composes the field getter with any Kleisli arrow `f` point-free.
+
+### `ApS` — Independent, Applicative Steps
+
+`ApS` uses **applicative** semantics: `fa` is evaluated without any access to the current state. Use it when steps have no dependency on each other — the library can choose to execute them concurrently.
+
+```go
+import (
+    RIO "github.com/IBM/fp-go/v2/context/readerioresult"
+    F   "github.com/IBM/fp-go/v2/function"
+    L   "github.com/IBM/fp-go/v2/optics/lens"
+)
+
+type Summary struct {
+    User    User
+    Weather Weather
+}
+
+var (
+    userLens    = L.MakeLens(func(s Summary) User    { return s.User },    func(s Summary, u User)    Summary { s.User = u; return s })
+    weatherLens = L.MakeLens(func(s Summary) Weather { return s.Weather }, func(s Summary, w Weather) Summary { s.Weather = w; return s })
+)
+
+// Both are independent — neither needs the other's result
+result := F.Pipe2(
+    RIO.Do(Summary{}),
+    RIO.ApS(userLens.Set,    fetchUser(42)),
+    RIO.ApS(weatherLens.Set, fetchWeather("NYC")),
+)
+```
+
+**Key difference from `Bind`:**
+
+| | `Bind(setter, f)` | `ApS(setter, fa)` |
+|-|---|---|
+| Second argument | `func(S1) M[T]` — a **function** of state | `M[T]` — a **fixed** monadic value |
+| Can read prior state? | Yes — receives `S1` | No — no access to state |
+| Semantics | Monadic (sequential) | Applicative (independent) |
+
+### `Let` and `LetTo` — Pure Additions
+
+`Let` adds a value computed by a **pure function** of the current state (no monad, cannot fail):
+
+```go
+import (
+    RIO "github.com/IBM/fp-go/v2/context/readerioresult"
+    F   "github.com/IBM/fp-go/v2/function"
+    L   "github.com/IBM/fp-go/v2/optics/lens"
+)
+
+type Enriched struct {
+    User     User
+    FullName string
+}
+
+var (
+    userLens     = L.MakeLens(func(s Enriched) User   { return s.User },     func(s Enriched, u User)   Enriched { s.User = u; return s })
+    fullNameLens  = L.MakeLens(func(s Enriched) string { return s.FullName }, func(s Enriched, n string) Enriched { s.FullName = n; return s })
+)
+
+fullName := func(u User) string { return u.FirstName + " " + u.LastName }
+
+result := F.Pipe2(
+    RIO.Do(Enriched{}),
+    RIO.Bind(userLens.Set,     func(_ Enriched) RIO.ReaderIOResult[User] { return fetchUser(42) }),
+    RIO.Let(fullNameLens.Set,  F.Flow2(userLens.Get, fullName)), // read s.User, compute pure string
+)
+```
+
+`LetTo` adds a **constant** with no computation:
+
+```go
+RIO.LetTo(setVersion, "v1.2.3")
+```
+
+### `BindTo` — Starting from an Existing Value
+
+When you have an existing `M[T]` and want to project it into a state struct rather than starting from `Do(empty)`:
+
+```go
+type State struct{ User User }
+
+result := F.Pipe1(
+    fetchUser(42),                                           // ReaderIOResult[User]
+    RIO.BindTo(func(u User) State { return State{User: u} }),// ReaderIOResult[State]
+)
+```
+
+### Lens Variants (`ApSL`, `BindL`, `LetL`, `LetToL`)
+
+If you have a `Lens[S, T]` (from the optics system or code generation), you can skip writing the setter function entirely:
+
+```go
+import (
+    RO "github.com/IBM/fp-go/v2/readeroption"
+    F  "github.com/IBM/fp-go/v2/function"
+)
+
+// Lenses generated by go:generate (see optics/README.md)
+// personLenses.Name : Lens[*Person, Name]
+// personLenses.Age  : Lens[*Person, Age]
+
+makePerson := F.Pipe2(
+    RO.Do[*PartialPerson](emptyPerson),
+    RO.ApSL(personLenses.Name, maybeName), // replaces: ApS(personLenses.Name.Set, maybeName)
+    RO.ApSL(personLenses.Age,  maybeAge),
+)
+```
+
+This exact pattern is used in [`samples/builder`](samples/builder/builder.go) to validate and construct a `Person` from an unvalidated `PartialPerson`.
+
+### Lifted Variants for Mixed Monads
+
+`context/readerioresult` provides `Bind*K` helpers that lift simpler computations directly into the do-chain:
+
+| Helper | Lifts |
+|--------|-------|
+| `BindResultK` / `BindEitherK` | `func(S1) (T, error)` — pure result |
+| `BindIOResultK` / `BindIOEitherK` | `func(S1) func() (T, error)` — lazy IO result |
+| `BindIOK` | `func(S1) func() T` — infallible IO |
+| `BindReaderK` | `func(S1) func(ctx) T` — context reader |
+
+```go
+RIO.BindResultK(setUser, func(s Pipeline) (User, error) {
+    return validateAndBuild(s)   // plain (value, error) function, no wrapping needed
+})
+```
+
+### Decision Guide
+
+```
+Does the new step need to read prior accumulated state?
+    YES  →  Bind   (monadic, sequential; f receives current S)
+    NO   →  ApS    (applicative, independent; fa is a fixed M[T])
+
+Is the new value derived purely from state, with no monad?
+    YES  →  Let    (pure function of S)
+
+Is the new value a compile-time or runtime constant?
+    YES  →  LetTo
+
+Starting from an existing M[T] rather than an empty struct?
+    YES  →  BindTo
+```
+
+### Complete Example — `result` Monad
+
+The same pattern works with simpler monads. Here with `result.Result[A]`:
+
+`Eitherize1` converts any standard `func(A) (B, error)` into `func(A) Result[B]`. Define these lifted functions once as variables. Then use lenses to focus on individual struct fields and compose with `F.Flow2(lens.Get, f)` — no inline lambdas, no manual error handling.
+
+```go
+import (
+    R    "github.com/IBM/fp-go/v2/result"
+    F    "github.com/IBM/fp-go/v2/function"
+    L    "github.com/IBM/fp-go/v2/optics/lens"
+    N    "github.com/IBM/fp-go/v2/number"
+    "strconv"
+)
+
+type Parsed struct {
+    Raw    string
+    Number int
+    Double int
+}
+
+// Lenses — focus on individual fields of Parsed.
+var (
+    rawLens    = L.MakeLens(
+        func(s Parsed) string { return s.Raw },
+        func(s Parsed, v string) Parsed { s.Raw = v; return s },
+    )
+    numberLens = L.MakeLens(
+        func(s Parsed) int { return s.Number },
+        func(s Parsed, v int) Parsed { s.Number = v; return s },
+    )
+    doubleLens = L.MakeLens(
+        func(s Parsed) int { return s.Double },
+        func(s Parsed, v int) Parsed { s.Double = v; return s },
+    )
+)
+
+// Lifted functions — convert standard (value, error) functions into Result-returning ones.
+var (
+    atoi = R.Eitherize1(strconv.Atoi) // func(string) Result[int]
+)
+
+parse := func(input string) R.Result[Parsed] {
+    return F.Pipe3(
+        R.Do(Parsed{}),
+        R.LetTo(rawLens.Set, input),                      // set Raw to constant input
+        R.Bind(numberLens.Set, F.Flow2(rawLens.Get, atoi)),           // get Raw, parse → Result[int]
+        R.Let(doubleLens.Set, F.Flow2(numberLens.Get, N.Mul(2))),   // get Number, multiply → int
+    )
+}
+
+parse("21")  // Ok(Parsed{Raw:"21", Number:21, Double:42})
+parse("abc") // Error(strconv parse error)
+```
+
+`rawLens.Set` is already `func(string) func(Parsed) Parsed`, matching the setter signature `Bind` and `LetTo` expect — no manual setter functions to write. `F.Flow2(rawLens.Get, atoi)` composes the field getter with the eitherized parse function into a `Kleisli[Parsed, int]` without any intermediate lambda.
+
+## Import Paths
+
+```go
+import (
+    "github.com/IBM/fp-go/v2/option"
+    "github.com/IBM/fp-go/v2/result"
+    "github.com/IBM/fp-go/v2/either"
+    "github.com/IBM/fp-go/v2/io"
+    "github.com/IBM/fp-go/v2/ioresult"
+    "github.com/IBM/fp-go/v2/context/readerioresult"
+    "github.com/IBM/fp-go/v2/effect"
+    F "github.com/IBM/fp-go/v2/function"
+    A "github.com/IBM/fp-go/v2/array"
+)
+```
+
+Requires Go 1.24+ (generic type aliases).

v2/AGENTS.md

@@ -0,0 +1,238 @@
+# Agent Guidelines for fp-go/v2
+
+This document provides guidelines for AI agents working on the fp-go/v2 project.
+
+## Table of Contents
+
+- [Documentation Standards](#documentation-standards)
+  - [Go Doc Comments](#go-doc-comments)
+  - [File Headers](#file-headers)
+- [Testing Standards](#testing-standards)
+  - [Test Structure](#test-structure)
+  - [Test Coverage](#test-coverage)
+  - [Example Test Pattern](#example-test-pattern)
+- [Code Style](#code-style)
+  - [Functional Patterns](#functional-patterns)
+  - [Error Handling](#error-handling)
+- [Checklist for New Code](#checklist-for-new-code)
+
+## Documentation Standards
+
+### Go Doc Comments
+
+1. **Use Standard Go Doc Format**
+   - Do NOT use markdown-style links like `[text](url)`
+   - Do NOT use markdown-style headers like `# Section` or `## Subsection`
+   - Use simple type references: `ReaderResult`, `Validate[I, A]`, `validation.Success`
+   - Go's documentation system will automatically create links
+   - Use plain text with blank lines to separate sections
+
+2. **Structure**
+   ```go
+   // FunctionName does something useful.
+   //
+   // Longer description explaining the purpose and behavior.
+   //
+   // Type Parameters:
+   //   - T: Description of type parameter
+   //
+   // Parameters:
+   //   - param: Description of parameter
+   //
+   // Returns:
+   //   - ReturnType: Description of return value
+   //
+   // Example:
+   //
+   //   code example here
+   //
+   // See Also:
+   //   - RelatedFunction: Brief description
+   func FunctionName[T any](param T) ReturnType {
+   ```
+
+3. **Code Examples**
+   - Use idiomatic Go patterns
+   - Prefer `result.Eitherize1(strconv.Atoi)` over manual error handling
+   - Show realistic, runnable examples
+   - Indent code examples with spaces (not tabs) for proper godoc rendering
+
+### File Headers
+
+Always include the Apache 2.0 license header:
+
+```go
+// 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.
+```
+
+## Testing Standards
+
+### Test Structure
+
+1. **Organize Tests by Category**
+   ```go
+   func TestFunctionName_Success(t *testing.T) {
+       t.Run("specific success case", func(t *testing.T) {
+           // test code
+       })
+   }
+   
+   func TestFunctionName_Failure(t *testing.T) {
+       t.Run("specific failure case", func(t *testing.T) {
+           // test code
+       })
+   }
+   
+   func TestFunctionName_EdgeCases(t *testing.T) {
+       // edge case tests
+   }
+   
+   func TestFunctionName_Integration(t *testing.T) {
+       // integration tests
+   }
+   ```
+
+2. **Use Direct Assertions**
+   - Prefer: `assert.Equal(t, validation.Success(expected), actual)`
+   - Avoid: Verbose `either.MonadFold` patterns unless necessary
+   - Exception: When you need to verify pointer is not nil or extract specific fields
+
+3. **Use Idiomatic Patterns**
+   - Use `result.Eitherize1` for converting `(T, error)` functions
+   - Use `result.Of` for success values
+   - Use `result.Left` for error values
+
+4. **Folding Either/Result Values in Tests**
+   - Use `F.Pipe1(result, Fold(onLeft, onRight))` — avoid the `_ = Fold(...)(result)` discard pattern
+   - Use `slices.Collect[T]` instead of a manual `for n := range seq { collected = append(...) }` loop
+   - Use `t.Fatal` in the unexpected branch to combine the `IsLeft`/`IsRight` check with value extraction:
+     ```go
+     // Good: single fold combines assertion and extraction
+     collected := F.Pipe1(result, Fold(
+         func(e error) []int { t.Fatal(e); return nil },
+         slices.Collect[int],
+     ))
+
+     // Avoid: separate IsRight check + manual loop
+     assert.True(t, IsRight(result))
+     var collected []int
+     _ = MonadFold(result,
+         func(e error) []int { return nil },
+         func(seq iter.Seq[int]) []int {
+             for n := range seq { collected = append(collected, n) }
+             return collected
+         },
+     )
+     ```
+   - Use `F.Identity[error]` as the Left branch when extracting an error value:
+     ```go
+     err := F.Pipe1(result, Fold(
+         F.Identity[error],
+         func(_ iter.Seq[int]) error { t.Fatal("expected Left but got Right"); return nil },
+     ))
+     ```
+   - Extract repeated fold patterns as local helper closures within the test function:
+     ```go
+     collectInts := func(r Result[iter.Seq[int]]) []int {
+         return F.Pipe1(r, Fold(
+             func(e error) []int { t.Fatal(e); return nil },
+             slices.Collect[int],
+         ))
+     }
+     ```
+
+5. **Other Test Style Details**
+   - Use `for i := range 10` instead of `for i := 0; i < 10; i++`
+   - Chain curried calls directly: `TraverseSeq(parse)(input)` — no need for an intermediate `traverseFn` variable
+   - Use direct slice literals (`[]string{"a", "b"}`) rather than `A.From("a", "b")` in tests
+
+### Test Coverage
+
+Include tests for:
+- **Success cases**: Normal operation with various input types
+- **Failure cases**: Error handling and error preservation
+- **Edge cases**: Nil, empty, zero values, boundary conditions
+- **Integration**: Composition with other functions
+- **Type safety**: Verify type parameters work correctly
+- **Benchmarks**: Performance-critical paths
+
+### Example Test Pattern
+
+```go
+func TestFromReaderResult_Success(t *testing.T) {
+    t.Run("converts successful ReaderResult", func(t *testing.T) {
+        // Arrange
+        parseIntRR := result.Eitherize1(strconv.Atoi)
+        validator := FromReaderResult[string, int](parseIntRR)
+        
+        // Act
+        result := validator("42")(nil)
+        
+        // Assert
+        assert.Equal(t, validation.Success(42), result)
+    })
+}
+```
+
+## Code Style
+
+### Functional Patterns
+
+1. **Prefer Composition**
+   ```go
+   validator := F.Pipe1(
+       FromReaderResult[string, int](parseIntRR),
+       Chain(validatePositive),
+   )
+   ```
+
+2. **Use Type-Safe Helpers**
+   - `result.Eitherize1` for `func(T) (R, error)`
+   - `result.Of` for wrapping success values
+   - `result.Left` for wrapping errors
+
+3. **Avoid Verbose Patterns**
+   - Don't manually handle `(value, error)` tuples when helpers exist
+   - Don't use `either.MonadFold` in tests unless necessary
+
+4. **Use Void Type for Unit Values**
+   - Use `function.Void` (or `F.Void`) instead of `struct{}`
+   - Use `function.VOID` (or `F.VOID`) instead of `struct{}{}`
+   - Example: `Empty[F.Void, F.Void, any](lazy.Of(pair.MakePair(F.VOID, F.VOID)))`
+
+### Error Handling
+
+1. **In Production Code**
+   - Use `validation.Success` for successful validations
+   - Use `validation.FailureWithMessage` for simple failures
+   - Use `validation.FailureWithError` to preserve error causes
+
+2. **In Tests**
+   - Verify error messages and causes
+   - Check error context is preserved
+   - Test error accumulation when applicable
+
+## Checklist for New Code
+
+- [ ] Apache 2.0 license header included
+- [ ] Go doc comments use standard format (no markdown links)
+- [ ] Code examples are idiomatic and concise
+- [ ] Tests cover success, failure, edge cases, and integration
+- [ ] Tests use direct assertions where possible
+- [ ] Benchmarks included for performance-critical code
+- [ ] All tests pass
+- [ ] Code uses functional composition patterns
+- [ ] Error handling preserves context and causes

v2/FUNCTIONAL_IO.md

@@ -1,6 +1,6 @@
 # Functional I/O in Go: Context, Errors, and the Reader Pattern
 
-This document explores how functional programming principles apply to I/O operations in Go, comparing traditional imperative approaches with functional patterns using the `context/readerioresult` and `idiomatic/context/readerresult` packages.
+This document explores how functional programming principles apply to I/O operations in Go, comparing traditional imperative approaches with functional patterns using the `context/readerioresult`, `idiomatic/context/readerresult`, and `effect` packages.
 
 ## Table of Contents
 
@@ -10,6 +10,7 @@ This document explores how functional programming principles apply to I/O operat
 - [Benefits of the Functional Approach](#benefits-of-the-functional-approach)
 - [Side-by-Side Comparison](#side-by-side-comparison)
 - [Advanced Patterns](#advanced-patterns)
+- [The Effect Package: Higher-Level Abstraction](#the-effect-package-higher-level-abstraction)
 - [When to Use Each Approach](#when-to-use-each-approach)
 
 ## Why Context in I/O Operations
@@ -775,6 +776,191 @@ func FetchWithRetry(url string, maxRetries int) RIO.ReaderIOResult[[]byte] {
 }
 ```
 
+## The Effect Package: Higher-Level Abstraction
+
+### What is Effect?
+
+The `effect` package provides a higher-level abstraction over `ReaderReaderIOResult`, offering a complete effect system for managing dependencies, errors, and side effects in a composable way. It's inspired by [effect-ts](https://effect.website/) and provides a cleaner API for complex workflows.
+
+### Core Type
+
+```go
+// Effect represents an effectful computation that:
+//   - Requires a context of type C (dependency injection)
+//   - Can perform I/O operations
+//   - Can fail with an error
+//   - Produces a value of type A on success
+type Effect[C, A any] = readerreaderioresult.ReaderReaderIOResult[C, A]
+```
+
+**Key difference from ReaderIOResult**: Effect adds an additional layer of dependency injection (the `C` type parameter) on top of the runtime `context.Context`, enabling type-safe dependency management.
+
+### When to Use Effect
+
+Use the Effect package when you need:
+
+1. **Type-Safe Dependency Injection**: Your application has typed dependencies (config, services, repositories) that need to be threaded through operations
+2. **Complex Workflows**: Multiple services and dependencies need to be composed
+3. **Testability**: You want to easily mock dependencies by providing different contexts
+4. **Separation of Concerns**: Clear separation between business logic, dependencies, and I/O
+
+### Effect vs ReaderIOResult
+
+```go
+// ReaderIOResult - depends only on runtime context
+type ReaderIOResult[A any] = func(context.Context) (A, error)
+
+// Effect - depends on typed context C AND runtime context
+type Effect[C, A any] = func(C) func(context.Context) (A, error)
+```
+
+**ReaderIOResult** is simpler and suitable when you only need runtime context (cancellation, deadlines, request-scoped values).
+
+**Effect** adds typed dependency injection, making it ideal for applications with complex service dependencies.
+
+### Basic Usage
+
+#### Creating Effects
+
+```go
+type AppConfig struct {
+    DatabaseURL string
+    APIKey      string
+}
+
+// Create a successful effect
+successEffect := effect.Succeed[AppConfig, string]("hello")
+
+// Create a failed effect
+failEffect := effect.Fail[AppConfig, string](errors.New("failed"))
+
+// Lift a pure value
+pureEffect := effect.Of[AppConfig, int](42)
+```
+
+#### Integrating Standard Go Functions
+
+The `Eitherize` function makes it easy to integrate standard Go functions that return `(value, error)`:
+
+```go
+type Database struct {
+    conn *sql.DB
+}
+
+// Convert a standard Go function to an Effect using Eitherize
+func fetchUser(id int) effect.Effect[Database, User] {
+    return effect.Eitherize(func(db Database, ctx context.Context) (User, error) {
+        var user User
+        err := db.conn.QueryRowContext(ctx, "SELECT * FROM users WHERE id = ?", id).Scan(&user)
+        return user, err
+    })
+}
+
+// Use Eitherize1 for Kleisli arrows (functions with an additional parameter)
+fetchUserKleisli := effect.Eitherize1(func(db Database, ctx context.Context, id int) (User, error) {
+    var user User
+    err := db.conn.QueryRowContext(ctx, "SELECT * FROM users WHERE id = ?", id).Scan(&user)
+    return user, err
+})
+// fetchUserKleisli has type: func(int) Effect[Database, User]
+```
+
+#### Composing Effects
+
+```go
+type Services struct {
+    UserRepo UserRepository
+    EmailSvc EmailService
+}
+
+// Compose multiple effects with typed dependencies
+func processUser(id int, newEmail string) effect.Effect[Services, User] {
+    return F.Pipe3(
+        // Fetch user from repository
+        effect.Eitherize(func(svc Services, ctx context.Context) (User, error) {
+            return svc.UserRepo.GetUser(ctx, id)
+        }),
+        // Validate user (pure function lifted into Effect)
+        effect.ChainEitherK[Services](validateUser),
+        // Update email
+        effect.Chain[Services](func(user User) effect.Effect[Services, User] {
+            return effect.Eitherize(func(svc Services, ctx context.Context) (User, error) {
+                if err := svc.EmailSvc.SendVerification(ctx, newEmail); err != nil {
+                    return User{}, err
+                }
+                return svc.UserRepo.UpdateEmail(ctx, user.ID, newEmail)
+            })
+        }),
+    )
+}
+```
+
+#### Running Effects
+
+```go
+func main() {
+    // Set up typed dependencies once
+    services := Services{
+        UserRepo: &PostgresUserRepo{db: db},
+        EmailSvc: &SMTPEmailService{host: "smtp.example.com"},
+    }
+
+    // Build the effect pipeline (no execution yet)
+    userEffect := processUser(42, "new@email.com")
+
+    // Provide dependencies - returns a Thunk (ReaderIOResult)
+    thunk := effect.Provide(services)(userEffect)
+
+    // Run synchronously - returns a func(context.Context) (User, error)
+    readerResult := effect.RunSync(thunk)
+
+    // Execute with runtime context
+    user, err := readerResult(context.Background())
+    if err != nil {
+        log.Fatal(err)
+    }
+
+    fmt.Printf("Updated user: %+v\n", user)
+}
+```
+
+### Comparison: Traditional vs ReaderIOResult vs Effect
+
+| Aspect | Traditional | ReaderIOResult | Effect |
+|---|---|---|---|
+| Error propagation | Manual | Automatic | Automatic |
+| Dependency injection | Function parameters | Closure / `context.Context` | Typed `C` parameter |
+| Testability | Requires mocking | Mock `ReaderIOResult` | Provide mock `C` |
+| Composability | Low | High | High |
+| Type-safe dependencies | No | No | Yes |
+| Complexity | Low | Medium | Medium-High |
+
+### Testing with Effect
+
+One of the key benefits of Effect is easy testing through dependency substitution:
+
+```go
+func TestProcessUser(t *testing.T) {
+    // Create mock services
+    mockServices := Services{
+        UserRepo: &MockUserRepo{
+            users: map[int]User{42: {ID: 42, Age: 25, Email: "old@email.com"}},
+        },
+        EmailSvc: &MockEmailService{},
+    }
+
+    // Run the effect with mock dependencies
+    user, err := effect.RunSync(
+        effect.Provide(mockServices)(processUser(42, "new@email.com")),
+    )(context.Background())
+
+    assert.NoError(t, err)
+    assert.Equal(t, "new@email.com", user.Email)
+}
+```
+
+No database or SMTP server needed — just provide mock implementations of the `Services` struct.
+
 ## When to Use Each Approach
 
 ### Use Traditional Go Style When:
@@ -794,6 +980,17 @@ func FetchWithRetry(url string, maxRetries int) RIO.ReaderIOResult[[]byte] {
 5. **Resource management**: Need guaranteed cleanup (Bracket)
 6. **Parallel execution**: Need to parallelize operations easily
 7. **Type safety**: Want the type system to track I/O effects
+8. **Simple dependencies**: Only need runtime `context.Context`, no typed dependencies
+
+### Use Effect When:
+
+1. **Type-safe dependency injection**: Application has typed dependencies (config, services, repositories)
+2. **Complex service architectures**: Multiple services need to be composed with clear dependency management
+3. **Testability with mocks**: Want to easily substitute dependencies for testing
+4. **Separation of concerns**: Need clear separation between business logic, dependencies, and I/O
+5. **Large applications**: Building applications where dependency management is critical
+6. **Team experience**: Team is comfortable with functional programming and effect systems
+7. **Integration with standard Go**: Need to integrate many standard `(value, error)` functions using `Eitherize`
 
 ### Use Idiomatic Functional Style (idiomatic/context/readerresult) When:
 
@@ -803,6 +1000,7 @@ func FetchWithRetry(url string, maxRetries int) RIO.ReaderIOResult[[]byte] {
 4. **Go integration**: Want seamless integration with existing Go code
 5. **Production services**: Building high-throughput services
 6. **Best of both worlds**: Want functional composition with Go's native patterns
+7. **Simple dependencies**: Only need runtime `context.Context`, no typed dependencies
 
 ## Summary
 
@@ -814,11 +1012,18 @@ The functional approach to I/O in Go offers significant advantages:
 4. **Type Safety**: I/O effects visible in the type system
 5. **Lazy Evaluation**: Build pipelines, execute when ready
 6. **Context Propagation**: Automatic threading of context
-7. **Performance**: Idiomatic version offers 2-10x speedup
+7. **Dependency Injection**: Type-safe dependency management with Effect
+8. **Performance**: Idiomatic version offers 2-10x speedup
 
-The key insight is that **I/O operations return descriptions of effects** (ReaderIOResult) rather than performing effects immediately. This enables powerful composition patterns while maintaining Go's idiomatic error handling through the `(value, error)` tuple pattern.
+The key insight is that **I/O operations return descriptions of effects** rather than performing effects immediately. This enables powerful composition patterns while maintaining Go's idiomatic error handling through the `(value, error)` tuple pattern.
 
-For production Go services, the **idiomatic/context/readerresult** package provides the best balance: full functional programming capabilities with native Go performance and familiar error handling patterns.
+### Choosing the Right Abstraction
+
+- **ReaderIOResult**: Best for simple I/O pipelines that only need runtime `context.Context`
+- **Effect**: Best for complex applications with typed dependencies and service architectures
+- **idiomatic/context/readerresult**: Best for production services needing high performance with functional patterns
+
+For production Go services, the **idiomatic/context/readerresult** package provides the best balance of performance and functional capabilities. For applications with complex dependency management, the **effect** package provides type-safe dependency injection with a clean, composable API.
 
 ## Further Reading
 
@@ -826,4 +1031,6 @@ For production Go services, the **idiomatic/context/readerresult** package provi
 - [IDIOMATIC_COMPARISON.md](./IDIOMATIC_COMPARISON.md) - Performance comparison
 - [idiomatic/doc.go](./idiomatic/doc.go) - Idiomatic package overview
 - [context/readerioresult](./context/readerioresult/) - ReaderIOResult package
-- [idiomatic/context/readerresult](./idiomatic/context/readerresult/) - Idiomatic ReaderResult package
+- [idiomatic/context/readerresult](./idiomatic/context/readerresult/) - Idiomatic ReaderResult package
+- [effect](./effect/) - Effect package for type-safe dependency injection
+- [effect-ts](https://effect.website/) - TypeScript effect system that inspired this package

v2/README.md

@@ -3,6 +3,7 @@
 [![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)
+[![Context7](https://img.shields.io/badge/context7-docs-blue)](https://context7.com/ibm/fp-go)
 
 **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.
 
@@ -461,7 +462,8 @@ func process() IOResult[string] {
 - **Result** - Simplified Either with error as left type (recommended for error handling)
 - **IO** - Lazy evaluation and side effect management
 - **IOOption** - Combine IO with Option for optional values with side effects
-- **IOResult** - Combine IO with Result for error handling (recommended over IOEither)
+- **IOResult** - Combine IO with Result for error handling (recommended over IOEither when using standard `error` type)
+- **Effect** - Composable effects with dependency injection and error handling
 - **Reader** - Dependency injection pattern
 - **ReaderOption** - Combine Reader with Option for optional values with dependency injection
 - **ReaderIOOption** - Combine Reader, IO, and Option for optional values with dependency injection and side effects

v2/array/array_nil_test.go

@@ -0,0 +1,522 @@
+// 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 (
+	"fmt"
+	"testing"
+
+	O "github.com/IBM/fp-go/v2/option"
+	P "github.com/IBM/fp-go/v2/pair"
+	S "github.com/IBM/fp-go/v2/string"
+	"github.com/stretchr/testify/assert"
+)
+
+// TestNilSlice_IsEmpty verifies that IsEmpty handles nil slices correctly
+func TestNilSlice_IsEmpty(t *testing.T) {
+	var nilSlice []int
+	assert.True(t, IsEmpty(nilSlice), "nil slice should be empty")
+}
+
+// TestNilSlice_IsNonEmpty verifies that IsNonEmpty handles nil slices correctly
+func TestNilSlice_IsNonEmpty(t *testing.T) {
+	var nilSlice []int
+	assert.False(t, IsNonEmpty(nilSlice), "nil slice should not be non-empty")
+}
+
+// TestNilSlice_MonadMap verifies that MonadMap handles nil slices correctly
+func TestNilSlice_MonadMap(t *testing.T) {
+	var nilSlice []int
+	result := MonadMap(nilSlice, func(v int) string {
+		return fmt.Sprintf("%d", v)
+	})
+	assert.NotNil(t, result, "MonadMap should return non-nil slice")
+	assert.Equal(t, 0, len(result), "MonadMap should return empty slice for nil input")
+}
+
+// TestNilSlice_MonadMapRef verifies that MonadMapRef handles nil slices correctly
+func TestNilSlice_MonadMapRef(t *testing.T) {
+	var nilSlice []int
+	result := MonadMapRef(nilSlice, func(v *int) string {
+		return fmt.Sprintf("%d", *v)
+	})
+	assert.NotNil(t, result, "MonadMapRef should return non-nil slice")
+	assert.Equal(t, 0, len(result), "MonadMapRef should return empty slice for nil input")
+}
+
+// TestNilSlice_Map verifies that Map handles nil slices correctly
+func TestNilSlice_Map(t *testing.T) {
+	var nilSlice []int
+	mapper := Map(func(v int) string {
+		return fmt.Sprintf("%d", v)
+	})
+	result := mapper(nilSlice)
+	assert.NotNil(t, result, "Map should return non-nil slice")
+	assert.Equal(t, 0, len(result), "Map should return empty slice for nil input")
+}
+
+// TestNilSlice_MapRef verifies that MapRef handles nil slices correctly
+func TestNilSlice_MapRef(t *testing.T) {
+	var nilSlice []int
+	mapper := MapRef(func(v *int) string {
+		return fmt.Sprintf("%d", *v)
+	})
+	result := mapper(nilSlice)
+	assert.NotNil(t, result, "MapRef should return non-nil slice")
+	assert.Equal(t, 0, len(result), "MapRef should return empty slice for nil input")
+}
+
+// TestNilSlice_MapWithIndex verifies that MapWithIndex handles nil slices correctly
+func TestNilSlice_MapWithIndex(t *testing.T) {
+	var nilSlice []int
+	mapper := MapWithIndex(func(i int, v int) string {
+		return fmt.Sprintf("%d:%d", i, v)
+	})
+	result := mapper(nilSlice)
+	assert.NotNil(t, result, "MapWithIndex should return non-nil slice")
+	assert.Equal(t, 0, len(result), "MapWithIndex should return empty slice for nil input")
+}
+
+// TestNilSlice_Filter verifies that Filter handles nil slices correctly
+func TestNilSlice_Filter(t *testing.T) {
+	var nilSlice []int
+	filter := Filter(func(v int) bool {
+		return v > 0
+	})
+	result := filter(nilSlice)
+	assert.NotNil(t, result, "Filter should return non-nil slice")
+	assert.Equal(t, 0, len(result), "Filter should return empty slice for nil input")
+}
+
+// TestNilSlice_FilterWithIndex verifies that FilterWithIndex handles nil slices correctly
+func TestNilSlice_FilterWithIndex(t *testing.T) {
+	var nilSlice []int
+	filter := FilterWithIndex(func(i int, v int) bool {
+		return v > 0
+	})
+	result := filter(nilSlice)
+	assert.NotNil(t, result, "FilterWithIndex should return non-nil slice")
+	assert.Equal(t, 0, len(result), "FilterWithIndex should return empty slice for nil input")
+}
+
+// TestNilSlice_FilterRef verifies that FilterRef handles nil slices correctly
+func TestNilSlice_FilterRef(t *testing.T) {
+	var nilSlice []int
+	filter := FilterRef(func(v *int) bool {
+		return *v > 0
+	})
+	result := filter(nilSlice)
+	assert.NotNil(t, result, "FilterRef should return non-nil slice")
+	assert.Equal(t, 0, len(result), "FilterRef should return empty slice for nil input")
+}
+
+// TestNilSlice_MonadFilterMap verifies that MonadFilterMap handles nil slices correctly
+func TestNilSlice_MonadFilterMap(t *testing.T) {
+	var nilSlice []int
+	result := MonadFilterMap(nilSlice, func(v int) O.Option[string] {
+		return O.Some(fmt.Sprintf("%d", v))
+	})
+	assert.NotNil(t, result, "MonadFilterMap should return non-nil slice")
+	assert.Equal(t, 0, len(result), "MonadFilterMap should return empty slice for nil input")
+}
+
+// TestNilSlice_MonadFilterMapWithIndex verifies that MonadFilterMapWithIndex handles nil slices correctly
+func TestNilSlice_MonadFilterMapWithIndex(t *testing.T) {
+	var nilSlice []int
+	result := MonadFilterMapWithIndex(nilSlice, func(i int, v int) O.Option[string] {
+		return O.Some(fmt.Sprintf("%d:%d", i, v))
+	})
+	assert.NotNil(t, result, "MonadFilterMapWithIndex should return non-nil slice")
+	assert.Equal(t, 0, len(result), "MonadFilterMapWithIndex should return empty slice for nil input")
+}
+
+// TestNilSlice_FilterMap verifies that FilterMap handles nil slices correctly
+func TestNilSlice_FilterMap(t *testing.T) {
+	var nilSlice []int
+	filter := FilterMap(func(v int) O.Option[string] {
+		return O.Some(fmt.Sprintf("%d", v))
+	})
+	result := filter(nilSlice)
+	assert.NotNil(t, result, "FilterMap should return non-nil slice")
+	assert.Equal(t, 0, len(result), "FilterMap should return empty slice for nil input")
+}
+
+// TestNilSlice_FilterMapWithIndex verifies that FilterMapWithIndex handles nil slices correctly
+func TestNilSlice_FilterMapWithIndex(t *testing.T) {
+	var nilSlice []int
+	filter := FilterMapWithIndex(func(i int, v int) O.Option[string] {
+		return O.Some(fmt.Sprintf("%d:%d", i, v))
+	})
+	result := filter(nilSlice)
+	assert.NotNil(t, result, "FilterMapWithIndex should return non-nil slice")
+	assert.Equal(t, 0, len(result), "FilterMapWithIndex should return empty slice for nil input")
+}
+
+// TestNilSlice_MonadReduce verifies that MonadReduce handles nil slices correctly
+func TestNilSlice_MonadReduce(t *testing.T) {
+	var nilSlice []int
+	result := MonadReduce(nilSlice, func(acc int, v int) int {
+		return acc + v
+	}, 10)
+	assert.Equal(t, 10, result, "MonadReduce should return initial value for nil slice")
+}
+
+// TestNilSlice_MonadReduceWithIndex verifies that MonadReduceWithIndex handles nil slices correctly
+func TestNilSlice_MonadReduceWithIndex(t *testing.T) {
+	var nilSlice []int
+	result := MonadReduceWithIndex(nilSlice, func(i int, acc int, v int) int {
+		return acc + v
+	}, 10)
+	assert.Equal(t, 10, result, "MonadReduceWithIndex should return initial value for nil slice")
+}
+
+// TestNilSlice_Reduce verifies that Reduce handles nil slices correctly
+func TestNilSlice_Reduce(t *testing.T) {
+	var nilSlice []int
+	reducer := Reduce(func(acc int, v int) int {
+		return acc + v
+	}, 10)
+	result := reducer(nilSlice)
+	assert.Equal(t, 10, result, "Reduce should return initial value for nil slice")
+}
+
+// TestNilSlice_ReduceWithIndex verifies that ReduceWithIndex handles nil slices correctly
+func TestNilSlice_ReduceWithIndex(t *testing.T) {
+	var nilSlice []int
+	reducer := ReduceWithIndex(func(i int, acc int, v int) int {
+		return acc + v
+	}, 10)
+	result := reducer(nilSlice)
+	assert.Equal(t, 10, result, "ReduceWithIndex should return initial value for nil slice")
+}
+
+// TestNilSlice_ReduceRight verifies that ReduceRight handles nil slices correctly
+func TestNilSlice_ReduceRight(t *testing.T) {
+	var nilSlice []int
+	reducer := ReduceRight(func(v int, acc int) int {
+		return acc + v
+	}, 10)
+	result := reducer(nilSlice)
+	assert.Equal(t, 10, result, "ReduceRight should return initial value for nil slice")
+}
+
+// TestNilSlice_ReduceRightWithIndex verifies that ReduceRightWithIndex handles nil slices correctly
+func TestNilSlice_ReduceRightWithIndex(t *testing.T) {
+	var nilSlice []int
+	reducer := ReduceRightWithIndex(func(i int, v int, acc int) int {
+		return acc + v
+	}, 10)
+	result := reducer(nilSlice)
+	assert.Equal(t, 10, result, "ReduceRightWithIndex should return initial value for nil slice")
+}
+
+// TestNilSlice_ReduceRef verifies that ReduceRef handles nil slices correctly
+func TestNilSlice_ReduceRef(t *testing.T) {
+	var nilSlice []int
+	reducer := ReduceRef(func(acc int, v *int) int {
+		return acc + *v
+	}, 10)
+	result := reducer(nilSlice)
+	assert.Equal(t, 10, result, "ReduceRef should return initial value for nil slice")
+}
+
+// TestNilSlice_Append verifies that Append handles nil slices correctly
+func TestNilSlice_Append(t *testing.T) {
+	var nilSlice []int
+	result := Append(nilSlice, 42)
+	assert.NotNil(t, result, "Append should return non-nil slice")
+	assert.Equal(t, 1, len(result), "Append should create slice with one element")
+	assert.Equal(t, 42, result[0], "Append should add element correctly")
+}
+
+// TestNilSlice_MonadChain verifies that MonadChain handles nil slices correctly
+func TestNilSlice_MonadChain(t *testing.T) {
+	var nilSlice []int
+	result := MonadChain(nilSlice, func(v int) []string {
+		return []string{fmt.Sprintf("%d", v)}
+	})
+	assert.NotNil(t, result, "MonadChain should return non-nil slice")
+	assert.Equal(t, 0, len(result), "MonadChain should return empty slice for nil input")
+}
+
+// TestNilSlice_Chain verifies that Chain handles nil slices correctly
+func TestNilSlice_Chain(t *testing.T) {
+	var nilSlice []int
+	chain := Chain(func(v int) []string {
+		return []string{fmt.Sprintf("%d", v)}
+	})
+	result := chain(nilSlice)
+	assert.NotNil(t, result, "Chain should return non-nil slice")
+	assert.Equal(t, 0, len(result), "Chain should return empty slice for nil input")
+}
+
+// TestNilSlice_MonadAp verifies that MonadAp handles nil slices correctly
+func TestNilSlice_MonadAp(t *testing.T) {
+	var nilFuncs []func(int) string
+	var nilValues []int
+
+	// nil functions, nil values
+	result1 := MonadAp(nilFuncs, nilValues)
+	assert.NotNil(t, result1, "MonadAp should return non-nil slice")
+	assert.Equal(t, 0, len(result1), "MonadAp should return empty slice for nil inputs")
+
+	// nil functions, non-nil values
+	nonNilValues := []int{1, 2, 3}
+	result2 := MonadAp(nilFuncs, nonNilValues)
+	assert.NotNil(t, result2, "MonadAp should return non-nil slice")
+	assert.Equal(t, 0, len(result2), "MonadAp should return empty slice when functions are nil")
+
+	// non-nil functions, nil values
+	nonNilFuncs := []func(int) string{func(v int) string { return fmt.Sprintf("%d", v) }}
+	result3 := MonadAp(nonNilFuncs, nilValues)
+	assert.NotNil(t, result3, "MonadAp should return non-nil slice")
+	assert.Equal(t, 0, len(result3), "MonadAp should return empty slice when values are nil")
+}
+
+// TestNilSlice_Ap verifies that Ap handles nil slices correctly
+func TestNilSlice_Ap(t *testing.T) {
+	var nilValues []int
+	ap := Ap[string](nilValues)
+
+	var nilFuncs []func(int) string
+	result := ap(nilFuncs)
+	assert.NotNil(t, result, "Ap should return non-nil slice")
+	assert.Equal(t, 0, len(result), "Ap should return empty slice for nil inputs")
+}
+
+// TestNilSlice_Head verifies that Head handles nil slices correctly
+func TestNilSlice_Head(t *testing.T) {
+	var nilSlice []int
+	result := Head(nilSlice)
+	assert.True(t, O.IsNone(result), "Head should return None for nil slice")
+}
+
+// TestNilSlice_First verifies that First handles nil slices correctly
+func TestNilSlice_First(t *testing.T) {
+	var nilSlice []int
+	result := First(nilSlice)
+	assert.True(t, O.IsNone(result), "First should return None for nil slice")
+}
+
+// TestNilSlice_Last verifies that Last handles nil slices correctly
+func TestNilSlice_Last(t *testing.T) {
+	var nilSlice []int
+	result := Last(nilSlice)
+	assert.True(t, O.IsNone(result), "Last should return None for nil slice")
+}
+
+// TestNilSlice_Tail verifies that Tail handles nil slices correctly
+func TestNilSlice_Tail(t *testing.T) {
+	var nilSlice []int
+	result := Tail(nilSlice)
+	assert.True(t, O.IsNone(result), "Tail should return None for nil slice")
+}
+
+// TestNilSlice_Flatten verifies that Flatten handles nil slices correctly
+func TestNilSlice_Flatten(t *testing.T) {
+	var nilSlice [][]int
+	result := Flatten(nilSlice)
+	assert.NotNil(t, result, "Flatten should return non-nil slice")
+	assert.Equal(t, 0, len(result), "Flatten should return empty slice for nil input")
+}
+
+// TestNilSlice_Lookup verifies that Lookup handles nil slices correctly
+func TestNilSlice_Lookup(t *testing.T) {
+	var nilSlice []int
+	lookup := Lookup[int](0)
+	result := lookup(nilSlice)
+	assert.True(t, O.IsNone(result), "Lookup should return None for nil slice")
+}
+
+// TestNilSlice_Size verifies that Size handles nil slices correctly
+func TestNilSlice_Size(t *testing.T) {
+	var nilSlice []int
+	result := Size(nilSlice)
+	assert.Equal(t, 0, result, "Size should return 0 for nil slice")
+}
+
+// TestNilSlice_MonadPartition verifies that MonadPartition handles nil slices correctly
+func TestNilSlice_MonadPartition(t *testing.T) {
+	var nilSlice []int
+	result := MonadPartition(nilSlice, func(v int) bool {
+		return v > 0
+	})
+	left := P.Head(result)
+	right := P.Tail(result)
+	assert.NotNil(t, left, "MonadPartition left should return non-nil slice")
+	assert.NotNil(t, right, "MonadPartition right should return non-nil slice")
+	assert.Equal(t, 0, len(left), "MonadPartition left should be empty for nil input")
+	assert.Equal(t, 0, len(right), "MonadPartition right should be empty for nil input")
+}
+
+// TestNilSlice_Partition verifies that Partition handles nil slices correctly
+func TestNilSlice_Partition(t *testing.T) {
+	var nilSlice []int
+	partition := Partition(func(v int) bool {
+		return v > 0
+	})
+	result := partition(nilSlice)
+	left := P.Head(result)
+	right := P.Tail(result)
+	assert.NotNil(t, left, "Partition left should return non-nil slice")
+	assert.NotNil(t, right, "Partition right should return non-nil slice")
+	assert.Equal(t, 0, len(left), "Partition left should be empty for nil input")
+	assert.Equal(t, 0, len(right), "Partition right should be empty for nil input")
+}
+
+// TestNilSlice_IsNil verifies that IsNil handles nil slices correctly
+func TestNilSlice_IsNil(t *testing.T) {
+	var nilSlice []int
+	assert.True(t, IsNil(nilSlice), "IsNil should return true for nil slice")
+
+	nonNilSlice := []int{}
+	assert.False(t, IsNil(nonNilSlice), "IsNil should return false for non-nil empty slice")
+}
+
+// TestNilSlice_IsNonNil verifies that IsNonNil handles nil slices correctly
+func TestNilSlice_IsNonNil(t *testing.T) {
+	var nilSlice []int
+	assert.False(t, IsNonNil(nilSlice), "IsNonNil should return false for nil slice")
+
+	nonNilSlice := []int{}
+	assert.True(t, IsNonNil(nonNilSlice), "IsNonNil should return true for non-nil empty slice")
+}
+
+// TestNilSlice_Copy verifies that Copy handles nil slices correctly
+func TestNilSlice_Copy(t *testing.T) {
+	var nilSlice []int
+	result := Copy(nilSlice)
+	assert.NotNil(t, result, "Copy should return non-nil slice")
+	assert.Equal(t, 0, len(result), "Copy should return empty slice for nil input")
+}
+
+// TestNilSlice_FoldMap verifies that FoldMap handles nil slices correctly
+func TestNilSlice_FoldMap(t *testing.T) {
+	var nilSlice []int
+	monoid := S.Monoid
+	foldMap := FoldMap[int](monoid)(func(v int) string {
+		return fmt.Sprintf("%d", v)
+	})
+	result := foldMap(nilSlice)
+	assert.Equal(t, "", result, "FoldMap should return empty value for nil slice")
+}
+
+// TestNilSlice_FoldMapWithIndex verifies that FoldMapWithIndex handles nil slices correctly
+func TestNilSlice_FoldMapWithIndex(t *testing.T) {
+	var nilSlice []int
+	monoid := S.Monoid
+	foldMap := FoldMapWithIndex[int](monoid)(func(i int, v int) string {
+		return fmt.Sprintf("%d:%d", i, v)
+	})
+	result := foldMap(nilSlice)
+	assert.Equal(t, "", result, "FoldMapWithIndex should return empty value for nil slice")
+}
+
+// TestNilSlice_Fold verifies that Fold handles nil slices correctly
+func TestNilSlice_Fold(t *testing.T) {
+	var nilSlice []string
+	monoid := S.Monoid
+	fold := Fold[string](monoid)
+	result := fold(nilSlice)
+	assert.Equal(t, "", result, "Fold should return empty value for nil slice")
+}
+
+// TestNilSlice_Concat verifies that Concat handles nil slices correctly
+func TestNilSlice_Concat(t *testing.T) {
+	var nilSlice []int
+	nonNilSlice := []int{1, 2, 3}
+
+	// nil concat non-nil
+	concat1 := Concat(nonNilSlice)
+	result1 := concat1(nilSlice)
+	assert.Equal(t, nonNilSlice, result1, "nil concat non-nil should return non-nil slice")
+
+	// non-nil concat nil
+	concat2 := Concat(nilSlice)
+	result2 := concat2(nonNilSlice)
+	assert.Equal(t, nonNilSlice, result2, "non-nil concat nil should return non-nil slice")
+
+	// nil concat nil
+	concat3 := Concat(nilSlice)
+	result3 := concat3(nilSlice)
+	assert.Nil(t, result3, "nil concat nil should return nil")
+}
+
+// TestNilSlice_MonadFlap verifies that MonadFlap handles nil slices correctly
+func TestNilSlice_MonadFlap(t *testing.T) {
+	var nilSlice []func(int) string
+	result := MonadFlap(nilSlice, 42)
+	assert.NotNil(t, result, "MonadFlap should return non-nil slice")
+	assert.Equal(t, 0, len(result), "MonadFlap should return empty slice for nil input")
+}
+
+// TestNilSlice_Flap verifies that Flap handles nil slices correctly
+func TestNilSlice_Flap(t *testing.T) {
+	var nilSlice []func(int) string
+	flap := Flap[string, int](42)
+	result := flap(nilSlice)
+	assert.NotNil(t, result, "Flap should return non-nil slice")
+	assert.Equal(t, 0, len(result), "Flap should return empty slice for nil input")
+}
+
+// TestNilSlice_Reverse verifies that Reverse handles nil slices correctly
+func TestNilSlice_Reverse(t *testing.T) {
+	var nilSlice []int
+	result := Reverse(nilSlice)
+	assert.Nil(t, result, "Reverse should return nil for nil slice")
+}
+
+// TestNilSlice_Extend verifies that Extend handles nil slices correctly
+func TestNilSlice_Extend(t *testing.T) {
+	var nilSlice []int
+	extend := Extend(func(as []int) string {
+		return fmt.Sprintf("%v", as)
+	})
+	result := extend(nilSlice)
+	assert.NotNil(t, result, "Extend should return non-nil slice")
+	assert.Equal(t, 0, len(result), "Extend should return empty slice for nil input")
+}
+
+// TestNilSlice_Empty verifies that Empty creates an empty non-nil slice
+func TestNilSlice_Empty(t *testing.T) {
+	result := Empty[int]()
+	assert.NotNil(t, result, "Empty should return non-nil slice")
+	assert.Equal(t, 0, len(result), "Empty should return empty slice")
+	assert.False(t, IsNil(result), "Empty should not return nil slice")
+}
+
+// TestNilSlice_Zero verifies that Zero creates an empty non-nil slice
+func TestNilSlice_Zero(t *testing.T) {
+	result := Zero[int]()
+	assert.NotNil(t, result, "Zero should return non-nil slice")
+	assert.Equal(t, 0, len(result), "Zero should return empty slice")
+	assert.False(t, IsNil(result), "Zero should not return nil slice")
+}
+
+// TestNilSlice_ConstNil verifies that ConstNil returns a nil slice
+func TestNilSlice_ConstNil(t *testing.T) {
+	result := ConstNil[int]()
+	assert.Nil(t, result, "ConstNil should return nil slice")
+	assert.True(t, IsNil(result), "ConstNil should return nil slice")
+}
+
+// TestNilSlice_Of verifies that Of creates a proper singleton slice
+func TestNilSlice_Of(t *testing.T) {
+	result := Of(42)
+	assert.NotNil(t, result, "Of should return non-nil slice")
+	assert.Equal(t, 1, len(result), "Of should create slice with one element")
+	assert.Equal(t, 42, result[0], "Of should set value correctly")
+}

v2/array/array_test.go

@@ -198,11 +198,228 @@ func TestFilterMap(t *testing.T) {
 }
 
 func TestFoldMap(t *testing.T) {
-	src := From("a", "b", "c")
+	t.Run("FoldMap with 0 items", func(t *testing.T) {
+		empty := []int{}
+		sumMonoid := N.MonoidSum[int]()
+		foldMap := FoldMap[int](sumMonoid)(N.Mul(2))
+		result := foldMap(empty)
+		assert.Equal(t, 0, result, "FoldMap should return monoid empty for 0 items")
+	})
 
-	fold := FoldMap[string](S.Monoid)(strings.ToUpper)
+	t.Run("FoldMap with 1 item", func(t *testing.T) {
+		single := From(5)
+		sumMonoid := N.MonoidSum[int]()
+		foldMap := FoldMap[int](sumMonoid)(N.Mul(2))
+		result := foldMap(single)
+		assert.Equal(t, 10, result, "FoldMap should map and return single item")
+	})
 
-	assert.Equal(t, "ABC", fold(src))
+	t.Run("FoldMap with 2 items", func(t *testing.T) {
+		two := From(3, 4)
+		sumMonoid := N.MonoidSum[int]()
+		foldMap := FoldMap[int](sumMonoid)(N.Mul(2))
+		result := foldMap(two)
+		assert.Equal(t, 14, result, "FoldMap should map and fold 2 items: (3*2) + (4*2) = 14")
+	})
+
+	t.Run("FoldMap with many items", func(t *testing.T) {
+		many := From(1, 2, 3, 4, 5)
+		sumMonoid := N.MonoidSum[int]()
+		foldMap := FoldMap[int](sumMonoid)(N.Mul(2))
+		result := foldMap(many)
+		assert.Equal(t, 30, result, "FoldMap should map and fold many items: (1*2) + (2*2) + (3*2) + (4*2) + (5*2) = 30")
+	})
+
+	t.Run("FoldMap with string concatenation - 0 items", func(t *testing.T) {
+		empty := []string{}
+		fold := FoldMap[string](S.Monoid)(strings.ToUpper)
+		result := fold(empty)
+		assert.Equal(t, "", result, "FoldMap should return empty string for 0 items")
+	})
+
+	t.Run("FoldMap with string concatenation - 1 item", func(t *testing.T) {
+		single := From("a")
+		fold := FoldMap[string](S.Monoid)(strings.ToUpper)
+		result := fold(single)
+		assert.Equal(t, "A", result, "FoldMap should map single string")
+	})
+
+	t.Run("FoldMap with string concatenation - 2 items", func(t *testing.T) {
+		two := From("a", "b")
+		fold := FoldMap[string](S.Monoid)(strings.ToUpper)
+		result := fold(two)
+		assert.Equal(t, "AB", result, "FoldMap should map and concatenate 2 strings")
+	})
+
+	t.Run("FoldMap with string concatenation - many items", func(t *testing.T) {
+		many := From("a", "b", "c", "d", "e")
+		fold := FoldMap[string](S.Monoid)(strings.ToUpper)
+		result := fold(many)
+		assert.Equal(t, "ABCDE", result, "FoldMap should map and concatenate many strings")
+	})
+}
+
+func TestFold(t *testing.T) {
+	t.Run("Fold with 0 items", func(t *testing.T) {
+		empty := []int{}
+		sumMonoid := N.MonoidSum[int]()
+		fold := Fold[int](sumMonoid)
+		result := fold(empty)
+		assert.Equal(t, 0, result, "Fold should return monoid empty for 0 items")
+	})
+
+	t.Run("Fold with 1 item", func(t *testing.T) {
+		single := From(42)
+		sumMonoid := N.MonoidSum[int]()
+		fold := Fold[int](sumMonoid)
+		result := fold(single)
+		assert.Equal(t, 42, result, "Fold should return single item")
+	})
+
+	t.Run("Fold with 2 items", func(t *testing.T) {
+		two := From(10, 20)
+		sumMonoid := N.MonoidSum[int]()
+		fold := Fold[int](sumMonoid)
+		result := fold(two)
+		assert.Equal(t, 30, result, "Fold should combine 2 items: 10 + 20 = 30")
+	})
+
+	t.Run("Fold with many items", func(t *testing.T) {
+		many := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+		sumMonoid := N.MonoidSum[int]()
+		fold := Fold[int](sumMonoid)
+		result := fold(many)
+		assert.Equal(t, 55, result, "Fold should combine many items: 1+2+3+4+5+6+7+8+9+10 = 55")
+	})
+
+	t.Run("Fold with string concatenation - 0 items", func(t *testing.T) {
+		empty := []string{}
+		fold := Fold[string](S.Monoid)
+		result := fold(empty)
+		assert.Equal(t, "", result, "Fold should return empty string for 0 items")
+	})
+
+	t.Run("Fold with string concatenation - 1 item", func(t *testing.T) {
+		single := From("hello")
+		fold := Fold[string](S.Monoid)
+		result := fold(single)
+		assert.Equal(t, "hello", result, "Fold should return single string")
+	})
+
+	t.Run("Fold with string concatenation - 2 items", func(t *testing.T) {
+		two := From("hello", "world")
+		fold := Fold[string](S.Monoid)
+		result := fold(two)
+		assert.Equal(t, "helloworld", result, "Fold should concatenate 2 strings")
+	})
+
+	t.Run("Fold with string concatenation - many items", func(t *testing.T) {
+		many := From("a", "b", "c", "d", "e", "f")
+		fold := Fold[string](S.Monoid)
+		result := fold(many)
+		assert.Equal(t, "abcdef", result, "Fold should concatenate many strings")
+	})
+
+	t.Run("Fold with product monoid - 0 items", func(t *testing.T) {
+		empty := []int{}
+		productMonoid := N.MonoidProduct[int]()
+		fold := Fold[int](productMonoid)
+		result := fold(empty)
+		assert.Equal(t, 1, result, "Fold should return monoid empty (1) for product with 0 items")
+	})
+
+	t.Run("Fold with product monoid - 1 item", func(t *testing.T) {
+		single := From(7)
+		productMonoid := N.MonoidProduct[int]()
+		fold := Fold[int](productMonoid)
+		result := fold(single)
+		assert.Equal(t, 7, result, "Fold should return single item for product")
+	})
+
+	t.Run("Fold with product monoid - 2 items", func(t *testing.T) {
+		two := From(3, 4)
+		productMonoid := N.MonoidProduct[int]()
+		fold := Fold[int](productMonoid)
+		result := fold(two)
+		assert.Equal(t, 12, result, "Fold should multiply 2 items: 3 * 4 = 12")
+	})
+
+	t.Run("Fold with product monoid - many items", func(t *testing.T) {
+		many := From(2, 3, 4, 5)
+		productMonoid := N.MonoidProduct[int]()
+		fold := Fold[int](productMonoid)
+		result := fold(many)
+		assert.Equal(t, 120, result, "Fold should multiply many items: 2*3*4*5 = 120")
+	})
+}
+func TestFoldMapWithIndex(t *testing.T) {
+	t.Run("FoldMapWithIndex with 0 items", func(t *testing.T) {
+		empty := []int{}
+		sumMonoid := N.MonoidSum[int]()
+		foldMap := FoldMapWithIndex[int](sumMonoid)(func(i, x int) int { return i + x })
+		result := foldMap(empty)
+		assert.Equal(t, 0, result, "FoldMapWithIndex should return monoid empty for 0 items")
+	})
+
+	t.Run("FoldMapWithIndex with 1 item", func(t *testing.T) {
+		single := From(10)
+		sumMonoid := N.MonoidSum[int]()
+		foldMap := FoldMapWithIndex[int](sumMonoid)(func(i, x int) int { return i + x })
+		result := foldMap(single)
+		assert.Equal(t, 10, result, "FoldMapWithIndex should map with index: 0 + 10 = 10")
+	})
+
+	t.Run("FoldMapWithIndex with 2 items", func(t *testing.T) {
+		two := From(10, 20)
+		sumMonoid := N.MonoidSum[int]()
+		foldMap := FoldMapWithIndex[int](sumMonoid)(func(i, x int) int { return i + x })
+		result := foldMap(two)
+		assert.Equal(t, 31, result, "FoldMapWithIndex should map with indices: (0+10) + (1+20) = 31")
+	})
+
+	t.Run("FoldMapWithIndex with many items", func(t *testing.T) {
+		many := From(5, 10, 15, 20)
+		sumMonoid := N.MonoidSum[int]()
+		foldMap := FoldMapWithIndex[int](sumMonoid)(func(i, x int) int { return i * x })
+		result := foldMap(many)
+		assert.Equal(t, 100, result, "FoldMapWithIndex should map with indices: (0*5) + (1*10) + (2*15) + (3*20) = 100")
+	})
+
+	t.Run("FoldMapWithIndex with string concatenation - 0 items", func(t *testing.T) {
+		empty := []string{}
+		foldMap := FoldMapWithIndex[string](S.Monoid)(func(i int, s string) string {
+			return fmt.Sprintf("%d:%s", i, s)
+		})
+		result := foldMap(empty)
+		assert.Equal(t, "", result, "FoldMapWithIndex should return empty string for 0 items")
+	})
+
+	t.Run("FoldMapWithIndex with string concatenation - 1 item", func(t *testing.T) {
+		single := From("a")
+		foldMap := FoldMapWithIndex[string](S.Monoid)(func(i int, s string) string {
+			return fmt.Sprintf("%d:%s", i, s)
+		})
+		result := foldMap(single)
+		assert.Equal(t, "0:a", result, "FoldMapWithIndex should format single item with index")
+	})
+
+	t.Run("FoldMapWithIndex with string concatenation - 2 items", func(t *testing.T) {
+		two := From("a", "b")
+		foldMap := FoldMapWithIndex[string](S.Monoid)(func(i int, s string) string {
+			return fmt.Sprintf("%d:%s,", i, s)
+		})
+		result := foldMap(two)
+		assert.Equal(t, "0:a,1:b,", result, "FoldMapWithIndex should format 2 items with indices")
+	})
+
+	t.Run("FoldMapWithIndex with string concatenation - many items", func(t *testing.T) {
+		many := From("a", "b", "c", "d")
+		foldMap := FoldMapWithIndex[string](S.Monoid)(func(i int, s string) string {
+			return fmt.Sprintf("[%d]%s", i, s)
+		})
+		result := foldMap(many)
+		assert.Equal(t, "[0]a[1]b[2]c[3]d", result, "FoldMapWithIndex should format many items with indices")
+	})
 }
 
 func ExampleFoldMap() {
@@ -767,6 +984,25 @@ func TestExtendUseCases(t *testing.T) {
 
 // TestConcat tests the Concat function
 func TestConcat(t *testing.T) {
+	t.Run("Semantic: Concat(b)(a) produces [a... b...]", func(t *testing.T) {
+		a := []int{1, 2, 3}
+		b := []int{4, 5, 6}
+
+		// Concat(b)(a) should produce [a... b...]
+		result := Concat(b)(a)
+		expected := []int{1, 2, 3, 4, 5, 6}
+
+		assert.Equal(t, expected, result, "Concat(b)(a) should produce [a... b...]")
+
+		// Verify order: a's elements come first, then b's elements
+		assert.Equal(t, a[0], result[0], "First element should be from a")
+		assert.Equal(t, a[1], result[1], "Second element should be from a")
+		assert.Equal(t, a[2], result[2], "Third element should be from a")
+		assert.Equal(t, b[0], result[3], "Fourth element should be from b")
+		assert.Equal(t, b[1], result[4], "Fifth element should be from b")
+		assert.Equal(t, b[2], result[5], "Sixth element should be from b")
+	})
+
 	t.Run("Concat two non-empty arrays", func(t *testing.T) {
 		base := []int{1, 2, 3}
 		toAppend := []int{4, 5, 6}
@@ -870,6 +1106,54 @@ func TestConcat(t *testing.T) {
 		expected := []int{1, 2, 3}
 		assert.Equal(t, expected, result)
 	})
+	t.Run("Explicit append semantic demonstration", func(t *testing.T) {
+		// Given a base array
+		base := []string{"A", "B", "C"}
+
+		// And a suffix to append
+		suffix := []string{"D", "E", "F"}
+
+		// When we apply Concat(suffix) to base
+		appendSuffix := Concat(suffix)
+		result := appendSuffix(base)
+
+		// Then the result should be base followed by suffix
+		expected := []string{"A", "B", "C", "D", "E", "F"}
+		assert.Equal(t, expected, result)
+
+		// And the base should be unchanged
+		assert.Equal(t, []string{"A", "B", "C"}, base)
+
+		// And the suffix should be unchanged
+		assert.Equal(t, []string{"D", "E", "F"}, suffix)
+	})
+
+	t.Run("Append semantic with different types", func(t *testing.T) {
+		// Integers
+		intResult := Concat([]int{4, 5})([]int{1, 2, 3})
+		assert.Equal(t, []int{1, 2, 3, 4, 5}, intResult)
+
+		// Strings
+		strResult := Concat([]string{"world"})([]string{"hello"})
+		assert.Equal(t, []string{"hello", "world"}, strResult)
+
+		// Floats
+		floatResult := Concat([]float64{3.3, 4.4})([]float64{1.1, 2.2})
+		assert.Equal(t, []float64{1.1, 2.2, 3.3, 4.4}, floatResult)
+	})
+
+	t.Run("Append semantic in pipeline", func(t *testing.T) {
+		// Start with [1, 2, 3]
+		// Append [4, 5] to get [1, 2, 3, 4, 5]
+		// Append [6, 7] to get [1, 2, 3, 4, 5, 6, 7]
+		result := F.Pipe2(
+			[]int{1, 2, 3},
+			Concat([]int{4, 5}),
+			Concat([]int{6, 7}),
+		)
+		expected := []int{1, 2, 3, 4, 5, 6, 7}
+		assert.Equal(t, expected, result)
+	})
 }
 
 // TestConcatComposition tests Concat with other array operations

v2/array/generic/array.go

@@ -323,34 +323,49 @@ func Clone[AS ~[]A, A any](f func(A) A) func(as AS) AS {
 }
 
 func FoldMap[AS ~[]A, A, B any](m M.Monoid[B]) func(func(A) B) func(AS) B {
-	empty := m.Empty()
 	concat := m.Concat
 	return func(f func(A) B) func(AS) B {
 		return func(as AS) B {
-			return array.Reduce(as, func(cur B, a A) B {
-				return concat(cur, f(a))
-			}, empty)
+			switch len(as) {
+			case 0:
+				return m.Empty()
+			case 1:
+				return f(as[0])
+			case 2:
+				return concat(f(as[0]), f(as[1]))
+			default:
+				return array.Reduce(as[1:], func(cur B, a A) B {
+					return concat(cur, f(a))
+				}, f(as[0]))
+			}
 		}
 	}
 }
 
 func FoldMapWithIndex[AS ~[]A, A, B any](m M.Monoid[B]) func(func(int, A) B) func(AS) B {
-	empty := m.Empty()
 	concat := m.Concat
 	return func(f func(int, A) B) func(AS) B {
 		return func(as AS) B {
 			return array.ReduceWithIndex(as, func(idx int, cur B, a A) B {
 				return concat(cur, f(idx, a))
-			}, empty)
+			}, m.Empty())
 		}
 	}
 }
 
 func Fold[AS ~[]A, A any](m M.Monoid[A]) func(AS) A {
-	empty := m.Empty()
 	concat := m.Concat
 	return func(as AS) A {
-		return array.Reduce(as, concat, empty)
+		switch len(as) {
+		case 0:
+			return m.Empty()
+		case 1:
+			return as[0]
+		case 2:
+			return concat(as[0], as[1])
+		default:
+			return array.Reduce(as[1:], concat, as[0])
+		}
 	}
 }
 

v2/array/sequence.go

@@ -25,7 +25,7 @@ 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)
+	return array.MonadSequence(fof, m.Empty, m.Concat, ma)
 }
 
 // Sequence takes an array where elements are HKT<A> (higher kinded type) and,
@@ -67,7 +67,7 @@ func Sequence[HKTA, HKTRA any](
 	fof func(HKTA) HKTRA,
 	m M.Monoid[HKTRA],
 ) func([]HKTA) HKTRA {
-	return array.Sequence[[]HKTA](fof, m.Empty(), m.Concat)
+	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.

v2/assert/from_test.go

@@ -29,7 +29,7 @@ func TestFromReaderIOResult(t *testing.T) {
 		ri := func(ctx context.Context) func() result.Result[Reader] {
 			return func() result.Result[Reader] {
 				// Return a Reader that always passes
-				return result.Of[Reader](func(t *testing.T) bool {
+				return result.Of(func(t *testing.T) bool {
 					return true
 				})
 			}
@@ -46,7 +46,7 @@ func TestFromReaderIOResult(t *testing.T) {
 		// Create a ReaderIOResult that returns a successful Equal assertion
 		ri := func(ctx context.Context) func() result.Result[Reader] {
 			return func() result.Result[Reader] {
-				return result.Of[Reader](Equal(42)(42))
+				return result.Of(Equal(42)(42))
 			}
 		}
 
@@ -80,7 +80,7 @@ func TestFromReaderIOResult(t *testing.T) {
 		// Create a ReaderIOResult that returns a failing assertion
 		ri := func(ctx context.Context) func() result.Result[Reader] {
 			return func() result.Result[Reader] {
-				return result.Of[Reader](Equal(42)(43))
+				return result.Of(Equal(42)(43))
 			}
 		}
 
@@ -100,7 +100,7 @@ func TestFromReaderIOResult(t *testing.T) {
 				contextUsed = true
 			}
 			return func() result.Result[Reader] {
-				return result.Of[Reader](func(t *testing.T) bool {
+				return result.Of(func(t *testing.T) bool {
 					return true
 				})
 			}
@@ -118,7 +118,7 @@ func TestFromReaderIOResult(t *testing.T) {
 		// Create a ReaderIOResult that returns NoError assertion
 		ri := func(ctx context.Context) func() result.Result[Reader] {
 			return func() result.Result[Reader] {
-				return result.Of[Reader](NoError(nil))
+				return result.Of(NoError(nil))
 			}
 		}
 
@@ -139,7 +139,7 @@ func TestFromReaderIOResult(t *testing.T) {
 					ArrayLength[int](3)(arr),
 					ArrayContains(2)(arr),
 				})
-				return result.Of[Reader](assertions)
+				return result.Of(assertions)
 			}
 		}
 
@@ -297,7 +297,7 @@ func TestFromReaderIO(t *testing.T) {
 		// Create a ReaderIO with Result assertions
 		ri := func(ctx context.Context) func() Reader {
 			return func() Reader {
-				successResult := result.Of[int](42)
+				successResult := result.Of(42)
 				return Success(successResult)
 			}
 		}
@@ -338,7 +338,7 @@ func TestFromReaderIOResultIntegration(t *testing.T) {
 				}
 
 				// Return a successful assertion
-				return result.Of[Reader](Equal("test")("test"))
+				return result.Of(Equal("test")("test"))
 			}
 		}
 

v2/cli/README.md

@@ -0,0 +1,273 @@
+# CLI Package - Functional Wrappers for urfave/cli/v3
+
+This package provides functional programming wrappers for the `github.com/urfave/cli/v3` library, enabling Effect-based command actions and type-safe flag handling through Prisms.
+
+## Features
+
+### 1. Effect-Based Command Actions
+
+Transform CLI command actions into composable Effects that follow functional programming principles.
+
+#### Key Functions
+
+- **`ToAction(effect CommandEffect) func(context.Context, *C.Command) error`**
+  - Converts a CommandEffect into a standard urfave/cli Action function
+  - Enables Effect-based command handlers to work with cli/v3 framework
+
+- **`FromAction(action func(context.Context, *C.Command) error) CommandEffect`**
+  - Lifts existing cli/v3 action handlers into the Effect type
+  - Allows gradual migration to functional style
+
+- **`MakeCommand(name, usage string, flags []C.Flag, effect CommandEffect) *C.Command`**
+  - Creates a new Command with an Effect-based action
+  - Convenience function combining command creation with Effect conversion
+
+- **`MakeCommandWithSubcommands(...) *C.Command`**
+  - Creates a Command with subcommands and an Effect-based action
+
+#### Example Usage
+
+```go
+import (
+    "context"
+    
+    E "github.com/IBM/fp-go/v2/effect"
+    F "github.com/IBM/fp-go/v2/function"
+    R "github.com/IBM/fp-go/v2/result"
+    "github.com/IBM/fp-go/v2/cli"
+    C "github.com/urfave/cli/v3"
+)
+
+// Define an Effect-based command action
+processEffect := func(cmd *C.Command) E.Thunk[F.Void] {
+    return func(ctx context.Context) E.IOResult[F.Void] {
+        return func() R.Result[F.Void] {
+            input := cmd.String("input")
+            // Process input...
+            return R.Of(F.Void{})
+        }
+    }
+}
+
+// Create command with Effect
+command := cli.MakeCommand(
+    "process",
+    "Process input files",
+    []C.Flag{
+        &C.StringFlag{Name: "input", Usage: "Input file path"},
+    },
+    processEffect,
+)
+
+// Or convert existing action to Effect
+existingAction := func(ctx context.Context, cmd *C.Command) error {
+    // Existing logic...
+    return nil
+}
+effect := cli.FromAction(existingAction)
+```
+
+### 2. Flag Type Prisms
+
+Type-safe extraction and manipulation of CLI flags using Prisms from the optics package.
+
+#### Available Prisms
+
+- `StringFlagPrism()` - Extract `*C.StringFlag` from `C.Flag`
+- `IntFlagPrism()` - Extract `*C.IntFlag` from `C.Flag`
+- `BoolFlagPrism()` - Extract `*C.BoolFlag` from `C.Flag`
+- `Float64FlagPrism()` - Extract `*C.Float64Flag` from `C.Flag`
+- `DurationFlagPrism()` - Extract `*C.DurationFlag` from `C.Flag`
+- `TimestampFlagPrism()` - Extract `*C.TimestampFlag` from `C.Flag`
+- `StringSliceFlagPrism()` - Extract `*C.StringSliceFlag` from `C.Flag`
+- `IntSliceFlagPrism()` - Extract `*C.IntSliceFlag` from `C.Flag`
+- `Float64SliceFlagPrism()` - Extract `*C.Float64SliceFlag` from `C.Flag`
+- `UintFlagPrism()` - Extract `*C.UintFlag` from `C.Flag`
+- `Uint64FlagPrism()` - Extract `*C.Uint64Flag` from `C.Flag`
+- `Int64FlagPrism()` - Extract `*C.Int64Flag` from `C.Flag`
+
+#### Example Usage
+
+```go
+import (
+    O "github.com/IBM/fp-go/v2/option"
+    "github.com/IBM/fp-go/v2/cli"
+    C "github.com/urfave/cli/v3"
+)
+
+// Extract a StringFlag from a Flag interface
+var flag C.Flag = &C.StringFlag{Name: "input", Value: "default"}
+prism := cli.StringFlagPrism()
+
+// Safe extraction returns Option
+result := prism.GetOption(flag)
+if O.IsSome(result) {
+    strFlag := O.MonadFold(result, 
+        func() *C.StringFlag { return nil },
+        func(f *C.StringFlag) *C.StringFlag { return f },
+    )
+    // Use strFlag...
+}
+
+// Type mismatch returns None
+var intFlag C.Flag = &C.IntFlag{Name: "count"}
+result = prism.GetOption(intFlag)  // Returns None
+
+// Convert back to Flag
+strFlag := &C.StringFlag{Name: "output"}
+flag = prism.ReverseGet(strFlag)
+```
+
+## Type Definitions
+
+### CommandEffect
+
+```go
+type CommandEffect = E.Effect[*C.Command, F.Void]
+```
+
+A CommandEffect represents a CLI command action as an Effect. It takes a `*C.Command` as context and produces a result wrapped in the Effect monad.
+
+The Effect structure is:
+```
+func(*C.Command) -> func(context.Context) -> func() -> Result[Void]
+```
+
+This allows for:
+- **Composability**: Effects can be composed using standard functional combinators
+- **Testability**: Pure functions are easier to test
+- **Error Handling**: Errors are explicitly represented in the Result type
+- **Context Management**: Context flows naturally through the Effect
+
+## Benefits
+
+### 1. Functional Composition
+
+Effects can be composed using standard functional programming patterns:
+
+```go
+import (
+    F "github.com/IBM/fp-go/v2/function"
+    RRIOE "github.com/IBM/fp-go/v2/context/readerreaderioresult"
+)
+
+// Compose multiple effects
+validateInput := func(cmd *C.Command) E.Thunk[F.Void] { /* ... */ }
+processData := func(cmd *C.Command) E.Thunk[F.Void] { /* ... */ }
+saveResults := func(cmd *C.Command) E.Thunk[F.Void] { /* ... */ }
+
+// Chain effects together
+pipeline := F.Pipe3(
+    validateInput,
+    RRIOE.Chain(func(F.Void) E.Effect[*C.Command, F.Void] { return processData }),
+    RRIOE.Chain(func(F.Void) E.Effect[*C.Command, F.Void] { return saveResults }),
+)
+```
+
+### 2. Type Safety
+
+Prisms provide compile-time type safety when working with flags:
+
+```go
+// Type-safe flag extraction
+flags := []C.Flag{
+    &C.StringFlag{Name: "input"},
+    &C.IntFlag{Name: "count"},
+}
+
+for _, flag := range flags {
+    // Safe extraction with pattern matching
+    O.MonadFold(
+        cli.StringFlagPrism().GetOption(flag),
+        func() { /* Not a string flag */ },
+        func(sf *C.StringFlag) { /* Handle string flag */ },
+    )
+}
+```
+
+### 3. Error Handling
+
+Errors are explicitly represented in the Result type:
+
+```go
+effect := func(cmd *C.Command) E.Thunk[F.Void] {
+    return func(ctx context.Context) E.IOResult[F.Void] {
+        return func() R.Result[F.Void] {
+            if err := validateInput(cmd); err != nil {
+                return R.Left[F.Void](err)  // Explicit error
+            }
+            return R.Of(F.Void{})  // Success
+        }
+    }
+}
+```
+
+### 4. Testability
+
+Pure functions are easier to test:
+
+```go
+func TestCommandEffect(t *testing.T) {
+    cmd := &C.Command{Name: "test"}
+    effect := myCommandEffect(cmd)
+    
+    // Execute effect
+    result := effect(context.Background())()
+    
+    // Assert on result
+    assert.True(t, R.IsRight(result))
+}
+```
+
+## Migration Guide
+
+### From Standard Actions to Effects
+
+**Before:**
+```go
+command := &C.Command{
+    Name: "process",
+    Action: func(ctx context.Context, cmd *C.Command) error {
+        input := cmd.String("input")
+        // Process...
+        return nil
+    },
+}
+```
+
+**After:**
+```go
+effect := func(cmd *C.Command) E.Thunk[F.Void] {
+    return func(ctx context.Context) E.IOResult[F.Void] {
+        return func() R.Result[F.Void] {
+            input := cmd.String("input")
+            // Process...
+            return R.Of(F.Void{})
+        }
+    }
+}
+
+command := cli.MakeCommand("process", "Process files", flags, effect)
+```
+
+### Gradual Migration
+
+You can mix both styles during migration:
+
+```go
+// Wrap existing action
+existingAction := func(ctx context.Context, cmd *C.Command) error {
+    // Legacy code...
+    return nil
+}
+
+// Use as Effect
+effect := cli.FromAction(existingAction)
+command := cli.MakeCommand("legacy", "Legacy command", flags, effect)
+```
+
+## See Also
+
+- [Effect Package](../effect/) - Core Effect type definitions
+- [Optics Package](../optics/) - Prism and other optics
+- [urfave/cli/v3](https://github.com/urfave/cli) - Underlying CLI framework

v2/cli/apply.go

@@ -21,7 +21,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-	"time"
 
 	C "github.com/urfave/cli/v3"
 )
@@ -388,8 +387,8 @@ func generateApplyHelpers(filename string, count int) error {
 
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 
 	fmt.Fprintf(f, "package %s\n\n", pkg)
 

v2/cli/bind.go

@@ -21,7 +21,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-	"time"
 
 	C "github.com/urfave/cli/v3"
 )
@@ -266,8 +265,8 @@ func generateBindHelpers(filename string, count int) error {
 
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 
 	fmt.Fprintf(f, "package %s\n", pkg)
 

v2/cli/di.go

@@ -21,7 +21,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-	"time"
 
 	C "github.com/urfave/cli/v3"
 )
@@ -189,8 +188,8 @@ func generateDIHelpers(filename string, count int) error {
 
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 
 	fmt.Fprintf(f, "package %s\n\n", pkg)
 

v2/cli/effect.go

@@ -0,0 +1,199 @@
+// 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 (
+	"context"
+
+	E "github.com/IBM/fp-go/v2/effect"
+	ET "github.com/IBM/fp-go/v2/either"
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/io"
+	R "github.com/IBM/fp-go/v2/result"
+	C "github.com/urfave/cli/v3"
+)
+
+// CommandEffect represents a CLI command action as an Effect.
+// The Effect takes a *C.Command as context and produces a result.
+type CommandEffect = E.Effect[*C.Command, F.Void]
+
+// ToAction converts a CommandEffect into a standard urfave/cli Action function.
+// This allows Effect-based command handlers to be used with the cli/v3 framework.
+//
+// The conversion process:
+//  1. Takes the Effect which expects a *C.Command context
+//  2. Executes it with the provided command
+//  3. Runs the resulting IO operation
+//  4. Converts the Result to either nil (success) or error (failure)
+//
+// # Parameters
+//
+//   - effect: The CommandEffect to convert
+//
+// # Returns
+//
+//   - A function compatible with C.Command.Action signature
+//
+// # Example Usage
+//
+//	effect := func(cmd *C.Command) E.Thunk[F.Void] {
+//	    return func(ctx context.Context) E.IOResult[F.Void] {
+//	        return func() R.Result[F.Void] {
+//	            // Command logic here
+//	            return R.Of(F.Void{})
+//	        }
+//	    }
+//	}
+//	action := ToAction(effect)
+//	command := &C.Command{
+//	    Name: "example",
+//	    Action: action,
+//	}
+func ToAction(effect CommandEffect) func(context.Context, *C.Command) error {
+	return func(ctx context.Context, cmd *C.Command) error {
+		// Execute the effect: cmd -> ctx -> IO -> Result
+		return F.Pipe3(
+			ctx,
+			effect(cmd),
+			io.Run,
+			// Convert Result[Void] to error
+			ET.Fold(F.Identity[error], F.Constant1[F.Void, error](nil)),
+		)
+	}
+}
+
+// FromAction converts a standard urfave/cli Action function into a CommandEffect.
+// This allows existing cli/v3 action handlers to be lifted into the Effect type.
+//
+// The conversion process:
+//  1. Takes a standard action function (context.Context, *C.Command) -> error
+//  2. Wraps it in the Effect structure
+//  3. Converts the error result to a Result type
+//
+// # Parameters
+//
+//   - action: The standard cli/v3 action function to convert
+//
+// # Returns
+//
+//   - A CommandEffect that wraps the original action
+//
+// # Example Usage
+//
+//	standardAction := func(ctx context.Context, cmd *C.Command) error {
+//	    // Existing command logic
+//	    return nil
+//	}
+//	effect := FromAction(standardAction)
+//	// Now can be composed with other Effects
+func FromAction(action func(context.Context, *C.Command) error) CommandEffect {
+	return func(cmd *C.Command) E.Thunk[F.Void] {
+		return func(ctx context.Context) E.IOResult[F.Void] {
+			return func() R.Result[F.Void] {
+				err := action(ctx, cmd)
+				if err != nil {
+					return R.Left[F.Void](err)
+				}
+				return R.Of(F.Void{})
+			}
+		}
+	}
+}
+
+// MakeCommand creates a new Command with an Effect-based action.
+// This is a convenience function that combines command creation with Effect conversion.
+//
+// # Parameters
+//
+//   - name: The command name
+//   - usage: The command usage description
+//   - flags: The command flags
+//   - effect: The CommandEffect to use as the action
+//
+// # Returns
+//
+//   - A *C.Command configured with the Effect-based action
+//
+// # Example Usage
+//
+//	cmd := MakeCommand(
+//	    "process",
+//	    "Process data files",
+//	    []C.Flag{
+//	        &C.StringFlag{Name: "input", Usage: "Input file"},
+//	    },
+//	    func(cmd *C.Command) E.Thunk[F.Void] {
+//	        return func(ctx context.Context) E.IOResult[F.Void] {
+//	            return func() R.Result[F.Void] {
+//	                input := cmd.String("input")
+//	                // Process input...
+//	                return R.Of(F.Void{})
+//	            }
+//	        }
+//	    },
+//	)
+func MakeCommand(
+	name string,
+	usage string,
+	flags []C.Flag,
+	effect CommandEffect,
+) *C.Command {
+	return &C.Command{
+		Name:   name,
+		Usage:  usage,
+		Flags:  flags,
+		Action: ToAction(effect),
+	}
+}
+
+// MakeCommandWithSubcommands creates a new Command with subcommands and an Effect-based action.
+//
+// # Parameters
+//
+//   - name: The command name
+//   - usage: The command usage description
+//   - flags: The command flags
+//   - commands: The subcommands
+//   - effect: The CommandEffect to use as the action
+//
+// # Returns
+//
+//   - A *C.Command configured with subcommands and the Effect-based action
+//
+// # Example Usage
+//
+//	cmd := MakeCommandWithSubcommands(
+//	    "app",
+//	    "Application commands",
+//	    []C.Flag{},
+//	    []*C.Command{subCmd1, subCmd2},
+//	    defaultEffect,
+//	)
+func MakeCommandWithSubcommands(
+	name string,
+	usage string,
+	flags []C.Flag,
+	commands []*C.Command,
+	effect CommandEffect,
+) *C.Command {
+	return &C.Command{
+		Name:     name,
+		Usage:    usage,
+		Flags:    flags,
+		Commands: commands,
+		Action:   ToAction(effect),
+	}
+}

v2/cli/effect_test.go

@@ -0,0 +1,204 @@
+// 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 (
+	"context"
+	"errors"
+	"testing"
+
+	E "github.com/IBM/fp-go/v2/effect"
+	F "github.com/IBM/fp-go/v2/function"
+	R "github.com/IBM/fp-go/v2/result"
+	"github.com/stretchr/testify/assert"
+	C "github.com/urfave/cli/v3"
+)
+
+func TestToAction_Success(t *testing.T) {
+	t.Run("converts successful Effect to action", func(t *testing.T) {
+		// Arrange
+		effect := func(cmd *C.Command) E.Thunk[F.Void] {
+			return func(ctx context.Context) E.IOResult[F.Void] {
+				return func() R.Result[F.Void] {
+					return R.Of(F.Void{})
+				}
+			}
+		}
+		action := ToAction(effect)
+		cmd := &C.Command{Name: "test"}
+
+		// Act
+		err := action(context.Background(), cmd)
+
+		// Assert
+		assert.NoError(t, err)
+	})
+}
+
+func TestToAction_Failure(t *testing.T) {
+	t.Run("converts failed Effect to error", func(t *testing.T) {
+		// Arrange
+		expectedErr := errors.New("test error")
+		effect := func(cmd *C.Command) E.Thunk[F.Void] {
+			return func(ctx context.Context) E.IOResult[F.Void] {
+				return func() R.Result[F.Void] {
+					return R.Left[F.Void](expectedErr)
+				}
+			}
+		}
+		action := ToAction(effect)
+		cmd := &C.Command{Name: "test"}
+
+		// Act
+		err := action(context.Background(), cmd)
+
+		// Assert
+		assert.Error(t, err)
+		assert.Equal(t, expectedErr, err)
+	})
+}
+
+func TestFromAction_Success(t *testing.T) {
+	t.Run("converts successful action to Effect", func(t *testing.T) {
+		// Arrange
+		action := func(ctx context.Context, cmd *C.Command) error {
+			return nil
+		}
+		effect := FromAction(action)
+		cmd := &C.Command{Name: "test"}
+
+		// Act
+		result := effect(cmd)(context.Background())()
+
+		// Assert
+		assert.True(t, R.IsRight(result))
+	})
+}
+
+func TestFromAction_Failure(t *testing.T) {
+	t.Run("converts failed action to Effect", func(t *testing.T) {
+		// Arrange
+		expectedErr := errors.New("test error")
+		action := func(ctx context.Context, cmd *C.Command) error {
+			return expectedErr
+		}
+		effect := FromAction(action)
+		cmd := &C.Command{Name: "test"}
+
+		// Act
+		result := effect(cmd)(context.Background())()
+
+		// Assert
+		assert.True(t, R.IsLeft(result))
+		err := R.MonadFold(result, F.Identity[error], func(F.Void) error { return nil })
+		assert.Equal(t, expectedErr, err)
+	})
+}
+
+func TestMakeCommand(t *testing.T) {
+	t.Run("creates command with Effect-based action", func(t *testing.T) {
+		// Arrange
+		effect := func(cmd *C.Command) E.Thunk[F.Void] {
+			return func(ctx context.Context) E.IOResult[F.Void] {
+				return func() R.Result[F.Void] {
+					return R.Of(F.Void{})
+				}
+			}
+		}
+
+		// Act
+		cmd := MakeCommand(
+			"test",
+			"Test command",
+			[]C.Flag{},
+			effect,
+		)
+
+		// Assert
+		assert.NotNil(t, cmd)
+		assert.Equal(t, "test", cmd.Name)
+		assert.Equal(t, "Test command", cmd.Usage)
+		assert.NotNil(t, cmd.Action)
+
+		// Test the action
+		err := cmd.Action(context.Background(), cmd)
+		assert.NoError(t, err)
+	})
+}
+
+func TestMakeCommandWithSubcommands(t *testing.T) {
+	t.Run("creates command with subcommands and Effect-based action", func(t *testing.T) {
+		// Arrange
+		subCmd := &C.Command{Name: "sub"}
+		effect := func(cmd *C.Command) E.Thunk[F.Void] {
+			return func(ctx context.Context) E.IOResult[F.Void] {
+				return func() R.Result[F.Void] {
+					return R.Of(F.Void{})
+				}
+			}
+		}
+
+		// Act
+		cmd := MakeCommandWithSubcommands(
+			"parent",
+			"Parent command",
+			[]C.Flag{},
+			[]*C.Command{subCmd},
+			effect,
+		)
+
+		// Assert
+		assert.NotNil(t, cmd)
+		assert.Equal(t, "parent", cmd.Name)
+		assert.Equal(t, "Parent command", cmd.Usage)
+		assert.Len(t, cmd.Commands, 1)
+		assert.Equal(t, "sub", cmd.Commands[0].Name)
+		assert.NotNil(t, cmd.Action)
+	})
+}
+
+func TestToAction_Integration(t *testing.T) {
+	t.Run("Effect can access command flags", func(t *testing.T) {
+		// Arrange
+		var capturedValue string
+		effect := func(cmd *C.Command) E.Thunk[F.Void] {
+			return func(ctx context.Context) E.IOResult[F.Void] {
+				return func() R.Result[F.Void] {
+					capturedValue = cmd.String("input")
+					return R.Of(F.Void{})
+				}
+			}
+		}
+
+		cmd := &C.Command{
+			Name: "test",
+			Flags: []C.Flag{
+				&C.StringFlag{
+					Name:  "input",
+					Value: "default-value",
+				},
+			},
+			Action: ToAction(effect),
+		}
+
+		// Act
+		err := cmd.Action(context.Background(), cmd)
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, "default-value", capturedValue)
+	})
+}

v2/cli/either.go

@@ -21,7 +21,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-	"time"
 
 	C "github.com/urfave/cli/v3"
 )
@@ -148,8 +147,8 @@ func generateEitherHelpers(filename string, count int) error {
 
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 
 	fmt.Fprintf(f, "package %s\n\n", pkg)
 

v2/cli/flags.go

@@ -0,0 +1,359 @@
+// 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 (
+	P "github.com/IBM/fp-go/v2/optics/prism"
+	O "github.com/IBM/fp-go/v2/option"
+	C "github.com/urfave/cli/v3"
+)
+
+// StringFlagPrism creates a Prism for extracting a StringFlag from a Flag.
+// This provides a type-safe way to work with string flags, handling type
+// mismatches gracefully through the Option type.
+//
+// The prism's GetOption attempts to cast a Flag to *C.StringFlag.
+// If the cast succeeds, it returns Some(*C.StringFlag); if it fails, it returns None.
+//
+// The prism's ReverseGet converts a *C.StringFlag back to a Flag.
+//
+// # Returns
+//
+//   - A Prism[C.Flag, *C.StringFlag] for safe StringFlag extraction
+//
+// # Example Usage
+//
+//	prism := StringFlagPrism()
+//
+//	// Extract StringFlag from Flag
+//	var flag C.Flag = &C.StringFlag{Name: "input", Value: "default"}
+//	result := prism.GetOption(flag)  // Some(*C.StringFlag{...})
+//
+//	// Type mismatch returns None
+//	var intFlag C.Flag = &C.IntFlag{Name: "count"}
+//	result = prism.GetOption(intFlag)  // None[*C.StringFlag]()
+//
+//	// Convert back to Flag
+//	strFlag := &C.StringFlag{Name: "output"}
+//	flag = prism.ReverseGet(strFlag)
+func StringFlagPrism() P.Prism[C.Flag, *C.StringFlag] {
+	return P.MakePrism(
+		func(flag C.Flag) O.Option[*C.StringFlag] {
+			if sf, ok := flag.(*C.StringFlag); ok {
+				return O.Some(sf)
+			}
+			return O.None[*C.StringFlag]()
+		},
+		func(f *C.StringFlag) C.Flag { return f },
+	)
+}
+
+// IntFlagPrism creates a Prism for extracting an IntFlag from a Flag.
+// This provides a type-safe way to work with integer flags, handling type
+// mismatches gracefully through the Option type.
+//
+// # Returns
+//
+//   - A Prism[C.Flag, *C.IntFlag] for safe IntFlag extraction
+//
+// # Example Usage
+//
+//	prism := IntFlagPrism()
+//
+//	// Extract IntFlag from Flag
+//	var flag C.Flag = &C.IntFlag{Name: "count", Value: 10}
+//	result := prism.GetOption(flag)  // Some(*C.IntFlag{...})
+func IntFlagPrism() P.Prism[C.Flag, *C.IntFlag] {
+	return P.MakePrism(
+		func(flag C.Flag) O.Option[*C.IntFlag] {
+			if f, ok := flag.(*C.IntFlag); ok {
+				return O.Some(f)
+			}
+			return O.None[*C.IntFlag]()
+		},
+		func(f *C.IntFlag) C.Flag { return f },
+	)
+}
+
+// BoolFlagPrism creates a Prism for extracting a BoolFlag from a Flag.
+// This provides a type-safe way to work with boolean flags, handling type
+// mismatches gracefully through the Option type.
+//
+// # Returns
+//
+//   - A Prism[C.Flag, *C.BoolFlag] for safe BoolFlag extraction
+//
+// # Example Usage
+//
+//	prism := BoolFlagPrism()
+//
+//	// Extract BoolFlag from Flag
+//	var flag C.Flag = &C.BoolFlag{Name: "verbose", Value: true}
+//	result := prism.GetOption(flag)  // Some(*C.BoolFlag{...})
+func BoolFlagPrism() P.Prism[C.Flag, *C.BoolFlag] {
+	return P.MakePrism(
+		func(flag C.Flag) O.Option[*C.BoolFlag] {
+			if f, ok := flag.(*C.BoolFlag); ok {
+				return O.Some(f)
+			}
+			return O.None[*C.BoolFlag]()
+		},
+		func(f *C.BoolFlag) C.Flag { return f },
+	)
+}
+
+// Float64FlagPrism creates a Prism for extracting a Float64Flag from a Flag.
+// This provides a type-safe way to work with float64 flags, handling type
+// mismatches gracefully through the Option type.
+//
+// # Returns
+//
+//   - A Prism[C.Flag, *C.Float64Flag] for safe Float64Flag extraction
+//
+// # Example Usage
+//
+//	prism := Float64FlagPrism()
+//
+//	// Extract Float64Flag from Flag
+//	var flag C.Flag = &C.Float64Flag{Name: "ratio", Value: 0.5}
+//	result := prism.GetOption(flag)  // Some(*C.Float64Flag{...})
+func Float64FlagPrism() P.Prism[C.Flag, *C.Float64Flag] {
+	return P.MakePrism(
+		func(flag C.Flag) O.Option[*C.Float64Flag] {
+			if f, ok := flag.(*C.Float64Flag); ok {
+				return O.Some(f)
+			}
+			return O.None[*C.Float64Flag]()
+		},
+		func(f *C.Float64Flag) C.Flag { return f },
+	)
+}
+
+// DurationFlagPrism creates a Prism for extracting a DurationFlag from a Flag.
+// This provides a type-safe way to work with duration flags, handling type
+// mismatches gracefully through the Option type.
+//
+// # Returns
+//
+//   - A Prism[C.Flag, *C.DurationFlag] for safe DurationFlag extraction
+//
+// # Example Usage
+//
+//	prism := DurationFlagPrism()
+//
+//	// Extract DurationFlag from Flag
+//	var flag C.Flag = &C.DurationFlag{Name: "timeout", Value: 30 * time.Second}
+//	result := prism.GetOption(flag)  // Some(*C.DurationFlag{...})
+func DurationFlagPrism() P.Prism[C.Flag, *C.DurationFlag] {
+	return P.MakePrism(
+		func(flag C.Flag) O.Option[*C.DurationFlag] {
+			if f, ok := flag.(*C.DurationFlag); ok {
+				return O.Some(f)
+			}
+			return O.None[*C.DurationFlag]()
+		},
+		func(f *C.DurationFlag) C.Flag { return f },
+	)
+}
+
+// TimestampFlagPrism creates a Prism for extracting a TimestampFlag from a Flag.
+// This provides a type-safe way to work with timestamp flags, handling type
+// mismatches gracefully through the Option type.
+//
+// # Returns
+//
+//   - A Prism[C.Flag, *C.TimestampFlag] for safe TimestampFlag extraction
+//
+// # Example Usage
+//
+//	prism := TimestampFlagPrism()
+//
+//	// Extract TimestampFlag from Flag
+//	var flag C.Flag = &C.TimestampFlag{Name: "created"}
+//	result := prism.GetOption(flag)  // Some(*C.TimestampFlag{...})
+func TimestampFlagPrism() P.Prism[C.Flag, *C.TimestampFlag] {
+	return P.MakePrism(
+		func(flag C.Flag) O.Option[*C.TimestampFlag] {
+			if f, ok := flag.(*C.TimestampFlag); ok {
+				return O.Some(f)
+			}
+			return O.None[*C.TimestampFlag]()
+		},
+		func(f *C.TimestampFlag) C.Flag { return f },
+	)
+}
+
+// StringSliceFlagPrism creates a Prism for extracting a StringSliceFlag from a Flag.
+// This provides a type-safe way to work with string slice flags, handling type
+// mismatches gracefully through the Option type.
+//
+// # Returns
+//
+//   - A Prism[C.Flag, *C.StringSliceFlag] for safe StringSliceFlag extraction
+//
+// # Example Usage
+//
+//	prism := StringSliceFlagPrism()
+//
+//	// Extract StringSliceFlag from Flag
+//	var flag C.Flag = &C.StringSliceFlag{Name: "tags"}
+//	result := prism.GetOption(flag)  // Some(*C.StringSliceFlag{...})
+func StringSliceFlagPrism() P.Prism[C.Flag, *C.StringSliceFlag] {
+	return P.MakePrism(
+		func(flag C.Flag) O.Option[*C.StringSliceFlag] {
+			if f, ok := flag.(*C.StringSliceFlag); ok {
+				return O.Some(f)
+			}
+			return O.None[*C.StringSliceFlag]()
+		},
+		func(f *C.StringSliceFlag) C.Flag { return f },
+	)
+}
+
+// IntSliceFlagPrism creates a Prism for extracting an IntSliceFlag from a Flag.
+// This provides a type-safe way to work with int slice flags, handling type
+// mismatches gracefully through the Option type.
+//
+// # Returns
+//
+//   - A Prism[C.Flag, *C.IntSliceFlag] for safe IntSliceFlag extraction
+//
+// # Example Usage
+//
+//	prism := IntSliceFlagPrism()
+//
+//	// Extract IntSliceFlag from Flag
+//	var flag C.Flag = &C.IntSliceFlag{Name: "ports"}
+//	result := prism.GetOption(flag)  // Some(*C.IntSliceFlag{...})
+func IntSliceFlagPrism() P.Prism[C.Flag, *C.IntSliceFlag] {
+	return P.MakePrism(
+		func(flag C.Flag) O.Option[*C.IntSliceFlag] {
+			if f, ok := flag.(*C.IntSliceFlag); ok {
+				return O.Some(f)
+			}
+			return O.None[*C.IntSliceFlag]()
+		},
+		func(f *C.IntSliceFlag) C.Flag { return f },
+	)
+}
+
+// Float64SliceFlagPrism creates a Prism for extracting a Float64SliceFlag from a Flag.
+// This provides a type-safe way to work with float64 slice flags, handling type
+// mismatches gracefully through the Option type.
+//
+// # Returns
+//
+//   - A Prism[C.Flag, *C.Float64SliceFlag] for safe Float64SliceFlag extraction
+//
+// # Example Usage
+//
+//	prism := Float64SliceFlagPrism()
+//
+//	// Extract Float64SliceFlag from Flag
+//	var flag C.Flag = &C.Float64SliceFlag{Name: "ratios"}
+//	result := prism.GetOption(flag)  // Some(*C.Float64SliceFlag{...})
+func Float64SliceFlagPrism() P.Prism[C.Flag, *C.Float64SliceFlag] {
+	return P.MakePrism(
+		func(flag C.Flag) O.Option[*C.Float64SliceFlag] {
+			if f, ok := flag.(*C.Float64SliceFlag); ok {
+				return O.Some(f)
+			}
+			return O.None[*C.Float64SliceFlag]()
+		},
+		func(f *C.Float64SliceFlag) C.Flag { return f },
+	)
+}
+
+// UintFlagPrism creates a Prism for extracting a UintFlag from a Flag.
+// This provides a type-safe way to work with unsigned integer flags, handling type
+// mismatches gracefully through the Option type.
+//
+// # Returns
+//
+//   - A Prism[C.Flag, *C.UintFlag] for safe UintFlag extraction
+//
+// # Example Usage
+//
+//	prism := UintFlagPrism()
+//
+//	// Extract UintFlag from Flag
+//	var flag C.Flag = &C.UintFlag{Name: "workers", Value: 4}
+//	result := prism.GetOption(flag)  // Some(*C.UintFlag{...})
+func UintFlagPrism() P.Prism[C.Flag, *C.UintFlag] {
+	return P.MakePrism(
+		func(flag C.Flag) O.Option[*C.UintFlag] {
+			if f, ok := flag.(*C.UintFlag); ok {
+				return O.Some(f)
+			}
+			return O.None[*C.UintFlag]()
+		},
+		func(f *C.UintFlag) C.Flag { return f },
+	)
+}
+
+// Uint64FlagPrism creates a Prism for extracting a Uint64Flag from a Flag.
+// This provides a type-safe way to work with uint64 flags, handling type
+// mismatches gracefully through the Option type.
+//
+// # Returns
+//
+//   - A Prism[C.Flag, *C.Uint64Flag] for safe Uint64Flag extraction
+//
+// # Example Usage
+//
+//	prism := Uint64FlagPrism()
+//
+//	// Extract Uint64Flag from Flag
+//	var flag C.Flag = &C.Uint64Flag{Name: "size"}
+//	result := prism.GetOption(flag)  // Some(*C.Uint64Flag{...})
+func Uint64FlagPrism() P.Prism[C.Flag, *C.Uint64Flag] {
+	return P.MakePrism(
+		func(flag C.Flag) O.Option[*C.Uint64Flag] {
+			if f, ok := flag.(*C.Uint64Flag); ok {
+				return O.Some(f)
+			}
+			return O.None[*C.Uint64Flag]()
+		},
+		func(f *C.Uint64Flag) C.Flag { return f },
+	)
+}
+
+// Int64FlagPrism creates a Prism for extracting an Int64Flag from a Flag.
+// This provides a type-safe way to work with int64 flags, handling type
+// mismatches gracefully through the Option type.
+//
+// # Returns
+//
+//   - A Prism[C.Flag, *C.Int64Flag] for safe Int64Flag extraction
+//
+// # Example Usage
+//
+//	prism := Int64FlagPrism()
+//
+//	// Extract Int64Flag from Flag
+//	var flag C.Flag = &C.Int64Flag{Name: "offset"}
+//	result := prism.GetOption(flag)  // Some(*C.Int64Flag{...})
+func Int64FlagPrism() P.Prism[C.Flag, *C.Int64Flag] {
+	return P.MakePrism(
+		func(flag C.Flag) O.Option[*C.Int64Flag] {
+			if f, ok := flag.(*C.Int64Flag); ok {
+				return O.Some(f)
+			}
+			return O.None[*C.Int64Flag]()
+		},
+		func(f *C.Int64Flag) C.Flag { return f },
+	)
+}

v2/cli/flags_test.go

@@ -0,0 +1,287 @@
+// 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 (
+	"testing"
+	"time"
+
+	O "github.com/IBM/fp-go/v2/option"
+	"github.com/stretchr/testify/assert"
+	C "github.com/urfave/cli/v3"
+)
+
+func TestStringFlagPrism_Success(t *testing.T) {
+	t.Run("extracts StringFlag from Flag", func(t *testing.T) {
+		// Arrange
+		prism := StringFlagPrism()
+		var flag C.Flag = &C.StringFlag{Name: "input", Value: "test"}
+
+		// Act
+		result := prism.GetOption(flag)
+
+		// Assert
+		assert.True(t, O.IsSome(result))
+		extracted := O.MonadFold(result, func() *C.StringFlag { return nil }, func(f *C.StringFlag) *C.StringFlag { return f })
+		assert.NotNil(t, extracted)
+		assert.Equal(t, "input", extracted.Name)
+		assert.Equal(t, "test", extracted.Value)
+	})
+}
+
+func TestStringFlagPrism_Failure(t *testing.T) {
+	t.Run("returns None for non-StringFlag", func(t *testing.T) {
+		// Arrange
+		prism := StringFlagPrism()
+		var flag C.Flag = &C.IntFlag{Name: "count"}
+
+		// Act
+		result := prism.GetOption(flag)
+
+		// Assert
+		assert.True(t, O.IsNone(result))
+	})
+}
+
+func TestStringFlagPrism_ReverseGet(t *testing.T) {
+	t.Run("converts StringFlag back to Flag", func(t *testing.T) {
+		// Arrange
+		prism := StringFlagPrism()
+		strFlag := &C.StringFlag{Name: "output", Value: "result"}
+
+		// Act
+		flag := prism.ReverseGet(strFlag)
+
+		// Assert
+		assert.NotNil(t, flag)
+		assert.IsType(t, &C.StringFlag{}, flag)
+	})
+}
+
+func TestIntFlagPrism_Success(t *testing.T) {
+	t.Run("extracts IntFlag from Flag", func(t *testing.T) {
+		// Arrange
+		prism := IntFlagPrism()
+		var flag C.Flag = &C.IntFlag{Name: "count", Value: 42}
+
+		// Act
+		result := prism.GetOption(flag)
+
+		// Assert
+		assert.True(t, O.IsSome(result))
+		extracted := O.MonadFold(result, func() *C.IntFlag { return nil }, func(f *C.IntFlag) *C.IntFlag { return f })
+		assert.NotNil(t, extracted)
+		assert.Equal(t, "count", extracted.Name)
+		assert.Equal(t, 42, extracted.Value)
+	})
+}
+
+func TestBoolFlagPrism_Success(t *testing.T) {
+	t.Run("extracts BoolFlag from Flag", func(t *testing.T) {
+		// Arrange
+		prism := BoolFlagPrism()
+		var flag C.Flag = &C.BoolFlag{Name: "verbose", Value: true}
+
+		// Act
+		result := prism.GetOption(flag)
+
+		// Assert
+		assert.True(t, O.IsSome(result))
+		extracted := O.MonadFold(result, func() *C.BoolFlag { return nil }, func(f *C.BoolFlag) *C.BoolFlag { return f })
+		assert.NotNil(t, extracted)
+		assert.Equal(t, "verbose", extracted.Name)
+		assert.Equal(t, true, extracted.Value)
+	})
+}
+
+func TestFloat64FlagPrism_Success(t *testing.T) {
+	t.Run("extracts Float64Flag from Flag", func(t *testing.T) {
+		// Arrange
+		prism := Float64FlagPrism()
+		var flag C.Flag = &C.Float64Flag{Name: "ratio", Value: 0.5}
+
+		// Act
+		result := prism.GetOption(flag)
+
+		// Assert
+		assert.True(t, O.IsSome(result))
+		extracted := O.MonadFold(result, func() *C.Float64Flag { return nil }, func(f *C.Float64Flag) *C.Float64Flag { return f })
+		assert.NotNil(t, extracted)
+		assert.Equal(t, "ratio", extracted.Name)
+		assert.Equal(t, 0.5, extracted.Value)
+	})
+}
+
+func TestDurationFlagPrism_Success(t *testing.T) {
+	t.Run("extracts DurationFlag from Flag", func(t *testing.T) {
+		// Arrange
+		prism := DurationFlagPrism()
+		duration := 30 * time.Second
+		var flag C.Flag = &C.DurationFlag{Name: "timeout", Value: duration}
+
+		// Act
+		result := prism.GetOption(flag)
+
+		// Assert
+		assert.True(t, O.IsSome(result))
+		extracted := O.MonadFold(result, func() *C.DurationFlag { return nil }, func(f *C.DurationFlag) *C.DurationFlag { return f })
+		assert.NotNil(t, extracted)
+		assert.Equal(t, "timeout", extracted.Name)
+		assert.Equal(t, duration, extracted.Value)
+	})
+}
+
+func TestTimestampFlagPrism_Success(t *testing.T) {
+	t.Run("extracts TimestampFlag from Flag", func(t *testing.T) {
+		// Arrange
+		prism := TimestampFlagPrism()
+		var flag C.Flag = &C.TimestampFlag{Name: "created"}
+
+		// Act
+		result := prism.GetOption(flag)
+
+		// Assert
+		assert.True(t, O.IsSome(result))
+		extracted := O.MonadFold(result, func() *C.TimestampFlag { return nil }, func(f *C.TimestampFlag) *C.TimestampFlag { return f })
+		assert.NotNil(t, extracted)
+		assert.Equal(t, "created", extracted.Name)
+	})
+}
+
+func TestStringSliceFlagPrism_Success(t *testing.T) {
+	t.Run("extracts StringSliceFlag from Flag", func(t *testing.T) {
+		// Arrange
+		prism := StringSliceFlagPrism()
+		var flag C.Flag = &C.StringSliceFlag{Name: "tags"}
+
+		// Act
+		result := prism.GetOption(flag)
+
+		// Assert
+		assert.True(t, O.IsSome(result))
+		extracted := O.MonadFold(result, func() *C.StringSliceFlag { return nil }, func(f *C.StringSliceFlag) *C.StringSliceFlag { return f })
+		assert.NotNil(t, extracted)
+		assert.Equal(t, "tags", extracted.Name)
+	})
+}
+
+func TestIntSliceFlagPrism_Success(t *testing.T) {
+	t.Run("extracts IntSliceFlag from Flag", func(t *testing.T) {
+		// Arrange
+		prism := IntSliceFlagPrism()
+		var flag C.Flag = &C.IntSliceFlag{Name: "ports"}
+
+		// Act
+		result := prism.GetOption(flag)
+
+		// Assert
+		assert.True(t, O.IsSome(result))
+		extracted := O.MonadFold(result, func() *C.IntSliceFlag { return nil }, func(f *C.IntSliceFlag) *C.IntSliceFlag { return f })
+		assert.NotNil(t, extracted)
+		assert.Equal(t, "ports", extracted.Name)
+	})
+}
+
+func TestFloat64SliceFlagPrism_Success(t *testing.T) {
+	t.Run("extracts Float64SliceFlag from Flag", func(t *testing.T) {
+		// Arrange
+		prism := Float64SliceFlagPrism()
+		var flag C.Flag = &C.Float64SliceFlag{Name: "ratios"}
+
+		// Act
+		result := prism.GetOption(flag)
+
+		// Assert
+		assert.True(t, O.IsSome(result))
+		extracted := O.MonadFold(result, func() *C.Float64SliceFlag { return nil }, func(f *C.Float64SliceFlag) *C.Float64SliceFlag { return f })
+		assert.NotNil(t, extracted)
+		assert.Equal(t, "ratios", extracted.Name)
+	})
+}
+
+func TestUintFlagPrism_Success(t *testing.T) {
+	t.Run("extracts UintFlag from Flag", func(t *testing.T) {
+		// Arrange
+		prism := UintFlagPrism()
+		var flag C.Flag = &C.UintFlag{Name: "workers", Value: 4}
+
+		// Act
+		result := prism.GetOption(flag)
+
+		// Assert
+		assert.True(t, O.IsSome(result))
+		extracted := O.MonadFold(result, func() *C.UintFlag { return nil }, func(f *C.UintFlag) *C.UintFlag { return f })
+		assert.NotNil(t, extracted)
+		assert.Equal(t, "workers", extracted.Name)
+		assert.Equal(t, uint(4), extracted.Value)
+	})
+}
+
+func TestUint64FlagPrism_Success(t *testing.T) {
+	t.Run("extracts Uint64Flag from Flag", func(t *testing.T) {
+		// Arrange
+		prism := Uint64FlagPrism()
+		var flag C.Flag = &C.Uint64Flag{Name: "size", Value: 1024}
+
+		// Act
+		result := prism.GetOption(flag)
+
+		// Assert
+		assert.True(t, O.IsSome(result))
+		extracted := O.MonadFold(result, func() *C.Uint64Flag { return nil }, func(f *C.Uint64Flag) *C.Uint64Flag { return f })
+		assert.NotNil(t, extracted)
+		assert.Equal(t, "size", extracted.Name)
+		assert.Equal(t, uint64(1024), extracted.Value)
+	})
+}
+
+func TestInt64FlagPrism_Success(t *testing.T) {
+	t.Run("extracts Int64Flag from Flag", func(t *testing.T) {
+		// Arrange
+		prism := Int64FlagPrism()
+		var flag C.Flag = &C.Int64Flag{Name: "offset", Value: -100}
+
+		// Act
+		result := prism.GetOption(flag)
+
+		// Assert
+		assert.True(t, O.IsSome(result))
+		extracted := O.MonadFold(result, func() *C.Int64Flag { return nil }, func(f *C.Int64Flag) *C.Int64Flag { return f })
+		assert.NotNil(t, extracted)
+		assert.Equal(t, "offset", extracted.Name)
+		assert.Equal(t, int64(-100), extracted.Value)
+	})
+}
+
+func TestPrisms_EdgeCases(t *testing.T) {
+	t.Run("all prisms return None for wrong type", func(t *testing.T) {
+		// Arrange
+		var flag C.Flag = &C.StringFlag{Name: "test"}
+
+		// Act & Assert
+		assert.True(t, O.IsNone(IntFlagPrism().GetOption(flag)))
+		assert.True(t, O.IsNone(BoolFlagPrism().GetOption(flag)))
+		assert.True(t, O.IsNone(Float64FlagPrism().GetOption(flag)))
+		assert.True(t, O.IsNone(DurationFlagPrism().GetOption(flag)))
+		assert.True(t, O.IsNone(TimestampFlagPrism().GetOption(flag)))
+		assert.True(t, O.IsNone(StringSliceFlagPrism().GetOption(flag)))
+		assert.True(t, O.IsNone(IntSliceFlagPrism().GetOption(flag)))
+		assert.True(t, O.IsNone(Float64SliceFlagPrism().GetOption(flag)))
+		assert.True(t, O.IsNone(UintFlagPrism().GetOption(flag)))
+		assert.True(t, O.IsNone(Uint64FlagPrism().GetOption(flag)))
+		assert.True(t, O.IsNone(Int64FlagPrism().GetOption(flag)))
+	})
+}

v2/cli/header.go

@@ -18,7 +18,6 @@ package cli
 import (
 	"fmt"
 	"os"
-	"time"
 )
 
 func writePackage(f *os.File, pkg string) {
@@ -26,6 +25,6 @@ func writePackage(f *os.File, pkg string) {
 	fmt.Fprintf(f, "package %s\n\n", pkg)
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 }

v2/cli/identity.go

@@ -21,7 +21,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-	"time"
 
 	C "github.com/urfave/cli/v3"
 )
@@ -62,8 +61,8 @@ func generateIdentityHelpers(filename string, count int) error {
 
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 
 	fmt.Fprintf(f, "package %s\n\n", pkg)
 

v2/cli/io.go

@@ -21,7 +21,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-	"time"
 
 	A "github.com/IBM/fp-go/v2/array"
 	C "github.com/urfave/cli/v3"
@@ -71,8 +70,8 @@ func generateIOHelpers(filename string, count int) error {
 
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 
 	fmt.Fprintf(f, "package %s\n\n", pkg)
 

v2/cli/ioeither.go

@@ -21,7 +21,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-	"time"
 
 	A "github.com/IBM/fp-go/v2/array"
 	C "github.com/urfave/cli/v3"
@@ -219,8 +218,8 @@ func generateIOEitherHelpers(filename string, count int) error {
 
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 
 	fmt.Fprintf(f, "package %s\n\n", pkg)
 
@@ -234,8 +233,7 @@ import (
 
 	// some header
 	fmt.Fprintln(fg, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(fg, "// This file was generated by robots at")
-	fmt.Fprintf(fg, "// %s\n", time.Now())
+	fmt.Fprintln(fg, "// This file was generated by robots.")
 
 	fmt.Fprintf(fg, "package generic\n\n")
 

v2/cli/iooption.go

@@ -21,7 +21,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-	"time"
 
 	A "github.com/IBM/fp-go/v2/array"
 	C "github.com/urfave/cli/v3"
@@ -76,8 +75,8 @@ func generateIOOptionHelpers(filename string, count int) error {
 
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 
 	fmt.Fprintf(f, "package %s\n\n", pkg)
 

v2/cli/option.go

@@ -21,7 +21,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-	"time"
 
 	C "github.com/urfave/cli/v3"
 )
@@ -148,8 +147,8 @@ func generateOptionHelpers(filename string, count int) error {
 
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 
 	fmt.Fprintf(f, "package %s\n\n", pkg)
 

v2/cli/pipe.go

@@ -21,7 +21,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-	"time"
 
 	C "github.com/urfave/cli/v3"
 )
@@ -378,8 +377,8 @@ func generatePipeHelpers(filename string, count int) error {
 
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 
 	fmt.Fprintf(f, "package %s\n", pkg)
 

v2/cli/reader.go

@@ -21,7 +21,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-	"time"
 
 	C "github.com/urfave/cli/v3"
 )
@@ -118,8 +117,8 @@ func generateReaderHelpers(filename string, count int) error {
 
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 
 	fmt.Fprintf(f, "package %s\n\n", pkg)
 
@@ -131,8 +130,7 @@ import (
 
 	// some header
 	fmt.Fprintln(fg, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(fg, "// This file was generated by robots at")
-	fmt.Fprintf(fg, "// %s\n", time.Now())
+	fmt.Fprintln(fg, "// This file was generated by robots.")
 
 	fmt.Fprintf(fg, "package generic\n\n")
 

v2/cli/readerioeither.go

@@ -21,7 +21,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-	"time"
 
 	C "github.com/urfave/cli/v3"
 )
@@ -233,8 +232,8 @@ func generateReaderIOEitherHelpers(filename string, count int) error {
 
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 
 	fmt.Fprintf(f, "package %s\n\n", pkg)
 
@@ -246,8 +245,7 @@ import (
 
 	// some header
 	fmt.Fprintln(fg, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(fg, "// This file was generated by robots at")
-	fmt.Fprintf(fg, "// %s\n", time.Now())
+	fmt.Fprintln(fg, "// This file was generated by robots.")
 
 	fmt.Fprintf(fg, "package generic\n\n")
 

v2/cli/tuple.go

@@ -22,7 +22,6 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
-	"time"
 
 	C "github.com/urfave/cli/v3"
 )
@@ -399,8 +398,8 @@ func generateTupleHelpers(filename string, count int) error {
 
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 
 	fmt.Fprintf(f, "package %s\n\n", pkg)
 

v2/constant/const.go

@@ -13,6 +13,37 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// Package constant provides the Const functor, a phantom type that ignores its second type parameter.
+//
+// The Const functor is a fundamental building block in functional programming that wraps a value
+// of type E while having a phantom type parameter A. This makes it useful for:
+//   - Accumulating values during traversals (e.g., collecting metadata)
+//   - Implementing optics (lenses, prisms) where you need to track information
+//   - Building applicative functors that combine values using a semigroup
+//
+// # The Const Functor
+//
+// Const[E, A] wraps a value of type E and has a phantom type parameter A that doesn't affect
+// the runtime value. This allows it to participate in functor and applicative operations while
+// maintaining the wrapped value unchanged.
+//
+// # Key Properties
+//
+//   - Map operations ignore the function and preserve the wrapped value
+//   - Ap operations combine wrapped values using a semigroup
+//   - The phantom type A allows type-safe composition with other functors
+//
+// # Example Usage
+//
+//	// Accumulate string values
+//	c1 := Make[string, int]("hello")
+//	c2 := Make[string, int]("world")
+//
+//	// Map doesn't change the wrapped value
+//	mapped := Map[string, int, string](strconv.Itoa)(c1)  // Still contains "hello"
+//
+//	// Ap combines values using a semigroup
+//	combined := Ap[string, int, int](S.Monoid)(c1)(c2)  // Contains "helloworld"
 package constant
 
 import (
@@ -21,36 +52,209 @@ import (
 	S "github.com/IBM/fp-go/v2/semigroup"
 )
 
+// Const is a functor that wraps a value of type E with a phantom type parameter A.
+//
+// The Const functor is useful for accumulating values during traversals or implementing
+// optics. The type parameter A is phantom - it doesn't affect the runtime value but allows
+// the type to participate in functor and applicative operations.
+//
+// Type Parameters:
+//   - E: The type of the wrapped value (the actual data)
+//   - A: The phantom type parameter (not stored, only used for type-level operations)
+//
+// Example:
+//
+//	// Create a Const that wraps a string
+//	c := Make[string, int]("metadata")
+//
+//	// The int type parameter is phantom - no int value is stored
+//	value := Unwrap(c)  // "metadata"
 type Const[E, A any] struct {
 	value E
 }
 
+// Make creates a Const value wrapping the given value.
+//
+// This is the primary constructor for Const values. The second type parameter A
+// is phantom and must be specified explicitly when needed for type inference.
+//
+// Type Parameters:
+//   - E: The type of the value to wrap
+//   - A: The phantom type parameter
+//
+// Parameters:
+//   - e: The value to wrap
+//
+// Returns:
+//   - A Const[E, A] wrapping the value
+//
+// Example:
+//
+//	c := Make[string, int]("hello")
+//	value := Unwrap(c)  // "hello"
 func Make[E, A any](e E) Const[E, A] {
 	return Const[E, A]{value: e}
 }
 
+// Unwrap extracts the wrapped value from a Const.
+//
+// This is the inverse of Make, retrieving the actual value stored in the Const.
+//
+// Type Parameters:
+//   - E: The type of the wrapped value
+//   - A: The phantom type parameter
+//
+// Parameters:
+//   - c: The Const to unwrap
+//
+// Returns:
+//   - The wrapped value of type E
+//
+// Example:
+//
+//	c := Make[string, int]("world")
+//	value := Unwrap(c)  // "world"
 func Unwrap[E, A any](c Const[E, A]) E {
 	return c.value
 }
 
+// Of creates a Const containing the monoid's empty value, ignoring the input.
+//
+// This implements the Applicative's "pure" operation for Const. It creates a Const
+// wrapping the monoid's identity element, regardless of the input value.
+//
+// Type Parameters:
+//   - E: The type of the wrapped value (must have a monoid)
+//   - A: The input type (ignored)
+//
+// Parameters:
+//   - m: The monoid providing the empty value
+//
+// Returns:
+//   - A function that ignores its input and returns Const[E, A] with the empty value
+//
+// Example:
+//
+//	import S "github.com/IBM/fp-go/v2/string"
+//
+//	of := Of[string, int](S.Monoid)
+//	c := of(42)  // Const[string, int] containing ""
+//	value := Unwrap(c)  // ""
 func Of[E, A any](m M.Monoid[E]) func(A) Const[E, A] {
 	return F.Constant1[A](Make[E, A](m.Empty()))
 }
 
+// MonadMap applies a function to the phantom type parameter without changing the wrapped value.
+//
+// This implements the Functor's map operation for Const. Since the type parameter A is phantom,
+// the function is never actually called - the wrapped value E remains unchanged.
+//
+// Type Parameters:
+//   - E: The type of the wrapped value
+//   - A: The input phantom type
+//   - B: The output phantom type
+//
+// Parameters:
+//   - fa: The Const to map over
+//   - _: The function to apply (ignored)
+//
+// Returns:
+//   - A Const[E, B] with the same wrapped value
+//
+// Example:
+//
+//	c := Make[string, int]("hello")
+//	mapped := MonadMap(c, func(i int) string { return strconv.Itoa(i) })
+//	// mapped still contains "hello", function was never called
 func MonadMap[E, A, B any](fa Const[E, A], _ func(A) B) Const[E, B] {
 	return Make[E, B](fa.value)
 }
 
+// MonadAp combines two Const values using a semigroup.
+//
+// This implements the Applicative's ap operation for Const. It combines the wrapped
+// values from both Const instances using the provided semigroup, ignoring the function
+// type in the first argument.
+//
+// Type Parameters:
+//   - E: The type of the wrapped values (must have a semigroup)
+//   - A: The input phantom type
+//   - B: The output phantom type
+//
+// Parameters:
+//   - s: The semigroup for combining wrapped values
+//
+// Returns:
+//   - A function that takes two Const values and combines their wrapped values
+//
+// Example:
+//
+//	import S "github.com/IBM/fp-go/v2/string"
+//
+//	ap := MonadAp[string, int, int](S.Monoid)
+//	c1 := Make[string, func(int) int]("hello")
+//	c2 := Make[string, int]("world")
+//	result := ap(c1, c2)  // Const containing "helloworld"
 func MonadAp[E, A, B any](s S.Semigroup[E]) func(fab Const[E, func(A) B], fa Const[E, A]) Const[E, B] {
 	return func(fab Const[E, func(A) B], fa Const[E, A]) Const[E, B] {
 		return Make[E, B](s.Concat(fab.value, fa.value))
 	}
 }
 
+// Map applies a function to the phantom type parameter without changing the wrapped value.
+//
+// This is the curried version of MonadMap, providing a more functional programming style.
+// The function is never actually called since A is a phantom type.
+//
+// Type Parameters:
+//   - E: The type of the wrapped value
+//   - A: The input phantom type
+//   - B: The output phantom type
+//
+// Parameters:
+//   - f: The function to apply (ignored)
+//
+// Returns:
+//   - A function that transforms Const[E, A] to Const[E, B]
+//
+// Example:
+//
+//	import F "github.com/IBM/fp-go/v2/function"
+//
+//	c := Make[string, int]("data")
+//	mapped := F.Pipe1(c, Map[string, int, string](strconv.Itoa))
+//	// mapped still contains "data"
 func Map[E, A, B any](f func(A) B) func(fa Const[E, A]) Const[E, B] {
 	return F.Bind2nd(MonadMap[E, A, B], f)
 }
 
+// Ap combines Const values using a semigroup in a curried style.
+//
+// This is the curried version of MonadAp, providing data-last style for better composition.
+// It combines the wrapped values from both Const instances using the provided semigroup.
+//
+// Type Parameters:
+//   - E: The type of the wrapped values (must have a semigroup)
+//   - A: The input phantom type
+//   - B: The output phantom type
+//
+// Parameters:
+//   - s: The semigroup for combining wrapped values
+//
+// Returns:
+//   - A curried function for combining Const values
+//
+// Example:
+//
+//	import (
+//	    F "github.com/IBM/fp-go/v2/function"
+//	    S "github.com/IBM/fp-go/v2/string"
+//	)
+//
+//	c1 := Make[string, int]("hello")
+//	c2 := Make[string, func(int) int]("world")
+//	result := F.Pipe1(c1, Ap[string, int, int](S.Monoid)(c2))
+//	// result contains "helloworld"
 func Ap[E, A, B any](s S.Semigroup[E]) func(fa Const[E, A]) func(fab Const[E, func(A) B]) Const[E, B] {
 	monadap := MonadAp[E, A, B](s)
 	return func(fa Const[E, A]) func(fab Const[E, func(A) B]) Const[E, B] {

v2/constant/const_test.go

@@ -16,25 +16,340 @@
 package constant
 
 import (
+	"strconv"
 	"testing"
 
 	F "github.com/IBM/fp-go/v2/function"
 	"github.com/IBM/fp-go/v2/internal/utils"
+	N "github.com/IBM/fp-go/v2/number"
 	S "github.com/IBM/fp-go/v2/string"
 
 	"github.com/stretchr/testify/assert"
 )
 
-func TestMap(t *testing.T) {
-	fa := Make[string, int]("foo")
-	assert.Equal(t, fa, F.Pipe1(fa, Map[string](utils.Double)))
+// TestMake tests the Make constructor
+func TestMake(t *testing.T) {
+	t.Run("creates Const with string value", func(t *testing.T) {
+		c := Make[string, int]("hello")
+		assert.Equal(t, "hello", Unwrap(c))
+	})
+
+	t.Run("creates Const with int value", func(t *testing.T) {
+		c := Make[int, string](42)
+		assert.Equal(t, 42, Unwrap(c))
+	})
+
+	t.Run("creates Const with struct value", func(t *testing.T) {
+		type Config struct {
+			Name string
+			Port int
+		}
+		cfg := Config{Name: "server", Port: 8080}
+		c := Make[Config, bool](cfg)
+		assert.Equal(t, cfg, Unwrap(c))
+	})
 }
 
+// TestUnwrap tests extracting values from Const
+func TestUnwrap(t *testing.T) {
+	t.Run("unwraps string value", func(t *testing.T) {
+		c := Make[string, int]("world")
+		value := Unwrap(c)
+		assert.Equal(t, "world", value)
+	})
+
+	t.Run("unwraps empty string", func(t *testing.T) {
+		c := Make[string, int]("")
+		value := Unwrap(c)
+		assert.Equal(t, "", value)
+	})
+
+	t.Run("unwraps zero value", func(t *testing.T) {
+		c := Make[int, string](0)
+		value := Unwrap(c)
+		assert.Equal(t, 0, value)
+	})
+}
+
+// TestOf tests the Of function
 func TestOf(t *testing.T) {
-	assert.Equal(t, Make[string, int](""), Of[string, int](S.Monoid)(1))
+	t.Run("creates Const with monoid empty value", func(t *testing.T) {
+		of := Of[string, int](S.Monoid)
+		c := of(42)
+		assert.Equal(t, "", Unwrap(c))
+	})
+
+	t.Run("ignores input value", func(t *testing.T) {
+		of := Of[string, int](S.Monoid)
+		c1 := of(1)
+		c2 := of(100)
+		assert.Equal(t, Unwrap(c1), Unwrap(c2))
+	})
+
+	t.Run("works with int monoid", func(t *testing.T) {
+		of := Of[int, string](N.MonoidSum[int]())
+		c := of("ignored")
+		assert.Equal(t, 0, Unwrap(c))
+	})
 }
 
-func TestAp(t *testing.T) {
-	fab := Make[string, int]("bar")
-	assert.Equal(t, Make[string, int]("foobar"), Ap[string, int, int](S.Monoid)(fab)(Make[string, func(int) int]("foo")))
+// TestMap tests the Map function
+func TestMap(t *testing.T) {
+	t.Run("preserves wrapped value", func(t *testing.T) {
+		fa := Make[string, int]("foo")
+		result := F.Pipe1(fa, Map[string](utils.Double))
+		assert.Equal(t, "foo", Unwrap(result))
+	})
+
+	t.Run("changes phantom type", func(t *testing.T) {
+		fa := Make[string, int]("data")
+		fb := Map[string, int, string](strconv.Itoa)(fa)
+		// Value unchanged, but type changed from Const[string, int] to Const[string, string]
+		assert.Equal(t, "data", Unwrap(fb))
+	})
+
+	t.Run("function is never called", func(t *testing.T) {
+		called := false
+		fa := Make[string, int]("test")
+		fb := Map[string, int, string](func(i int) string {
+			called = true
+			return strconv.Itoa(i)
+		})(fa)
+		assert.False(t, called, "Map function should not be called")
+		assert.Equal(t, "test", Unwrap(fb))
+	})
+}
+
+// TestMonadMap tests the MonadMap function
+func TestMonadMap(t *testing.T) {
+	t.Run("preserves wrapped value", func(t *testing.T) {
+		fa := Make[string, int]("original")
+		fb := MonadMap(fa, func(i int) string { return strconv.Itoa(i) })
+		assert.Equal(t, "original", Unwrap(fb))
+	})
+
+	t.Run("works with different types", func(t *testing.T) {
+		fa := Make[int, string](42)
+		fb := MonadMap(fa, func(s string) bool { return len(s) > 0 })
+		assert.Equal(t, 42, Unwrap(fb))
+	})
+}
+
+// TestAp tests the Ap function
+func TestAp(t *testing.T) {
+	t.Run("combines string values", func(t *testing.T) {
+		fab := Make[string, int]("bar")
+		fa := Make[string, func(int) int]("foo")
+		result := Ap[string, int, int](S.Monoid)(fab)(fa)
+		assert.Equal(t, "foobar", Unwrap(result))
+	})
+
+	t.Run("combines int values with sum", func(t *testing.T) {
+		fab := Make[int, string](10)
+		fa := Make[int, func(string) string](5)
+		result := Ap[int, string, string](N.SemigroupSum[int]())(fab)(fa)
+		assert.Equal(t, 15, Unwrap(result))
+	})
+
+	t.Run("combines int values with product", func(t *testing.T) {
+		fab := Make[int, bool](3)
+		fa := Make[int, func(bool) bool](4)
+		result := Ap[int, bool, bool](N.SemigroupProduct[int]())(fab)(fa)
+		assert.Equal(t, 12, Unwrap(result))
+	})
+}
+
+// TestMonadAp tests the MonadAp function
+func TestMonadAp(t *testing.T) {
+	t.Run("combines values using semigroup", func(t *testing.T) {
+		ap := MonadAp[string, int, int](S.Monoid)
+		fab := Make[string, func(int) int]("hello")
+		fa := Make[string, int]("world")
+		result := ap(fab, fa)
+		assert.Equal(t, "helloworld", Unwrap(result))
+	})
+
+	t.Run("works with empty strings", func(t *testing.T) {
+		ap := MonadAp[string, int, int](S.Monoid)
+		fab := Make[string, func(int) int]("")
+		fa := Make[string, int]("test")
+		result := ap(fab, fa)
+		assert.Equal(t, "test", Unwrap(result))
+	})
+}
+
+// TestMonoid tests the Monoid function
+func TestMonoid(t *testing.T) {
+	t.Run("always returns constant value", func(t *testing.T) {
+		m := Monoid(42)
+		assert.Equal(t, 42, m.Concat(1, 2))
+		assert.Equal(t, 42, m.Concat(100, 200))
+		assert.Equal(t, 42, m.Empty())
+	})
+
+	t.Run("works with strings", func(t *testing.T) {
+		m := Monoid("constant")
+		assert.Equal(t, "constant", m.Concat("a", "b"))
+		assert.Equal(t, "constant", m.Empty())
+	})
+
+	t.Run("works with structs", func(t *testing.T) {
+		type Point struct{ X, Y int }
+		p := Point{X: 1, Y: 2}
+		m := Monoid(p)
+		assert.Equal(t, p, m.Concat(Point{X: 3, Y: 4}, Point{X: 5, Y: 6}))
+		assert.Equal(t, p, m.Empty())
+	})
+
+	t.Run("satisfies monoid laws", func(t *testing.T) {
+		m := Monoid(10)
+
+		// Left identity: Concat(Empty(), x) = x (both return constant)
+		assert.Equal(t, 10, m.Concat(m.Empty(), 5))
+
+		// Right identity: Concat(x, Empty()) = x (both return constant)
+		assert.Equal(t, 10, m.Concat(5, m.Empty()))
+
+		// Associativity: Concat(Concat(x, y), z) = Concat(x, Concat(y, z))
+		left := m.Concat(m.Concat(1, 2), 3)
+		right := m.Concat(1, m.Concat(2, 3))
+		assert.Equal(t, left, right)
+		assert.Equal(t, 10, left)
+	})
+}
+
+// TestConstFunctorLaws tests functor laws for Const
+func TestConstFunctorLaws(t *testing.T) {
+	t.Run("identity law", func(t *testing.T) {
+		// map id = id
+		fa := Make[string, int]("test")
+		mapped := Map[string, int, int](F.Identity[int])(fa)
+		assert.Equal(t, Unwrap(fa), Unwrap(mapped))
+	})
+
+	t.Run("composition law", func(t *testing.T) {
+		// map (g . f) = map g . map f
+		fa := Make[string, int]("data")
+		f := func(i int) string { return strconv.Itoa(i) }
+		g := func(s string) bool { return len(s) > 0 }
+
+		// map (g . f)
+		composed := Map[string, int, bool](func(i int) bool { return g(f(i)) })(fa)
+
+		// map g . map f
+		intermediate := F.Pipe1(fa, Map[string, int, string](f))
+		chained := Map[string, string, bool](g)(intermediate)
+
+		assert.Equal(t, Unwrap(composed), Unwrap(chained))
+	})
+}
+
+// TestConstApplicativeLaws tests applicative laws for Const
+func TestConstApplicativeLaws(t *testing.T) {
+	t.Run("identity law", func(t *testing.T) {
+		// For Const, ap combines the wrapped values using the semigroup
+		// ap (of id) v combines empty (from of) with v's value
+		v := Make[string, int]("value")
+		ofId := Of[string, func(int) int](S.Monoid)(F.Identity[int])
+		result := Ap[string, int, int](S.Monoid)(v)(ofId)
+		// Result combines "" (from Of) with "value" using string monoid
+		assert.Equal(t, "value", Unwrap(result))
+	})
+
+	t.Run("homomorphism law", func(t *testing.T) {
+		// ap (of f) (of x) = of (f x)
+		f := func(i int) string { return strconv.Itoa(i) }
+		x := 42
+
+		ofF := Of[string, func(int) string](S.Monoid)(f)
+		ofX := Of[string, int](S.Monoid)(x)
+		left := Ap[string, int, string](S.Monoid)(ofX)(ofF)
+
+		right := Of[string, string](S.Monoid)(f(x))
+
+		assert.Equal(t, Unwrap(left), Unwrap(right))
+	})
+}
+
+// TestConstEdgeCases tests edge cases
+func TestConstEdgeCases(t *testing.T) {
+	t.Run("empty string values", func(t *testing.T) {
+		c := Make[string, int]("")
+		assert.Equal(t, "", Unwrap(c))
+
+		mapped := Map[string, int, string](strconv.Itoa)(c)
+		assert.Equal(t, "", Unwrap(mapped))
+	})
+
+	t.Run("zero values", func(t *testing.T) {
+		c := Make[int, string](0)
+		assert.Equal(t, 0, Unwrap(c))
+	})
+
+	t.Run("nil pointer", func(t *testing.T) {
+		var ptr *int
+		c := Make[*int, string](ptr)
+		assert.Nil(t, Unwrap(c))
+	})
+
+	t.Run("multiple map operations", func(t *testing.T) {
+		c := Make[string, int]("original")
+		// Chain multiple map operations
+		step1 := Map[string, int, string](strconv.Itoa)(c)
+		step2 := Map[string, string, bool](func(s string) bool { return len(s) > 0 })(step1)
+		result := Map[string, bool, int](func(b bool) int {
+			if b {
+				return 1
+			}
+			return 0
+		})(step2)
+		assert.Equal(t, "original", Unwrap(result))
+	})
+}
+
+// BenchmarkMake benchmarks the Make constructor
+func BenchmarkMake(b *testing.B) {
+	b.ResetTimer()
+	for b.Loop() {
+		_ = Make[string, int]("test")
+	}
+}
+
+// BenchmarkUnwrap benchmarks the Unwrap function
+func BenchmarkUnwrap(b *testing.B) {
+	c := Make[string, int]("test")
+	b.ResetTimer()
+	for b.Loop() {
+		_ = Unwrap(c)
+	}
+}
+
+// BenchmarkMap benchmarks the Map function
+func BenchmarkMap(b *testing.B) {
+	c := Make[string, int]("test")
+	mapFn := Map[string, int, string](strconv.Itoa)
+	b.ResetTimer()
+	for b.Loop() {
+		_ = mapFn(c)
+	}
+}
+
+// BenchmarkAp benchmarks the Ap function
+func BenchmarkAp(b *testing.B) {
+	fab := Make[string, int]("hello")
+	fa := Make[string, func(int) int]("world")
+	apFn := Ap[string, int, int](S.Monoid)
+	b.ResetTimer()
+	for b.Loop() {
+		_ = apFn(fab)(fa)
+	}
+}
+
+// BenchmarkMonoid benchmarks the Monoid function
+func BenchmarkMonoid(b *testing.B) {
+	m := Monoid(42)
+	b.ResetTimer()
+	for b.Loop() {
+		_ = m.Concat(1, 2)
+	}
 }

v2/constant/monoid.go

@@ -1,3 +1,18 @@
+// 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 constant
 
 import (
@@ -5,7 +20,47 @@ import (
 	M "github.com/IBM/fp-go/v2/monoid"
 )
 
-// Monoid returns a [M.Monoid] that returns a constant value in all operations
+// Monoid creates a monoid that always returns a constant value.
+//
+// This creates a trivial monoid where both the Concat operation and Empty
+// always return the same constant value, regardless of inputs. This is useful
+// for testing, placeholder implementations, or when you need a monoid instance
+// but the actual combining behavior doesn't matter.
+//
+// # Monoid Laws
+//
+// The constant monoid satisfies all monoid laws trivially:
+//   - Associativity: Concat(Concat(x, y), z) = Concat(x, Concat(y, z)) - always returns 'a'
+//   - Left Identity: Concat(Empty(), x) = x - both return 'a'
+//   - Right Identity: Concat(x, Empty()) = x - both return 'a'
+//
+// Type Parameters:
+//   - A: The type of the constant value
+//
+// Parameters:
+//   - a: The constant value to return in all operations
+//
+// Returns:
+//   - A Monoid[A] that always returns the constant value
+//
+// Example:
+//
+//	// Create a monoid that always returns 42
+//	m := Monoid(42)
+//	result := m.Concat(1, 2)  // 42
+//	empty := m.Empty()         // 42
+//
+//	// Useful for testing or placeholder implementations
+//	type Config struct {
+//	    Timeout int
+//	}
+//	defaultConfig := Monoid(Config{Timeout: 30})
+//	config := defaultConfig.Concat(Config{Timeout: 10}, Config{Timeout: 20})
+//	// config is Config{Timeout: 30}
+//
+// See also:
+//   - function.Constant2: The underlying constant function
+//   - M.MakeMonoid: The monoid constructor
 func Monoid[A any](a A) M.Monoid[A] {
 	return M.MakeMonoid(function.Constant2[A, A](a), a)
 }

v2/constraints/constraints.go

@@ -13,28 +13,218 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+/*
+Package constraints defines a set of useful type constraints for generic programming in Go.
+
+# Overview
+
+This package provides type constraints that can be used with Go generics to restrict
+type parameters to specific categories of types. These constraints are similar to those
+in Go's standard constraints package but are defined here for consistency within the
+fp-go project.
+
+# Type Constraints
+
+Ordered - Types that support comparison operators:
+
+	type Ordered interface {
+	    Integer | Float | ~string
+	}
+
+Used for types that can be compared using <, <=, >, >= operators.
+
+Integer - All integer types (signed and unsigned):
+
+	type Integer interface {
+	    Signed | Unsigned
+	}
+
+Signed - Signed integer types:
+
+	type Signed interface {
+	    ~int | ~int8 | ~int16 | ~int32 | ~int64
+	}
+
+Unsigned - Unsigned integer types:
+
+	type Unsigned interface {
+	    ~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr
+	}
+
+Float - Floating-point types:
+
+	type Float interface {
+	    ~float32 | ~float64
+	}
+
+Complex - Complex number types:
+
+	type Complex interface {
+	    ~complex64 | ~complex128
+	}
+
+# Usage Examples
+
+Using Ordered constraint for comparison:
+
+	import C "github.com/IBM/fp-go/v2/constraints"
+
+	func Min[T C.Ordered](a, b T) T {
+	    if a < b {
+	        return a
+	    }
+	    return b
+	}
+
+	result := Min(5, 3)        // 3
+	result := Min(3.14, 2.71)  // 2.71
+	result := Min("apple", "banana")  // "apple"
+
+Using Integer constraint:
+
+	func Abs[T C.Integer](n T) T {
+	    if n < 0 {
+	        return -n
+	    }
+	    return n
+	}
+
+	result := Abs(-42)  // 42
+	result := Abs(uint(10))  // 10
+
+Using Float constraint:
+
+	func Average[T C.Float](a, b T) T {
+	    return (a + b) / 2
+	}
+
+	result := Average(3.14, 2.86)  // 3.0
+
+Using Complex constraint:
+
+	func Magnitude[T C.Complex](c T) float64 {
+	    r, i := real(c), imag(c)
+	    return math.Sqrt(r*r + i*i)
+	}
+
+	c := complex(3, 4)
+	result := Magnitude(c)  // 5.0
+
+# Combining Constraints
+
+Constraints can be combined to create more specific type restrictions:
+
+	type Number interface {
+	    C.Integer | C.Float | C.Complex
+	}
+
+	func Add[T Number](a, b T) T {
+	    return a + b
+	}
+
+# Tilde Operator
+
+The ~ operator in type constraints means "underlying type". For example, ~int
+matches not only int but also any type whose underlying type is int:
+
+	type MyInt int
+
+	func Double[T C.Integer](n T) T {
+	    return n * 2
+	}
+
+	var x MyInt = 5
+	result := Double(x)  // Works because MyInt's underlying type is int
+
+# Related Packages
+
+  - number: Provides algebraic structures and utilities for numeric types
+  - ord: Provides ordering operations using these constraints
+  - eq: Provides equality operations for comparable types
+*/
 package constraints
 
+// Ordered is a constraint that permits any ordered type: any type that supports
+// the operators < <= >= >. Ordered types include integers, floats, and strings.
+//
+// This constraint is commonly used for comparison operations, sorting, and
+// finding minimum/maximum values.
+//
+// Example:
+//
+//	func Max[T Ordered](a, b T) T {
+//	    if a > b {
+//	        return a
+//	    }
+//	    return b
+//	}
 type Ordered interface {
 	Integer | Float | ~string
 }
 
+// Signed is a constraint that permits any signed integer type.
+// This includes int, int8, int16, int32, and int64, as well as any
+// types whose underlying type is one of these.
+//
+// Example:
+//
+//	func Negate[T Signed](n T) T {
+//	    return -n
+//	}
 type Signed interface {
 	~int | ~int8 | ~int16 | ~int32 | ~int64
 }
 
+// Unsigned is a constraint that permits any unsigned integer type.
+// This includes uint, uint8, uint16, uint32, uint64, and uintptr, as well
+// as any types whose underlying type is one of these.
+//
+// Example:
+//
+//	func IsEven[T Unsigned](n T) bool {
+//	    return n%2 == 0
+//	}
 type Unsigned interface {
 	~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr
 }
 
+// Integer is a constraint that permits any integer type, both signed and unsigned.
+// This is a union of the Signed and Unsigned constraints.
+//
+// Example:
+//
+//	func Abs[T Integer](n T) T {
+//	    if n < 0 {
+//	        return -n
+//	    }
+//	    return n
+//	}
 type Integer interface {
 	Signed | Unsigned
 }
 
+// Float is a constraint that permits any floating-point type.
+// This includes float32 and float64, as well as any types whose
+// underlying type is one of these.
+//
+// Example:
+//
+//	func Round[T Float](f T) T {
+//	    return T(math.Round(float64(f)))
+//	}
 type Float interface {
 	~float32 | ~float64
 }
 
+// Complex is a constraint that permits any complex numeric type.
+// This includes complex64 and complex128, as well as any types whose
+// underlying type is one of these.
+//
+// Example:
+//
+//	func Conjugate[T Complex](c T) T {
+//	    return complex(real(c), -imag(c))
+//	}
 type Complex interface {
 	~complex64 | ~complex128
 }

v2/context/reader/reader.go

@@ -0,0 +1,130 @@
+// 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 reader provides a specialization of the Reader monad for [context.Context].
+//
+// This package offers a context-aware Reader monad that simplifies working with
+// Go's [context.Context] in a functional programming style. It eliminates the need
+// to explicitly thread context through function calls while maintaining type safety
+// and composability.
+//
+// # Core Concept
+//
+// The Reader monad represents computations that depend on a shared environment.
+// In this package, that environment is fixed to [context.Context], making it
+// particularly useful for:
+//
+//   - Request-scoped data propagation
+//   - Cancellation and timeout handling
+//   - Dependency injection via context values
+//   - Avoiding explicit context parameter threading
+//
+// # Type Definitions
+//
+//   - Reader[A]: A computation that depends on context.Context and produces A
+//   - Kleisli[A, B]: A function from A to Reader[B] for composing computations
+//   - Operator[A, B]: A transformation from Reader[A] to Reader[B]
+//
+// # Usage Pattern
+//
+// Instead of passing context explicitly through every function:
+//
+//	func processUser(ctx context.Context, userID string) (User, error) {
+//	    user := fetchUser(ctx, userID)
+//	    profile := fetchProfile(ctx, user.ProfileID)
+//	    return enrichUser(ctx, user, profile), nil
+//	}
+//
+// You can use Reader to compose context-dependent operations:
+//
+//	fetchUser := func(userID string) Reader[User] {
+//	    return func(ctx context.Context) User {
+//	        // Use ctx for database access, cancellation, etc.
+//	        return queryDatabase(ctx, userID)
+//	    }
+//	}
+//
+//	processUser := func(userID string) Reader[User] {
+//	    return F.Pipe2(
+//	        fetchUser(userID),
+//	        reader.Chain(func(user User) Reader[Profile] {
+//	            return fetchProfile(user.ProfileID)
+//	        }),
+//	        reader.Map(func(profile Profile) User {
+//	            return enrichUser(user, profile)
+//	        }),
+//	    )
+//	}
+//
+//	// Execute with context
+//	ctx := context.Background()
+//	user := processUser("user123")(ctx)
+//
+// # Integration with Standard Library
+//
+// This package works seamlessly with Go's standard [context] package:
+//
+//   - Context cancellation and deadlines are preserved
+//   - Context values can be accessed within Reader computations
+//   - Readers can be composed with context-aware libraries
+//
+// # Relationship to Other Packages
+//
+// This package is a specialization of [github.com/IBM/fp-go/v2/reader] where
+// the environment type R is fixed to [context.Context]. For more general
+// Reader operations, see the base reader package.
+//
+// For combining Reader with other monads:
+//   - [github.com/IBM/fp-go/v2/context/readerio]: Reader + IO effects
+//   - [github.com/IBM/fp-go/v2/readeroption]: Reader + Option
+//   - [github.com/IBM/fp-go/v2/readerresult]: Reader + Result (Either)
+//
+// # Example: HTTP Request Handler
+//
+//	type RequestContext struct {
+//	    UserID    string
+//	    RequestID string
+//	}
+//
+//	// Extract request context from context.Context
+//	getRequestContext := func(ctx context.Context) RequestContext {
+//	    return RequestContext{
+//	        UserID:    ctx.Value("userID").(string),
+//	        RequestID: ctx.Value("requestID").(string),
+//	    }
+//	}
+//
+//	// A Reader that logs with request context
+//	logInfo := func(message string) Reader[function.Void] {
+//	    return func(ctx context.Context) function.Void {
+//	        reqCtx := getRequestContext(ctx)
+//	        log.Printf("[%s] User %s: %s", reqCtx.RequestID, reqCtx.UserID, message)
+//	        return function.VOID
+//	    }
+//	}
+//
+//	// Compose operations
+//	handleRequest := func(data string) Reader[Response] {
+//	    return F.Pipe2(
+//	        logInfo("Processing request"),
+//	        reader.Chain(func(_ function.Void) Reader[Result] {
+//	            return processData(data)
+//	        }),
+//	        reader.Map(func(result Result) Response {
+//	            return Response{Data: result}
+//	        }),
+//	    )
+//	}
+package reader

v2/context/reader/types.go

@@ -0,0 +1,142 @@
+// 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 reader
+
+import (
+	"context"
+
+	R "github.com/IBM/fp-go/v2/reader"
+)
+
+type (
+	// Reader represents a computation that depends on a [context.Context] and produces a value of type A.
+	//
+	// This is a specialization of the generic Reader monad where the environment type is fixed
+	// to [context.Context]. This is particularly useful for Go applications that need to thread
+	// context through computations for cancellation, deadlines, and request-scoped values.
+	//
+	// Type Parameters:
+	//   - A: The result type produced by the computation
+	//
+	// Reader[A] is equivalent to func(context.Context) A
+	//
+	// The Reader monad enables:
+	//   - Dependency injection using context values
+	//   - Cancellation and timeout handling
+	//   - Request-scoped data propagation
+	//   - Avoiding explicit context parameter threading
+	//
+	// Example:
+	//
+	//	// A Reader that extracts a user ID from context
+	//	getUserID := func(ctx context.Context) string {
+	//	    if userID, ok := ctx.Value("userID").(string); ok {
+	//	        return userID
+	//	    }
+	//	    return "anonymous"
+	//	}
+	//
+	//	// A Reader that checks if context is cancelled
+	//	isCancelled := func(ctx context.Context) bool {
+	//	    select {
+	//	    case <-ctx.Done():
+	//	        return true
+	//	    default:
+	//	        return false
+	//	    }
+	//	}
+	//
+	//	// Use the readers with a context
+	//	ctx := context.WithValue(context.Background(), "userID", "user123")
+	//	userID := getUserID(ctx)      // "user123"
+	//	cancelled := isCancelled(ctx) // false
+	Reader[A any] = R.Reader[context.Context, A]
+
+	// Kleisli represents a Kleisli arrow for the context-based Reader monad.
+	//
+	// It's a function from A to Reader[B], used for composing Reader computations
+	// that all depend on the same [context.Context].
+	//
+	// Type Parameters:
+	//   - A: The input type
+	//   - B: The output type wrapped in Reader
+	//
+	// Kleisli[A, B] is equivalent to func(A) func(context.Context) B
+	//
+	// Kleisli arrows are fundamental for monadic composition, allowing you to chain
+	// operations that depend on context without explicitly passing the context through
+	// each function call.
+	//
+	// Example:
+	//
+	//	// A Kleisli arrow that creates a greeting Reader from a name
+	//	greet := func(name string) Reader[string] {
+	//	    return func(ctx context.Context) string {
+	//	        if deadline, ok := ctx.Deadline(); ok {
+	//	            return fmt.Sprintf("Hello %s (deadline: %v)", name, deadline)
+	//	        }
+	//	        return fmt.Sprintf("Hello %s", name)
+	//	    }
+	//	}
+	//
+	//	// Use the Kleisli arrow
+	//	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	//	defer cancel()
+	//	greeting := greet("Alice")(ctx) // "Hello Alice (deadline: ...)"
+	Kleisli[A, B any] = R.Reader[A, Reader[B]]
+
+	// Operator represents a transformation from one Reader to another.
+	//
+	// It takes a Reader[A] and produces a Reader[B], where both readers depend on
+	// the same [context.Context]. This type is commonly used for operations like
+	// Map, Chain, and other transformations that convert readers while preserving
+	// the context dependency.
+	//
+	// Type Parameters:
+	//   - A: The input Reader's result type
+	//   - B: The output Reader's result type
+	//
+	// Operator[A, B] is equivalent to func(Reader[A]) func(context.Context) B
+	//
+	// Operators enable building pipelines of context-dependent computations where
+	// each step can transform the result of the previous computation while maintaining
+	// access to the shared context.
+	//
+	// Example:
+	//
+	//	// An operator that transforms int readers to string readers
+	//	intToString := func(r Reader[int]) Reader[string] {
+	//	    return func(ctx context.Context) string {
+	//	        value := r(ctx)
+	//	        return strconv.Itoa(value)
+	//	    }
+	//	}
+	//
+	//	// A Reader that extracts a timeout value from context
+	//	getTimeout := func(ctx context.Context) int {
+	//	    if deadline, ok := ctx.Deadline(); ok {
+	//	        return int(time.Until(deadline).Seconds())
+	//	    }
+	//	    return 0
+	//	}
+	//
+	//	// Transform the Reader
+	//	getTimeoutStr := intToString(getTimeout)
+	//	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	//	defer cancel()
+	//	result := getTimeoutStr(ctx) // "30" (approximately)
+	Operator[A, B any] = Kleisli[Reader[A], B]
+)

v2/context/readerio/bracket.go

@@ -1,6 +1,7 @@
 package readerio
 
 import (
+	"github.com/IBM/fp-go/v2/function"
 	RIO "github.com/IBM/fp-go/v2/readerio"
 )
 
@@ -73,3 +74,117 @@ func Bracket[
 ) ReaderIO[B] {
 	return RIO.Bracket(acquire, use, release)
 }
+
+// WithResource creates a higher-order function that manages a resource lifecycle for any operation.
+// It returns a Kleisli arrow that takes a use function and automatically handles resource
+// acquisition and cleanup using the bracket pattern.
+//
+// This is a more composable alternative to Bracket, allowing you to define resource management
+// once and reuse it with different use functions. The resource is acquired when the returned
+// Kleisli arrow is invoked, used by the provided function, and then released regardless of
+// success or failure.
+//
+// Type Parameters:
+//   - A: The type of the resource to be managed
+//   - B: The type of the result produced by the use function
+//   - ANY: The type returned by the release function (typically ignored)
+//
+// Parameters:
+//   - onCreate: A ReaderIO that acquires/creates the resource
+//   - onRelease: A Kleisli arrow that releases/cleans up the resource
+//
+// Returns:
+//   - A Kleisli arrow that takes a use function and returns a ReaderIO managing the full lifecycle
+//
+// Example with database connection:
+//
+//	// Define resource management once
+//	withDB := WithResource(
+//	    // Acquire connection
+//	    func(ctx context.Context) IO[*sql.DB] {
+//	        return func() *sql.DB {
+//	            db, _ := sql.Open("postgres", "connection-string")
+//	            return db
+//	        }
+//	    },
+//	    // Release connection
+//	    func(db *sql.DB) ReaderIO[any] {
+//	        return func(ctx context.Context) IO[any] {
+//	            return func() any {
+//	                db.Close()
+//	                return nil
+//	            }
+//	        }
+//	    },
+//	)
+//
+//	// Reuse with different operations
+//	queryUsers := withDB(func(db *sql.DB) ReaderIO[[]User] {
+//	    return func(ctx context.Context) IO[[]User] {
+//	        return func() []User {
+//	            // Query users from db
+//	            return users
+//	        }
+//	    }
+//	})
+//
+//	insertUser := withDB(func(db *sql.DB) ReaderIO[int64] {
+//	    return func(ctx context.Context) IO[int64] {
+//	        return func() int64 {
+//	            // Insert user into db
+//	            return userID
+//	        }
+//	    }
+//	})
+//
+// Example with file handling:
+//
+//	withFile := WithResource(
+//	    func(ctx context.Context) IO[*os.File] {
+//	        return func() *os.File {
+//	            f, _ := os.Open("data.txt")
+//	            return f
+//	        }
+//	    },
+//	    func(f *os.File) ReaderIO[any] {
+//	        return func(ctx context.Context) IO[any] {
+//	            return func() any {
+//	                f.Close()
+//	                return nil
+//	            }
+//	        }
+//	    },
+//	)
+//
+//	// Use for reading
+//	readContent := withFile(func(f *os.File) ReaderIO[string] {
+//	    return func(ctx context.Context) IO[string] {
+//	        return func() string {
+//	            data, _ := io.ReadAll(f)
+//	            return string(data)
+//	        }
+//	    }
+//	})
+//
+//	// Use for getting file info
+//	getSize := withFile(func(f *os.File) ReaderIO[int64] {
+//	    return func(ctx context.Context) IO[int64] {
+//	        return func() int64 {
+//	            info, _ := f.Stat()
+//	            return info.Size()
+//	        }
+//	    }
+//	})
+//
+// Use Cases:
+//   - Database connections: Acquire connection, execute queries, close connection
+//   - File handles: Open file, read/write, close file
+//   - Network connections: Establish connection, transfer data, close connection
+//   - Locks: Acquire lock, perform critical section, release lock
+//   - Temporary resources: Create temp file/directory, use it, clean up
+//
+//go:inline
+func WithResource[A, B, ANY any](
+	onCreate ReaderIO[A], onRelease Kleisli[A, ANY]) Kleisli[Kleisli[A, B], B] {
+	return function.Bind13of3(Bracket[A, B, ANY])(onCreate, function.Ignore2of2[B](onRelease))
+}

v2/context/readerio/bracket_test.go

@@ -0,0 +1,454 @@
+// 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"
+	"errors"
+	"testing"
+
+	"github.com/IBM/fp-go/v2/io"
+	"github.com/stretchr/testify/assert"
+)
+
+// mockResource simulates a resource that tracks its lifecycle
+type mockResource struct {
+	id       int
+	acquired bool
+	released bool
+	used     bool
+}
+
+// TestBracket_Success tests that Bracket properly manages resource lifecycle on success
+func TestBracket_Success(t *testing.T) {
+	resource := &mockResource{id: 1}
+
+	// Acquire resource
+	acquire := func(ctx context.Context) io.IO[*mockResource] {
+		return func() *mockResource {
+			resource.acquired = true
+			return resource
+		}
+	}
+
+	// Use resource
+	use := func(r *mockResource) ReaderIO[string] {
+		return func(ctx context.Context) io.IO[string] {
+			return func() string {
+				r.used = true
+				return "success"
+			}
+		}
+	}
+
+	// Release resource
+	release := func(r *mockResource, result string) ReaderIO[any] {
+		return func(ctx context.Context) io.IO[any] {
+			return func() any {
+				r.released = true
+				return nil
+			}
+		}
+	}
+
+	// Execute bracket
+	operation := Bracket(acquire, use, release)
+	result := operation(context.Background())()
+
+	// Verify lifecycle
+	assert.True(t, resource.acquired, "Resource should be acquired")
+	assert.True(t, resource.used, "Resource should be used")
+	assert.True(t, resource.released, "Resource should be released")
+	assert.Equal(t, "success", result)
+}
+
+// TestBracket_MultipleResources tests managing multiple resources
+func TestBracket_MultipleResources(t *testing.T) {
+	resource1 := &mockResource{id: 1}
+	resource2 := &mockResource{id: 2}
+
+	acquire1 := func(ctx context.Context) io.IO[*mockResource] {
+		return func() *mockResource {
+			resource1.acquired = true
+			return resource1
+		}
+	}
+
+	use1 := func(r1 *mockResource) ReaderIO[*mockResource] {
+		return func(ctx context.Context) io.IO[*mockResource] {
+			return func() *mockResource {
+				r1.used = true
+				resource2.acquired = true
+				return resource2
+			}
+		}
+	}
+
+	release1 := func(r1 *mockResource, result string) ReaderIO[any] {
+		return func(ctx context.Context) io.IO[any] {
+			return func() any {
+				r1.released = true
+				return nil
+			}
+		}
+	}
+
+	// Nested bracket for second resource
+	use2 := func(r2 *mockResource) ReaderIO[string] {
+		return func(ctx context.Context) io.IO[string] {
+			return func() string {
+				r2.used = true
+				return "both used"
+			}
+		}
+	}
+
+	release2 := func(r2 *mockResource, result string) ReaderIO[any] {
+		return func(ctx context.Context) io.IO[any] {
+			return func() any {
+				r2.released = true
+				return nil
+			}
+		}
+	}
+
+	// Compose brackets
+	operation := Bracket(acquire1, func(r1 *mockResource) ReaderIO[string] {
+		return func(ctx context.Context) io.IO[string] {
+			r2 := use1(r1)(ctx)()
+			return Bracket(
+				func(ctx context.Context) io.IO[*mockResource] {
+					return func() *mockResource { return r2 }
+				},
+				use2,
+				release2,
+			)(ctx)
+		}
+	}, release1)
+
+	result := operation(context.Background())()
+
+	assert.True(t, resource1.acquired)
+	assert.True(t, resource1.used)
+	assert.True(t, resource1.released)
+	assert.True(t, resource2.acquired)
+	assert.True(t, resource2.used)
+	assert.True(t, resource2.released)
+	assert.Equal(t, "both used", result)
+}
+
+// TestWithResource_Success tests WithResource with successful operation
+func TestWithResource_Success(t *testing.T) {
+	resource := &mockResource{id: 1}
+
+	// Define resource management
+	withResource := WithResource[*mockResource, string, any](
+		func(ctx context.Context) io.IO[*mockResource] {
+			return func() *mockResource {
+				resource.acquired = true
+				return resource
+			}
+		},
+		func(r *mockResource) ReaderIO[any] {
+			return func(ctx context.Context) io.IO[any] {
+				return func() any {
+					r.released = true
+					return nil
+				}
+			}
+		},
+	)
+
+	// Use resource
+	operation := withResource(func(r *mockResource) ReaderIO[string] {
+		return func(ctx context.Context) io.IO[string] {
+			return func() string {
+				r.used = true
+				return "result"
+			}
+		}
+	})
+
+	result := operation(context.Background())()
+
+	assert.True(t, resource.acquired)
+	assert.True(t, resource.used)
+	assert.True(t, resource.released)
+	assert.Equal(t, "result", result)
+}
+
+// TestWithResource_Reusability tests that WithResource can be reused with different operations
+func TestWithResource_Reusability(t *testing.T) {
+	callCount := 0
+
+	withResource := WithResource[*mockResource, int, any](
+		func(ctx context.Context) io.IO[*mockResource] {
+			return func() *mockResource {
+				callCount++
+				return &mockResource{id: callCount, acquired: true}
+			}
+		},
+		func(r *mockResource) ReaderIO[any] {
+			return func(ctx context.Context) io.IO[any] {
+				return func() any {
+					r.released = true
+					return nil
+				}
+			}
+		},
+	)
+
+	// First operation
+	op1 := withResource(func(r *mockResource) ReaderIO[int] {
+		return func(ctx context.Context) io.IO[int] {
+			return func() int {
+				r.used = true
+				return r.id * 2
+			}
+		}
+	})
+
+	result1 := op1(context.Background())()
+	assert.Equal(t, 2, result1)
+	assert.Equal(t, 1, callCount)
+
+	// Second operation (should create new resource)
+	op2 := withResource(func(r *mockResource) ReaderIO[int] {
+		return func(ctx context.Context) io.IO[int] {
+			return func() int {
+				r.used = true
+				return r.id * 3
+			}
+		}
+	})
+
+	result2 := op2(context.Background())()
+	assert.Equal(t, 6, result2)
+	assert.Equal(t, 2, callCount)
+}
+
+// TestWithResource_DifferentResultTypes tests WithResource with different result types
+func TestWithResource_DifferentResultTypes(t *testing.T) {
+	resource := &mockResource{id: 42}
+
+	withResourceInt := WithResource[*mockResource, int, any](
+		func(ctx context.Context) io.IO[*mockResource] {
+			return func() *mockResource {
+				resource.acquired = true
+				return resource
+			}
+		},
+		func(r *mockResource) ReaderIO[any] {
+			return func(ctx context.Context) io.IO[any] {
+				return func() any {
+					r.released = true
+					return nil
+				}
+			}
+		},
+	)
+
+	// Operation returning int
+	opInt := withResourceInt(func(r *mockResource) ReaderIO[int] {
+		return func(ctx context.Context) io.IO[int] {
+			return func() int {
+				return r.id
+			}
+		}
+	})
+
+	resultInt := opInt(context.Background())()
+	assert.Equal(t, 42, resultInt)
+
+	// Reset resource state
+	resource.acquired = false
+	resource.released = false
+
+	// Create new WithResource for string type
+	withResourceString := WithResource[*mockResource, string, any](
+		func(ctx context.Context) io.IO[*mockResource] {
+			return func() *mockResource {
+				resource.acquired = true
+				return resource
+			}
+		},
+		func(r *mockResource) ReaderIO[any] {
+			return func(ctx context.Context) io.IO[any] {
+				return func() any {
+					r.released = true
+					return nil
+				}
+			}
+		},
+	)
+
+	// Operation returning string
+	opString := withResourceString(func(r *mockResource) ReaderIO[string] {
+		return func(ctx context.Context) io.IO[string] {
+			return func() string {
+				return "value"
+			}
+		}
+	})
+
+	resultString := opString(context.Background())()
+	assert.Equal(t, "value", resultString)
+	assert.True(t, resource.released)
+}
+
+// TestWithResource_ContextPropagation tests that context is properly propagated
+func TestWithResource_ContextPropagation(t *testing.T) {
+	type contextKey string
+	const key contextKey = "test-key"
+
+	withResource := WithResource[string, string, any](
+		func(ctx context.Context) io.IO[string] {
+			return func() string {
+				value := ctx.Value(key)
+				if value != nil {
+					return value.(string)
+				}
+				return "no-value"
+			}
+		},
+		func(r string) ReaderIO[any] {
+			return func(ctx context.Context) io.IO[any] {
+				return func() any {
+					return nil
+				}
+			}
+		},
+	)
+
+	operation := withResource(func(r string) ReaderIO[string] {
+		return func(ctx context.Context) io.IO[string] {
+			return func() string {
+				return r + "-processed"
+			}
+		}
+	})
+
+	ctx := context.WithValue(context.Background(), key, "test-value")
+	result := operation(ctx)()
+
+	assert.Equal(t, "test-value-processed", result)
+}
+
+// TestWithResource_ErrorInRelease tests behavior when release function encounters an error
+func TestWithResource_ErrorInRelease(t *testing.T) {
+	resource := &mockResource{id: 1}
+	releaseError := errors.New("release failed")
+
+	withResource := WithResource[*mockResource, string, error](
+		func(ctx context.Context) io.IO[*mockResource] {
+			return func() *mockResource {
+				resource.acquired = true
+				return resource
+			}
+		},
+		func(r *mockResource) ReaderIO[error] {
+			return func(ctx context.Context) io.IO[error] {
+				return func() error {
+					r.released = true
+					return releaseError
+				}
+			}
+		},
+	)
+
+	operation := withResource(func(r *mockResource) ReaderIO[string] {
+		return func(ctx context.Context) io.IO[string] {
+			return func() string {
+				r.used = true
+				return "success"
+			}
+		}
+	})
+
+	result := operation(context.Background())()
+
+	// Operation should succeed even if release returns error
+	assert.Equal(t, "success", result)
+	assert.True(t, resource.acquired)
+	assert.True(t, resource.used)
+	assert.True(t, resource.released)
+}
+
+// BenchmarkBracket benchmarks the Bracket function
+func BenchmarkBracket(b *testing.B) {
+	acquire := func(ctx context.Context) io.IO[int] {
+		return func() int {
+			return 42
+		}
+	}
+
+	use := func(n int) ReaderIO[int] {
+		return func(ctx context.Context) io.IO[int] {
+			return func() int {
+				return n * 2
+			}
+		}
+	}
+
+	release := func(n int, result int) ReaderIO[any] {
+		return func(ctx context.Context) io.IO[any] {
+			return func() any {
+				return nil
+			}
+		}
+	}
+
+	operation := Bracket(acquire, use, release)
+	ctx := context.Background()
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		operation(ctx)()
+	}
+}
+
+// BenchmarkWithResource benchmarks the WithResource function
+func BenchmarkWithResource(b *testing.B) {
+	withResource := WithResource[int, int, any](
+		func(ctx context.Context) io.IO[int] {
+			return func() int {
+				return 42
+			}
+		},
+		func(n int) ReaderIO[any] {
+			return func(ctx context.Context) io.IO[any] {
+				return func() any {
+					return nil
+				}
+			}
+		},
+	)
+
+	operation := withResource(func(n int) ReaderIO[int] {
+		return func(ctx context.Context) io.IO[int] {
+			return func() int {
+				return n * 2
+			}
+		}
+	})
+
+	ctx := context.Background()
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		operation(ctx)()
+	}
+}

v2/context/readerio/profunctor.go

@@ -19,6 +19,9 @@ import (
 	"context"
 
 	"github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/io"
+	"github.com/IBM/fp-go/v2/pair"
+	RIO "github.com/IBM/fp-go/v2/readerio"
 )
 
 // Promap is the profunctor map operation that transforms both the input and output of a context-based ReaderIO.
@@ -33,21 +36,24 @@ import (
 // The function f returns both a new context and a CancelFunc that should be called to release resources.
 //
 // Type Parameters:
+//   - R: The input environment type that f transforms into context.Context
 //   - A: The original result type produced by the ReaderIO
 //   - B: The new output result type
 //
 // Parameters:
-//   - f: Function to transform the input context (contravariant)
+//   - f: Function to transform the input environment R into context.Context (contravariant)
 //   - g: Function to transform the output value from A to B (covariant)
 //
 // Returns:
-//   - An Operator that takes a ReaderIO[A] and returns a ReaderIO[B]
+//   - A Kleisli arrow that takes a ReaderIO[A] and returns a function from R to B
+//
+// Note: When R is context.Context, this simplifies to an Operator[A, B]
 //
 //go:inline
-func Promap[A, B any](f func(context.Context) (context.Context, context.CancelFunc), g func(A) B) Operator[A, B] {
+func Promap[R, A, B any](f pair.Kleisli[context.CancelFunc, R, context.Context], g func(A) B) RIO.Kleisli[R, ReaderIO[A], B] {
 	return function.Flow2(
 		Local[A](f),
-		Map(g),
+		RIO.Map[R](g),
 	)
 }
 
@@ -61,14 +67,87 @@ func Promap[A, B any](f func(context.Context) (context.Context, context.CancelFu
 //
 // Type Parameters:
 //   - A: The result type (unchanged)
+//   - R: The input environment type that f transforms into context.Context
 //
 // Parameters:
-//   - f: Function to transform the context, returning a new context and CancelFunc
+//   - f: Function to transform the input environment R into context.Context, returning a new context and CancelFunc
 //
 // Returns:
-//   - An Operator that takes a ReaderIO[A] and returns a ReaderIO[A]
+//   - A Kleisli arrow that takes a ReaderIO[A] and returns a function from R to A
+//
+// Note: When R is context.Context, this simplifies to an Operator[A, A]
 //
 //go:inline
-func Contramap[A any](f func(context.Context) (context.Context, context.CancelFunc)) Operator[A, A] {
+func Contramap[A, R any](f pair.Kleisli[context.CancelFunc, R, context.Context]) RIO.Kleisli[R, ReaderIO[A], A] {
 	return Local[A](f)
 }
+
+// LocalIOK transforms the context using an IO effect before passing it to a ReaderIO computation.
+//
+// This is similar to Local, but the context transformation itself is wrapped in an IO effect,
+// allowing for side-effectful context transformations. The transformation function receives
+// the current context and returns an IO effect that produces 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:
+//   - Context transformations that require side effects (e.g., loading configuration)
+//   - Lazy initialization of context values
+//   - Context transformations that may fail or need to perform I/O
+//   - Composing effectful context setup with computations
+//
+// Type Parameters:
+//   - A: The value type of the ReaderIO
+//
+// Parameters:
+//   - f: An IO Kleisli arrow that transforms the context with side effects
+//
+// Returns:
+//   - An Operator that runs the computation with the effectfully transformed context
+//
+// Example:
+//
+//	import (
+//	    "context"
+//	    G "github.com/IBM/fp-go/v2/io"
+//	    F "github.com/IBM/fp-go/v2/function"
+//	)
+//
+//	// Context transformation with side effects (e.g., loading config)
+//	loadConfig := func(ctx context.Context) G.IO[ContextCancel] {
+//	    return func() ContextCancel {
+//	        // Simulate loading configuration
+//	        config := loadConfigFromFile()
+//	        newCtx := context.WithValue(ctx, "config", config)
+//	        return pair.MakePair[context.CancelFunc](func() {}, newCtx)
+//	    }
+//	}
+//
+//	getValue := readerio.FromReader(func(ctx context.Context) string {
+//	    if cfg := ctx.Value("config"); cfg != nil {
+//	        return cfg.(string)
+//	    }
+//	    return "default"
+//	})
+//
+//	result := F.Pipe1(
+//	    getValue,
+//	    readerio.LocalIOK[string](loadConfig),
+//	)
+//	value := result(t.Context())()  // Loads config and uses it
+//
+// Comparison with Local:
+//   - Local: Takes a pure function that transforms the context
+//   - LocalIOK: Takes an IO effect that transforms the context, allowing side effects
+func LocalIOK[A any](f io.Kleisli[context.Context, ContextCancel]) Operator[A, A] {
+	return func(r ReaderIO[A]) ReaderIO[A] {
+		return func(ctx context.Context) IO[A] {
+			p := f(ctx)
+			return func() A {
+				otherCancel, otherCtx := pair.Unpack(p())
+				defer otherCancel()
+				return r(otherCtx)()
+			}
+		}
+	}
+}

v2/context/readerio/profunctor_test.go

@@ -21,6 +21,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/IBM/fp-go/v2/pair"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -38,9 +39,9 @@ func TestPromapBasic(t *testing.T) {
 		}
 
 		// Transform context and result
-		addKey := func(ctx context.Context) (context.Context, context.CancelFunc) {
+		addKey := func(ctx context.Context) ContextCancel {
 			newCtx := context.WithValue(ctx, "key", 42)
-			return newCtx, func() {}
+			return pair.MakePair[context.CancelFunc](func() {}, newCtx)
 		}
 		toString := strconv.Itoa
 
@@ -63,9 +64,9 @@ func TestContramapBasic(t *testing.T) {
 			}
 		}
 
-		addKey := func(ctx context.Context) (context.Context, context.CancelFunc) {
+		addKey := func(ctx context.Context) ContextCancel {
 			newCtx := context.WithValue(ctx, "key", 100)
-			return newCtx, func() {}
+			return pair.MakePair[context.CancelFunc](func() {}, newCtx)
 		}
 
 		adapted := Contramap[int](addKey)(getValue)
@@ -85,8 +86,9 @@ func TestLocalBasic(t *testing.T) {
 			}
 		}
 
-		addTimeout := func(ctx context.Context) (context.Context, context.CancelFunc) {
-			return context.WithTimeout(ctx, time.Second)
+		addTimeout := func(ctx context.Context) ContextCancel {
+			newCtx, cancelFct := context.WithTimeout(ctx, time.Second)
+			return pair.MakePair(cancelFct, newCtx)
 		}
 
 		adapted := Local[bool](addTimeout)(getValue)
@@ -95,3 +97,81 @@ func TestLocalBasic(t *testing.T) {
 		assert.True(t, result)
 	})
 }
+
+// TestLocalIOKBasic tests basic LocalIOK functionality
+func TestLocalIOKBasic(t *testing.T) {
+	t.Run("context transformation with IO effect", func(t *testing.T) {
+		getValue := func(ctx context.Context) IO[string] {
+			return func() string {
+				if v := ctx.Value("key"); v != nil {
+					return v.(string)
+				}
+				return "default"
+			}
+		}
+
+		// Context transformation wrapped in IO effect
+		addKeyIO := func(ctx context.Context) IO[ContextCancel] {
+			return func() ContextCancel {
+				// Simulate side effect (e.g., loading config)
+				newCtx := context.WithValue(ctx, "key", "loaded-value")
+				return pair.MakePair[context.CancelFunc](func() {}, newCtx)
+			}
+		}
+
+		adapted := LocalIOK[string](addKeyIO)(getValue)
+		result := adapted(t.Context())()
+
+		assert.Equal(t, "loaded-value", result)
+	})
+
+	t.Run("cleanup function is called", func(t *testing.T) {
+		cleanupCalled := false
+
+		getValue := func(ctx context.Context) IO[int] {
+			return func() int {
+				if v := ctx.Value("value"); v != nil {
+					return v.(int)
+				}
+				return 0
+			}
+		}
+
+		addValueIO := func(ctx context.Context) IO[ContextCancel] {
+			return func() ContextCancel {
+				newCtx := context.WithValue(ctx, "value", 42)
+				cleanup := context.CancelFunc(func() {
+					cleanupCalled = true
+				})
+				return pair.MakePair(cleanup, newCtx)
+			}
+		}
+
+		adapted := LocalIOK[int](addValueIO)(getValue)
+		result := adapted(t.Context())()
+
+		assert.Equal(t, 42, result)
+		assert.True(t, cleanupCalled, "cleanup function should be called")
+	})
+
+	t.Run("works with timeout context", func(t *testing.T) {
+		getValue := func(ctx context.Context) IO[bool] {
+			return func() bool {
+				_, hasDeadline := ctx.Deadline()
+				return hasDeadline
+			}
+		}
+
+		addTimeoutIO := func(ctx context.Context) IO[ContextCancel] {
+			return func() ContextCancel {
+				newCtx, cancelFct := context.WithTimeout(ctx, time.Second)
+				return pair.MakePair(cancelFct, newCtx)
+			}
+		}
+
+		adapted := LocalIOK[bool](addTimeoutIO)(getValue)
+		result := adapted(t.Context())()
+
+		assert.True(t, result, "context should have deadline")
+	})
+}

v2/context/readerio/reader.go

@@ -20,6 +20,7 @@ import (
 	"time"
 
 	"github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/pair"
 	"github.com/IBM/fp-go/v2/reader"
 	RIO "github.com/IBM/fp-go/v2/readerio"
 )
@@ -633,12 +634,15 @@ func ReadIO[A any](r IO[context.Context]) func(ReaderIO[A]) IO[A] {
 //
 // Type Parameters:
 //   - A: The value type of the ReaderIO
+//   - R: The input environment type that f transforms into context.Context
 //
 // Parameters:
-//   - f: A function that transforms the context and returns a cancel function
+//   - f: A function that transforms the input environment R into context.Context and returns a cancel function
 //
 // Returns:
-//   - An Operator that runs the computation with the transformed context
+//   - A Kleisli arrow that runs the computation with the transformed context
+//
+// Note: When R is context.Context, this simplifies to an Operator[A, A]
 //
 // Example:
 //
@@ -648,9 +652,9 @@ func ReadIO[A any](r IO[context.Context]) func(ReaderIO[A]) IO[A] {
 //	type key int
 //	const userKey key = 0
 //
-//	addUser := readerio.Local[string](func(ctx context.Context) (context.Context, context.CancelFunc) {
+//	addUser := readerio.Local[string, context.Context](func(ctx context.Context) pair.Pair[context.CancelFunc, context.Context] {
 //	    newCtx := context.WithValue(ctx, userKey, "Alice")
-//	    return newCtx, func() {} // No-op cancel
+//	    return pair.MakePair(func() {}, newCtx) // No-op cancel
 //	})
 //
 //	getUser := readerio.FromReader(func(ctx context.Context) string {
@@ -669,19 +673,20 @@ func ReadIO[A any](r IO[context.Context]) func(ReaderIO[A]) IO[A] {
 // 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)
+//	withTimeout := readerio.Local[Data, context.Context](func(ctx context.Context) pair.Pair[context.CancelFunc, context.Context] {
+//	    newCtx, cancel := context.WithTimeout(ctx, 5*time.Second)
+//	    return pair.MakePair(cancel, newCtx)
 //	})
 //
 //	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] {
+func Local[A, R any](f pair.Kleisli[context.CancelFunc, R, context.Context]) RIO.Kleisli[R, ReaderIO[A], A] {
+	return func(rr ReaderIO[A]) RIO.ReaderIO[R, A] {
+		return func(r R) IO[A] {
 			return func() A {
-				otherCtx, otherCancel := f(ctx)
+				otherCancel, otherCtx := pair.Unpack(f(r))
 				defer otherCancel()
 				return rr(otherCtx)()
 			}
@@ -742,8 +747,9 @@ func Local[A any](f func(context.Context) (context.Context, context.CancelFunc))
 //	)
 //	data := result(t.Context())()  // 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)
+	return Local[A](func(ctx context.Context) ContextCancel {
+		newCtx, cancelFct := context.WithTimeout(ctx, timeout)
+		return pair.MakePair(cancelFct, newCtx)
 	})
 }
 
@@ -806,8 +812,9 @@ func WithTimeout[A any](timeout time.Duration) Operator[A, A] {
 //	)
 //	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)
+	return Local[A](func(ctx context.Context) ContextCancel {
+		newCtx, cancelFct := context.WithDeadline(ctx, deadline)
+		return pair.MakePair(cancelFct, newCtx)
 	})
 }
 

v2/context/readerio/type.go

@@ -23,6 +23,7 @@ import (
 	"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/pair"
 	"github.com/IBM/fp-go/v2/predicate"
 	"github.com/IBM/fp-go/v2/reader"
 	"github.com/IBM/fp-go/v2/readerio"
@@ -81,4 +82,15 @@ type (
 	Predicate[A any] = predicate.Predicate[A]
 
 	Void = function.Void
+
+	// Pair represents a tuple of two values of types A and B.
+	// It is used to group two related values together.
+	Pair[A, B any] = pair.Pair[A, B]
+
+	// ContextCancel represents a pair of a cancel function and a context.
+	// It is used in operations that create new contexts with cancellation capabilities.
+	//
+	// The first element is the CancelFunc that should be called to release resources.
+	// The second element is the new Context that was created.
+	ContextCancel = Pair[context.CancelFunc, context.Context]
 )

v2/context/readerioresult/cancel.go

@@ -21,6 +21,7 @@ import (
 	CIOE "github.com/IBM/fp-go/v2/context/ioresult"
 	F "github.com/IBM/fp-go/v2/function"
 	"github.com/IBM/fp-go/v2/ioeither"
+	"github.com/IBM/fp-go/v2/pair"
 )
 
 // WithContext wraps an existing [ReaderIOResult] and performs a context check for cancellation before delegating.
@@ -85,3 +86,7 @@ func WithContextK[A, B any](f Kleisli[A, B]) Kleisli[A, B] {
 		WithContext,
 	)
 }
+
+func pairFromContextCancel(newCtx context.Context, cancelFct context.CancelFunc) ContextCancel {
+	return pair.MakePair(cancelFct, newCtx)
+}

v2/context/readerioresult/file/file.go

@@ -13,6 +13,25 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// Package file provides context-aware file operations that integrate with the ReaderIOResult monad.
+// It offers safe, composable file I/O operations that respect context cancellation and properly
+// manage resources using the RAII pattern.
+//
+// All operations in this package:
+//   - Respect context.Context for cancellation and timeouts
+//   - Return ReaderIOResult for composable error handling
+//   - Automatically manage resource cleanup
+//   - Are safe to use in concurrent environments
+//
+// # Example Usage
+//
+//	// Read a file with automatic resource management
+//	readOp := ReadFile("data.txt")
+//	result := readOp(ctx)()
+//
+//	// Open and manually manage a file
+//	fileOp := Open("config.json")
+//	fileResult := fileOp(ctx)()
 package file
 
 import (
@@ -29,32 +48,181 @@ import (
 )
 
 var (
-	// Open opens a file for reading within the given context
+	// Open opens a file for reading within the given context.
+	// The operation respects context cancellation and returns a ReaderIOResult
+	// that produces an os.File handle on success.
+	//
+	// The returned file handle should be closed using the Close function when no longer needed,
+	// or managed automatically using WithResource or ReadFile.
+	//
+	// Parameters:
+	//   - path: The path to the file to open
+	//
+	// Returns:
+	//   - ReaderIOResult[*os.File]: A context-aware computation that opens the file
+	//
+	// Example:
+	//
+	//	openFile := Open("data.txt")
+	//	result := openFile(ctx)()
+	//	either.Fold(
+	//	    result,
+	//	    func(err error) { log.Printf("Error: %v", err) },
+	//	    func(f *os.File) {
+	//	        defer f.Close()
+	//	        // Use file...
+	//	    },
+	//	)
+	//
+	// See Also:
+	//   - ReadFile: For reading entire file contents with automatic resource management
+	//   - Close: For closing file handles
 	Open = F.Flow3(
 		IOEF.Open,
 		RIOE.FromIOEither[*os.File],
 		RIOE.WithContext[*os.File],
 	)
 
-	// Remove removes a file by name
+	// Create creates or truncates a file for writing within the given context.
+	// If the file already exists, it is truncated. If it doesn't exist, it is created
+	// with mode 0666 (before umask).
+	//
+	// The operation respects context cancellation and returns a ReaderIOResult
+	// that produces an os.File handle on success.
+	//
+	// The returned file handle should be closed using the Close function when no longer needed,
+	// or managed automatically using WithResource or WriteFile.
+	//
+	// Parameters:
+	//   - path: The path to the file to create or truncate
+	//
+	// Returns:
+	//   - ReaderIOResult[*os.File]: A context-aware computation that creates the file
+	//
+	// Example:
+	//
+	//	createFile := Create("output.txt")
+	//	result := createFile(ctx)()
+	//	either.Fold(
+	//	    result,
+	//	    func(err error) { log.Printf("Error: %v", err) },
+	//	    func(f *os.File) {
+	//	        defer f.Close()
+	//	        f.WriteString("Hello, World!")
+	//	    },
+	//	)
+	//
+	// See Also:
+	//   - WriteFile: For writing data to a file with automatic resource management
+	//   - Open: For opening files for reading
+	//   - Close: For closing file handles
+	Create = F.Flow3(
+		IOEF.Create,
+		RIOE.FromIOEither[*os.File],
+		RIOE.WithContext[*os.File],
+	)
+
+	// Remove removes a file by name.
+	// The operation returns the filename on success, allowing for easy composition
+	// with other file operations.
+	//
+	// Parameters:
+	//   - name: The path to the file to remove
+	//
+	// Returns:
+	//   - ReaderIOResult[string]: A computation that removes the file and returns its name
+	//
+	// Example:
+	//
+	//	removeOp := Remove("temp.txt")
+	//	result := removeOp(ctx)()
+	//	either.Fold(
+	//	    result,
+	//	    func(err error) { log.Printf("Failed to remove: %v", err) },
+	//	    func(name string) { log.Printf("Removed: %s", name) },
+	//	)
+	//
+	// See Also:
+	//   - Open: For opening files
+	//   - ReadFile: For reading file contents
 	Remove = F.Flow2(
 		IOEF.Remove,
 		RIOE.FromIOEither[string],
 	)
 )
 
-// Close closes an object
-func Close[C io.Closer](c C) RIOE.ReaderIOResult[struct{}] {
+// Close closes an io.Closer resource and returns a ReaderIOResult.
+// This function is generic and works with any type that implements io.Closer,
+// including os.File, network connections, and other closeable resources.
+//
+// The function captures any error that occurs during closing and returns it
+// as part of the ReaderIOResult. On success, it returns Void (empty struct).
+//
+// Type Parameters:
+//   - C: Any type that implements io.Closer
+//
+// Parameters:
+//   - c: The resource to close
+//
+// Returns:
+//   - ReaderIOResult[Void]: A computation that closes the resource
+//
+// Example:
+//
+//	file, _ := os.Open("data.txt")
+//	closeOp := Close(file)
+//	result := closeOp(ctx)()
+//
+// Note: This function is typically used with WithResource for automatic resource management
+// rather than being called directly.
+//
+// See Also:
+//   - Open: For opening files
+//   - ReadFile: For reading files with automatic closing
+func Close[C io.Closer](c C) ReaderIOResult[Void] {
 	return F.Pipe2(
 		c,
 		IOEF.Close[C],
-		RIOE.FromIOEither[struct{}],
+		RIOE.FromIOEither[Void],
 	)
 }
 
-// ReadFile reads a file in the scope of a context
-func ReadFile(path string) RIOE.ReaderIOResult[[]byte] {
-	return RIOE.WithResource[[]byte](Open(path), Close[*os.File])(func(r *os.File) RIOE.ReaderIOResult[[]byte] {
+// ReadFile reads the entire contents of a file in a context-aware manner.
+// This function automatically manages the file resource using the RAII pattern,
+// ensuring the file is properly closed even if an error occurs or the context is canceled.
+//
+// The operation:
+//   - Opens the file for reading
+//   - Reads all contents into a byte slice
+//   - Automatically closes the file when done
+//   - Respects context cancellation during the read operation
+//
+// Parameters:
+//   - path: The path to the file to read
+//
+// Returns:
+//   - ReaderIOResult[[]byte]: A computation that reads the file contents
+//
+// Example:
+//
+//	readOp := ReadFile("config.json")
+//	result := readOp(ctx)()
+//	either.Fold(
+//	    result,
+//	    func(err error) { log.Printf("Read error: %v", err) },
+//	    func(data []byte) { log.Printf("Read %d bytes", len(data)) },
+//	)
+//
+// The function uses WithResource internally to ensure proper cleanup:
+//
+//	ReadFile(path) = WithResource(Open(path), Close)(readAllBytes)
+//
+// See Also:
+//   - Open: For opening files without automatic reading
+//   - Close: For closing file handles
+//   - WithResource: For custom resource management patterns
+func ReadFile(path string) ReaderIOResult[[]byte] {
+	return RIOE.WithResource[[]byte](Open(path), Close[*os.File])(func(r *os.File) ReaderIOResult[[]byte] {
 		return func(ctx context.Context) IOE.IOEither[error, []byte] {
 			return func() ET.Either[error, []byte] {
 				return file.ReadAll(ctx, r)
@@ -62,3 +230,48 @@ func ReadFile(path string) RIOE.ReaderIOResult[[]byte] {
 		}
 	})
 }
+
+// WriteFile writes data to a file in a context-aware manner.
+// This function automatically manages the file resource using the RAII pattern,
+// ensuring the file is properly closed even if an error occurs or the context is canceled.
+//
+// If the file doesn't exist, it is created with mode 0666 (before umask).
+// If the file already exists, it is truncated before writing.
+//
+// The operation:
+//   - Creates or truncates the file for writing
+//   - Writes all data to the file
+//   - Automatically closes the file when done
+//   - Respects context cancellation during the write operation
+//
+// Parameters:
+//   - data: The byte slice to write to the file
+//
+// Returns:
+//   - Kleisli[string, []byte]: A function that takes a file path and returns a computation
+//     that writes the data and returns the written bytes on success
+//
+// Example:
+//
+//	writeOp := WriteFile([]byte("Hello, World!"))
+//	result := writeOp("output.txt")(ctx)()
+//	either.Fold(
+//	    result,
+//	    func(err error) { log.Printf("Write error: %v", err) },
+//	    func(data []byte) { log.Printf("Wrote %d bytes", len(data)) },
+//	)
+//
+// The function uses WithResource internally to ensure proper cleanup:
+//
+//	WriteFile(data) = Create >> WriteAll(data) >> Close
+//
+// See Also:
+//   - ReadFile: For reading file contents with automatic resource management
+//   - Create: For creating files without automatic writing
+//   - WriteAll: For writing to an already-open file handle
+func WriteFile(data []byte) Kleisli[string, []byte] {
+	return F.Flow2(
+		Create,
+		WriteAll[*os.File](data),
+	)
+}

v2/context/readerioresult/file/file_test.go

@@ -18,11 +18,16 @@ package file
 import (
 	"context"
 	"fmt"
+	"os"
+	"path/filepath"
+	"testing"
 
 	R "github.com/IBM/fp-go/v2/context/readerioresult"
+	E "github.com/IBM/fp-go/v2/either"
 	F "github.com/IBM/fp-go/v2/function"
 	"github.com/IBM/fp-go/v2/io"
 	J "github.com/IBM/fp-go/v2/json"
+	"github.com/stretchr/testify/assert"
 )
 
 type RecordType struct {
@@ -49,3 +54,267 @@ func ExampleReadFile() {
 	// Output:
 	// Right[string](Carsten)
 }
+
+func TestCreate(t *testing.T) {
+	ctx := context.Background()
+
+	t.Run("Success - creates new file", func(t *testing.T) {
+		tempFile := filepath.Join(t.TempDir(), "test_create.txt")
+
+		createOp := Create(tempFile)
+		result := createOp(ctx)()
+
+		assert.True(t, E.IsRight(result))
+
+		// Verify file was created
+		_, err := os.Stat(tempFile)
+		assert.NoError(t, err)
+
+		// Clean up file handle
+		E.MonadFold(result,
+			func(error) *os.File { return nil },
+			func(f *os.File) *os.File { f.Close(); return f },
+		)
+	})
+
+	t.Run("Success - truncates existing file", func(t *testing.T) {
+		tempFile := filepath.Join(t.TempDir(), "test_truncate.txt")
+
+		// Create file with initial content
+		err := os.WriteFile(tempFile, []byte("initial content"), 0644)
+		assert.NoError(t, err)
+
+		// Create should truncate
+		createOp := Create(tempFile)
+		result := createOp(ctx)()
+
+		assert.True(t, E.IsRight(result))
+
+		// Close the file
+		E.MonadFold(result,
+			func(error) *os.File { return nil },
+			func(f *os.File) *os.File { f.Close(); return f },
+		)
+
+		// Verify file was truncated
+		content, err := os.ReadFile(tempFile)
+		assert.NoError(t, err)
+		assert.Empty(t, content)
+	})
+
+	t.Run("Failure - invalid path", func(t *testing.T) {
+		// Try to create file in non-existent directory
+		invalidPath := filepath.Join(t.TempDir(), "nonexistent", "test.txt")
+
+		createOp := Create(invalidPath)
+		result := createOp(ctx)()
+
+		assert.True(t, E.IsLeft(result))
+	})
+
+	t.Run("Success - file can be written to", func(t *testing.T) {
+		tempFile := filepath.Join(t.TempDir(), "test_write.txt")
+
+		createOp := Create(tempFile)
+		result := createOp(ctx)()
+
+		assert.True(t, E.IsRight(result))
+
+		// Write to the file
+		E.MonadFold(result,
+			func(err error) *os.File { t.Fatalf("Unexpected error: %v", err); return nil },
+			func(f *os.File) *os.File {
+				defer f.Close()
+				_, err := f.WriteString("test content")
+				assert.NoError(t, err)
+				return f
+			},
+		)
+
+		// Verify content was written
+		content, err := os.ReadFile(tempFile)
+		assert.NoError(t, err)
+		assert.Equal(t, "test content", string(content))
+	})
+
+	t.Run("Context cancellation", func(t *testing.T) {
+		cancelCtx, cancel := context.WithCancel(context.Background())
+		cancel() // Cancel immediately
+
+		tempFile := filepath.Join(t.TempDir(), "test_cancel.txt")
+
+		createOp := Create(tempFile)
+		result := createOp(cancelCtx)()
+
+		// Note: File creation itself doesn't check context, but this tests the pattern
+		// In practice, context cancellation would affect subsequent operations
+		_ = result
+	})
+}
+
+func TestWriteFile(t *testing.T) {
+	ctx := context.Background()
+
+	t.Run("Success - writes data to new file", func(t *testing.T) {
+		tempFile := filepath.Join(t.TempDir(), "test_write.txt")
+		testData := []byte("Hello, World!")
+
+		writeOp := WriteFile(testData)
+		result := writeOp(tempFile)(ctx)()
+
+		assert.True(t, E.IsRight(result))
+
+		// Verify returned data
+		E.MonadFold(result,
+			func(err error) []byte { t.Fatalf("Unexpected error: %v", err); return nil },
+			func(data []byte) []byte {
+				assert.Equal(t, testData, data)
+				return data
+			},
+		)
+
+		// Verify file content
+		content, err := os.ReadFile(tempFile)
+		assert.NoError(t, err)
+		assert.Equal(t, testData, content)
+	})
+
+	t.Run("Success - overwrites existing file", func(t *testing.T) {
+		tempFile := filepath.Join(t.TempDir(), "test_overwrite.txt")
+
+		// Write initial content
+		err := os.WriteFile(tempFile, []byte("old content"), 0644)
+		assert.NoError(t, err)
+
+		// Overwrite with new content
+		newData := []byte("new content")
+		writeOp := WriteFile(newData)
+		result := writeOp(tempFile)(ctx)()
+
+		assert.True(t, E.IsRight(result))
+
+		// Verify file was overwritten
+		content, err := os.ReadFile(tempFile)
+		assert.NoError(t, err)
+		assert.Equal(t, newData, content)
+	})
+
+	t.Run("Success - writes empty data", func(t *testing.T) {
+		tempFile := filepath.Join(t.TempDir(), "test_empty.txt")
+		emptyData := []byte{}
+
+		writeOp := WriteFile(emptyData)
+		result := writeOp(tempFile)(ctx)()
+
+		assert.True(t, E.IsRight(result))
+
+		// Verify file is empty
+		content, err := os.ReadFile(tempFile)
+		assert.NoError(t, err)
+		assert.Empty(t, content)
+	})
+
+	t.Run("Success - writes large data", func(t *testing.T) {
+		tempFile := filepath.Join(t.TempDir(), "test_large.txt")
+		largeData := make([]byte, 1024*1024) // 1MB
+		for i := range largeData {
+			largeData[i] = byte(i % 256)
+		}
+
+		writeOp := WriteFile(largeData)
+		result := writeOp(tempFile)(ctx)()
+
+		assert.True(t, E.IsRight(result))
+
+		// Verify file content
+		content, err := os.ReadFile(tempFile)
+		assert.NoError(t, err)
+		assert.Equal(t, largeData, content)
+	})
+
+	t.Run("Failure - invalid path", func(t *testing.T) {
+		invalidPath := filepath.Join(t.TempDir(), "nonexistent", "test.txt")
+		testData := []byte("test")
+
+		writeOp := WriteFile(testData)
+		result := writeOp(invalidPath)(ctx)()
+
+		assert.True(t, E.IsLeft(result))
+	})
+
+	t.Run("Success - writes binary data", func(t *testing.T) {
+		tempFile := filepath.Join(t.TempDir(), "test_binary.bin")
+		binaryData := []byte{0x00, 0x01, 0x02, 0xFF, 0xFE, 0xFD}
+
+		writeOp := WriteFile(binaryData)
+		result := writeOp(tempFile)(ctx)()
+
+		assert.True(t, E.IsRight(result))
+
+		// Verify binary content
+		content, err := os.ReadFile(tempFile)
+		assert.NoError(t, err)
+		assert.Equal(t, binaryData, content)
+	})
+
+	t.Run("Integration - write then read", func(t *testing.T) {
+		tempFile := filepath.Join(t.TempDir(), "test_roundtrip.txt")
+		testData := []byte("Round trip test data")
+
+		// Write data
+		writeOp := WriteFile(testData)
+		writeResult := writeOp(tempFile)(ctx)()
+		assert.True(t, E.IsRight(writeResult))
+
+		// Read data back
+		readOp := ReadFile(tempFile)
+		readResult := readOp(ctx)()
+		assert.True(t, E.IsRight(readResult))
+
+		// Verify data matches
+		E.MonadFold(readResult,
+			func(err error) []byte { t.Fatalf("Unexpected error: %v", err); return nil },
+			func(data []byte) []byte {
+				assert.Equal(t, testData, data)
+				return data
+			},
+		)
+	})
+
+	t.Run("Composition with Map", func(t *testing.T) {
+		tempFile := filepath.Join(t.TempDir(), "test_compose.txt")
+		testData := []byte("test data")
+
+		// Write and transform result
+		pipeline := F.Pipe1(
+			WriteFile(testData)(tempFile),
+			R.Map(func(data []byte) int { return len(data) }),
+		)
+
+		result := pipeline(ctx)()
+		assert.True(t, E.IsRight(result))
+
+		E.MonadFold(result,
+			func(err error) int { t.Fatalf("Unexpected error: %v", err); return 0 },
+			func(length int) int {
+				assert.Equal(t, len(testData), length)
+				return length
+			},
+		)
+	})
+
+	t.Run("Context cancellation during write", func(t *testing.T) {
+		cancelCtx, cancel := context.WithCancel(context.Background())
+		cancel() // Cancel immediately
+
+		tempFile := filepath.Join(t.TempDir(), "test_cancel.txt")
+		testData := []byte("test")
+
+		writeOp := WriteFile(testData)
+		result := writeOp(tempFile)(cancelCtx)()
+
+		// Note: The actual write may complete before cancellation is checked
+		// This test verifies the pattern works with cancelled contexts
+		_ = result
+	})
+}

v2/context/readerioresult/file/tempfile.go

@@ -38,7 +38,7 @@ var (
 )
 
 // CreateTemp created a temp file with proper parametrization
-func CreateTemp(dir, pattern string) RIOE.ReaderIOResult[*os.File] {
+func CreateTemp(dir, pattern string) ReaderIOResult[*os.File] {
 	return F.Pipe2(
 		IOEF.CreateTemp(dir, pattern),
 		RIOE.FromIOEither[*os.File],
@@ -47,6 +47,6 @@ func CreateTemp(dir, pattern string) RIOE.ReaderIOResult[*os.File] {
 }
 
 // WithTempFile creates a temporary file, then invokes a callback to create a resource based on the file, then close and remove the temp file
-func WithTempFile[A any](f func(*os.File) RIOE.ReaderIOResult[A]) RIOE.ReaderIOResult[A] {
+func WithTempFile[A any](f Kleisli[*os.File, A]) ReaderIOResult[A] {
 	return RIOE.WithResource[A](onCreateTempFile, onReleaseTempFile)(f)
 }

v2/context/readerioresult/file/tempfile_test.go

@@ -34,7 +34,7 @@ func TestWithTempFile(t *testing.T) {
 
 func TestWithTempFileOnClosedFile(t *testing.T) {
 
-	res := WithTempFile(func(f *os.File) RIOE.ReaderIOResult[[]byte] {
+	res := WithTempFile(func(f *os.File) ReaderIOResult[[]byte] {
 		return F.Pipe2(
 			f,
 			onWriteAll[*os.File]([]byte("Carsten")),

v2/context/readerioresult/file/types.go

@@ -0,0 +1,90 @@
+// 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 file
+
+import (
+	"github.com/IBM/fp-go/v2/context/readerioresult"
+	"github.com/IBM/fp-go/v2/function"
+)
+
+type (
+	// ReaderIOResult represents a context-aware computation that performs side effects
+	// and can fail with an error. This is the main type used throughout the file package
+	// for all file operations.
+	//
+	// ReaderIOResult[A] is equivalent to:
+	//   func(context.Context) func() Either[error, A]
+	//
+	// The computation:
+	//   - Takes a context.Context for cancellation and timeouts
+	//   - Performs side effects (IO operations)
+	//   - Returns Either an error or a value of type A
+	//
+	// See Also:
+	//   - readerioresult.ReaderIOResult: The underlying type definition
+	ReaderIOResult[A any] = readerioresult.ReaderIOResult[A]
+
+	// Void represents the absence of a meaningful value, similar to unit type in other languages.
+	// It is used when a function performs side effects but doesn't return a meaningful result.
+	//
+	// Void is typically used as the success type in operations like Close that perform
+	// an action but don't produce a useful value.
+	//
+	// Example:
+	//   Close[*os.File](file) // Returns ReaderIOResult[Void]
+	//
+	// See Also:
+	//   - function.Void: The underlying type definition
+	Void = function.Void
+
+	// Kleisli represents a Kleisli arrow for ReaderIOResult.
+	// It is a function that takes a value of type A and returns a ReaderIOResult[B].
+	//
+	// Kleisli arrows are used for monadic composition, allowing you to chain operations
+	// that produce ReaderIOResults. They are particularly useful with Chain and Bind operations.
+	//
+	// Kleisli[A, B] is equivalent to:
+	//   func(A) ReaderIOResult[B]
+	//
+	// Example:
+	//   // A Kleisli arrow that reads a file given its path
+	//   var readFileK Kleisli[string, []byte] = ReadFile
+	//
+	// See Also:
+	//   - readerioresult.Kleisli: The underlying type definition
+	//   - Operator: For transforming ReaderIOResults
+	Kleisli[A, B any] = readerioresult.Kleisli[A, B]
+
+	// Operator represents a transformation from one ReaderIOResult to another.
+	// This is useful for point-free style composition and building reusable transformations.
+	//
+	// Operator[A, B] is equivalent to:
+	//   func(ReaderIOResult[A]) ReaderIOResult[B]
+	//
+	// Operators are used to transform computations without executing them, enabling
+	// powerful composition patterns.
+	//
+	// Example:
+	//   // An operator that maps over file contents
+	//   var toUpper Operator[[]byte, string] = Map(func(data []byte) string {
+	//       return strings.ToUpper(string(data))
+	//   })
+	//
+	// See Also:
+	//   - readerioresult.Operator: The underlying type definition
+	//   - Kleisli: For functions that produce ReaderIOResults
+	Operator[A, B any] = readerioresult.Operator[A, B]
+)

v2/context/readerioresult/file/write.go

@@ -23,8 +23,8 @@ import (
 	F "github.com/IBM/fp-go/v2/function"
 )
 
-func onWriteAll[W io.Writer](data []byte) func(w W) RIOE.ReaderIOResult[[]byte] {
-	return func(w W) RIOE.ReaderIOResult[[]byte] {
+func onWriteAll[W io.Writer](data []byte) Kleisli[W, []byte] {
+	return func(w W) ReaderIOResult[[]byte] {
 		return F.Pipe1(
 			RIOE.TryCatch(func(_ context.Context) func() ([]byte, error) {
 				return func() ([]byte, error) {
@@ -38,9 +38,9 @@ func onWriteAll[W io.Writer](data []byte) func(w W) RIOE.ReaderIOResult[[]byte]
 }
 
 // WriteAll uses a generator function to create a stream, writes data to it and closes it
-func WriteAll[W io.WriteCloser](data []byte) func(acquire RIOE.ReaderIOResult[W]) RIOE.ReaderIOResult[[]byte] {
+func WriteAll[W io.WriteCloser](data []byte) Operator[W, []byte] {
 	onWrite := onWriteAll[W](data)
-	return func(onCreate RIOE.ReaderIOResult[W]) RIOE.ReaderIOResult[[]byte] {
+	return func(onCreate ReaderIOResult[W]) ReaderIOResult[[]byte] {
 		return RIOE.WithResource[[]byte](
 			onCreate,
 			Close[W])(
@@ -50,7 +50,7 @@ func WriteAll[W io.WriteCloser](data []byte) func(acquire RIOE.ReaderIOResult[W]
 }
 
 // Write uses a generator function to create a stream, writes data to it and closes it
-func Write[R any, W io.WriteCloser](acquire RIOE.ReaderIOResult[W]) func(use func(W) RIOE.ReaderIOResult[R]) RIOE.ReaderIOResult[R] {
+func Write[R any, W io.WriteCloser](acquire ReaderIOResult[W]) Kleisli[Kleisli[W, R], R] {
 	return RIOE.WithResource[R](
 		acquire,
 		Close[W])

v2/context/readerioresult/filter.go

@@ -18,6 +18,10 @@ package readerioresult
 import (
 	"context"
 
+	"github.com/IBM/fp-go/v2/array"
+	"github.com/IBM/fp-go/v2/internal/witherable"
+	"github.com/IBM/fp-go/v2/iterator/iter"
+	"github.com/IBM/fp-go/v2/option"
 	RIOR "github.com/IBM/fp-go/v2/readerioresult"
 )
 
@@ -49,3 +53,43 @@ import (
 func FilterOrElse[A any](pred Predicate[A], onFalse func(A) error) Operator[A, A] {
 	return RIOR.FilterOrElse[context.Context](pred, onFalse)
 }
+
+//go:inline
+func Filter[HKTA, A any](
+	filter func(Predicate[A]) Endomorphism[HKTA],
+) func(Predicate[A]) Operator[HKTA, HKTA] {
+	return witherable.Filter(
+		Map,
+		filter,
+	)
+}
+
+//go:inline
+func FilterArray[A any](p Predicate[A]) Operator[[]A, []A] {
+	return Filter(array.Filter[A])(p)
+}
+
+//go:inline
+func FilterIter[A any](p Predicate[A]) Operator[Seq[A], Seq[A]] {
+	return Filter(iter.Filter[A])(p)
+}
+
+//go:inline
+func FilterMap[HKTA, HKTB, A, B any](
+	filter func(option.Kleisli[A, B]) Reader[HKTA, HKTB],
+) func(option.Kleisli[A, B]) Operator[HKTA, HKTB] {
+	return witherable.FilterMap(
+		Map,
+		filter,
+	)
+}
+
+//go:inline
+func FilterMapArray[A, B any](p option.Kleisli[A, B]) Operator[[]A, []B] {
+	return FilterMap(array.FilterMap[A, B])(p)
+}
+
+//go:inline
+func FilterMapIter[A, B any](p option.Kleisli[A, B]) Operator[Seq[A], Seq[B]] {
+	return FilterMap(iter.FilterMap[A, B])(p)
+}

v2/context/readerioresult/logging.go

@@ -28,6 +28,7 @@ import (
 	"github.com/IBM/fp-go/v2/io"
 	"github.com/IBM/fp-go/v2/logging"
 	"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/result"
 )
@@ -90,132 +91,7 @@ func withLoggingContext(lctx loggingContext) Endomorphism[context.Context] {
 	return F.Bind2nd(withLoggingContextValue, any(lctx))
 }
 
-// LogEntryExitF creates a customizable operator that wraps a ReaderIOResult computation with entry/exit callbacks.
-//
-// This is a more flexible version of LogEntryExit that allows you to provide custom callbacks for
-// entry and exit events. The onEntry callback receives the current context and can return a modified
-// context (e.g., with additional logging information). The onExit callback receives the computation
-// result and can perform custom logging, metrics collection, or cleanup.
-//
-// The function uses the bracket pattern to ensure that:
-//   - The onEntry callback is executed before the computation starts
-//   - The computation runs with the context returned by onEntry
-//   - The onExit callback is executed after the computation completes (success or failure)
-//   - The original result is preserved and returned unchanged
-//   - Cleanup happens even if the computation fails
-//
-// Type Parameters:
-//   - A: The success type of the ReaderIOResult
-//   - ANY: The return type of the onExit callback (typically any)
-//
-// Parameters:
-//   - onEntry: A ReaderIO that receives the current context and returns a (possibly modified) context.
-//     This is executed before the computation starts. Use this for logging entry, adding context values,
-//     starting timers, or initialization logic.
-//   - onExit: A Kleisli function that receives the Result[A] and returns a ReaderIO[ANY].
-//     This is executed after the computation completes, regardless of success or failure.
-//     Use this for logging exit, recording metrics, cleanup, or finalization logic.
-//
-// Returns:
-//   - An Operator that wraps the ReaderIOResult computation with the custom entry/exit callbacks
-//
-// Example with custom context modification:
-//
-//	type RequestID string
-//
-//	logOp := LogEntryExitF[User, any](
-//	    func(ctx context.Context) IO[context.Context] {
-//	        return func() context.Context {
-//	            reqID := RequestID(uuid.New().String())
-//	            log.Printf("[%s] Starting operation", reqID)
-//	            return context.WithValue(ctx, "requestID", reqID)
-//	        }
-//	    },
-//	    func(res Result[User]) ReaderIO[any] {
-//	        return func(ctx context.Context) IO[any] {
-//	            return func() any {
-//	                reqID := ctx.Value("requestID").(RequestID)
-//	                return F.Pipe1(
-//	                    res,
-//	                    result.Fold(
-//	                        func(err error) any {
-//	                            log.Printf("[%s] Operation failed: %v", reqID, err)
-//	                            return nil
-//	                        },
-//	                        func(_ User) any {
-//	                            log.Printf("[%s] Operation succeeded", reqID)
-//	                            return nil
-//	                        },
-//	                    ),
-//	                )
-//	            }
-//	        }
-//	    },
-//	)
-//
-//	wrapped := logOp(fetchUser(123))
-//
-// Example with metrics collection:
-//
-//	import "github.com/prometheus/client_golang/prometheus"
-//
-//	metricsOp := LogEntryExitF[Response, any](
-//	    func(ctx context.Context) IO[context.Context] {
-//	        return func() context.Context {
-//	            requestCount.WithLabelValues("api_call", "started").Inc()
-//	            return context.WithValue(ctx, "startTime", time.Now())
-//	        }
-//	    },
-//	    func(res Result[Response]) ReaderIO[any] {
-//	        return func(ctx context.Context) IO[any] {
-//	            return func() any {
-//	                startTime := ctx.Value("startTime").(time.Time)
-//	                duration := time.Since(startTime).Seconds()
-//
-//	                return F.Pipe1(
-//	                    res,
-//	                    result.Fold(
-//	                        func(err error) any {
-//	                            requestCount.WithLabelValues("api_call", "error").Inc()
-//	                            requestDuration.WithLabelValues("api_call", "error").Observe(duration)
-//	                            return nil
-//	                        },
-//	                        func(_ Response) any {
-//	                            requestCount.WithLabelValues("api_call", "success").Inc()
-//	                            requestDuration.WithLabelValues("api_call", "success").Observe(duration)
-//	                            return nil
-//	                        },
-//	                    ),
-//	                )
-//	            }
-//	        }
-//	    },
-//	)
-//
-// Use Cases:
-//   - Custom context modification: Adding request IDs, trace IDs, or other context values
-//   - Structured logging: Integration with zap, logrus, or other structured loggers
-//   - Metrics collection: Recording operation durations, success/failure rates
-//   - Distributed tracing: OpenTelemetry, Jaeger integration
-//   - Custom monitoring: Application-specific monitoring and alerting
-//
-// Note: LogEntryExit is implemented using LogEntryExitF with standard logging and context management.
-// Use LogEntryExitF when you need more control over the entry/exit behavior or context modification.
-func LogEntryExitF[A, ANY any](
-	onEntry ReaderIO[context.Context],
-	onExit readerio.Kleisli[Result[A], ANY],
-) Operator[A, A] {
-	bracket := F.Bind13of3(readerio.Bracket[context.Context, Result[A], ANY])(onEntry, func(newCtx context.Context, res Result[A]) ReaderIO[ANY] {
-		return readerio.FromIO(onExit(res)(newCtx)) // Get the exit callback for this result
-	})
-
-	return func(src ReaderIOResult[A]) ReaderIOResult[A] {
-		return bracket(F.Flow2(
-			src,
-			FromIOResult,
-		))
-	}
-}
+func noop() {}
 
 // onEntry creates a ReaderIO that handles the entry logging for an operation.
 // It generates a unique logging ID, captures the start time, and logs the entry message.
@@ -230,15 +106,15 @@ func LogEntryExitF[A, ANY any](
 //   - A ReaderIO that prepares the context with logging information and logs the entry
 func onEntry(
 	logLevel slog.Level,
-	cb func(context.Context) *slog.Logger,
+	cb Reader[context.Context, *slog.Logger],
 	nameAttr slog.Attr,
-) ReaderIO[context.Context] {
+) ReaderIO[ContextCancel] {
 
-	return func(ctx context.Context) IO[context.Context] {
+	return func(ctx context.Context) IO[ContextCancel] {
 		// logger
 		logger := cb(ctx)
 
-		return func() context.Context {
+		return func() ContextCancel {
 			// check if the logger is enabled
 			if logger.Enabled(ctx, logLevel) {
 				// Generate unique logging ID and capture start time
@@ -258,19 +134,23 @@ func onEntry(
 				})
 				withLogger := logging.WithLogger(newLogger)
 
-				return withCtx(withLogger(ctx))
+				return F.Pipe2(
+					ctx,
+					withLogger,
+					pair.Map[context.CancelFunc](withCtx),
+				)
 			}
 			// logging disabled
 			withCtx := withLoggingContext(loggingContext{
 				logger:    logger,
 				isEnabled: false,
 			})
-			return withCtx(ctx)
+			return pair.MakePair[context.CancelFunc](noop, withCtx(ctx))
 		}
 	}
 }
 
-// onExitAny creates a Kleisli function that handles exit logging for an operation.
+// onExitVoid creates a Kleisli function that handles exit logging for an operation.
 // It logs either success or error based on the Result, including the operation duration.
 // Only logs if logging was enabled during entry (checked via loggingContext.isEnabled).
 //
@@ -280,33 +160,33 @@ func onEntry(
 //
 // Returns:
 //   - A Kleisli function that logs the exit/error and returns nil
-func onExitAny(
+func onExitVoid(
 	logLevel slog.Level,
 	nameAttr slog.Attr,
-) readerio.Kleisli[Result[any], any] {
-	return func(res Result[any]) ReaderIO[any] {
-		return func(ctx context.Context) IO[any] {
+) readerio.Kleisli[Result[Void], Void] {
+	return func(res Result[Void]) ReaderIO[Void] {
+		return func(ctx context.Context) IO[Void] {
 			value := getLoggingContext(ctx)
 
 			if value.isEnabled {
 
-				return func() any {
+				return func() Void {
 					// Retrieve logging information from context
 					durationAttr := slog.Duration("duration", time.Since(value.startTime))
 
 					// Log error with ID and duration
-					onError := func(err error) any {
+					onError := func(err error) Void {
 						value.logger.LogAttrs(ctx, logLevel, "[throwing]",
 							nameAttr,
 							durationAttr,
 							slog.Any("error", err))
-						return nil
+						return F.VOID
 					}
 
 					// Log success with ID and duration
-					onSuccess := func(_ any) any {
+					onSuccess := func(v Void) Void {
 						value.logger.LogAttrs(ctx, logLevel, "[exiting ]", nameAttr, durationAttr)
-						return nil
+						return v
 					}
 
 					return F.Pipe1(
@@ -316,7 +196,7 @@ func onExitAny(
 				}
 			}
 			// nothing to do
-			return io.Of[any](nil)
+			return io.Of(F.VOID)
 		}
 	}
 }
@@ -369,18 +249,26 @@ func onExitAny(
 //	)
 func LogEntryExitWithCallback[A any](
 	logLevel slog.Level,
-	cb func(context.Context) *slog.Logger,
+	cb Reader[context.Context, *slog.Logger],
 	name string) Operator[A, A] {
 
 	nameAttr := slog.String("name", name)
 
-	return LogEntryExitF(
+	entry := F.Pipe1(
 		onEntry(logLevel, cb, nameAttr),
-		F.Flow2(
-			result.MapTo[A, any](nil),
-			onExitAny(logLevel, nameAttr),
-		),
+		readerio.LocalIOK[Result[A]],
 	)
+
+	exit := readerio.Tap(F.Flow2(
+		result.MapTo[A](F.VOID),
+		onExitVoid(logLevel, nameAttr),
+	))
+
+	return F.Flow2(
+		exit,
+		entry,
+	)
+
 }
 
 // LogEntryExit creates an operator that logs the entry and exit of a ReaderIOResult computation with timing and correlation IDs.
@@ -499,12 +387,12 @@ func LogEntryExit[A any](name string) Operator[A, A] {
 func curriedLog(
 	logLevel slog.Level,
 	cb func(context.Context) *slog.Logger,
-	message string) func(slog.Attr) func(context.Context) func() struct{} {
-	return F.Curry2(func(a slog.Attr, ctx context.Context) func() struct{} {
+	message string) func(slog.Attr) ReaderIO[Void] {
+	return F.Curry2(func(a slog.Attr, ctx context.Context) IO[Void] {
 		logger := cb(ctx)
-		return func() struct{} {
+		return func() Void {
 			logger.LogAttrs(ctx, logLevel, message, a)
-			return struct{}{}
+			return F.VOID
 		}
 	})
 }
@@ -571,7 +459,7 @@ func curriedLog(
 //   - Conditional logging: Enable/disable logging based on logger configuration
 func SLogWithCallback[A any](
 	logLevel slog.Level,
-	cb func(context.Context) *slog.Logger,
+	cb Reader[context.Context, *slog.Logger],
 	message string) Kleisli[Result[A], A] {
 
 	return F.Pipe1(
@@ -582,18 +470,23 @@ func SLogWithCallback[A any](
 			curriedLog(logLevel, cb, message),
 		),
 		// preserve the original context
-		reader.Chain(reader.Sequence(readerio.MapTo[struct{}, Result[A]])),
+		reader.Chain(reader.Sequence(readerio.MapTo[Void, Result[A]])),
 	)
 }
 
 // SLog creates a Kleisli arrow that logs a Result value (success or error) with a message.
 //
-// This function logs both successful values and errors at Info level using the logger from the context.
+// This function logs both successful values and errors at Info level. It retrieves the logger
+// using logging.GetLoggerFromContext, which returns either:
+//   - The logger stored in the context via logging.WithLogger, or
+//   - The global logger (set via logging.SetLogger or slog.Default())
+//
 // It's a convenience wrapper around SLogWithCallback with standard settings.
 //
-// The logged output includes:
-//   - For success: The message with the value as a structured "value" attribute
-//   - For error: The message with the error as a structured "error" attribute
+// The message parameter becomes the main log message text, and the Result value or error
+// is attached as a structured logging attribute:
+//   - For success: Logs message with attribute value=<the actual value>
+//   - For error: Logs message with attribute error=<the error>
 //
 // The Result is passed through unchanged after logging, making this function transparent in the
 // computation pipeline.
@@ -647,25 +540,47 @@ func SLog[A any](message string) Kleisli[Result[A], A] {
 	return SLogWithCallback[A](slog.LevelInfo, logging.GetLoggerFromContext, message)
 }
 
-// TapSLog creates an operator that logs only successful values with a message and passes them through unchanged.
+// TapSLog creates an operator that logs both successful values and errors with a message,
+// and passes the ReaderIOResult through unchanged.
 //
 // This function is useful for debugging and monitoring values as they flow through a ReaderIOResult
-// computation chain. Unlike SLog which logs both successes and errors, TapSLog only logs when the
-// computation is successful. If the computation contains an error, no logging occurs and the error
-// is propagated unchanged.
+// computation chain. It logs both successful values and errors at Info level. It retrieves the logger
+// using logging.GetLoggerFromContext, which returns either:
+//   - The logger stored in the context via logging.WithLogger, or
+//   - The global logger (set via logging.SetLogger or slog.Default())
 //
-// The logged output includes:
-//   - The provided message
-//   - The value being passed through (as a structured "value" attribute)
+// The ReaderIOResult is returned unchanged after logging.
+//
+// The difference between TapSLog and SLog is their position in the pipeline:
+//   - SLog is a Kleisli[Result[A], A] used with Chain to intercept the raw Result
+//   - TapSLog is an Operator[A, A] used directly in a pipe on a ReaderIOResult[A]
+//
+// Both log the same information (success value or error), but TapSLog is more ergonomic
+// when composing ReaderIOResult pipelines with F.Pipe.
+//
+// The message parameter becomes the main log message text, and the Result value or error
+// is attached as a structured logging attribute:
+//   - For success: Logs message with attribute value=<the actual value>
+//   - For error: Logs message with attribute error=<the error>
+//
+// For example, TapSLog[User]("Fetched user") with a successful result produces:
+//
+//	Log message: "Fetched user"
+//	Structured attribute: value={ID:123 Name:"Alice"}
+//
+// With an error result, it produces:
+//
+//	Log message: "Fetched user"
+//	Structured attribute: error="user not found"
 //
 // Type Parameters:
-//   - A: The type of the value to log and pass through
+//   - A: The success type of the ReaderIOResult to log and pass through
 //
 // Parameters:
 //   - message: A descriptive message to include in the log entry
 //
 // Returns:
-//   - An Operator that logs successful values and returns them unchanged
+//   - An Operator that logs the Result (value or error) and returns the ReaderIOResult unchanged
 //
 // Example with simple value logging:
 //
@@ -680,7 +595,7 @@ func SLog[A any](message string) Kleisli[Result[A], A] {
 //	)
 //
 //	result := pipeline(t.Context())()
-//	// Logs: "Fetched user" value={ID:123 Name:"Alice"}
+//	// If successful, logs: "Fetched user" value={ID:123 Name:"Alice"}
 //	// Returns: result.Of("Alice")
 //
 // Example in a processing pipeline:
@@ -695,36 +610,36 @@ func SLog[A any](message string) Kleisli[Result[A], A] {
 //	)
 //
 //	result := processOrder(t.Context())()
-//	// Logs each successful step with the intermediate values
-//	// If any step fails, subsequent TapSLog calls don't log
+//	// Logs each step with its value or error
 //
 // Example with error handling:
 //
 //	pipeline := F.Pipe3(
 //	    fetchData(id),
 //	    TapSLog[Data]("Data fetched"),
-//	    Chain(func(d Data) ReaderIOResult[Result] {
+//	    Chain(func(d Data) ReaderIOResult[Data] {
 //	        if d.IsValid() {
 //	            return Of(processData(d))
 //	        }
-//	        return Left[Result](errors.New("invalid data"))
+//	        return Left[Data](errors.New("invalid data"))
 //	    }),
-//	    TapSLog[Result]("Data processed"),
+//	    TapSLog[Data]("Data processed"),
 //	)
 //
-//	// If fetchData succeeds: logs "Data fetched" with the data
-//	// If processing succeeds: logs "Data processed" with the result
-//	// If processing fails: "Data processed" is NOT logged (error propagates)
+//	// If fetchData succeeds: logs "Data fetched" value={...}
+//	// If fetchData fails: logs "Data fetched" error="..."
+//	// If processing succeeds: logs "Data processed" value={...}
+//	// If processing fails: logs "Data processed" error="invalid data"
 //
 // Use Cases:
-//   - Debugging: Inspect intermediate successful values in a computation pipeline
-//   - Monitoring: Track successful data flow through complex operations
-//   - Troubleshooting: Identify where successful computations stop (last logged value before error)
-//   - Auditing: Log important successful values for compliance or security
-//   - Development: Understand data transformations during development
+//   - Debugging: Inspect intermediate values and errors in a computation pipeline
+//   - Monitoring: Track both successful and failed data flow through complex operations
+//   - Troubleshooting: Identify where errors are introduced or propagated
+//   - Auditing: Log important values and failures for compliance or security
+//   - Development: Understand data transformations and error paths during development
 //
-// Note: This function only logs successful values. Errors are silently propagated without logging.
-// For logging both successes and errors, use SLog instead.
+// Note: This function logs both successful values and errors. It is equivalent to SLog
+// but expressed as an Operator for direct use in F.Pipe pipelines on ReaderIOResult values.
 //
 //go:inline
 func TapSLog[A any](message string) Operator[A, A] {

v2/context/readerioresult/logging_comprehensive_test.go

@@ -0,0 +1,415 @@
+// 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 readerioresult
+
+import (
+	"bytes"
+	"errors"
+	"log/slog"
+	"strings"
+	"testing"
+
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/logging"
+	N "github.com/IBM/fp-go/v2/number"
+	"github.com/IBM/fp-go/v2/result"
+	"github.com/stretchr/testify/assert"
+)
+
+// TestTapSLogComprehensive_Success verifies TapSLog logs successful values
+func TestTapSLogComprehensive_Success(t *testing.T) {
+	var buf bytes.Buffer
+	logger := slog.New(slog.NewTextHandler(&buf, &slog.HandlerOptions{
+		Level: slog.LevelInfo,
+	}))
+	oldLogger := logging.SetLogger(logger)
+	defer logging.SetLogger(oldLogger)
+
+	t.Run("logs integer success value", func(t *testing.T) {
+		buf.Reset()
+
+		pipeline := F.Pipe2(
+			Of(42),
+			TapSLog[int]("Integer value"),
+			Map(N.Mul(2)),
+		)
+
+		res := pipeline(t.Context())()
+
+		// Verify result is correct
+		assert.Equal(t, 84, F.Pipe1(res, getOrZero))
+
+		// Verify logging occurred
+		logOutput := buf.String()
+		assert.Contains(t, logOutput, "Integer value", "Should log the message")
+		assert.Contains(t, logOutput, "value=42", "Should log the success value")
+		assert.NotContains(t, logOutput, "error", "Should not contain error keyword for success")
+	})
+
+	t.Run("logs string success value", func(t *testing.T) {
+		buf.Reset()
+
+		pipeline := F.Pipe1(
+			Of("hello world"),
+			TapSLog[string]("String value"),
+		)
+
+		res := pipeline(t.Context())()
+
+		// Verify result is correct
+		assert.True(t, F.Pipe1(res, isRight[string]))
+
+		// Verify logging occurred
+		logOutput := buf.String()
+		assert.Contains(t, logOutput, "String value")
+		assert.Contains(t, logOutput, `value="hello world"`)
+	})
+
+	t.Run("logs struct success value", func(t *testing.T) {
+		buf.Reset()
+
+		type User struct {
+			ID   int
+			Name string
+		}
+
+		user := User{ID: 123, Name: "Alice"}
+		pipeline := F.Pipe1(
+			Of(user),
+			TapSLog[User]("User struct"),
+		)
+
+		res := pipeline(t.Context())()
+
+		// Verify result is correct
+		assert.True(t, F.Pipe1(res, isRight[User]))
+
+		// Verify logging occurred with struct fields
+		logOutput := buf.String()
+		assert.Contains(t, logOutput, "User struct")
+		assert.Contains(t, logOutput, "ID:123")
+		assert.Contains(t, logOutput, "Name:Alice")
+	})
+
+	t.Run("logs multiple success values in pipeline", func(t *testing.T) {
+		buf.Reset()
+
+		step1 := F.Pipe2(
+			Of(10),
+			TapSLog[int]("Initial value"),
+			Map(N.Mul(2)),
+		)
+
+		pipeline := F.Pipe2(
+			step1,
+			TapSLog[int]("After doubling"),
+			Map(N.Add(5)),
+		)
+
+		res := pipeline(t.Context())()
+
+		// Verify result is correct
+		assert.Equal(t, 25, getOrZero(res))
+
+		// Verify both log entries
+		logOutput := buf.String()
+		assert.Contains(t, logOutput, "Initial value")
+		assert.Contains(t, logOutput, "value=10")
+		assert.Contains(t, logOutput, "After doubling")
+		assert.Contains(t, logOutput, "value=20")
+	})
+}
+
+// TestTapSLogComprehensive_Error verifies TapSLog behavior with errors
+func TestTapSLogComprehensive_Error(t *testing.T) {
+	var buf bytes.Buffer
+	logger := slog.New(slog.NewTextHandler(&buf, &slog.HandlerOptions{
+		Level: slog.LevelInfo,
+	}))
+	oldLogger := logging.SetLogger(logger)
+	defer logging.SetLogger(oldLogger)
+
+	t.Run("logs error values", func(t *testing.T) {
+		buf.Reset()
+
+		testErr := errors.New("test error")
+		pipeline := F.Pipe2(
+			Left[int](testErr),
+			TapSLog[int]("Error case"),
+			Map(N.Mul(2)),
+		)
+
+		res := pipeline(t.Context())()
+
+		// Verify error is preserved
+		assert.True(t, F.Pipe1(res, isLeft[int]))
+
+		// Verify logging occurred for error
+		logOutput := buf.String()
+		assert.Contains(t, logOutput, "Error case", "Should log the message")
+		assert.Contains(t, logOutput, "error", "Should contain error keyword")
+		assert.Contains(t, logOutput, "test error", "Should log the error message")
+		assert.NotContains(t, logOutput, "value=", "Should not log 'value=' for errors")
+	})
+
+	t.Run("preserves error through pipeline", func(t *testing.T) {
+		buf.Reset()
+
+		originalErr := errors.New("original error")
+		step1 := F.Pipe2(
+			Left[int](originalErr),
+			TapSLog[int]("First tap"),
+			Map(N.Mul(2)),
+		)
+
+		pipeline := F.Pipe2(
+			step1,
+			TapSLog[int]("Second tap"),
+			Map(N.Add(5)),
+		)
+
+		res := pipeline(t.Context())()
+
+		// Verify error is preserved
+		assert.True(t, isLeft(res))
+
+		// Verify both taps logged the error
+		logOutput := buf.String()
+		errorCount := strings.Count(logOutput, "original error")
+		assert.Equal(t, 2, errorCount, "Both TapSLog calls should log the error")
+		assert.Contains(t, logOutput, "First tap")
+		assert.Contains(t, logOutput, "Second tap")
+	})
+
+	t.Run("logs error after successful operation", func(t *testing.T) {
+		buf.Reset()
+
+		pipeline := F.Pipe3(
+			Of(10),
+			TapSLog[int]("Before error"),
+			Chain(func(n int) ReaderIOResult[int] {
+				return Left[int](errors.New("chain error"))
+			}),
+			TapSLog[int]("After error"),
+		)
+
+		res := pipeline(t.Context())()
+
+		// Verify error is present
+		assert.True(t, F.Pipe1(res, isLeft[int]))
+
+		// Verify both logs
+		logOutput := buf.String()
+		assert.Contains(t, logOutput, "Before error")
+		assert.Contains(t, logOutput, "value=10")
+		assert.Contains(t, logOutput, "After error")
+		assert.Contains(t, logOutput, "chain error")
+	})
+}
+
+// TestTapSLogComprehensive_EdgeCases verifies TapSLog with edge cases
+func TestTapSLogComprehensive_EdgeCases(t *testing.T) {
+	var buf bytes.Buffer
+	logger := slog.New(slog.NewTextHandler(&buf, &slog.HandlerOptions{
+		Level: slog.LevelInfo,
+	}))
+	oldLogger := logging.SetLogger(logger)
+	defer logging.SetLogger(oldLogger)
+
+	t.Run("logs zero value", func(t *testing.T) {
+		buf.Reset()
+
+		pipeline := F.Pipe1(
+			Of(0),
+			TapSLog[int]("Zero value"),
+		)
+
+		res := pipeline(t.Context())()
+
+		assert.Equal(t, 0, F.Pipe1(res, getOrZero))
+
+		logOutput := buf.String()
+		assert.Contains(t, logOutput, "Zero value")
+		assert.Contains(t, logOutput, "value=0")
+	})
+
+	t.Run("logs empty string", func(t *testing.T) {
+		buf.Reset()
+
+		pipeline := F.Pipe1(
+			Of(""),
+			TapSLog[string]("Empty string"),
+		)
+
+		res := pipeline(t.Context())()
+
+		assert.True(t, F.Pipe1(res, isRight[string]))
+
+		logOutput := buf.String()
+		assert.Contains(t, logOutput, "Empty string")
+		assert.Contains(t, logOutput, `value=""`)
+	})
+
+	t.Run("logs nil pointer", func(t *testing.T) {
+		buf.Reset()
+
+		type Data struct {
+			Value string
+		}
+
+		var nilData *Data
+		pipeline := F.Pipe1(
+			Of(nilData),
+			TapSLog[*Data]("Nil pointer"),
+		)
+
+		res := pipeline(t.Context())()
+
+		assert.True(t, F.Pipe1(res, isRight[*Data]))
+
+		logOutput := buf.String()
+		assert.Contains(t, logOutput, "Nil pointer")
+		// Nil representation may vary, but should be logged
+		assert.NotEmpty(t, logOutput)
+	})
+
+	t.Run("respects logger level - disabled", func(t *testing.T) {
+		buf.Reset()
+
+		// Create logger that only logs errors
+		errorLogger := slog.New(slog.NewTextHandler(&buf, &slog.HandlerOptions{
+			Level: slog.LevelError,
+		}))
+		oldLogger := logging.SetLogger(errorLogger)
+		defer logging.SetLogger(oldLogger)
+
+		pipeline := F.Pipe1(
+			Of(42),
+			TapSLog[int]("Should not log"),
+		)
+
+		res := pipeline(t.Context())()
+
+		assert.Equal(t, 42, F.Pipe1(res, getOrZero))
+
+		// Should have no logs since level is ERROR
+		logOutput := buf.String()
+		assert.Empty(t, logOutput, "Should not log when level is disabled")
+	})
+}
+
+// TestTapSLogComprehensive_Integration verifies TapSLog in realistic scenarios
+func TestTapSLogComprehensive_Integration(t *testing.T) {
+	var buf bytes.Buffer
+	logger := slog.New(slog.NewTextHandler(&buf, &slog.HandlerOptions{
+		Level: slog.LevelInfo,
+	}))
+	oldLogger := logging.SetLogger(logger)
+	defer logging.SetLogger(oldLogger)
+
+	t.Run("complex pipeline with mixed success and error", func(t *testing.T) {
+		buf.Reset()
+
+		// Simulate a data processing pipeline
+		validatePositive := func(n int) ReaderIOResult[int] {
+			if n > 0 {
+				return Of(n)
+			}
+			return Left[int](errors.New("number must be positive"))
+		}
+
+		step1 := F.Pipe3(
+			Of(5),
+			TapSLog[int]("Input received"),
+			Map(N.Mul(2)),
+			TapSLog[int]("After multiplication"),
+		)
+
+		pipeline := F.Pipe2(
+			step1,
+			Chain(validatePositive),
+			TapSLog[int]("After validation"),
+		)
+
+		res := pipeline(t.Context())()
+
+		assert.Equal(t, 10, getOrZero(res))
+
+		logOutput := buf.String()
+		assert.Contains(t, logOutput, "Input received")
+		assert.Contains(t, logOutput, "value=5")
+		assert.Contains(t, logOutput, "After multiplication")
+		assert.Contains(t, logOutput, "value=10")
+		assert.Contains(t, logOutput, "After validation")
+		assert.Contains(t, logOutput, "value=10")
+	})
+
+	t.Run("error propagation with logging", func(t *testing.T) {
+		buf.Reset()
+
+		validatePositive := func(n int) ReaderIOResult[int] {
+			if n > 0 {
+				return Of(n)
+			}
+			return Left[int](errors.New("number must be positive"))
+		}
+
+		step1 := F.Pipe3(
+			Of(-5),
+			TapSLog[int]("Input received"),
+			Map(N.Mul(2)),
+			TapSLog[int]("After multiplication"),
+		)
+
+		pipeline := F.Pipe2(
+			step1,
+			Chain(validatePositive),
+			TapSLog[int]("After validation"),
+		)
+
+		res := pipeline(t.Context())()
+
+		assert.True(t, isLeft(res))
+
+		logOutput := buf.String()
+		// First two taps should log success
+		assert.Contains(t, logOutput, "Input received")
+		assert.Contains(t, logOutput, "value=-5")
+		assert.Contains(t, logOutput, "After multiplication")
+		assert.Contains(t, logOutput, "value=-10")
+		// Last tap should log error
+		assert.Contains(t, logOutput, "After validation")
+		assert.Contains(t, logOutput, "number must be positive")
+	})
+}
+
+// Helper functions for tests
+
+func getOrZero(res Result[int]) int {
+	val, err := result.Unwrap(res)
+	if err == nil {
+		return val
+	}
+	return 0
+}
+
+func isRight[A any](res Result[A]) bool {
+	return result.IsRight(res)
+}
+
+func isLeft[A any](res Result[A]) bool {
+	return result.IsLeft(res)
+}

v2/context/readerioresult/logging_test.go

@@ -13,6 +13,7 @@ import (
 	F "github.com/IBM/fp-go/v2/function"
 	"github.com/IBM/fp-go/v2/logging"
 	N "github.com/IBM/fp-go/v2/number"
+	"github.com/IBM/fp-go/v2/pair"
 	"github.com/IBM/fp-go/v2/result"
 	S "github.com/IBM/fp-go/v2/string"
 	"github.com/stretchr/testify/assert"
@@ -53,6 +54,11 @@ func TestLogEntryExitSuccess(t *testing.T) {
 	assert.Contains(t, logOutput, "TestOperation")
 	assert.Contains(t, logOutput, "ID=")
 	assert.Contains(t, logOutput, "duration=")
+
+	// Verify entry log appears before exit log
+	enteringIdx := strings.Index(logOutput, "[entering]")
+	exitingIdx := strings.Index(logOutput, "[exiting ]")
+	assert.Greater(t, exitingIdx, enteringIdx, "Exit log should appear after entry log")
 }
 
 // TestLogEntryExitError tests error operation logging
@@ -81,6 +87,11 @@ func TestLogEntryExitError(t *testing.T) {
 	assert.Contains(t, logOutput, "test error")
 	assert.Contains(t, logOutput, "ID=")
 	assert.Contains(t, logOutput, "duration=")
+
+	// Verify entry log appears before error log
+	enteringIdx := strings.Index(logOutput, "[entering]")
+	throwingIdx := strings.Index(logOutput, "[throwing]")
+	assert.Greater(t, throwingIdx, enteringIdx, "Error log should appear after entry log")
 }
 
 // TestLogEntryExitNested tests nested operations with different IDs
@@ -119,6 +130,48 @@ func TestLogEntryExitNested(t *testing.T) {
 	exitCount := strings.Count(logOutput, "[exiting ]")
 	assert.Equal(t, 2, enterCount, "Should have 2 entering logs")
 	assert.Equal(t, 2, exitCount, "Should have 2 exiting logs")
+
+	// Verify log ordering: Each operation logs entry before exit
+	// Note: Due to Chain semantics, OuterOp completes before InnerOp starts
+	lines := strings.Split(logOutput, "\n")
+	var logSequence []string
+	for _, line := range lines {
+		if strings.Contains(line, "OuterOp") && strings.Contains(line, "[entering]") {
+			logSequence = append(logSequence, "OuterOp-entering")
+		} else if strings.Contains(line, "OuterOp") && strings.Contains(line, "[exiting ]") {
+			logSequence = append(logSequence, "OuterOp-exiting")
+		} else if strings.Contains(line, "InnerOp") && strings.Contains(line, "[entering]") {
+			logSequence = append(logSequence, "InnerOp-entering")
+		} else if strings.Contains(line, "InnerOp") && strings.Contains(line, "[exiting ]") {
+			logSequence = append(logSequence, "InnerOp-exiting")
+		}
+	}
+
+	// Verify each operation's entry comes before its exit
+	assert.Equal(t, 4, len(logSequence), "Should have 4 log entries")
+
+	// Find indices
+	outerEnterIdx := -1
+	outerExitIdx := -1
+	innerEnterIdx := -1
+	innerExitIdx := -1
+
+	for i, log := range logSequence {
+		switch log {
+		case "OuterOp-entering":
+			outerEnterIdx = i
+		case "OuterOp-exiting":
+			outerExitIdx = i
+		case "InnerOp-entering":
+			innerEnterIdx = i
+		case "InnerOp-exiting":
+			innerExitIdx = i
+		}
+	}
+
+	// Verify entry before exit for each operation
+	assert.Greater(t, outerExitIdx, outerEnterIdx, "OuterOp exit should come after OuterOp entry")
+	assert.Greater(t, innerExitIdx, innerEnterIdx, "InnerOp exit should come after InnerOp entry")
 }
 
 // TestLogEntryExitWithCallback tests custom log level and callback
@@ -172,76 +225,6 @@ func TestLogEntryExitDisabled(t *testing.T) {
 	assert.Empty(t, logOutput, "Should have no logs when logging is disabled")
 }
 
-// TestLogEntryExitF tests custom entry/exit callbacks
-func TestLogEntryExitF(t *testing.T) {
-	var entryCount, exitCount int
-
-	onEntry := func(ctx context.Context) IO[context.Context] {
-		return func() context.Context {
-			entryCount++
-			return ctx
-		}
-	}
-
-	onExit := func(res Result[string]) ReaderIO[any] {
-		return func(ctx context.Context) IO[any] {
-			return func() any {
-				exitCount++
-				return nil
-			}
-		}
-	}
-
-	operation := F.Pipe1(
-		Of("test"),
-		LogEntryExitF(onEntry, onExit),
-	)
-
-	res := operation(t.Context())()
-
-	assert.True(t, result.IsRight(res))
-	assert.Equal(t, 1, entryCount, "Entry callback should be called once")
-	assert.Equal(t, 1, exitCount, "Exit callback should be called once")
-}
-
-// TestLogEntryExitFWithError tests custom callbacks with error
-func TestLogEntryExitFWithError(t *testing.T) {
-	var entryCount, exitCount int
-	var capturedError error
-
-	onEntry := func(ctx context.Context) IO[context.Context] {
-		return func() context.Context {
-			entryCount++
-			return ctx
-		}
-	}
-
-	onExit := func(res Result[string]) ReaderIO[any] {
-		return func(ctx context.Context) IO[any] {
-			return func() any {
-				exitCount++
-				if result.IsLeft(res) {
-					_, capturedError = result.Unwrap(res)
-				}
-				return nil
-			}
-		}
-	}
-
-	testErr := errors.New("custom error")
-	operation := F.Pipe1(
-		Left[string](testErr),
-		LogEntryExitF(onEntry, onExit),
-	)
-
-	res := operation(t.Context())()
-
-	assert.True(t, result.IsLeft(res))
-	assert.Equal(t, 1, entryCount, "Entry callback should be called once")
-	assert.Equal(t, 1, exitCount, "Exit callback should be called once")
-	assert.Equal(t, testErr, capturedError, "Should capture the error")
-}
-
 // TestLoggingIDUniqueness tests that logging IDs are unique
 func TestLoggingIDUniqueness(t *testing.T) {
 	var buf bytes.Buffer
@@ -287,7 +270,8 @@ func TestLogEntryExitWithContextLogger(t *testing.T) {
 		Level: slog.LevelInfo,
 	}))
 
-	ctx := logging.WithLogger(contextLogger)(t.Context())
+	cancelFct, ctx := pair.Unpack(logging.WithLogger(contextLogger)(t.Context()))
+	defer cancelFct()
 
 	operation := F.Pipe1(
 		Of("context value"),
@@ -546,7 +530,8 @@ func TestTapSLogWithContextLogger(t *testing.T) {
 		Level: slog.LevelInfo,
 	}))
 
-	ctx := logging.WithLogger(contextLogger)(t.Context())
+	cancelFct, ctx := pair.Unpack(logging.WithLogger(contextLogger)(t.Context()))
+	defer cancelFct()
 
 	operation := F.Pipe2(
 		Of("test value"),
@@ -660,3 +645,138 @@ func TestSLogWithCallbackLogsError(t *testing.T) {
 	assert.Contains(t, logOutput, "warning error")
 	assert.Contains(t, logOutput, "level=WARN")
 }
+
+// TestTapSLogPreservesResult tests that TapSLog doesn't modify the result
+func TestTapSLogPreservesResult(t *testing.T) {
+	var buf bytes.Buffer
+	logger := slog.New(slog.NewTextHandler(&buf, &slog.HandlerOptions{
+		Level: slog.LevelInfo,
+	}))
+	oldLogger := logging.SetLogger(logger)
+	defer logging.SetLogger(oldLogger)
+
+	// Test with success value
+	successOp := F.Pipe2(
+		Of(42),
+		TapSLog[int]("Success value"),
+		Map(N.Mul(2)),
+	)
+
+	successRes := successOp(t.Context())()
+	assert.Equal(t, result.Of(84), successRes)
+
+	// Test with error value
+	testErr := errors.New("test error")
+	errorOp := F.Pipe2(
+		Left[int](testErr),
+		TapSLog[int]("Error value"),
+		Map(N.Mul(2)),
+	)
+
+	errorRes := errorOp(t.Context())()
+	assert.True(t, result.IsLeft(errorRes))
+
+	// Verify the error is preserved
+	_, err := result.Unwrap(errorRes)
+	assert.Equal(t, testErr, err)
+}
+
+// TestTapSLogChainBehavior tests that TapSLog properly chains with other operations
+func TestTapSLogChainBehavior(t *testing.T) {
+	var buf bytes.Buffer
+	logger := slog.New(slog.NewTextHandler(&buf, &slog.HandlerOptions{
+		Level: slog.LevelInfo,
+	}))
+	oldLogger := logging.SetLogger(logger)
+	defer logging.SetLogger(oldLogger)
+
+	// Create a pipeline with multiple TapSLog calls
+	step1 := F.Pipe2(
+		Of(1),
+		TapSLog[int]("Step 1"),
+		Map(N.Mul(2)),
+	)
+
+	step2 := F.Pipe2(
+		step1,
+		TapSLog[int]("Step 2"),
+		Map(N.Mul(3)),
+	)
+
+	pipeline := F.Pipe1(
+		step2,
+		TapSLog[int]("Step 3"),
+	)
+
+	res := pipeline(t.Context())()
+	assert.Equal(t, result.Of(6), res)
+
+	logOutput := buf.String()
+
+	// Verify all steps were logged
+	assert.Contains(t, logOutput, "Step 1")
+	assert.Contains(t, logOutput, "value=1")
+	assert.Contains(t, logOutput, "Step 2")
+	assert.Contains(t, logOutput, "value=2")
+	assert.Contains(t, logOutput, "Step 3")
+	assert.Contains(t, logOutput, "value=6")
+}
+
+// TestTapSLogWithNilValue tests TapSLog with nil pointer values
+func TestTapSLogWithNilValue(t *testing.T) {
+	var buf bytes.Buffer
+	logger := slog.New(slog.NewTextHandler(&buf, &slog.HandlerOptions{
+		Level: slog.LevelInfo,
+	}))
+	oldLogger := logging.SetLogger(logger)
+	defer logging.SetLogger(oldLogger)
+
+	type Data struct {
+		Value string
+	}
+
+	// Test with nil pointer
+	var nilData *Data
+	operation := F.Pipe1(
+		Of(nilData),
+		TapSLog[*Data]("Nil pointer value"),
+	)
+
+	res := operation(t.Context())()
+	assert.True(t, result.IsRight(res))
+
+	logOutput := buf.String()
+	assert.Contains(t, logOutput, "Nil pointer value")
+	// The exact representation of nil may vary, but it should be logged
+	assert.NotEmpty(t, logOutput)
+}
+
+// TestTapSLogLogsErrors verifies that TapSLog DOES log errors
+// TapSLog uses SLog internally, which logs both success values and errors
+func TestTapSLogLogsErrors(t *testing.T) {
+	var buf bytes.Buffer
+	logger := slog.New(slog.NewTextHandler(&buf, &slog.HandlerOptions{
+		Level: slog.LevelInfo,
+	}))
+	oldLogger := logging.SetLogger(logger)
+	defer logging.SetLogger(oldLogger)
+
+	testErr := errors.New("test error message")
+	pipeline := F.Pipe2(
+		Left[int](testErr),
+		TapSLog[int]("Error logging test"),
+		Map(N.Mul(2)),
+	)
+
+	res := pipeline(t.Context())()
+
+	// Verify the error is preserved
+	assert.True(t, result.IsLeft(res))
+
+	// Verify logging occurred for the error
+	logOutput := buf.String()
+	assert.NotEmpty(t, logOutput, "TapSLog should log when the Result is an error")
+	assert.Contains(t, logOutput, "Error logging test")
+	assert.Contains(t, logOutput, "error")
+	assert.Contains(t, logOutput, "test error message")
+}

v2/context/readerioresult/profunctor.go

@@ -19,6 +19,11 @@ import (
 	"context"
 
 	"github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/io"
+	"github.com/IBM/fp-go/v2/ioresult"
+	"github.com/IBM/fp-go/v2/pair"
+	RIOR "github.com/IBM/fp-go/v2/readerioresult"
+	"github.com/IBM/fp-go/v2/result"
 )
 
 // Promap is the profunctor map operation that transforms both the input and output of a context-based ReaderIOResult.
@@ -34,21 +39,24 @@ import (
 // The error type is fixed as error and remains unchanged through the transformation.
 //
 // Type Parameters:
+//   - R: The input environment type that f transforms into context.Context
 //   - A: The original success type produced by the ReaderIOResult
 //   - B: The new output success type
 //
 // Parameters:
-//   - f: Function to transform the input context (contravariant)
+//   - f: Function to transform the input environment R into context.Context (contravariant)
 //   - g: Function to transform the output success value from A to B (covariant)
 //
 // Returns:
-//   - An Operator that takes a ReaderIOResult[A] and returns a ReaderIOResult[B]
+//   - A Kleisli arrow that takes a ReaderIOResult[A] and returns a function from R to B
+//
+// Note: When R is context.Context, this simplifies to an Operator[A, B]
 //
 //go:inline
-func Promap[A, B any](f func(context.Context) (context.Context, context.CancelFunc), g func(A) B) Operator[A, B] {
+func Promap[R, A, B any](f pair.Kleisli[context.CancelFunc, R, context.Context], g func(A) B) RIOR.Kleisli[R, ReaderIOResult[A], B] {
 	return function.Flow2(
 		Local[A](f),
-		Map(g),
+		RIOR.Map[R](g),
 	)
 }
 
@@ -62,14 +70,168 @@ func Promap[A, B any](f func(context.Context) (context.Context, context.CancelFu
 //
 // Type Parameters:
 //   - A: The success type (unchanged)
+//   - R: The input environment type that f transforms into context.Context
 //
 // Parameters:
-//   - f: Function to transform the context, returning a new context and CancelFunc
+//   - f: Function to transform the input environment R into context.Context, returning a new context and CancelFunc
+//
+// Returns:
+//   - A Kleisli arrow that takes a ReaderIOResult[A] and returns a function from R to A
+//
+// Note: When R is context.Context, this simplifies to an Operator[A, A]
+//
+//go:inline
+func Contramap[A, R any](f pair.Kleisli[context.CancelFunc, R, context.Context]) RIOR.Kleisli[R, ReaderIOResult[A], A] {
+	return Local[A](f)
+}
+
+// ContramapIOK changes the context during the execution of a ReaderIOResult using an IO effect.
+// This is the contravariant functor operation with IO effects.
+//
+// ContramapIOK is an alias for LocalIOK and is useful for adapting a ReaderIOResult to work with
+// a modified context when the transformation itself requires side effects.
+//
+// Type Parameters:
+//   - A: The success type (unchanged)
+//
+// Parameters:
+//   - f: An IO Kleisli arrow that transforms the context with side effects
 //
 // Returns:
 //   - An Operator that takes a ReaderIOResult[A] and returns a ReaderIOResult[A]
 //
+// See Also:
+//   - Contramap: For pure context transformations
+//   - LocalIOK: The underlying implementation
+//
 //go:inline
-func Contramap[A any](f func(context.Context) (context.Context, context.CancelFunc)) Operator[A, A] {
-	return Local[A](f)
+func ContramapIOK[A any](f io.Kleisli[context.Context, ContextCancel]) Operator[A, A] {
+	return LocalIOK[A](f)
+}
+
+// LocalIOK transforms the context using an IO-based function before passing it to a ReaderIOResult.
+// This is similar to Local but the context transformation itself is wrapped in an IO effect.
+//
+// The function f takes a context and returns an IO effect that produces a ContextCancel
+// (a pair of CancelFunc and the new Context). This allows the context transformation to
+// perform side effects.
+//
+// # Use Cases
+//
+// This function is useful for sharing information via the Context that is computed through
+// side effects that cannot fail, such as:
+//   - Generating unique request IDs or trace IDs
+//   - Recording timestamps or metrics
+//   - Logging context information
+//   - Computing derived values from existing context data
+//
+// The side effect is executed during the context transformation, and the resulting data is
+// stored in the context for downstream computations to access.
+//
+// # Type Parameters
+//
+//   - A: The success type (unchanged through the transformation)
+//
+// # Parameters
+//
+//   - f: An IO-based Kleisli function that transforms the context
+//
+// # Returns
+//
+//   - An Operator that applies the context transformation before executing the ReaderIOResult
+//
+// # Example Usage
+//
+//	// Generate a request ID via side effect and add to context
+//	addRequestID := func(ctx context.Context) io.IO[ContextCancel] {
+//	    return func() ContextCancel {
+//	        // Side effect: generate unique ID
+//	        requestID := uuid.New().String()
+//	        // Share the ID via context
+//	        newCtx := context.WithValue(ctx, "requestID", requestID)
+//	        return pair.MakePair(func() {}, newCtx)
+//	    }
+//	}
+//	adapted := LocalIOK[int](addRequestID)(computation)
+//
+// # See Also
+//
+//   - Local: For pure context transformations
+//   - LocalIOResultK: For context transformations that can fail
+//
+//go:inline
+func LocalIOK[A any](f io.Kleisli[context.Context, ContextCancel]) Operator[A, A] {
+	return LocalIOResultK[A](function.Flow2(f, ioresult.FromIO))
+}
+
+// LocalIOResultK transforms the context using an IOResult-based function before passing it to a ReaderIOResult.
+// This is similar to Local but the context transformation can fail with an error.
+//
+// The function f takes a context and returns an IOResult that produces either an error or a ContextCancel
+// (a pair of CancelFunc and the new Context). If the transformation fails, the error is propagated
+// and the original ReaderIOResult is not executed.
+//
+// # Use Cases
+//
+// This function is particularly useful for sharing information via the Context that is computed
+// through side effects, such as:
+//   - Loading configuration from a file or database
+//   - Fetching authentication tokens from an external service
+//   - Computing derived values that require I/O operations
+//   - Validating and enriching context with data from external sources
+//
+// The side effect is executed during the context transformation, and the resulting data is
+// stored in the context for downstream computations to access.
+//
+// # Type Parameters
+//
+//   - A: The success type (unchanged through the transformation)
+//
+// # Parameters
+//
+//   - f: An IOResult-based Kleisli function that transforms the context and may fail
+//
+// # Returns
+//
+//   - An Operator that applies the context transformation before executing the ReaderIOResult
+//
+// # Example Usage
+//
+//	// Load configuration via side effect and add to context
+//	loadConfig := func(ctx context.Context) ioresult.IOResult[ContextCancel] {
+//	    return func() result.Result[ContextCancel] {
+//	        // Side effect: read from file system
+//	        config, err := os.ReadFile("config.json")
+//	        if err != nil {
+//	            return result.Left[ContextCancel](err)
+//	        }
+//	        // Share the loaded config via context
+//	        newCtx := context.WithValue(ctx, "config", config)
+//	        return result.Of(pair.MakePair(func() {}, newCtx))
+//	    }
+//	}
+//	adapted := LocalIOResultK[int](loadConfig)(computation)
+//
+// # See Also
+//
+//   - Local: For pure context transformations
+//   - LocalIOK: For context transformations with side effects that cannot fail
+func LocalIOResultK[A any](f ioresult.Kleisli[context.Context, ContextCancel]) Operator[A, A] {
+	return func(rr ReaderIOResult[A]) ReaderIOResult[A] {
+		return func(ctx context.Context) IOResult[A] {
+			return func() Result[A] {
+				if ctx.Err() != nil {
+					return result.Left[A](context.Cause(ctx))
+				}
+				p, err := result.Unwrap(f(ctx)())
+				if err != nil {
+					return result.Left[A](err)
+				}
+				// unwrap
+				otherCancel, otherCtx := pair.Unpack(p)
+				defer otherCancel()
+				return rr(otherCtx)()
+			}
+		}
+	}
 }

v2/context/readerioresult/profunctor_test.go

@@ -20,6 +20,10 @@ import (
 	"strconv"
 	"testing"
 
+	"github.com/IBM/fp-go/v2/context/ioresult"
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/io"
+	"github.com/IBM/fp-go/v2/pair"
 	R "github.com/IBM/fp-go/v2/result"
 	"github.com/stretchr/testify/assert"
 )
@@ -36,9 +40,9 @@ func TestPromapBasic(t *testing.T) {
 			}
 		}
 
-		addKey := func(ctx context.Context) (context.Context, context.CancelFunc) {
+		addKey := func(ctx context.Context) pair.Pair[context.CancelFunc, context.Context] {
 			newCtx := context.WithValue(ctx, "key", 42)
-			return newCtx, func() {}
+			return pair.MakePair(context.CancelFunc(func() {}), newCtx)
 		}
 		toString := strconv.Itoa
 
@@ -61,9 +65,9 @@ func TestContramapBasic(t *testing.T) {
 			}
 		}
 
-		addKey := func(ctx context.Context) (context.Context, context.CancelFunc) {
+		addKey := func(ctx context.Context) pair.Pair[context.CancelFunc, context.Context] {
 			newCtx := context.WithValue(ctx, "key", 100)
-			return newCtx, func() {}
+			return pair.MakePair(context.CancelFunc(func() {}), newCtx)
 		}
 
 		adapted := Contramap[int](addKey)(getValue)
@@ -73,6 +77,105 @@ func TestContramapBasic(t *testing.T) {
 	})
 }
 
+// TestContramapIOK tests ContramapIOK functionality
+func TestContramapIOK(t *testing.T) {
+	t.Run("transforms context with IO effect", func(t *testing.T) {
+		getValue := func(ctx context.Context) IOResult[string] {
+			return func() R.Result[string] {
+				if v := ctx.Value("requestID"); v != nil {
+					return R.Of(v.(string))
+				}
+				return R.Of("no-id")
+			}
+		}
+
+		addRequestID := func(ctx context.Context) io.IO[ContextCancel] {
+			return func() ContextCancel {
+				// Simulate generating a request ID via side effect
+				requestID := "req-12345"
+				newCtx := context.WithValue(ctx, "requestID", requestID)
+				return pair.MakePair(context.CancelFunc(func() {}), newCtx)
+			}
+		}
+
+		adapted := ContramapIOK[string](addRequestID)(getValue)
+		result := adapted(t.Context())()
+
+		assert.Equal(t, R.Of("req-12345"), result)
+	})
+
+	t.Run("preserves value type", func(t *testing.T) {
+		getValue := func(ctx context.Context) IOResult[int] {
+			return func() R.Result[int] {
+				if v := ctx.Value("counter"); v != nil {
+					return R.Of(v.(int))
+				}
+				return R.Of(0)
+			}
+		}
+
+		addCounter := func(ctx context.Context) io.IO[ContextCancel] {
+			return func() ContextCancel {
+				newCtx := context.WithValue(ctx, "counter", 999)
+				return pair.MakePair(context.CancelFunc(func() {}), newCtx)
+			}
+		}
+
+		adapted := ContramapIOK[int](addCounter)(getValue)
+		result := adapted(t.Context())()
+
+		assert.Equal(t, R.Of(999), result)
+	})
+
+	t.Run("calls cancel function", func(t *testing.T) {
+		cancelCalled := false
+
+		getValue := func(ctx context.Context) IOResult[string] {
+			return func() R.Result[string] {
+				return R.Of("test")
+			}
+		}
+
+		addData := func(ctx context.Context) io.IO[ContextCancel] {
+			return func() ContextCancel {
+				newCtx := context.WithValue(ctx, "data", "value")
+				cancelFunc := context.CancelFunc(func() {
+					cancelCalled = true
+				})
+				return pair.MakePair(cancelFunc, newCtx)
+			}
+		}
+
+		adapted := ContramapIOK[string](addData)(getValue)
+		_ = adapted(t.Context())()
+
+		assert.True(t, cancelCalled, "cancel function should be called")
+	})
+
+	t.Run("handles cancelled context", func(t *testing.T) {
+		getValue := func(ctx context.Context) IOResult[string] {
+			return func() R.Result[string] {
+				return R.Of("should not reach here")
+			}
+		}
+
+		addData := func(ctx context.Context) io.IO[ContextCancel] {
+			return func() ContextCancel {
+				newCtx := context.WithValue(ctx, "data", "value")
+				return pair.MakePair(context.CancelFunc(func() {}), newCtx)
+			}
+		}
+
+		ctx, cancel := context.WithCancel(t.Context())
+		cancel()
+
+		adapted := ContramapIOK[string](addData)(getValue)
+		result := adapted(ctx)()
+
+		assert.True(t, R.IsLeft(result))
+	})
+}
+
 // TestLocalBasic tests basic Local functionality
 func TestLocalBasic(t *testing.T) {
 	t.Run("adds value to context", func(t *testing.T) {
@@ -85,9 +188,9 @@ func TestLocalBasic(t *testing.T) {
 			}
 		}
 
-		addUser := func(ctx context.Context) (context.Context, context.CancelFunc) {
+		addUser := func(ctx context.Context) pair.Pair[context.CancelFunc, context.Context] {
 			newCtx := context.WithValue(ctx, "user", "Alice")
-			return newCtx, func() {}
+			return pair.MakePair(context.CancelFunc(func() {}), newCtx)
 		}
 
 		adapted := Local[string](addUser)(getValue)
@@ -96,3 +199,311 @@ func TestLocalBasic(t *testing.T) {
 		assert.Equal(t, R.Of("Alice"), result)
 	})
 }
+
+// TestLocalIOK_Success tests LocalIOK with successful context transformation
+func TestLocalIOK_Success(t *testing.T) {
+	t.Run("transforms context with IO effect", func(t *testing.T) {
+		getValue := func(ctx context.Context) IOResult[string] {
+			return func() R.Result[string] {
+				if v := ctx.Value("user"); v != nil {
+					return R.Of(v.(string))
+				}
+				return R.Of("unknown")
+			}
+		}
+
+		addUser := func(ctx context.Context) io.IO[ContextCancel] {
+			return func() ContextCancel {
+				newCtx := context.WithValue(ctx, "user", "Bob")
+				return pair.MakePair(context.CancelFunc(func() {}), newCtx)
+			}
+		}
+
+		adapted := LocalIOK[string](addUser)(getValue)
+		result := adapted(t.Context())()
+
+		assert.Equal(t, R.Of("Bob"), result)
+	})
+
+	t.Run("preserves original value type", func(t *testing.T) {
+		getValue := func(ctx context.Context) IOResult[int] {
+			return func() R.Result[int] {
+				if v := ctx.Value("count"); v != nil {
+					return R.Of(v.(int))
+				}
+				return R.Of(0)
+			}
+		}
+
+		addCount := func(ctx context.Context) io.IO[ContextCancel] {
+			return func() ContextCancel {
+				newCtx := context.WithValue(ctx, "count", 42)
+				return pair.MakePair(context.CancelFunc(func() {}), newCtx)
+			}
+		}
+
+		adapted := LocalIOK[int](addCount)(getValue)
+		result := adapted(t.Context())()
+
+		assert.Equal(t, R.Of(42), result)
+	})
+}
+
+// TestLocalIOK_CancelledContext tests LocalIOK with cancelled context
+func TestLocalIOK_CancelledContext(t *testing.T) {
+	t.Run("returns error when context is cancelled", func(t *testing.T) {
+		getValue := func(ctx context.Context) IOResult[string] {
+			return func() R.Result[string] {
+				return R.Of("should not reach here")
+			}
+		}
+
+		addUser := func(ctx context.Context) io.IO[ContextCancel] {
+			return func() ContextCancel {
+				newCtx := context.WithValue(ctx, "user", "Charlie")
+				return pair.MakePair(context.CancelFunc(func() {}), newCtx)
+			}
+		}
+
+		ctx, cancel := context.WithCancel(t.Context())
+		cancel()
+
+		adapted := LocalIOK[string](addUser)(getValue)
+		result := adapted(ctx)()
+
+		assert.True(t, R.IsLeft(result))
+	})
+}
+
+// TestLocalIOK_CancelFuncCalled tests that CancelFunc is properly called
+func TestLocalIOK_CancelFuncCalled(t *testing.T) {
+	t.Run("calls cancel function after execution", func(t *testing.T) {
+		cancelCalled := false
+
+		getValue := func(ctx context.Context) IOResult[string] {
+			return func() R.Result[string] {
+				return R.Of("test")
+			}
+		}
+
+		addUser := func(ctx context.Context) io.IO[ContextCancel] {
+			return func() ContextCancel {
+				newCtx := context.WithValue(ctx, "user", "Dave")
+				cancelFunc := context.CancelFunc(func() {
+					cancelCalled = true
+				})
+				return pair.MakePair(cancelFunc, newCtx)
+			}
+		}
+
+		adapted := LocalIOK[string](addUser)(getValue)
+		_ = adapted(t.Context())()
+
+		assert.True(t, cancelCalled, "cancel function should be called")
+	})
+}
+
+// TestLocalIOResultK_Success tests LocalIOResultK with successful context transformation
+func TestLocalIOResultK_Success(t *testing.T) {
+	t.Run("transforms context with IOResult effect", func(t *testing.T) {
+		getValue := func(ctx context.Context) IOResult[string] {
+			return func() R.Result[string] {
+				if v := ctx.Value("role"); v != nil {
+					return R.Of(v.(string))
+				}
+				return R.Of("guest")
+			}
+		}
+
+		addRole := func(ctx context.Context) ioresult.IOResult[ContextCancel] {
+			return func() R.Result[ContextCancel] {
+				newCtx := context.WithValue(ctx, "role", "admin")
+				return R.Of(pair.MakePair(context.CancelFunc(func() {}), newCtx))
+			}
+		}
+
+		adapted := LocalIOResultK[string](addRole)(getValue)
+		result := adapted(t.Context())()
+
+		assert.Equal(t, R.Of("admin"), result)
+	})
+
+	t.Run("preserves original value type", func(t *testing.T) {
+		getValue := func(ctx context.Context) IOResult[int] {
+			return func() R.Result[int] {
+				if v := ctx.Value("score"); v != nil {
+					return R.Of(v.(int))
+				}
+				return R.Of(0)
+			}
+		}
+
+		addScore := func(ctx context.Context) ioresult.IOResult[ContextCancel] {
+			return func() R.Result[ContextCancel] {
+				newCtx := context.WithValue(ctx, "score", 100)
+				return R.Of(pair.MakePair(context.CancelFunc(func() {}), newCtx))
+			}
+		}
+
+		adapted := LocalIOResultK[int](addScore)(getValue)
+		result := adapted(t.Context())()
+
+		assert.Equal(t, R.Of(100), result)
+	})
+}
+
+// TestLocalIOResultK_Failure tests LocalIOResultK with failed context transformation
+func TestLocalIOResultK_Failure(t *testing.T) {
+	t.Run("propagates transformation error", func(t *testing.T) {
+		getValue := func(ctx context.Context) IOResult[string] {
+			return func() R.Result[string] {
+				return R.Of("should not reach here")
+			}
+		}
+
+		failTransform := func(ctx context.Context) ioresult.IOResult[ContextCancel] {
+			return func() R.Result[ContextCancel] {
+				return R.Left[ContextCancel](assert.AnError)
+			}
+		}
+
+		adapted := LocalIOResultK[string](failTransform)(getValue)
+		result := adapted(t.Context())()
+
+		assert.True(t, R.IsLeft(result))
+		_, err := R.UnwrapError(result)
+		assert.Equal(t, assert.AnError, err)
+	})
+
+	t.Run("does not execute original computation on transformation failure", func(t *testing.T) {
+		executed := false
+
+		getValue := func(ctx context.Context) IOResult[string] {
+			return func() R.Result[string] {
+				executed = true
+				return R.Of("should not execute")
+			}
+		}
+
+		failTransform := func(ctx context.Context) ioresult.IOResult[ContextCancel] {
+			return func() R.Result[ContextCancel] {
+				return R.Left[ContextCancel](assert.AnError)
+			}
+		}
+
+		adapted := LocalIOResultK[string](failTransform)(getValue)
+		_ = adapted(t.Context())()
+
+		assert.False(t, executed, "original computation should not execute")
+	})
+}
+
+// TestLocalIOResultK_CancelledContext tests LocalIOResultK with cancelled context
+func TestLocalIOResultK_CancelledContext(t *testing.T) {
+	t.Run("returns error when context is cancelled", func(t *testing.T) {
+		getValue := func(ctx context.Context) IOResult[string] {
+			return func() R.Result[string] {
+				return R.Of("should not reach here")
+			}
+		}
+
+		addRole := func(ctx context.Context) ioresult.IOResult[ContextCancel] {
+			return func() R.Result[ContextCancel] {
+				newCtx := context.WithValue(ctx, "role", "user")
+				return R.Of(pair.MakePair(context.CancelFunc(func() {}), newCtx))
+			}
+		}
+
+		ctx, cancel := context.WithCancel(t.Context())
+		cancel()
+
+		adapted := LocalIOResultK[string](addRole)(getValue)
+		result := adapted(ctx)()
+
+		assert.True(t, R.IsLeft(result))
+	})
+}
+
+// TestLocalIOResultK_CancelFuncCalled tests that CancelFunc is properly called
+func TestLocalIOResultK_CancelFuncCalled(t *testing.T) {
+	t.Run("calls cancel function after successful execution", func(t *testing.T) {
+		cancelCalled := false
+
+		getValue := func(ctx context.Context) IOResult[string] {
+			return func() R.Result[string] {
+				return R.Of("test")
+			}
+		}
+
+		addRole := func(ctx context.Context) ioresult.IOResult[ContextCancel] {
+			return func() R.Result[ContextCancel] {
+				newCtx := context.WithValue(ctx, "role", "user")
+				cancelFunc := context.CancelFunc(func() {
+					cancelCalled = true
+				})
+				return R.Of(pair.MakePair(cancelFunc, newCtx))
+			}
+		}
+
+		adapted := LocalIOResultK[string](addRole)(getValue)
+		_ = adapted(t.Context())()
+
+		assert.True(t, cancelCalled, "cancel function should be called")
+	})
+
+	t.Run("does not call cancel function on transformation failure", func(t *testing.T) {
+		cancelCalled := false
+
+		getValue := func(ctx context.Context) IOResult[string] {
+			return func() R.Result[string] {
+				return R.Of("test")
+			}
+		}
+
+		failTransform := func(ctx context.Context) ioresult.IOResult[ContextCancel] {
+			return func() R.Result[ContextCancel] {
+				cancelFunc := context.CancelFunc(func() {
+					cancelCalled = true
+				})
+				_ = cancelFunc // avoid unused warning
+				return R.Left[ContextCancel](assert.AnError)
+			}
+		}
+
+		adapted := LocalIOResultK[string](failTransform)(getValue)
+		_ = adapted(t.Context())()
+
+		assert.False(t, cancelCalled, "cancel function should not be called on failure")
+	})
+}
+
+// TestLocalIOResultK_Integration tests integration with other operations
+func TestLocalIOResultK_Integration(t *testing.T) {
+	t.Run("composes with Map", func(t *testing.T) {
+		getValue := func(ctx context.Context) IOResult[int] {
+			return func() R.Result[int] {
+				if v := ctx.Value("value"); v != nil {
+					return R.Of(v.(int))
+				}
+				return R.Of(0)
+			}
+		}
+
+		addValue := func(ctx context.Context) ioresult.IOResult[ContextCancel] {
+			return func() R.Result[ContextCancel] {
+				newCtx := context.WithValue(ctx, "value", 10)
+				return R.Of(pair.MakePair(context.CancelFunc(func() {}), newCtx))
+			}
+		}
+
+		double := func(x int) int { return x * 2 }
+
+		adapted := F.Flow2(
+			LocalIOResultK[int](addValue),
+			Map(double),
+		)(getValue)
+		result := adapted(t.Context())()
+
+		assert.Equal(t, R.Of(20), result)
+	})
+}

v2/context/readerioresult/reader.go

@@ -28,10 +28,10 @@ import (
 	"github.com/IBM/fp-go/v2/ioeither"
 	"github.com/IBM/fp-go/v2/ioresult"
 	"github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/pair"
 	"github.com/IBM/fp-go/v2/reader"
 	RIOR "github.com/IBM/fp-go/v2/readerioresult"
 	"github.com/IBM/fp-go/v2/readeroption"
-	"github.com/IBM/fp-go/v2/result"
 )
 
 const (
@@ -1010,12 +1010,15 @@ func TapLeftIOK[A, B any](f io.Kleisli[error, B]) Operator[A, A] {
 //
 // Type Parameters:
 //   - A: The value type of the ReaderIOResult
+//   - R: The input environment type that f transforms into context.Context
 //
 // Parameters:
-//   - f: A function that transforms the context and returns a cancel function
+//   - f: A function that transforms the input environment R into context.Context and returns a cancel function
 //
 // Returns:
-//   - An Operator that runs the computation with the transformed context
+//   - A Kleisli arrow that runs the computation with the transformed context
+//
+// Note: When R is context.Context, this simplifies to an Operator[A, A]
 //
 // Example:
 //
@@ -1025,9 +1028,9 @@ func TapLeftIOK[A, B any](f io.Kleisli[error, B]) Operator[A, A] {
 //	type key int
 //	const userKey key = 0
 //
-//	addUser := readerioresult.Local[string](func(ctx context.Context) (context.Context, context.CancelFunc) {
+//	addUser := readerioresult.Local[string, context.Context](func(ctx context.Context) pair.Pair[context.CancelFunc, context.Context] {
 //	    newCtx := context.WithValue(ctx, userKey, "Alice")
-//	    return newCtx, func() {} // No-op cancel
+//	    return pair.MakePair(func() {}, newCtx) // No-op cancel
 //	})
 //
 //	getUser := readerioresult.FromReader(func(ctx context.Context) string {
@@ -1046,27 +1049,19 @@ func TapLeftIOK[A, B any](f io.Kleisli[error, B]) Operator[A, A] {
 // Timeout Example:
 //
 //	// Add a 5-second timeout to a specific operation
-//	withTimeout := readerioresult.Local[Data](func(ctx context.Context) (context.Context, context.CancelFunc) {
-//	    return context.WithTimeout(ctx, 5*time.Second)
+//	withTimeout := readerioresult.Local[Data, context.Context](func(ctx context.Context) pair.Pair[context.CancelFunc, context.Context] {
+//	    newCtx, cancel := context.WithTimeout(ctx, 5*time.Second)
+//	    return pair.MakePair(cancel, newCtx)
 //	})
 //
 //	result := F.Pipe1(
 //	    fetchData,
 //	    withTimeout,
 //	)
-func Local[A any](f func(context.Context) (context.Context, context.CancelFunc)) Operator[A, A] {
-	return func(rr ReaderIOResult[A]) ReaderIOResult[A] {
-		return func(ctx context.Context) IOResult[A] {
-			return func() Result[A] {
-				if ctx.Err() != nil {
-					return result.Left[A](context.Cause(ctx))
-				}
-				otherCtx, otherCancel := f(ctx)
-				defer otherCancel()
-				return rr(otherCtx)()
-			}
-		}
-	}
+//
+//go:inline
+func Local[A, R any](f pair.Kleisli[context.CancelFunc, R, context.Context]) RIOR.Kleisli[R, ReaderIOResult[A], A] {
+	return readerio.Local[Result[A]](f)
 }
 
 // WithTimeout adds a timeout to the context for a ReaderIOResult computation.
@@ -1123,9 +1118,10 @@ func Local[A any](f func(context.Context) (context.Context, context.CancelFunc))
 //	)
 //	value, err := result(t.Context())()  // Returns (Data{Value: "quick"}, nil)
 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)
-	})
+	return Local[A](
+		func(ctx context.Context) ContextCancel {
+			return pairFromContextCancel(context.WithTimeout(ctx, timeout))
+		})
 }
 
 // WithDeadline adds an absolute deadline to the context for a ReaderIOResult computation.
@@ -1188,7 +1184,7 @@ func WithTimeout[A any](timeout time.Duration) Operator[A, A] {
 //	)
 //	value, err := 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)
+	return Local[A](func(ctx context.Context) ContextCancel {
+		return pairFromContextCancel(context.WithDeadline(ctx, deadline))
 	})
 }

v2/context/readerioresult/rec.go

@@ -52,7 +52,7 @@ import (
 //
 //   - f: A Kleisli arrow (A => ReaderIOResult[Trampoline[A, B]]) that:
 //   - Takes the current state A
-//   - Returns a ReaderIOResult that depends on [context.Context]
+//   - Returns a ReaderIOResult that depends on context.Context
 //   - Can fail with error (Left in the outer Either)
 //   - Produces Trampoline[A, B] to control recursion flow (Right in the outer Either)
 //
@@ -60,13 +60,13 @@ import (
 //
 // A Kleisli arrow (A => ReaderIOResult[B]) that:
 //   - Takes an initial state A
-//   - Returns a ReaderIOResult that requires [context.Context]
+//   - Returns a ReaderIOResult that requires context.Context
 //   - Can fail with error or context cancellation
 //   - Produces the final result B after recursion completes
 //
 // # Context Cancellation
 //
-// Unlike the base [readerioresult.TailRec], this version automatically integrates
+// Unlike the base readerioresult.TailRec, this version automatically integrates
 // context cancellation checking:
 //   - Each recursive iteration checks if the context is cancelled
 //   - If cancelled, recursion terminates immediately with a cancellation error
@@ -92,9 +92,9 @@ import (
 //
 // # Example: Cancellable Countdown
 //
-//	countdownStep := func(n int) readerioresult.ReaderIOResult[tailrec.Trampoline[int, string]] {
-//	    return func(ctx context.Context) ioeither.IOEither[error, tailrec.Trampoline[int, string]] {
-//	        return func() either.Either[error, tailrec.Trampoline[int, string]] {
+//	countdownStep := func(n int) ReaderIOResult[Trampoline[int, string]] {
+//	    return func(ctx context.Context) IOEither[Trampoline[int, string]] {
+//	        return func() Either[Trampoline[int, string]] {
 //	            if n <= 0 {
 //	                return either.Right[error](tailrec.Land[int]("Done!"))
 //	            }
@@ -105,7 +105,7 @@ import (
 //	    }
 //	}
 //
-//	countdown := readerioresult.TailRec(countdownStep)
+//	countdown := TailRec(countdownStep)
 //
 //	// With cancellation
 //	ctx, cancel := context.WithTimeout(t.Context(), 500*time.Millisecond)
@@ -119,9 +119,9 @@ import (
 //	    processed []string
 //	}
 //
-//	processStep := func(state ProcessState) readerioresult.ReaderIOResult[tailrec.Trampoline[ProcessState, []string]] {
-//	    return func(ctx context.Context) ioeither.IOEither[error, tailrec.Trampoline[ProcessState, []string]] {
-//	        return func() either.Either[error, tailrec.Trampoline[ProcessState, []string]] {
+//	processStep := func(state ProcessState) ReaderIOResult[Trampoline[ProcessState, []string]] {
+//	    return func(ctx context.Context) IOEither[Trampoline[ProcessState, []string]] {
+//	        return func() Either[Trampoline[ProcessState, []string]] {
 //	            if len(state.files) == 0 {
 //	                return either.Right[error](tailrec.Land[ProcessState](state.processed))
 //	            }
@@ -140,7 +140,7 @@ import (
 //	    }
 //	}
 //
-//	processFiles := readerioresult.TailRec(processStep)
+//	processFiles := TailRec(processStep)
 //	ctx, cancel := context.WithCancel(t.Context())
 //
 //	// Can be cancelled at any point during processing
@@ -158,7 +158,7 @@ import (
 // still respecting context cancellation:
 //
 //	// Safe for very large inputs with cancellation support
-//	largeCountdown := readerioresult.TailRec(countdownStep)
+//	largeCountdown := TailRec(countdownStep)
 //	ctx := t.Context()
 //	result := largeCountdown(1000000)(ctx)() // Safe, no stack overflow
 //
@@ -171,11 +171,11 @@ import (
 //
 // # See Also
 //
-//   - [readerioresult.TailRec]: Base tail recursion without automatic context checking
-//   - [WithContext]: Context cancellation wrapper used internally
-//   - [Chain]: For sequencing ReaderIOResult computations
-//   - [Ask]: For accessing the context
-//   - [Left]/[Right]: For creating error/success values
+//   - readerioresult.TailRec: Base tail recursion without automatic context checking
+//   - WithContext: Context cancellation wrapper used internally
+//   - Chain: For sequencing ReaderIOResult computations
+//   - Ask: For accessing the context
+//   - Left/Right: For creating error/success values
 //
 //go:inline
 func TailRec[A, B any](f Kleisli[A, Trampoline[A, B]]) Kleisli[A, B] {

v2/context/readerioresult/rec_test.go

@@ -30,6 +30,16 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+// CustomError is a test error type
+type CustomError struct {
+	Code    int
+	Message string
+}
+
+func (e *CustomError) Error() string {
+	return fmt.Sprintf("error %d: %s", e.Code, e.Message)
+}
+
 func TestTailRec_BasicRecursion(t *testing.T) {
 	// Test basic countdown recursion
 	countdownStep := func(n int) ReaderIOResult[Trampoline[int, string]] {
@@ -432,3 +442,237 @@ func TestTailRec_ContextWithValue(t *testing.T) {
 
 	assert.Equal(t, E.Of[error]("Done!"), result)
 }
+
+func TestTailRec_MultipleErrorTypes(t *testing.T) {
+	// Test that different error types are properly handled
+	errorStep := func(n int) ReaderIOResult[Trampoline[int, string]] {
+		return func(ctx context.Context) IOEither[Trampoline[int, string]] {
+			return func() Either[Trampoline[int, string]] {
+				if n == 5 {
+					customErr := &CustomError{Code: 500, Message: "custom error"}
+					return E.Left[Trampoline[int, string]](error(customErr))
+				}
+				if n <= 0 {
+					return E.Right[error](tailrec.Land[int]("Done!"))
+				}
+				return E.Right[error](tailrec.Bounce[string](n - 1))
+			}
+		}
+	}
+
+	errorRecursion := TailRec(errorStep)
+	result := errorRecursion(10)(t.Context())()
+
+	assert.True(t, E.IsLeft(result))
+	err := E.ToError(result)
+	customErr, ok := err.(*CustomError)
+	require.True(t, ok, "Expected CustomError type")
+	assert.Equal(t, 500, customErr.Code)
+	assert.Equal(t, "custom error", customErr.Message)
+}
+
+func TestTailRec_ContextCancelDuringBounce(t *testing.T) {
+	// Test cancellation happens between bounces, not during computation
+	var iterationCount int32
+	ctx, cancel := context.WithCancel(t.Context())
+
+	slowStep := func(n int) ReaderIOResult[Trampoline[int, string]] {
+		return func(ctx context.Context) IOEither[Trampoline[int, string]] {
+			return func() Either[Trampoline[int, string]] {
+				count := atomic.AddInt32(&iterationCount, 1)
+
+				// Cancel after 3 iterations
+				if count == 3 {
+					cancel()
+				}
+
+				if n <= 0 {
+					return E.Right[error](tailrec.Land[int]("Done!"))
+				}
+				return E.Right[error](tailrec.Bounce[string](n - 1))
+			}
+		}
+	}
+
+	slowRecursion := TailRec(slowStep)
+	result := slowRecursion(10)(ctx)()
+
+	// Should be cancelled after a few iterations
+	assert.True(t, E.IsLeft(result))
+	iterations := atomic.LoadInt32(&iterationCount)
+	assert.Greater(t, iterations, int32(2))
+	assert.Less(t, iterations, int32(10))
+}
+
+func TestTailRec_EmptyState(t *testing.T) {
+	// Test with empty/zero-value state
+	type EmptyState struct{}
+
+	emptyStep := func(state EmptyState) ReaderIOResult[Trampoline[EmptyState, int]] {
+		return func(ctx context.Context) IOEither[Trampoline[EmptyState, int]] {
+			return func() Either[Trampoline[EmptyState, int]] {
+				return E.Right[error](tailrec.Land[EmptyState](42))
+			}
+		}
+	}
+
+	emptyRecursion := TailRec(emptyStep)
+	result := emptyRecursion(EmptyState{})(t.Context())()
+
+	assert.Equal(t, E.Of[error](42), result)
+}
+
+func TestTailRec_PointerState(t *testing.T) {
+	// Test with pointer state to ensure proper handling
+	type Node struct {
+		Value int
+		Next  *Node
+	}
+
+	// Create a linked list: 1 -> 2 -> 3 -> nil
+	list := &Node{Value: 1, Next: &Node{Value: 2, Next: &Node{Value: 3, Next: nil}}}
+
+	sumStep := func(node *Node) ReaderIOResult[Trampoline[*Node, int]] {
+		return func(ctx context.Context) IOEither[Trampoline[*Node, int]] {
+			return func() Either[Trampoline[*Node, int]] {
+				if node == nil {
+					return E.Right[error](tailrec.Land[*Node](0))
+				}
+				if node.Next == nil {
+					return E.Right[error](tailrec.Land[*Node](node.Value))
+				}
+				// Accumulate value and continue
+				node.Next.Value += node.Value
+				return E.Right[error](tailrec.Bounce[int](node.Next))
+			}
+		}
+	}
+
+	sumList := TailRec(sumStep)
+	result := sumList(list)(t.Context())()
+
+	assert.Equal(t, E.Of[error](6), result) // 1 + 2 + 3 = 6
+}
+
+func TestTailRec_ConcurrentCancellation(t *testing.T) {
+	// Test that cancellation works correctly with concurrent operations
+	var iterationCount int32
+	ctx, cancel := context.WithCancel(t.Context())
+
+	slowStep := func(n int) ReaderIOResult[Trampoline[int, string]] {
+		return func(ctx context.Context) IOEither[Trampoline[int, string]] {
+			return func() Either[Trampoline[int, string]] {
+				atomic.AddInt32(&iterationCount, 1)
+				time.Sleep(10 * time.Millisecond)
+
+				if n <= 0 {
+					return E.Right[error](tailrec.Land[int]("Done!"))
+				}
+				return E.Right[error](tailrec.Bounce[string](n - 1))
+			}
+		}
+	}
+
+	slowRecursion := TailRec(slowStep)
+
+	// Cancel from another goroutine after 50ms
+	go func() {
+		time.Sleep(50 * time.Millisecond)
+		cancel()
+	}()
+
+	start := time.Now()
+	result := slowRecursion(20)(ctx)()
+	elapsed := time.Since(start)
+
+	// Should be cancelled
+	assert.True(t, E.IsLeft(result))
+
+	// Should complete quickly due to cancellation
+	assert.Less(t, elapsed, 100*time.Millisecond)
+
+	// Should have executed some but not all iterations
+	iterations := atomic.LoadInt32(&iterationCount)
+	assert.Greater(t, iterations, int32(0))
+	assert.Less(t, iterations, int32(20))
+}
+
+func TestTailRec_NestedContextValues(t *testing.T) {
+	// Test that nested context values are preserved
+	type contextKey string
+	const (
+		key1 contextKey = "key1"
+		key2 contextKey = "key2"
+	)
+
+	nestedStep := func(n int) ReaderIOResult[Trampoline[int, string]] {
+		return func(ctx context.Context) IOEither[Trampoline[int, string]] {
+			return func() Either[Trampoline[int, string]] {
+				val1 := ctx.Value(key1)
+				val2 := ctx.Value(key2)
+
+				require.NotNil(t, val1)
+				require.NotNil(t, val2)
+				assert.Equal(t, "value1", val1.(string))
+				assert.Equal(t, "value2", val2.(string))
+
+				if n <= 0 {
+					return E.Right[error](tailrec.Land[int]("Done!"))
+				}
+				return E.Right[error](tailrec.Bounce[string](n - 1))
+			}
+		}
+	}
+
+	nestedRecursion := TailRec(nestedStep)
+
+	ctx := context.WithValue(t.Context(), key1, "value1")
+	ctx = context.WithValue(ctx, key2, "value2")
+
+	result := nestedRecursion(3)(ctx)()
+
+	assert.Equal(t, E.Of[error]("Done!"), result)
+}
+
+func BenchmarkTailRec_SimpleCountdown(b *testing.B) {
+	countdownStep := func(n int) ReaderIOResult[Trampoline[int, int]] {
+		return func(ctx context.Context) IOEither[Trampoline[int, int]] {
+			return func() Either[Trampoline[int, int]] {
+				if n <= 0 {
+					return E.Right[error](tailrec.Land[int](0))
+				}
+				return E.Right[error](tailrec.Bounce[int](n - 1))
+			}
+		}
+	}
+
+	countdown := TailRec(countdownStep)
+	ctx := context.Background()
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_ = countdown(1000)(ctx)()
+	}
+}
+
+func BenchmarkTailRec_WithCancellation(b *testing.B) {
+	countdownStep := func(n int) ReaderIOResult[Trampoline[int, int]] {
+		return func(ctx context.Context) IOEither[Trampoline[int, int]] {
+			return func() Either[Trampoline[int, int]] {
+				if n <= 0 {
+					return E.Right[error](tailrec.Land[int](0))
+				}
+				return E.Right[error](tailrec.Bounce[int](n - 1))
+			}
+		}
+	}
+
+	countdown := TailRec(countdownStep)
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_ = countdown(1000)(ctx)()
+	}
+}

v2/context/readerioresult/type.go

@@ -17,6 +17,7 @@ package readerioresult
 
 import (
 	"context"
+	"iter"
 
 	"github.com/IBM/fp-go/v2/consumer"
 	"github.com/IBM/fp-go/v2/context/ioresult"
@@ -55,6 +56,10 @@ type (
 	// Either[A] is equivalent to Either[error, A] from the either package.
 	Either[A any] = either.Either[error, A]
 
+	// Result represents a computation that can either succeed with a value of type A
+	// or fail with an error. This is an alias for result.Result[A].
+	//
+	// Result[A] is equivalent to Either[error, A]
 	Result[A any] = result.Result[A]
 
 	// Lazy represents a deferred computation that produces a value of type A when executed.
@@ -73,6 +78,10 @@ type (
 	// IOEither[A] is equivalent to func() Either[error, A]
 	IOEither[A any] = ioeither.IOEither[error, A]
 
+	// IOResult represents a side-effectful computation that can fail with an error.
+	// This combines IO (side effects) with Result (error handling).
+	//
+	// IOResult[A] is equivalent to func() Result[A]
 	IOResult[A any] = ioresult.IOResult[A]
 
 	// Reader represents a computation that depends on a context of type R.
@@ -118,6 +127,13 @@ type (
 	//   result := fetchUser("123")(ctx)()
 	ReaderIOResult[A any] = RIOR.ReaderIOResult[context.Context, A]
 
+	// Kleisli represents a Kleisli arrow for ReaderIOResult.
+	// It is a function that takes a value of type A and returns a ReaderIOResult[B].
+	//
+	// Kleisli arrows are used for monadic composition, allowing you to chain operations
+	// that produce ReaderIOResults. They are particularly useful with Chain operations.
+	//
+	// Kleisli[A, B] is equivalent to func(A) ReaderIOResult[B]
 	Kleisli[A, B any] = reader.Reader[A, ReaderIOResult[B]]
 
 	// Operator represents a transformation from one ReaderIOResult to another.
@@ -133,26 +149,82 @@ type (
 	//   result := toUpper(computation)
 	Operator[A, B any] = Kleisli[ReaderIOResult[A], B]
 
-	ReaderResult[A any]       = readerresult.ReaderResult[A]
-	ReaderEither[R, E, A any] = readereither.ReaderEither[R, E, A]
-	ReaderOption[R, A any]    = readeroption.ReaderOption[R, A]
+	// ReaderResult represents a context-dependent computation that can fail.
+	// This is specialized to use context.Context as the context type.
+	//
+	// ReaderResult[A] is equivalent to func(context.Context) Result[A]
+	ReaderResult[A any] = readerresult.ReaderResult[A]
 
+	// ReaderEither represents a context-dependent computation that can fail.
+	// It takes a context of type R and produces an Either[E, A].
+	//
+	// ReaderEither[R, E, A] is equivalent to func(R) Either[E, A]
+	ReaderEither[R, E, A any] = readereither.ReaderEither[R, E, A]
+
+	// ReaderOption represents a context-dependent computation that may not produce a value.
+	// It takes a context of type R and produces an Option[A].
+	//
+	// ReaderOption[R, A] is equivalent to func(R) Option[A]
+	ReaderOption[R, A any] = readeroption.ReaderOption[R, A]
+
+	// Endomorphism represents a function from a type to itself.
+	// It is used for transformations that preserve the type.
+	//
+	// Endomorphism[A] is equivalent to func(A) A
 	Endomorphism[A any] = endomorphism.Endomorphism[A]
 
+	// Consumer represents a function that consumes a value without producing a result.
+	// It is used for side effects like logging or updating state.
+	//
+	// Consumer[A] is equivalent to func(A)
 	Consumer[A any] = consumer.Consumer[A]
 
+	// Prism represents an optic for working with sum types (tagged unions).
+	// It provides a way to focus on a specific variant of a sum type.
 	Prism[S, T any] = prism.Prism[S, T]
-	Lens[S, T any]  = lens.Lens[S, T]
 
+	// Lens represents an optic for working with product types (records/structs).
+	// It provides a way to focus on a specific field of a product type.
+	Lens[S, T any] = lens.Lens[S, T]
+
+	// Trampoline represents a computation that can be executed in a stack-safe manner.
+	// It is used for tail-recursive computations that would otherwise overflow the stack.
 	Trampoline[B, L any] = tailrec.Trampoline[B, L]
 
+	// Predicate represents a function that tests a value of type A.
+	// It returns true if the value satisfies the predicate, false otherwise.
+	//
+	// Predicate[A] is equivalent to func(A) bool
 	Predicate[A any] = predicate.Predicate[A]
 
+	// Pair represents a tuple of two values of types A and B.
+	// It is used to group two related values together.
 	Pair[A, B any] = pair.Pair[A, B]
 
+	// IORef represents a mutable reference that can be safely accessed in IO computations.
+	// It provides thread-safe read and write operations.
 	IORef[A any] = ioref.IORef[A]
 
+	// State represents a stateful computation that transforms a state of type S
+	// and produces a value of type A.
+	//
+	// State[S, A] is equivalent to func(S) Pair[A, S]
 	State[S, A any] = state.State[S, A]
 
+	// Void represents the absence of a value, similar to unit type in other languages.
+	// It is used when a function performs side effects but doesn't return a meaningful value.
 	Void = function.Void
+
+	// ContextCancel represents a pair of a cancel function and a context.
+	// It is used in operations that create new contexts with cancellation capabilities.
+	//
+	// The first element is the CancelFunc that should be called to release resources.
+	// The second element is the new Context that was created.
+	ContextCancel = Pair[context.CancelFunc, context.Context]
+
+	// Seq is an iterator over sequences of individual values.
+	// When called as seq(yield), seq calls yield(v) for each value v in the sequence,
+	// stopping early if yield returns false.
+	// See the [iter] package documentation for more details.
+	Seq[A any] = iter.Seq[A]
 )

v2/context/readerreaderioresult/eitherize.go

@@ -0,0 +1,213 @@
+// 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 readerreaderioresult
+
+import (
+	"context"
+
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/ioresult"
+)
+
+// Eitherize converts a function that returns a value and error into a ReaderReaderIOResult.
+//
+// This function takes a function that accepts an outer context R and context.Context,
+// returning a value T and an error, and converts it into a ReaderReaderIOResult[R, T].
+// The error is automatically converted into the Left case of the Result, while successful
+// values become the Right case.
+//
+// This is particularly useful for integrating standard Go error-handling patterns into
+// the functional programming style of ReaderReaderIOResult. It is especially helpful
+// for adapting interface member functions that accept a context. When you have an
+// interface method with signature (receiver, context.Context) (T, error), you can
+// use Eitherize to convert it into a ReaderReaderIOResult where the receiver becomes
+// the outer reader context R.
+//
+// # Type Parameters
+//
+//   - R: The outer reader context type (e.g., application configuration)
+//   - T: The success value type
+//
+// # Parameters
+//
+//   - f: A function that takes R and context.Context and returns (T, error)
+//
+// # Returns
+//
+//   - ReaderReaderIOResult[R, T]: A computation that depends on R and context.Context,
+//     performs IO, and produces a Result[T]
+//
+// # Example Usage
+//
+//	type AppConfig struct {
+//	    DatabaseURL string
+//	}
+//
+//	// A function using standard Go error handling
+//	func fetchUser(cfg AppConfig, ctx context.Context) (*User, error) {
+//	    // Implementation that may return an error
+//	    return &User{ID: 1, Name: "Alice"}, nil
+//	}
+//
+//	// Convert to ReaderReaderIOResult
+//	fetchUserRR := Eitherize(fetchUser)
+//
+//	// Use in functional composition
+//	result := F.Pipe1(
+//	    fetchUserRR,
+//	    Map[AppConfig](func(u *User) string { return u.Name }),
+//	)
+//
+//	// Execute with config and context
+//	cfg := AppConfig{DatabaseURL: "postgres://localhost"}
+//	outcome := result(cfg)(context.Background())()
+//
+// # Adapting Interface Methods
+//
+// Eitherize is particularly useful for adapting interface member functions:
+//
+//	type UserRepository interface {
+//	    GetUser(ctx context.Context, id int) (*User, error)
+//	}
+//
+//	type UserRepo struct {
+//	    db *sql.DB
+//	}
+//
+//	func (r *UserRepo) GetUser(ctx context.Context, id int) (*User, error) {
+//	    // Implementation
+//	    return &User{ID: id}, nil
+//	}
+//
+//	// Adapt the method by binding the first parameter (receiver)
+//	repo := &UserRepo{db: db}
+//	getUserRR := Eitherize(func(id int, ctx context.Context) (*User, error) {
+//	    return repo.GetUser(ctx, id)
+//	})
+//
+//	// Now getUserRR has type: ReaderReaderIOResult[int, *User]
+//	// The receiver (repo) is captured in the closure
+//	// The id becomes the outer reader context R
+//
+// # See Also
+//
+//   - Eitherize1: For functions that take an additional parameter
+//   - ioresult.Eitherize2: The underlying conversion function
+func Eitherize[R, T any](f func(R, context.Context) (T, error)) ReaderReaderIOResult[R, T] {
+	return F.Pipe1(
+		ioresult.Eitherize2(f),
+		F.Curry2,
+	)
+}
+
+// Eitherize1 converts a function that takes an additional parameter and returns a value
+// and error into a Kleisli arrow.
+//
+// This function takes a function that accepts an outer context R, context.Context, and
+// an additional parameter A, returning a value T and an error, and converts it into a
+// Kleisli arrow (A -> ReaderReaderIOResult[R, T]). The error is automatically converted
+// into the Left case of the Result, while successful values become the Right case.
+//
+// This is useful for creating composable operations that depend on both contexts and
+// an input value, following standard Go error-handling patterns. It is especially helpful
+// for adapting interface member functions that accept a context and additional parameters.
+// When you have an interface method with signature (receiver, context.Context, A) (T, error),
+// you can use Eitherize1 to convert it into a Kleisli arrow where the receiver becomes
+// the outer reader context R and A becomes the input parameter.
+//
+// # Type Parameters
+//
+//   - R: The outer reader context type (e.g., application configuration)
+//   - A: The input parameter type
+//   - T: The success value type
+//
+// # Parameters
+//
+//   - f: A function that takes R, context.Context, and A, returning (T, error)
+//
+// # Returns
+//
+//   - Kleisli[R, A, T]: A function from A to ReaderReaderIOResult[R, T]
+//
+// # Example Usage
+//
+//	type AppConfig struct {
+//	    DatabaseURL string
+//	}
+//
+//	// A function using standard Go error handling
+//	func fetchUserByID(cfg AppConfig, ctx context.Context, id int) (*User, error) {
+//	    // Implementation that may return an error
+//	    return &User{ID: id, Name: "Alice"}, nil
+//	}
+//
+//	// Convert to Kleisli arrow
+//	fetchUserKleisli := Eitherize1(fetchUserByID)
+//
+//	// Use in functional composition with Chain
+//	pipeline := F.Pipe1(
+//	    Of[AppConfig](123),
+//	    Chain[AppConfig](fetchUserKleisli),
+//	)
+//
+//	// Execute with config and context
+//	cfg := AppConfig{DatabaseURL: "postgres://localhost"}
+//	outcome := pipeline(cfg)(context.Background())()
+//
+// # Adapting Interface Methods
+//
+// Eitherize1 is particularly useful for adapting interface member functions with parameters:
+//
+//	type UserRepository interface {
+//	    GetUserByID(ctx context.Context, id int) (*User, error)
+//	    UpdateUser(ctx context.Context, user *User) error
+//	}
+//
+//	type UserRepo struct {
+//	    db *sql.DB
+//	}
+//
+//	func (r *UserRepo) GetUserByID(ctx context.Context, id int) (*User, error) {
+//	    // Implementation
+//	    return &User{ID: id}, nil
+//	}
+//
+//	// Adapt the method - receiver becomes R, id becomes A
+//	repo := &UserRepo{db: db}
+//	getUserKleisli := Eitherize1(func(r *UserRepo, ctx context.Context, id int) (*User, error) {
+//	    return r.GetUserByID(ctx, id)
+//	})
+//
+//	// Now getUserKleisli has type: Kleisli[*UserRepo, int, *User]
+//	// Which is: func(int) ReaderReaderIOResult[*UserRepo, *User]
+//	// Use it in composition:
+//	pipeline := F.Pipe1(
+//	    Of[*UserRepo](123),
+//	    Chain[*UserRepo](getUserKleisli),
+//	)
+//	result := pipeline(repo)(context.Background())()
+//
+// # See Also
+//
+//   - Eitherize: For functions without an additional parameter
+//   - Chain: For composing Kleisli arrows
+//   - ioresult.Eitherize3: The underlying conversion function
+func Eitherize1[R, A, T any](f func(R, context.Context, A) (T, error)) Kleisli[R, A, T] {
+	return F.Flow2(
+		F.Bind3of3(ioresult.Eitherize3(f)),
+		F.Curry2,
+	)
+}

v2/context/readerreaderioresult/eitherize_test.go

@@ -0,0 +1,507 @@
+// 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 readerreaderioresult
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"strconv"
+	"testing"
+
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/result"
+	"github.com/stretchr/testify/assert"
+)
+
+type TestConfig struct {
+	Prefix string
+	MaxLen int
+}
+
+var testConfig = TestConfig{
+	Prefix: "test",
+	MaxLen: 100,
+}
+
+// TestEitherize_Success tests successful conversion with Eitherize
+func TestEitherize_Success(t *testing.T) {
+	t.Run("converts successful function to ReaderReaderIOResult", func(t *testing.T) {
+		// Arrange
+		successFunc := func(cfg TestConfig, ctx context.Context) (string, error) {
+			return cfg.Prefix + "-success", nil
+		}
+		rr := Eitherize(successFunc)
+
+		// Act
+		outcome := rr(testConfig)(context.Background())()
+
+		// Assert
+		assert.Equal(t, result.Of("test-success"), outcome)
+	})
+
+	t.Run("preserves context values", func(t *testing.T) {
+		// Arrange
+		type ctxKey string
+		key := ctxKey("testKey")
+		expectedValue := "contextValue"
+
+		contextFunc := func(cfg TestConfig, ctx context.Context) (string, error) {
+			value := ctx.Value(key)
+			if value == nil {
+				return "", errors.New("context value not found")
+			}
+			return value.(string), nil
+		}
+		rr := Eitherize(contextFunc)
+
+		ctx := context.WithValue(context.Background(), key, expectedValue)
+
+		// Act
+		outcome := rr(testConfig)(ctx)()
+
+		// Assert
+		assert.Equal(t, result.Of(expectedValue), outcome)
+	})
+
+	t.Run("works with different types", func(t *testing.T) {
+		// Arrange
+		intFunc := func(cfg TestConfig, ctx context.Context) (int, error) {
+			return cfg.MaxLen, nil
+		}
+		rr := Eitherize(intFunc)
+
+		// Act
+		outcome := rr(testConfig)(context.Background())()
+
+		// Assert
+		assert.Equal(t, result.Of(100), outcome)
+	})
+}
+
+// TestEitherize_Failure tests error handling with Eitherize
+func TestEitherize_Failure(t *testing.T) {
+	t.Run("converts error to Left", func(t *testing.T) {
+		// Arrange
+		expectedErr := errors.New("operation failed")
+		failFunc := func(cfg TestConfig, ctx context.Context) (string, error) {
+			return "", expectedErr
+		}
+		rr := Eitherize(failFunc)
+
+		// Act
+		outcome := rr(testConfig)(context.Background())()
+
+		// Assert
+		assert.True(t, result.IsLeft(outcome))
+		assert.Equal(t, result.Left[string](expectedErr), outcome)
+	})
+
+	t.Run("preserves error message", func(t *testing.T) {
+		// Arrange
+		expectedErr := fmt.Errorf("validation error: field is required")
+		failFunc := func(cfg TestConfig, ctx context.Context) (int, error) {
+			return 0, expectedErr
+		}
+		rr := Eitherize(failFunc)
+
+		// Act
+		outcome := rr(testConfig)(context.Background())()
+
+		// Assert
+		assert.True(t, result.IsLeft(outcome))
+		leftValue := result.MonadFold(outcome,
+			F.Identity[error],
+			func(int) error { return nil },
+		)
+		assert.Equal(t, expectedErr, leftValue)
+	})
+}
+
+// TestEitherize_EdgeCases tests edge cases for Eitherize
+func TestEitherize_EdgeCases(t *testing.T) {
+	t.Run("handles nil context", func(t *testing.T) {
+		// Arrange
+		nilCtxFunc := func(cfg TestConfig, ctx context.Context) (string, error) {
+			if ctx == nil {
+				return "nil-context", nil
+			}
+			return "non-nil-context", nil
+		}
+		rr := Eitherize(nilCtxFunc)
+
+		// Act
+		outcome := rr(testConfig)(nil)()
+
+		// Assert
+		assert.Equal(t, result.Of("nil-context"), outcome)
+	})
+
+	t.Run("handles zero value config", func(t *testing.T) {
+		// Arrange
+		zeroFunc := func(cfg TestConfig, ctx context.Context) (string, error) {
+			return cfg.Prefix, nil
+		}
+		rr := Eitherize(zeroFunc)
+
+		// Act
+		outcome := rr(TestConfig{})(context.Background())()
+
+		// Assert
+		assert.Equal(t, result.Of(""), outcome)
+	})
+
+	t.Run("handles pointer types", func(t *testing.T) {
+		// Arrange
+		type User struct {
+			Name string
+		}
+		ptrFunc := func(cfg TestConfig, ctx context.Context) (*User, error) {
+			return &User{Name: "Alice"}, nil
+		}
+		rr := Eitherize(ptrFunc)
+
+		// Act
+		outcome := rr(testConfig)(context.Background())()
+
+		// Assert
+		assert.True(t, result.IsRight(outcome))
+		user := result.MonadFold(outcome,
+			func(error) *User { return nil },
+			F.Identity[*User],
+		)
+		assert.NotNil(t, user)
+		assert.Equal(t, "Alice", user.Name)
+	})
+}
+
+// TestEitherize_Integration tests integration with other operations
+func TestEitherize_Integration(t *testing.T) {
+	t.Run("composes with Map", func(t *testing.T) {
+		// Arrange
+		baseFunc := func(cfg TestConfig, ctx context.Context) (int, error) {
+			return 42, nil
+		}
+		rr := Eitherize(baseFunc)
+
+		// Act
+		pipeline := F.Pipe1(
+			rr,
+			Map[TestConfig](func(n int) string { return strconv.Itoa(n) }),
+		)
+		outcome := pipeline(testConfig)(context.Background())()
+
+		// Assert
+		assert.Equal(t, result.Of("42"), outcome)
+	})
+
+	t.Run("composes with Chain", func(t *testing.T) {
+		// Arrange
+		firstFunc := func(cfg TestConfig, ctx context.Context) (int, error) {
+			return 10, nil
+		}
+		secondFunc := func(n int) ReaderReaderIOResult[TestConfig, string] {
+			return Of[TestConfig](fmt.Sprintf("value: %d", n))
+		}
+
+		// Act
+		pipeline := F.Pipe1(
+			Eitherize(firstFunc),
+			Chain[TestConfig](secondFunc),
+		)
+		outcome := pipeline(testConfig)(context.Background())()
+
+		// Assert
+		assert.Equal(t, result.Of("value: 10"), outcome)
+	})
+}
+
+// TestEitherize1_Success tests successful conversion with Eitherize1
+func TestEitherize1_Success(t *testing.T) {
+	t.Run("converts successful function to Kleisli", func(t *testing.T) {
+		// Arrange
+		addFunc := func(cfg TestConfig, ctx context.Context, n int) (int, error) {
+			return n + cfg.MaxLen, nil
+		}
+		kleisli := Eitherize1(addFunc)
+
+		// Act
+		outcome := kleisli(10)(testConfig)(context.Background())()
+
+		// Assert
+		assert.Equal(t, result.Of(110), outcome)
+	})
+
+	t.Run("works with string input", func(t *testing.T) {
+		// Arrange
+		concatFunc := func(cfg TestConfig, ctx context.Context, s string) (string, error) {
+			return cfg.Prefix + "-" + s, nil
+		}
+		kleisli := Eitherize1(concatFunc)
+
+		// Act
+		outcome := kleisli("input")(testConfig)(context.Background())()
+
+		// Assert
+		assert.Equal(t, result.Of("test-input"), outcome)
+	})
+
+	t.Run("preserves context in Kleisli", func(t *testing.T) {
+		// Arrange
+		type ctxKey string
+		key := ctxKey("multiplier")
+
+		multiplyFunc := func(cfg TestConfig, ctx context.Context, n int) (int, error) {
+			multiplier := ctx.Value(key)
+			if multiplier == nil {
+				return n, nil
+			}
+			return n * multiplier.(int), nil
+		}
+		kleisli := Eitherize1(multiplyFunc)
+
+		ctx := context.WithValue(context.Background(), key, 3)
+
+		// Act
+		outcome := kleisli(5)(testConfig)(ctx)()
+
+		// Assert
+		assert.Equal(t, result.Of(15), outcome)
+	})
+}
+
+// TestEitherize1_Failure tests error handling with Eitherize1
+func TestEitherize1_Failure(t *testing.T) {
+	t.Run("converts error to Left in Kleisli", func(t *testing.T) {
+		// Arrange
+		expectedErr := errors.New("division by zero")
+		divideFunc := func(cfg TestConfig, ctx context.Context, n int) (int, error) {
+			if n == 0 {
+				return 0, expectedErr
+			}
+			return 100 / n, nil
+		}
+		kleisli := Eitherize1(divideFunc)
+
+		// Act
+		outcome := kleisli(0)(testConfig)(context.Background())()
+
+		// Assert
+		assert.True(t, result.IsLeft(outcome))
+		assert.Equal(t, result.Left[int](expectedErr), outcome)
+	})
+
+	t.Run("preserves error context", func(t *testing.T) {
+		// Arrange
+		validateFunc := func(cfg TestConfig, ctx context.Context, s string) (string, error) {
+			if len(s) > cfg.MaxLen {
+				return "", fmt.Errorf("string too long: %d > %d", len(s), cfg.MaxLen)
+			}
+			return s, nil
+		}
+		kleisli := Eitherize1(validateFunc)
+
+		longString := string(make([]byte, 200))
+
+		// Act
+		outcome := kleisli(longString)(testConfig)(context.Background())()
+
+		// Assert
+		assert.True(t, result.IsLeft(outcome))
+		leftValue := result.MonadFold(outcome,
+			F.Identity[error],
+			func(string) error { return nil },
+		)
+		assert.Contains(t, leftValue.Error(), "string too long")
+	})
+}
+
+// TestEitherize1_EdgeCases tests edge cases for Eitherize1
+func TestEitherize1_EdgeCases(t *testing.T) {
+	t.Run("handles zero value input", func(t *testing.T) {
+		// Arrange
+		zeroFunc := func(cfg TestConfig, ctx context.Context, n int) (int, error) {
+			return n, nil
+		}
+		kleisli := Eitherize1(zeroFunc)
+
+		// Act
+		outcome := kleisli(0)(testConfig)(context.Background())()
+
+		// Assert
+		assert.Equal(t, result.Of(0), outcome)
+	})
+
+	t.Run("handles pointer input", func(t *testing.T) {
+		// Arrange
+		type Input struct {
+			Value int
+		}
+		ptrFunc := func(cfg TestConfig, ctx context.Context, in *Input) (int, error) {
+			if in == nil {
+				return 0, errors.New("nil input")
+			}
+			return in.Value, nil
+		}
+		kleisli := Eitherize1(ptrFunc)
+
+		// Act
+		outcome := kleisli(&Input{Value: 42})(testConfig)(context.Background())()
+
+		// Assert
+		assert.Equal(t, result.Of(42), outcome)
+	})
+
+	t.Run("handles nil pointer input", func(t *testing.T) {
+		// Arrange
+		type Input struct {
+			Value int
+		}
+		ptrFunc := func(cfg TestConfig, ctx context.Context, in *Input) (int, error) {
+			if in == nil {
+				return 0, errors.New("nil input")
+			}
+			return in.Value, nil
+		}
+		kleisli := Eitherize1(ptrFunc)
+
+		// Act
+		outcome := kleisli((*Input)(nil))(testConfig)(context.Background())()
+
+		// Assert
+		assert.True(t, result.IsLeft(outcome))
+	})
+}
+
+// TestEitherize1_Integration tests integration with other operations
+func TestEitherize1_Integration(t *testing.T) {
+	t.Run("composes with Chain", func(t *testing.T) {
+		// Arrange
+		parseFunc := func(cfg TestConfig, ctx context.Context, s string) (int, error) {
+			return strconv.Atoi(s)
+		}
+		doubleFunc := func(n int) ReaderReaderIOResult[TestConfig, int] {
+			return Of[TestConfig](n * 2)
+		}
+
+		parseKleisli := Eitherize1(parseFunc)
+
+		// Act
+		pipeline := F.Pipe2(
+			Of[TestConfig]("42"),
+			Chain[TestConfig](parseKleisli),
+			Chain[TestConfig](doubleFunc),
+		)
+		outcome := pipeline(testConfig)(context.Background())()
+
+		// Assert
+		assert.Equal(t, result.Of(84), outcome)
+	})
+
+	t.Run("handles error in chain", func(t *testing.T) {
+		// Arrange
+		parseFunc := func(cfg TestConfig, ctx context.Context, s string) (int, error) {
+			return strconv.Atoi(s)
+		}
+		parseKleisli := Eitherize1(parseFunc)
+
+		// Act
+		pipeline := F.Pipe1(
+			Of[TestConfig]("not-a-number"),
+			Chain(parseKleisli),
+		)
+		outcome := pipeline(testConfig)(context.Background())()
+
+		// Assert
+		assert.True(t, result.IsLeft(outcome))
+	})
+
+	t.Run("composes multiple Kleisli arrows", func(t *testing.T) {
+		// Arrange
+		parseFunc := func(cfg TestConfig, ctx context.Context, s string) (int, error) {
+			return strconv.Atoi(s)
+		}
+		formatFunc := func(cfg TestConfig, ctx context.Context, n int) (string, error) {
+			return fmt.Sprintf("%s-%d", cfg.Prefix, n), nil
+		}
+
+		parseKleisli := Eitherize1(parseFunc)
+		formatKleisli := Eitherize1(formatFunc)
+
+		// Act
+		pipeline := F.Pipe2(
+			Of[TestConfig]("123"),
+			Chain[TestConfig](parseKleisli),
+			Chain[TestConfig](formatKleisli),
+		)
+		outcome := pipeline(testConfig)(context.Background())()
+
+		// Assert
+		assert.Equal(t, result.Of("test-123"), outcome)
+	})
+}
+
+// TestEitherize_TypeSafety tests type safety across different scenarios
+func TestEitherize_TypeSafety(t *testing.T) {
+	t.Run("Eitherize with complex types", func(t *testing.T) {
+		// Arrange
+		type ComplexResult struct {
+			Data  map[string]int
+			Count int
+		}
+
+		complexFunc := func(cfg TestConfig, ctx context.Context) (ComplexResult, error) {
+			return ComplexResult{
+				Data:  map[string]int{"key": 42},
+				Count: 1,
+			}, nil
+		}
+		rr := Eitherize(complexFunc)
+
+		// Act
+		outcome := rr(testConfig)(context.Background())()
+
+		// Assert
+		assert.True(t, result.IsRight(outcome))
+		value := result.MonadFold(outcome,
+			func(error) ComplexResult { return ComplexResult{} },
+			F.Identity[ComplexResult],
+		)
+		assert.Equal(t, 42, value.Data["key"])
+		assert.Equal(t, 1, value.Count)
+	})
+
+	t.Run("Eitherize1 with different input and output types", func(t *testing.T) {
+		// Arrange
+		type Input struct {
+			ID int
+		}
+		type Output struct {
+			Name string
+		}
+
+		convertFunc := func(cfg TestConfig, ctx context.Context, in Input) (Output, error) {
+			return Output{Name: fmt.Sprintf("%s-%d", cfg.Prefix, in.ID)}, nil
+		}
+		kleisli := Eitherize1(convertFunc)
+
+		// Act
+		outcome := kleisli(Input{ID: 99})(testConfig)(context.Background())()
+
+		// Assert
+		assert.Equal(t, result.Of(Output{Name: "test-99"}), outcome)
+	})
+}

v2/context/readerreaderioresult/filter.go

@@ -0,0 +1,48 @@
+package readerreaderioresult
+
+import (
+	"github.com/IBM/fp-go/v2/array"
+	"github.com/IBM/fp-go/v2/internal/witherable"
+	"github.com/IBM/fp-go/v2/iterator/iter"
+	"github.com/IBM/fp-go/v2/option"
+)
+
+//go:inline
+func Filter[C, HKTA, A any](
+	filter func(Predicate[A]) Endomorphism[HKTA],
+) func(Predicate[A]) Operator[C, HKTA, HKTA] {
+	return witherable.Filter(
+		Map[C],
+		filter,
+	)
+}
+
+//go:inline
+func FilterArray[C, A any](p Predicate[A]) Operator[C, []A, []A] {
+	return Filter[C](array.Filter[A])(p)
+}
+
+//go:inline
+func FilterIter[C, A any](p Predicate[A]) Operator[C, Seq[A], Seq[A]] {
+	return Filter[C](iter.Filter[A])(p)
+}
+
+//go:inline
+func FilterMap[C, HKTA, HKTB, A, B any](
+	filter func(option.Kleisli[A, B]) Reader[HKTA, HKTB],
+) func(option.Kleisli[A, B]) Operator[C, HKTA, HKTB] {
+	return witherable.FilterMap(
+		Map[C],
+		filter,
+	)
+}
+
+//go:inline
+func FilterMapArray[C, A, B any](p option.Kleisli[A, B]) Operator[C, []A, []B] {
+	return FilterMap[C](array.FilterMap[A, B])(p)
+}
+
+//go:inline
+func FilterMapIter[C, A, B any](p option.Kleisli[A, B]) Operator[C, Seq[A], Seq[B]] {
+	return FilterMap[C](iter.FilterMap[A, B])(p)
+}

v2/context/readerreaderioresult/promap.go

@@ -3,6 +3,7 @@ package readerreaderioresult
 import (
 	"context"
 
+	"github.com/IBM/fp-go/v2/context/reader"
 	"github.com/IBM/fp-go/v2/context/readerioresult"
 	"github.com/IBM/fp-go/v2/io"
 	"github.com/IBM/fp-go/v2/ioresult"
@@ -13,6 +14,17 @@ import (
 // Local modifies the outer environment before passing it to a computation.
 // Useful for providing different configurations to sub-computations.
 //
+// Type Parameters:
+//   - A: The success type produced by the ReaderReaderIOResult
+//   - R1: The original outer environment type expected by the ReaderReaderIOResult
+//   - R2: The new input outer environment type
+//
+// Parameters:
+//   - f: A function that transforms R2 to R1
+//
+// Returns:
+//   - A function that takes a ReaderReaderIOResult[R1, A] and returns a ReaderReaderIOResult[R2, A]
+//
 //go:inline
 func Local[A, R1, R2 any](f func(R2) R1) func(ReaderReaderIOResult[R1, A]) ReaderReaderIOResult[R2, A] {
 	return RRIOE.Local[context.Context, error, A](f)
@@ -102,6 +114,29 @@ func LocalIOResultK[A, R1, R2 any](f ioresult.Kleisli[R2, R1]) func(ReaderReader
 	return RRIOE.LocalIOEitherK[context.Context, A](f)
 }
 
+// LocalResultK transforms the outer environment of a ReaderReaderIOResult using a Result-based Kleisli arrow.
+// It allows you to modify the outer environment through a pure computation that can fail before
+// passing it to the ReaderReaderIOResult.
+//
+// This is useful when the outer environment transformation is a pure computation that can fail,
+// such as parsing, validation, or data transformation that doesn't require IO effects.
+//
+// The transformation happens in two stages:
+//  1. The Result function f is executed with the R2 environment to produce Result[R1]
+//  2. If successful (Ok), the R1 value is passed as the outer environment to the ReaderReaderIOResult[R1, A]
+//  3. If failed (Err), the error is propagated without executing the ReaderReaderIOResult
+//
+// Type Parameters:
+//   - A: The success type produced by the ReaderReaderIOResult
+//   - R1: The original outer environment type expected by the ReaderReaderIOResult
+//   - R2: The new input outer environment type
+//
+// Parameters:
+//   - f: A Result Kleisli arrow that transforms R2 to R1 with pure computation that can fail
+//
+// Returns:
+//   - A function that takes a ReaderReaderIOResult[R1, A] and returns a ReaderReaderIOResult[R2, A]
+//
 //go:inline
 func LocalResultK[A, R1, R2 any](f result.Kleisli[R2, R1]) func(ReaderReaderIOResult[R1, A]) ReaderReaderIOResult[R2, A] {
 	return RRIOE.LocalEitherK[context.Context, A](f)
@@ -162,6 +197,90 @@ func LocalReaderIOResultK[A, R1, R2 any](f readerioresult.Kleisli[R2, R1]) func(
 	return RRIOE.LocalReaderIOEitherK[A](f)
 }
 
+// LocalReaderK transforms the outer environment of a ReaderReaderIOResult using a Reader-based Kleisli arrow.
+// It allows you to modify the outer environment through a pure computation that depends on the inner context
+// before passing it to the ReaderReaderIOResult.
+//
+// This is useful when the outer environment transformation is a pure computation that requires access
+// to the inner context (e.g., context.Context) but cannot fail. Common use cases include:
+//   - Extracting configuration from context values
+//   - Computing derived environment values based on context
+//   - Transforming environment based on context metadata
+//
+// The transformation happens in two stages:
+//  1. The Reader function f is executed with the R2 outer environment and inner context to produce an R1 value
+//  2. The resulting R1 value is passed as the outer environment to the ReaderReaderIOResult[R1, A]
+//
+// Type Parameters:
+//   - A: The success type produced by the ReaderReaderIOResult
+//   - R1: The original outer environment type expected by the ReaderReaderIOResult
+//   - R2: The new input outer environment type
+//
+// Parameters:
+//   - f: A Reader Kleisli arrow that transforms R2 to R1 using the inner context
+//
+// Returns:
+//   - A function that takes a ReaderReaderIOResult[R1, A] and returns a ReaderReaderIOResult[R2, A]
+//
+// Example Usage:
+//
+//	type ctxKey string
+//	const configKey ctxKey = "config"
+//
+//	// Extract config from context and transform environment
+//	extractConfig := func(path string) reader.Reader[DetailedConfig] {
+//	    return func(ctx context.Context) DetailedConfig {
+//	        if cfg, ok := ctx.Value(configKey).(DetailedConfig); ok {
+//	            return cfg
+//	        }
+//	        return DetailedConfig{Host: "localhost", Port: 8080}
+//	    }
+//	}
+//
+//	// Use the config
+//	useConfig := func(cfg DetailedConfig) readerioresult.ReaderIOResult[string] {
+//	    return func(ctx context.Context) ioresult.IOResult[string] {
+//	        return func() result.Result[string] {
+//	            return result.Of(fmt.Sprintf("%s:%d", cfg.Host, cfg.Port))
+//	        }
+//	    }
+//	}
+//
+//	// Compose using LocalReaderK
+//	adapted := LocalReaderK[string](extractConfig)(useConfig)
+//	ctx := context.WithValue(context.Background(), configKey, DetailedConfig{Host: "api.example.com", Port: 443})
+//	result := adapted("config.json")(ctx)() // Result: "api.example.com:443"
+//
+//go:inline
+func LocalReaderK[A, R1, R2 any](f reader.Kleisli[R2, R1]) func(ReaderReaderIOResult[R1, A]) ReaderReaderIOResult[R2, A] {
+	return RRIOE.LocalReaderK[error, A](f)
+}
+
+// LocalReaderReaderIOEitherK transforms the outer environment of a ReaderReaderIOResult using a ReaderReaderIOResult-based Kleisli arrow.
+// It allows you to modify the outer environment through a computation that depends on both the outer environment
+// and the inner context, and can perform IO effects that may fail.
+//
+// This is the most powerful Local variant, useful when the outer environment transformation requires:
+//   - Access to both the outer environment (R2) and inner context (context.Context)
+//   - IO operations that can fail
+//   - Complex transformations that need the full computational context
+//
+// The transformation happens in three stages:
+//  1. The ReaderReaderIOResult effect f is executed with the R2 outer environment and inner context
+//  2. If successful (Ok), the R1 value is passed as the outer environment to the ReaderReaderIOResult[R1, A]
+//  3. If failed (Err), the error is propagated without executing the ReaderReaderIOResult
+//
+// Type Parameters:
+//   - A: The success type produced by the ReaderReaderIOResult
+//   - R1: The original outer environment type expected by the ReaderReaderIOResult
+//   - R2: The new input outer environment type
+//
+// Parameters:
+//   - f: A ReaderReaderIOResult Kleisli arrow that transforms R2 to R1 with full context-aware IO effects that can fail
+//
+// Returns:
+//   - A function that takes a ReaderReaderIOResult[R1, A] and returns a ReaderReaderIOResult[R2, A]
+//
 //go:inline
 func LocalReaderReaderIOEitherK[A, R1, R2 any](f Kleisli[R2, R2, R1]) func(ReaderReaderIOResult[R1, A]) ReaderReaderIOResult[R2, A] {
 	return RRIOE.LocalReaderReaderIOEitherK[A](f)

v2/context/readerreaderioresult/promap_test.go

@@ -21,6 +21,7 @@ import (
 	"fmt"
 	"testing"
 
+	"github.com/IBM/fp-go/v2/context/reader"
 	"github.com/IBM/fp-go/v2/context/readerioresult"
 	"github.com/IBM/fp-go/v2/io"
 	"github.com/IBM/fp-go/v2/ioresult"
@@ -426,3 +427,226 @@ func TestLocalReaderIOResultK(t *testing.T) {
 		assert.True(t, result.IsLeft(resErr))
 	})
 }
+
+// TestLocalReaderK tests LocalReaderK functionality
+func TestLocalReaderK(t *testing.T) {
+	ctx := context.Background()
+
+	t.Run("basic Reader transformation", func(t *testing.T) {
+		// Reader that transforms string path to SimpleConfig using context
+		loadConfig := func(path string) reader.Reader[SimpleConfig] {
+			return func(ctx context.Context) SimpleConfig {
+				// Could extract values from context here
+				return SimpleConfig{Port: 8080}
+			}
+		}
+
+		// ReaderReaderIOResult that uses the config
+		useConfig := func(cfg SimpleConfig) readerioresult.ReaderIOResult[string] {
+			return func(ctx context.Context) ioresult.IOResult[string] {
+				return func() result.Result[string] {
+					return result.Of(fmt.Sprintf("Port: %d", cfg.Port))
+				}
+			}
+		}
+
+		// Compose using LocalReaderK
+		adapted := LocalReaderK[string](loadConfig)(useConfig)
+		res := adapted("config.json")(ctx)()
+
+		assert.Equal(t, result.Of("Port: 8080"), res)
+	})
+
+	t.Run("extract config from context", func(t *testing.T) {
+		type ctxKey string
+		const configKey ctxKey = "config"
+
+		// Reader that extracts config from context
+		extractConfig := func(path string) reader.Reader[DetailedConfig] {
+			return func(ctx context.Context) DetailedConfig {
+				if cfg, ok := ctx.Value(configKey).(DetailedConfig); ok {
+					return cfg
+				}
+				// Default config if not in context
+				return DetailedConfig{Host: "localhost", Port: 8080}
+			}
+		}
+
+		// Use the config
+		useConfig := func(cfg DetailedConfig) readerioresult.ReaderIOResult[string] {
+			return func(ctx context.Context) ioresult.IOResult[string] {
+				return func() result.Result[string] {
+					return result.Of(fmt.Sprintf("%s:%d", cfg.Host, cfg.Port))
+				}
+			}
+		}
+
+		adapted := LocalReaderK[string](extractConfig)(useConfig)
+
+		// With context value
+		ctxWithConfig := context.WithValue(ctx, configKey, DetailedConfig{Host: "api.example.com", Port: 443})
+		res := adapted("ignored")(ctxWithConfig)()
+		assert.Equal(t, result.Of("api.example.com:443"), res)
+
+		// Without context value (uses default)
+		resDefault := adapted("ignored")(ctx)()
+		assert.Equal(t, result.Of("localhost:8080"), resDefault)
+	})
+
+	t.Run("context-aware transformation", func(t *testing.T) {
+		type ctxKey string
+		const multiplierKey ctxKey = "multiplier"
+
+		// Reader that uses context to compute environment
+		computeValue := func(base int) reader.Reader[int] {
+			return func(ctx context.Context) int {
+				if mult, ok := ctx.Value(multiplierKey).(int); ok {
+					return base * mult
+				}
+				return base
+			}
+		}
+
+		// Use the computed value
+		formatValue := func(val int) readerioresult.ReaderIOResult[string] {
+			return func(ctx context.Context) ioresult.IOResult[string] {
+				return func() result.Result[string] {
+					return result.Of(fmt.Sprintf("Value: %d", val))
+				}
+			}
+		}
+
+		adapted := LocalReaderK[string](computeValue)(formatValue)
+
+		// With multiplier in context
+		ctxWithMult := context.WithValue(ctx, multiplierKey, 10)
+		res := adapted(5)(ctxWithMult)()
+		assert.Equal(t, result.Of("Value: 50"), res)
+
+		// Without multiplier (uses base value)
+		resBase := adapted(5)(ctx)()
+		assert.Equal(t, result.Of("Value: 5"), resBase)
+	})
+
+	t.Run("compose multiple LocalReaderK", func(t *testing.T) {
+		type ctxKey string
+		const prefixKey ctxKey = "prefix"
+
+		// First transformation: int -> string using context
+		intToString := func(n int) reader.Reader[string] {
+			return func(ctx context.Context) string {
+				if prefix, ok := ctx.Value(prefixKey).(string); ok {
+					return fmt.Sprintf("%s-%d", prefix, n)
+				}
+				return fmt.Sprintf("%d", n)
+			}
+		}
+
+		// Second transformation: string -> SimpleConfig
+		stringToConfig := func(s string) reader.Reader[SimpleConfig] {
+			return func(ctx context.Context) SimpleConfig {
+				return SimpleConfig{Port: len(s) * 100}
+			}
+		}
+
+		// Use the config
+		formatConfig := func(cfg SimpleConfig) readerioresult.ReaderIOResult[string] {
+			return func(ctx context.Context) ioresult.IOResult[string] {
+				return func() result.Result[string] {
+					return result.Of(fmt.Sprintf("Port: %d", cfg.Port))
+				}
+			}
+		}
+
+		// Compose transformations
+		step1 := LocalReaderK[string](stringToConfig)(formatConfig)
+		step2 := LocalReaderK[string](intToString)(step1)
+
+		// With prefix in context
+		ctxWithPrefix := context.WithValue(ctx, prefixKey, "test")
+		res := step2(42)(ctxWithPrefix)()
+		// "test-42" has length 7, so port = 700
+		assert.Equal(t, result.Of("Port: 700"), res)
+
+		// Without prefix
+		resNoPrefix := step2(42)(ctx)()
+		// "42" has length 2, so port = 200
+		assert.Equal(t, result.Of("Port: 200"), resNoPrefix)
+	})
+
+	t.Run("error propagation in ReaderReaderIOResult", func(t *testing.T) {
+		// Reader transformation (pure, cannot fail)
+		loadConfig := func(path string) reader.Reader[SimpleConfig] {
+			return func(ctx context.Context) SimpleConfig {
+				return SimpleConfig{Port: 8080}
+			}
+		}
+
+		// ReaderReaderIOResult that returns an error
+		failingOperation := func(cfg SimpleConfig) readerioresult.ReaderIOResult[string] {
+			return func(ctx context.Context) ioresult.IOResult[string] {
+				return func() result.Result[string] {
+					return result.Left[string](errors.New("operation failed"))
+				}
+			}
+		}
+
+		adapted := LocalReaderK[string](loadConfig)(failingOperation)
+		res := adapted("config.json")(ctx)()
+
+		// Error from the ReaderReaderIOResult should propagate
+		assert.True(t, result.IsLeft(res))
+	})
+
+	t.Run("real-world: environment selection based on context", func(t *testing.T) {
+		type Environment string
+		const (
+			Dev  Environment = "dev"
+			Prod Environment = "prod"
+		)
+
+		type ctxKey string
+		const envKey ctxKey = "environment"
+
+		type EnvConfig struct {
+			Name string
+		}
+
+		// Reader that selects config based on context environment
+		selectConfig := func(envName EnvConfig) reader.Reader[DetailedConfig] {
+			return func(ctx context.Context) DetailedConfig {
+				env := Dev
+				if e, ok := ctx.Value(envKey).(Environment); ok {
+					env = e
+				}
+
+				switch env {
+				case Prod:
+					return DetailedConfig{Host: "api.production.com", Port: 443}
+				default:
+					return DetailedConfig{Host: "localhost", Port: 8080}
+				}
+			}
+		}
+
+		// Use the selected config
+		useConfig := func(cfg DetailedConfig) readerioresult.ReaderIOResult[string] {
+			return func(ctx context.Context) ioresult.IOResult[string] {
+				return func() result.Result[string] {
+					return result.Of(fmt.Sprintf("Connecting to %s:%d", cfg.Host, cfg.Port))
+				}
+			}
+		}
+
+		adapted := LocalReaderK[string](selectConfig)(useConfig)
+
+		// Production environment
+		ctxProd := context.WithValue(ctx, envKey, Prod)
+		resProd := adapted(EnvConfig{Name: "app"})(ctxProd)()
+		assert.Equal(t, result.Of("Connecting to api.production.com:443"), resProd)
+
+		// Development environment (default)
+		resDev := adapted(EnvConfig{Name: "app"})(ctx)()
+		assert.Equal(t, result.Of("Connecting to localhost:8080"), resDev)
+	})
+}

v2/context/readerreaderioresult/reader.go

@@ -834,7 +834,7 @@ func Flap[R, B, A any](a A) Operator[R, func(A) B, B] {
 // This is the monadic version that takes the computation as the first parameter.
 //
 //go:inline
-func MonadMapLeft[R, A any](fa ReaderReaderIOResult[R, A], f Endmorphism[error]) ReaderReaderIOResult[R, A] {
+func MonadMapLeft[R, A any](fa ReaderReaderIOResult[R, A], f Endomorphism[error]) ReaderReaderIOResult[R, A] {
 	return RRIOE.MonadMapLeft(fa, f)
 }
 
@@ -843,7 +843,7 @@ func MonadMapLeft[R, A any](fa ReaderReaderIOResult[R, A], f Endmorphism[error])
 // This is the curried version that returns an operator.
 //
 //go:inline
-func MapLeft[R, A any](f Endmorphism[error]) Operator[R, A, A] {
+func MapLeft[R, A any](f Endomorphism[error]) Operator[R, A, A] {
 	return RRIOE.MapLeft[R, context.Context, A](f)
 }
 

v2/context/readerreaderioresult/types.go

@@ -24,6 +24,7 @@ import (
 	"github.com/IBM/fp-go/v2/io"
 	"github.com/IBM/fp-go/v2/ioeither"
 	"github.com/IBM/fp-go/v2/ioresult"
+	"github.com/IBM/fp-go/v2/iterator/iter"
 	"github.com/IBM/fp-go/v2/lazy"
 	"github.com/IBM/fp-go/v2/optics/lens"
 	"github.com/IBM/fp-go/v2/optics/traversal/result"
@@ -146,9 +147,15 @@ type (
 	// It's an alias for predicate.Predicate[A].
 	Predicate[A any] = predicate.Predicate[A]
 
-	// Endmorphism represents a function from type A to type A.
+	// Endomorphism represents a function from type A to type A.
 	// It's an alias for endomorphism.Endomorphism[A].
-	Endmorphism[A any] = endomorphism.Endomorphism[A]
+	Endomorphism[A any] = endomorphism.Endomorphism[A]
+
+	// Seq is an iterator over sequences of individual values.
+	// When called as seq(yield), seq calls yield(v) for each value v in the sequence,
+	// stopping early if yield returns false.
+	// See the [iter] package documentation for more details.
+	Seq[A any] = iter.Seq[A]
 
 	Void = function.Void
 )

v2/context/readerresult/logging_test.go

@@ -24,6 +24,7 @@ import (
 
 	"github.com/IBM/fp-go/v2/logging"
 	N "github.com/IBM/fp-go/v2/number"
+	"github.com/IBM/fp-go/v2/pair"
 	"github.com/IBM/fp-go/v2/result"
 	"github.com/stretchr/testify/assert"
 )
@@ -104,7 +105,8 @@ func TestSLogWithContextLogger(t *testing.T) {
 		Level: slog.LevelInfo,
 	}))
 
-	ctx := logging.WithLogger(contextLogger)(t.Context())
+	cancelFct, ctx := pair.Unpack(logging.WithLogger(contextLogger)(t.Context()))
+	defer cancelFct()
 
 	res1 := result.Of("test value")
 	logged := SLog[string]("Context logger test")(res1)(ctx)

v2/context/readerresult/profunctor.go

@@ -19,6 +19,8 @@ import (
 	"context"
 
 	"github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/pair"
+	RR "github.com/IBM/fp-go/v2/readerresult"
 )
 
 // Promap is the profunctor map operation that transforms both the input and output of a context-based ReaderResult.
@@ -34,21 +36,24 @@ import (
 // The error type is fixed as error and remains unchanged through the transformation.
 //
 // Type Parameters:
+//   - R: The input environment type that f transforms into context.Context
 //   - A: The original success type produced by the ReaderResult
 //   - B: The new output success type
 //
 // Parameters:
-//   - f: Function to transform the input context (contravariant)
+//   - f: Function to transform the input environment R into context.Context (contravariant)
 //   - g: Function to transform the output success value from A to B (covariant)
 //
 // Returns:
-//   - An Operator that takes a ReaderResult[A] and returns a ReaderResult[B]
+//   - A Kleisli arrow that takes a ReaderResult[A] and returns a function from R to B
+//
+// Note: When R is context.Context, this simplifies to an Operator[A, B]
 //
 //go:inline
-func Promap[A, B any](f func(context.Context) (context.Context, context.CancelFunc), g func(A) B) Operator[A, B] {
+func Promap[R, A, B any](f pair.Kleisli[context.CancelFunc, R, context.Context], g func(A) B) RR.Kleisli[R, ReaderResult[A], B] {
 	return function.Flow2(
 		Local[A](f),
-		Map(g),
+		RR.Map[R](g),
 	)
 }
 
@@ -62,15 +67,18 @@ func Promap[A, B any](f func(context.Context) (context.Context, context.CancelFu
 //
 // Type Parameters:
 //   - A: The success type (unchanged)
+//   - R: The input environment type that f transforms into context.Context
 //
 // Parameters:
-//   - f: Function to transform the context, returning a new context and CancelFunc
+//   - f: Function to transform the input environment R into context.Context, returning a new context and CancelFunc
 //
 // Returns:
-//   - An Operator that takes a ReaderResult[A] and returns a ReaderResult[A]
+//   - A Kleisli arrow that takes a ReaderResult[A] and returns a function from R to A
+//
+// Note: When R is context.Context, this simplifies to an Operator[A, A]
 //
 //go:inline
-func Contramap[A any](f func(context.Context) (context.Context, context.CancelFunc)) Operator[A, A] {
+func Contramap[A, R any](f pair.Kleisli[context.CancelFunc, R, context.Context]) RR.Kleisli[R, ReaderResult[A], A] {
 	return Local[A](f)
 }
 
@@ -89,16 +97,19 @@ func Contramap[A any](f func(context.Context) (context.Context, context.CancelFu
 //
 // Type Parameters:
 //   - A: The result type (unchanged)
+//   - R: The input environment type that f transforms into context.Context
 //
 // Parameters:
-//   - f: Function to transform the context, returning a new context and CancelFunc
+//   - f: Function to transform the input environment R into context.Context, returning a new context and CancelFunc
 //
 // Returns:
-//   - An Operator that takes a ReaderResult[A] and returns a ReaderResult[A]
-func Local[A any](f func(context.Context) (context.Context, context.CancelFunc)) Operator[A, A] {
-	return func(rr ReaderResult[A]) ReaderResult[A] {
-		return func(ctx context.Context) Result[A] {
-			otherCtx, otherCancel := f(ctx)
+//   - A Kleisli arrow that takes a ReaderResult[A] and returns a function from R to A
+//
+// Note: When R is context.Context, this simplifies to an Operator[A, A]
+func Local[A, R any](f pair.Kleisli[context.CancelFunc, R, context.Context]) RR.Kleisli[R, ReaderResult[A], A] {
+	return func(rr ReaderResult[A]) RR.ReaderResult[R, A] {
+		return func(r R) Result[A] {
+			otherCancel, otherCtx := pair.Unpack(f(r))
 			defer otherCancel()
 			return rr(otherCtx)
 		}

v2/context/readerresult/profunctor_test.go

@@ -20,6 +20,7 @@ import (
 	"strconv"
 	"testing"
 
+	"github.com/IBM/fp-go/v2/pair"
 	R "github.com/IBM/fp-go/v2/result"
 	"github.com/stretchr/testify/assert"
 )
@@ -34,9 +35,9 @@ func TestPromapBasic(t *testing.T) {
 			return R.Of(0)
 		}
 
-		addKey := func(ctx context.Context) (context.Context, context.CancelFunc) {
+		addKey := func(ctx context.Context) pair.Pair[context.CancelFunc, context.Context] {
 			newCtx := context.WithValue(ctx, "key", 42)
-			return newCtx, func() {}
+			return pair.MakePair[context.CancelFunc](func() {}, newCtx)
 		}
 		toString := strconv.Itoa
 
@@ -57,9 +58,9 @@ func TestContramapBasic(t *testing.T) {
 			return R.Of(0)
 		}
 
-		addKey := func(ctx context.Context) (context.Context, context.CancelFunc) {
+		addKey := func(ctx context.Context) pair.Pair[context.CancelFunc, context.Context] {
 			newCtx := context.WithValue(ctx, "key", 100)
-			return newCtx, func() {}
+			return pair.MakePair[context.CancelFunc](func() {}, newCtx)
 		}
 
 		adapted := Contramap[int](addKey)(getValue)
@@ -79,9 +80,9 @@ func TestLocalBasic(t *testing.T) {
 			return R.Of("unknown")
 		}
 
-		addUser := func(ctx context.Context) (context.Context, context.CancelFunc) {
+		addUser := func(ctx context.Context) pair.Pair[context.CancelFunc, context.Context] {
 			newCtx := context.WithValue(ctx, "user", "Alice")
-			return newCtx, func() {}
+			return pair.MakePair[context.CancelFunc](func() {}, newCtx)
 		}
 
 		adapted := Local[string](addUser)(getValue)

v2/context/statereaderioresult/state.go

@@ -21,8 +21,9 @@ import (
 	RIORES "github.com/IBM/fp-go/v2/context/readerioresult"
 	"github.com/IBM/fp-go/v2/function"
 	"github.com/IBM/fp-go/v2/internal/statet"
-	RIOR "github.com/IBM/fp-go/v2/readerioresult"
+	"github.com/IBM/fp-go/v2/pair"
 	"github.com/IBM/fp-go/v2/result"
+	SRIOE "github.com/IBM/fp-go/v2/statereaderioeither"
 )
 
 // Left creates a StateReaderIOResult that represents a failed computation with the given error.
@@ -202,21 +203,42 @@ func FromResult[S, A any](ma Result[A]) StateReaderIOResult[S, A] {
 // Combinators
 
 // Local runs a computation with a modified context.
-// The function f transforms the context before passing it to the computation.
+// The function f transforms the context before passing it to the computation,
+// returning both a new context and a CancelFunc that should be called to release resources.
+//
+// This is useful for:
+//   - Adding values to the context
+//   - Setting timeouts or deadlines
+//   - Modifying context metadata
+//
+// The CancelFunc is automatically called after the computation completes to ensure proper cleanup.
+//
+// Type Parameters:
+//   - S: The state type
+//   - A: The result type
+//   - R: The input environment type that f transforms into context.Context
+//
+// Parameters:
+//   - f: Function to transform the input environment R into context.Context, returning a new context and CancelFunc
+//
+// Returns:
+//   - A Kleisli arrow that takes a StateReaderIOResult[S, A] and returns a StateReaderIOEither[S, R, error, A]
+//
+// Note: When R is context.Context, the return type simplifies to func(StateReaderIOResult[S, A]) StateReaderIOResult[S, A]
 //
 // Example:
 //
-//	// Modify context before running computation
-//	withTimeout := statereaderioresult.Local[AppState](
-//	    func(ctx context.Context) context.Context {
-//	        ctx, _ = context.WithTimeout(ctx, 60*time.Second)
-//	        return ctx
-//	    }
+//	// Add a timeout to a specific operation
+//	withTimeout := statereaderioresult.Local[AppState, Data, context.Context](
+//	    func(ctx context.Context) pair.Pair[context.CancelFunc, context.Context] {
+//	        newCtx, cancel := context.WithTimeout(ctx, 60*time.Second)
+//	        return pair.MakePair(cancel, newCtx)
+//	    },
 //	)
 //	result := withTimeout(computation)
-func Local[S, A any](f func(context.Context) context.Context) func(StateReaderIOResult[S, A]) StateReaderIOResult[S, A] {
-	return func(ma StateReaderIOResult[S, A]) StateReaderIOResult[S, A] {
-		return function.Flow2(ma, RIOR.Local[Pair[S, A]](f))
+func Local[S, A, R any](f pair.Kleisli[context.CancelFunc, R, context.Context]) SRIOE.Kleisli[S, R, error, StateReaderIOResult[S, A], A] {
+	return func(ma StateReaderIOResult[S, A]) SRIOE.StateReaderIOEither[S, R, error, A] {
+		return function.Flow2(ma, RIORES.Local[Pair[S, A]](f))
 	}
 }
 

v2/context/statereaderioresult/statereaderioresult_test.go

@@ -25,6 +25,7 @@ import (
 	"github.com/IBM/fp-go/v2/io"
 	IOR "github.com/IBM/fp-go/v2/ioresult"
 	N "github.com/IBM/fp-go/v2/number"
+	"github.com/IBM/fp-go/v2/pair"
 	P "github.com/IBM/fp-go/v2/pair"
 	RES "github.com/IBM/fp-go/v2/result"
 	"github.com/stretchr/testify/assert"
@@ -264,8 +265,8 @@ func TestLocal(t *testing.T) {
 
 	// Modify context before running computation
 	result := Local[testState, string](
-		func(c context.Context) context.Context {
-			return context.WithValue(c, "key", "value2")
+		func(c context.Context) ContextCancel {
+			return pair.MakePair[context.CancelFunc](func() {}, context.WithValue(c, "key", "value2"))
 		},
 	)(comp)
 

v2/context/statereaderioresult/type.go

@@ -16,6 +16,8 @@
 package statereaderioresult
 
 import (
+	"context"
+
 	RIORES "github.com/IBM/fp-go/v2/context/readerioresult"
 	"github.com/IBM/fp-go/v2/endomorphism"
 	"github.com/IBM/fp-go/v2/io"
@@ -84,4 +86,11 @@ type (
 	Operator[S, A, B any] = Reader[StateReaderIOResult[S, A], StateReaderIOResult[S, B]]
 
 	Predicate[A any] = predicate.Predicate[A]
+
+	// ContextCancel represents a pair of a cancel function and a context.
+	// It is used in operations that create new contexts with cancellation capabilities.
+	//
+	// The first element is the CancelFunc that should be called to release resources.
+	// The second element is the new Context that was created.
+	ContextCancel = Pair[context.CancelFunc, context.Context]
 )

v2/effect/bind.go

@@ -25,6 +25,31 @@ import (
 	"github.com/IBM/fp-go/v2/readerio"
 )
 
+// Do creates an Effect with an initial state value.
+// This is the starting point for do-notation style effect composition,
+// allowing you to build up complex state transformations step by step.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - S: The state type
+//
+// # Parameters
+//
+//   - empty: The initial state value
+//
+// # Returns
+//
+//   - Effect[C, S]: An effect that produces the initial state
+//
+// # Example
+//
+//	type State struct {
+//		Name string
+//		Age  int
+//	}
+//	eff := effect.Do[MyContext](State{})
+//
 //go:inline
 func Do[C, S any](
 	empty S,
@@ -32,6 +57,40 @@ func Do[C, S any](
 	return readerreaderioresult.Of[C](empty)
 }
 
+// Bind executes an effectful computation and binds its result to the state.
+// This is the core operation for do-notation, allowing you to sequence effects
+// while accumulating results in a state structure.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effects
+//   - S1: The input state type
+//   - S2: The output state type
+//   - T: The type of value produced by the effect
+//
+// # Parameters
+//
+//   - setter: A function that takes the effect result and returns a state updater
+//   - f: An effectful computation that depends on the current state
+//
+// # Returns
+//
+//   - Operator[C, S1, S2]: A function that transforms the state effect
+//
+// # Example
+//
+//	eff := effect.Bind(
+//		func(age int) func(State) State {
+//			return func(s State) State {
+//				s.Age = age
+//				return s
+//			}
+//		},
+//		func(s State) Effect[MyContext, int] {
+//			return effect.Of[MyContext](30)
+//		},
+//	)(effect.Do[MyContext](State{}))
+//
 //go:inline
 func Bind[C, S1, S2, T any](
 	setter func(T) func(S1) S2,
@@ -40,6 +99,39 @@ func Bind[C, S1, S2, T any](
 	return readerreaderioresult.Bind(setter, f)
 }
 
+// Let computes a pure value from the current state and binds it to the state.
+// Unlike Bind, this doesn't perform any effects - it's for pure computations.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - S1: The input state type
+//   - S2: The output state type
+//   - T: The type of computed value
+//
+// # Parameters
+//
+//   - setter: A function that takes the computed value and returns a state updater
+//   - f: A pure function that computes a value from the current state
+//
+// # Returns
+//
+//   - Operator[C, S1, S2]: A function that transforms the state effect
+//
+// # Example
+//
+//	eff := effect.Let[MyContext](
+//		func(nameLen int) func(State) State {
+//			return func(s State) State {
+//				s.NameLength = nameLen
+//				return s
+//			}
+//		},
+//		func(s State) int {
+//			return len(s.Name)
+//		},
+//	)(stateEff)
+//
 //go:inline
 func Let[C, S1, S2, T any](
 	setter func(T) func(S1) S2,
@@ -48,6 +140,37 @@ func Let[C, S1, S2, T any](
 	return readerreaderioresult.Let[C](setter, f)
 }
 
+// LetTo binds a constant value to the state.
+// This is useful for setting fixed values in your state structure.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - S1: The input state type
+//   - S2: The output state type
+//   - T: The type of the constant value
+//
+// # Parameters
+//
+//   - setter: A function that takes the constant and returns a state updater
+//   - b: The constant value to bind
+//
+// # Returns
+//
+//   - Operator[C, S1, S2]: A function that transforms the state effect
+//
+// # Example
+//
+//	eff := effect.LetTo[MyContext](
+//		func(age int) func(State) State {
+//			return func(s State) State {
+//				s.Age = age
+//				return s
+//			}
+//		},
+//		42,
+//	)(stateEff)
+//
 //go:inline
 func LetTo[C, S1, S2, T any](
 	setter func(T) func(S1) S2,
@@ -56,6 +179,30 @@ func LetTo[C, S1, S2, T any](
 	return readerreaderioresult.LetTo[C](setter, b)
 }
 
+// BindTo wraps a value in an initial state structure.
+// This is typically used to start a bind chain by converting a simple value
+// into a state structure.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - S1: The state type to create
+//   - T: The type of the input value
+//
+// # Parameters
+//
+//   - setter: A function that creates a state from the value
+//
+// # Returns
+//
+//   - Operator[C, T, S1]: A function that wraps the value in state
+//
+// # Example
+//
+//	eff := effect.BindTo[MyContext](func(name string) State {
+//		return State{Name: name}
+//	})(effect.Of[MyContext]("Alice"))
+//
 //go:inline
 func BindTo[C, S1, T any](
 	setter func(T) S1,
@@ -63,6 +210,39 @@ func BindTo[C, S1, T any](
 	return readerreaderioresult.BindTo[C](setter)
 }
 
+// ApS applies an effect and binds its result to the state using a setter function.
+// This is similar to Bind but takes a pre-existing effect rather than a function
+// that creates an effect from the state.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effects
+//   - S1: The input state type
+//   - S2: The output state type
+//   - T: The type of value produced by the effect
+//
+// # Parameters
+//
+//   - setter: A function that takes the effect result and returns a state updater
+//   - fa: The effect to apply
+//
+// # Returns
+//
+//   - Operator[C, S1, S2]: A function that transforms the state effect
+//
+// # Example
+//
+//	ageEffect := effect.Of[MyContext](30)
+//	eff := effect.ApS(
+//		func(age int) func(State) State {
+//			return func(s State) State {
+//				s.Age = age
+//				return s
+//			}
+//		},
+//		ageEffect,
+//	)(stateEff)
+//
 //go:inline
 func ApS[C, S1, S2, T any](
 	setter func(T) func(S1) S2,
@@ -71,6 +251,33 @@ func ApS[C, S1, S2, T any](
 	return readerreaderioresult.ApS(setter, fa)
 }
 
+// ApSL applies an effect and updates a field in the state using a lens.
+// This provides a more ergonomic way to update nested state structures.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effects
+//   - S: The state type
+//   - T: The type of the field being updated
+//
+// # Parameters
+//
+//   - lens: A lens focusing on the field to update
+//   - fa: The effect producing the new field value
+//
+// # Returns
+//
+//   - Operator[C, S, S]: A function that updates the state field
+//
+// # Example
+//
+//	ageLens := lens.MakeLens(
+//		func(s State) int { return s.Age },
+//		func(s State, age int) State { s.Age = age; return s },
+//	)
+//	ageEffect := effect.Of[MyContext](30)
+//	eff := effect.ApSL(ageLens, ageEffect)(stateEff)
+//
 //go:inline
 func ApSL[C, S, T any](
 	lens Lens[S, T],
@@ -79,6 +286,37 @@ func ApSL[C, S, T any](
 	return readerreaderioresult.ApSL(lens, fa)
 }
 
+// BindL executes an effectful computation on a field and updates it using a lens.
+// The effect function receives the current field value and produces a new value.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effects
+//   - S: The state type
+//   - T: The type of the field being updated
+//
+// # Parameters
+//
+//   - lens: A lens focusing on the field to update
+//   - f: An effectful function that transforms the field value
+//
+// # Returns
+//
+//   - Operator[C, S, S]: A function that updates the state field
+//
+// # Example
+//
+//	ageLens := lens.MakeLens(
+//		func(s State) int { return s.Age },
+//		func(s State, age int) State { s.Age = age; return s },
+//	)
+//	eff := effect.BindL(
+//		ageLens,
+//		func(age int) Effect[MyContext, int] {
+//			return effect.Of[MyContext](age + 1)
+//		},
+//	)(stateEff)
+//
 //go:inline
 func BindL[C, S, T any](
 	lens Lens[S, T],
@@ -87,6 +325,35 @@ func BindL[C, S, T any](
 	return readerreaderioresult.BindL(lens, f)
 }
 
+// LetL computes a new field value from the current value using a lens.
+// This is a pure transformation of a field within the state.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - S: The state type
+//   - T: The type of the field being updated
+//
+// # Parameters
+//
+//   - lens: A lens focusing on the field to update
+//   - f: A pure function that transforms the field value
+//
+// # Returns
+//
+//   - Operator[C, S, S]: A function that updates the state field
+//
+// # Example
+//
+//	ageLens := lens.MakeLens(
+//		func(s State) int { return s.Age },
+//		func(s State, age int) State { s.Age = age; return s },
+//	)
+//	eff := effect.LetL[MyContext](
+//		ageLens,
+//		func(age int) int { return age * 2 },
+//	)(stateEff)
+//
 //go:inline
 func LetL[C, S, T any](
 	lens Lens[S, T],
@@ -95,6 +362,31 @@ func LetL[C, S, T any](
 	return readerreaderioresult.LetL[C](lens, f)
 }
 
+// LetToL sets a field to a constant value using a lens.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - S: The state type
+//   - T: The type of the field being updated
+//
+// # Parameters
+//
+//   - lens: A lens focusing on the field to update
+//   - b: The constant value to set
+//
+// # Returns
+//
+//   - Operator[C, S, S]: A function that updates the state field
+//
+// # Example
+//
+//	ageLens := lens.MakeLens(
+//		func(s State) int { return s.Age },
+//		func(s State, age int) State { s.Age = age; return s },
+//	)
+//	eff := effect.LetToL[MyContext](ageLens, 42)(stateEff)
+//
 //go:inline
 func LetToL[C, S, T any](
 	lens Lens[S, T],

v2/effect/common_test.go

@@ -0,0 +1,32 @@
+// 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 effect
+
+import (
+	"context"
+)
+
+// TestContext is a common test context type used across effect tests
+type TestContext struct {
+	Value string
+}
+
+// runEffect is a helper function to run an effect with a context and return the result
+func runEffect[C, A any](eff Effect[C, A], ctx C) (A, error) {
+	ioResult := Provide[A](ctx)(eff)
+	readerResult := RunSync(ioResult)
+	return readerResult(context.Background())
+}

v2/effect/dependencies.go

@@ -1,6 +1,22 @@
+// 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 effect
 
 import (
+	"github.com/IBM/fp-go/v2/context/reader"
 	thunk "github.com/IBM/fp-go/v2/context/readerioresult"
 	"github.com/IBM/fp-go/v2/context/readerreaderioresult"
 	"github.com/IBM/fp-go/v2/io"
@@ -8,31 +24,182 @@ import (
 	"github.com/IBM/fp-go/v2/result"
 )
 
+// Local transforms the context required by an effect using a pure function.
+// This allows you to adapt an effect that requires one context type to work
+// with a different context type by providing a transformation function.
+//
+// # Type Parameters
+//
+//   - C1: The outer context type (what you have)
+//   - C2: The inner context type (what the effect needs)
+//   - A: The value type produced by the effect
+//
+// # Parameters
+//
+//   - acc: A pure function that transforms C1 to C2
+//
+// # Returns
+//
+//   - Kleisli[C1, Effect[C2, A], A]: A function that adapts the effect to use C1
+//
+// # Example
+//
+//	type AppConfig struct { DB DatabaseConfig }
+//	type DatabaseConfig struct { Host string }
+//	dbEffect := effect.Of[DatabaseConfig]("connected")
+//	appEffect := effect.Local[AppConfig, DatabaseConfig, string](
+//		func(app AppConfig) DatabaseConfig { return app.DB },
+//	)(dbEffect)
+//
 //go:inline
-func Local[C1, C2, A any](acc Reader[C1, C2]) Kleisli[C1, Effect[C2, A], A] {
+func Local[A, C1, C2 any](acc Reader[C1, C2]) Kleisli[C1, Effect[C2, A], A] {
 	return readerreaderioresult.Local[A](acc)
 }
 
+// Contramap is an alias for Local, following the contravariant functor naming convention.
+// It transforms the context required by an effect using a pure function.
+//
+// # Type Parameters
+//
+//   - C1: The outer context type (what you have)
+//   - C2: The inner context type (what the effect needs)
+//   - A: The value type produced by the effect
+//
+// # Parameters
+//
+//   - acc: A pure function that transforms C1 to C2
+//
+// # Returns
+//
+//   - Kleisli[C1, Effect[C2, A], A]: A function that adapts the effect to use C1
+//
 //go:inline
-func Contramap[C1, C2, A any](acc Reader[C1, C2]) Kleisli[C1, Effect[C2, A], A] {
+func Contramap[A, C1, C2 any](acc Reader[C1, C2]) Kleisli[C1, Effect[C2, A], A] {
 	return readerreaderioresult.Local[A](acc)
 }
 
+// LocalIOK transforms the context using an IO-based function.
+// This allows the context transformation itself to perform I/O operations.
+//
+// # Type Parameters
+//
+//   - A: The value type produced by the effect
+//   - C1: The inner context type (what the effect needs)
+//   - C2: The outer context type (what you have)
+//
+// # Parameters
+//
+//   - f: An IO function that transforms C2 to C1
+//
+// # Returns
+//
+//   - func(Effect[C1, A]) Effect[C2, A]: A function that adapts the effect
+//
+// # Example
+//
+//	loadConfig := func(path string) io.IO[Config] {
+//		return func() Config { /* load from file */ }
+//	}
+//	transform := effect.LocalIOK[string](loadConfig)
+//	adapted := transform(configEffect)
+//
 //go:inline
 func LocalIOK[A, C1, C2 any](f io.Kleisli[C2, C1]) func(Effect[C1, A]) Effect[C2, A] {
 	return readerreaderioresult.LocalIOK[A](f)
 }
 
+// LocalIOResultK transforms the context using an IOResult-based function.
+// This allows the context transformation to perform I/O and handle errors.
+//
+// # Type Parameters
+//
+//   - A: The value type produced by the effect
+//   - C1: The inner context type (what the effect needs)
+//   - C2: The outer context type (what you have)
+//
+// # Parameters
+//
+//   - f: An IOResult function that transforms C2 to C1
+//
+// # Returns
+//
+//   - func(Effect[C1, A]) Effect[C2, A]: A function that adapts the effect
+//
+// # Example
+//
+//	loadConfig := func(path string) ioresult.IOResult[Config] {
+//		return func() result.Result[Config] {
+//			// load from file, may fail
+//		}
+//	}
+//	transform := effect.LocalIOResultK[string](loadConfig)
+//	adapted := transform(configEffect)
+//
 //go:inline
 func LocalIOResultK[A, C1, C2 any](f ioresult.Kleisli[C2, C1]) func(Effect[C1, A]) Effect[C2, A] {
 	return readerreaderioresult.LocalIOResultK[A](f)
 }
 
+// LocalResultK transforms the context using a Result-based function.
+// This allows the context transformation to fail with an error.
+//
+// # Type Parameters
+//
+//   - A: The value type produced by the effect
+//   - C1: The inner context type (what the effect needs)
+//   - C2: The outer context type (what you have)
+//
+// # Parameters
+//
+//   - f: A Result function that transforms C2 to C1
+//
+// # Returns
+//
+//   - func(Effect[C1, A]) Effect[C2, A]: A function that adapts the effect
+//
+// # Example
+//
+//	validateConfig := func(raw RawConfig) result.Result[Config] {
+//		if raw.IsValid() {
+//			return result.Of(raw.ToConfig())
+//		}
+//		return result.Left[Config](errors.New("invalid"))
+//	}
+//	transform := effect.LocalResultK[string](validateConfig)
+//	adapted := transform(configEffect)
+//
 //go:inline
 func LocalResultK[A, C1, C2 any](f result.Kleisli[C2, C1]) func(Effect[C1, A]) Effect[C2, A] {
 	return readerreaderioresult.LocalResultK[A](f)
 }
 
+// LocalThunkK transforms the context using a Thunk (ReaderIOResult) function.
+// This allows the context transformation to depend on context.Context, perform I/O, and handle errors.
+//
+// # Type Parameters
+//
+//   - A: The value type produced by the effect
+//   - C1: The inner context type (what the effect needs)
+//   - C2: The outer context type (what you have)
+//
+// # Parameters
+//
+//   - f: A Thunk function that transforms C2 to C1
+//
+// # Returns
+//
+//   - func(Effect[C1, A]) Effect[C2, A]: A function that adapts the effect
+//
+// # Example
+//
+//	loadConfig := func(path string) readerioresult.ReaderIOResult[Config] {
+//		return func(ctx context.Context) ioresult.IOResult[Config] {
+//			// load from file with context, may fail
+//		}
+//	}
+//	transform := effect.LocalThunkK[string](loadConfig)
+//	adapted := transform(configEffect)
+//
 //go:inline
 func LocalThunkK[A, C1, C2 any](f thunk.Kleisli[C2, C1]) func(Effect[C1, A]) Effect[C2, A] {
 	return readerreaderioresult.LocalReaderIOResultK[A](f)
@@ -101,10 +268,106 @@ func LocalThunkK[A, C1, C2 any](f thunk.Kleisli[C2, C1]) func(Effect[C1, A]) Eff
 //   - Local/Contramap: Pure context transformation (C2 -> C1)
 //   - LocalIOK: IO-based transformation (C2 -> IO[C1])
 //   - LocalIOResultK: IO with error handling (C2 -> IOResult[C1])
-//   - LocalReaderIOResultK: Reader-based with IO and errors (C2 -> ReaderIOResult[C1])
+//   - LocalThunkK: Reader-based with IO and errors (C2 -> ReaderIOResult[C1])
 //   - LocalEffectK: Full Effect transformation (C2 -> Effect[C2, C1])
 //
 //go:inline
 func LocalEffectK[A, C1, C2 any](f Kleisli[C2, C2, C1]) func(Effect[C1, A]) Effect[C2, A] {
 	return readerreaderioresult.LocalReaderReaderIOEitherK[A](f)
 }
+
+// LocalReaderK transforms the context of an Effect using a Reader-based Kleisli arrow.
+// It allows you to modify the context through a pure computation that depends on the runtime context
+// before passing it to the Effect.
+//
+// This is useful when the context transformation is a pure computation that requires access
+// to the runtime context (context.Context) but cannot fail. Common use cases include:
+//   - Extracting configuration from context values
+//   - Computing derived context values based on runtime context
+//   - Transforming context based on runtime metadata
+//
+// The transformation happens in two stages:
+//  1. The Reader function f is executed with the C2 context and runtime context to produce a C1 value
+//  2. The resulting C1 value is passed as the context to the Effect[C1, A]
+//
+// # Type Parameters
+//
+//   - A: The value type produced by the effect
+//   - C1: The inner context type (required by the original effect)
+//   - C2: The outer context type (provided to the transformed effect)
+//
+// # Parameters
+//
+//   - f: A Reader Kleisli arrow that transforms C2 to C1 using the runtime context
+//
+// # Returns
+//
+//   - func(Effect[C1, A]) Effect[C2, A]: A function that adapts the effect to use C2
+//
+// # Example
+//
+//	type ctxKey string
+//	const configKey ctxKey = "config"
+//
+//	type DetailedConfig struct {
+//		Host string
+//		Port int
+//	}
+//
+//	type SimpleConfig struct {
+//		Port int
+//	}
+//
+//	// Extract config from runtime context and transform
+//	extractConfig := func(path string) reader.Reader[DetailedConfig] {
+//		return func(ctx context.Context) DetailedConfig {
+//			if cfg, ok := ctx.Value(configKey).(DetailedConfig); ok {
+//				return cfg
+//			}
+//			return DetailedConfig{Host: "localhost", Port: 8080}
+//		}
+//	}
+//
+//	// Effect that uses DetailedConfig
+//	configEffect := effect.Of[DetailedConfig]("connected")
+//
+//	// Transform to use string path instead
+//	transform := effect.LocalReaderK[string](extractConfig)
+//	pathEffect := transform(configEffect)
+//
+//	// Run with runtime context containing config
+//	ctx := context.WithValue(context.Background(), configKey, DetailedConfig{Host: "api.example.com", Port: 443})
+//	ioResult := effect.Provide[string]("config.json")(pathEffect)
+//	readerResult := effect.RunSync(ioResult)
+//	result, err := readerResult(ctx) // Uses config from context
+//
+// # Comparison with other Local functions
+//
+//   - Local/Contramap: Pure context transformation (C2 -> C1)
+//   - LocalIOK: IO-based transformation (C2 -> IO[C1])
+//   - LocalIOResultK: IO with error handling (C2 -> IOResult[C1])
+//   - LocalReaderK: Reader-based pure transformation with runtime context access (C2 -> Reader[C1])
+//   - LocalThunkK: Reader-based with IO and errors (C2 -> ReaderIOResult[C1])
+//   - LocalEffectK: Full Effect transformation (C2 -> Effect[C2, C1])
+//
+//go:inline
+func LocalReaderK[A, C1, C2 any](f reader.Kleisli[C2, C1]) func(Effect[C1, A]) Effect[C2, A] {
+	return readerreaderioresult.LocalReaderK[A](f)
+}
+
+// Ask returns an Effect that produces the context C as its success value.
+// This is the fundamental operation of the reader/environment monad,
+// allowing effects to access their own context.
+//
+// # Type Parameters
+//
+//   - C: The context type (also the produced value type)
+//
+// # Returns
+//
+//   - Effect[C, C]: An effect that succeeds with its own context value
+//
+//go:inline
+func Ask[C any]() Effect[C, C] {
+	return readerreaderioresult.Ask[C]()
+}

v2/effect/dependencies_test.go

@@ -19,7 +19,7 @@ import (
 	"context"
 	"fmt"
 	"testing"
-
+	"github.com/IBM/fp-go/v2/context/reader"
 	"github.com/IBM/fp-go/v2/context/readerreaderioresult"
 	"github.com/stretchr/testify/assert"
 )
@@ -44,11 +44,11 @@ func TestLocal(t *testing.T) {
 		}
 
 		// Apply Local to transform the context
-		kleisli := Local[OuterContext, InnerContext, string](accessor)
+		kleisli := Local[string](accessor)
 		outerEffect := kleisli(innerEffect)
 
 		// Run with OuterContext
-		ioResult := Provide[OuterContext, string](OuterContext{
+		ioResult := Provide[string](OuterContext{
 			Value:  "test",
 			Number: 42,
 		})(outerEffect)
@@ -70,11 +70,11 @@ func TestLocal(t *testing.T) {
 			return InnerContext{Value: outer.Value + " transformed"}
 		}
 
-		kleisli := Local[OuterContext, InnerContext, string](accessor)
+		kleisli := Local[string](accessor)
 		outerEffect := kleisli(innerEffect)
 
 		// Run with OuterContext
-		ioResult := Provide[OuterContext, string](OuterContext{
+		ioResult := Provide[string](OuterContext{
 			Value:  "original",
 			Number: 100,
 		})(outerEffect)
@@ -93,10 +93,10 @@ func TestLocal(t *testing.T) {
 			return InnerContext{Value: outer.Value}
 		}
 
-		kleisli := Local[OuterContext, InnerContext, string](accessor)
+		kleisli := Local[string](accessor)
 		outerEffect := kleisli(innerEffect)
 
-		ioResult := Provide[OuterContext, string](OuterContext{
+		ioResult := Provide[string](OuterContext{
 			Value:  "test",
 			Number: 42,
 		})(outerEffect)
@@ -122,12 +122,12 @@ func TestLocal(t *testing.T) {
 		level3Effect := Of[Level3]("deep result")
 
 		// Transform Level2 -> Level3
-		local23 := Local[Level2, Level3, string](func(l2 Level2) Level3 {
+		local23 := Local[string](func(l2 Level2) Level3 {
 			return Level3{C: l2.B + "-c"}
 		})
 
 		// Transform Level1 -> Level2
-		local12 := Local[Level1, Level2, string](func(l1 Level1) Level2 {
+		local12 := Local[string](func(l1 Level1) Level2 {
 			return Level2{B: l1.A + "-b"}
 		})
 
@@ -136,7 +136,7 @@ func TestLocal(t *testing.T) {
 		level1Effect := local12(level2Effect)
 
 		// Run with Level1 context
-		ioResult := Provide[Level1, string](Level1{A: "a"})(level1Effect)
+		ioResult := Provide[string](Level1{A: "a"})(level1Effect)
 		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
@@ -165,11 +165,11 @@ func TestLocal(t *testing.T) {
 			return app.DB
 		}
 
-		kleisli := Local[AppConfig, DatabaseConfig, string](accessor)
+		kleisli := Local[string](accessor)
 		appEffect := kleisli(dbEffect)
 
 		// Run with full AppConfig
-		ioResult := Provide[AppConfig, string](AppConfig{
+		ioResult := Provide[string](AppConfig{
 			DB: DatabaseConfig{
 				Host:     "localhost",
 				Port:     5432,
@@ -195,21 +195,21 @@ func TestContramap(t *testing.T) {
 		}
 
 		// Test Local
-		localKleisli := Local[OuterContext, InnerContext, int](accessor)
+		localKleisli := Local[int](accessor)
 		localEffect := localKleisli(innerEffect)
 
 		// Test Contramap
-		contramapKleisli := Contramap[OuterContext, InnerContext, int](accessor)
+		contramapKleisli := Contramap[int](accessor)
 		contramapEffect := contramapKleisli(innerEffect)
 
 		outerCtx := OuterContext{Value: "test", Number: 100}
 
 		// Run both
-		localIO := Provide[OuterContext, int](outerCtx)(localEffect)
+		localIO := Provide[int](outerCtx)(localEffect)
 		localReader := RunSync(localIO)
 		localResult, localErr := localReader(context.Background())
 
-		contramapIO := Provide[OuterContext, int](outerCtx)(contramapEffect)
+		contramapIO := Provide[int](outerCtx)(contramapEffect)
 		contramapReader := RunSync(contramapIO)
 		contramapResult, contramapErr := contramapReader(context.Background())
 
@@ -225,10 +225,10 @@ func TestContramap(t *testing.T) {
 			return InnerContext{Value: outer.Value + " modified"}
 		}
 
-		kleisli := Contramap[OuterContext, InnerContext, string](accessor)
+		kleisli := Contramap[string](accessor)
 		outerEffect := kleisli(innerEffect)
 
-		ioResult := Provide[OuterContext, string](OuterContext{
+		ioResult := Provide[string](OuterContext{
 			Value:  "original",
 			Number: 50,
 		})(outerEffect)
@@ -247,10 +247,10 @@ func TestContramap(t *testing.T) {
 			return InnerContext{Value: outer.Value}
 		}
 
-		kleisli := Contramap[OuterContext, InnerContext, int](accessor)
+		kleisli := Contramap[int](accessor)
 		outerEffect := kleisli(innerEffect)
 
-		ioResult := Provide[OuterContext, int](OuterContext{
+		ioResult := Provide[int](OuterContext{
 			Value:  "test",
 			Number: 42,
 		})(outerEffect)
@@ -278,12 +278,12 @@ func TestLocalAndContramapInteroperability(t *testing.T) {
 		effect3 := Of[Config3]("result")
 
 		// Use Local for first transformation
-		local23 := Local[Config2, Config3, string](func(c2 Config2) Config3 {
+		local23 := Local[string](func(c2 Config2) Config3 {
 			return Config3{Info: c2.Data}
 		})
 
 		// Use Contramap for second transformation
-		contramap12 := Contramap[Config1, Config2, string](func(c1 Config1) Config2 {
+		contramap12 := Contramap[string](func(c1 Config1) Config2 {
 			return Config2{Data: c1.Value}
 		})
 
@@ -292,7 +292,7 @@ func TestLocalAndContramapInteroperability(t *testing.T) {
 		effect1 := contramap12(effect2)
 
 		// Run
-		ioResult := Provide[Config1, string](Config1{Value: "test"})(effect1)
+		ioResult := Provide[string](Config1{Value: "test"})(effect1)
 		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
@@ -326,7 +326,7 @@ func TestLocalEffectK(t *testing.T) {
 		appEffect := transform(dbEffect)
 
 		// Run with AppConfig
-		ioResult := Provide[AppConfig, string](AppConfig{
+		ioResult := Provide[string](AppConfig{
 			ConfigPath: "/etc/app.conf",
 		})(appEffect)
 		readerResult := RunSync(ioResult)
@@ -356,7 +356,7 @@ func TestLocalEffectK(t *testing.T) {
 		transform := LocalEffectK[string](failingTransform)
 		outerEffect := transform(innerEffect)
 
-		ioResult := Provide[OuterCtx, string](OuterCtx{Path: "test"})(outerEffect)
+		ioResult := Provide[string](OuterCtx{Path: "test"})(outerEffect)
 		readerResult := RunSync(ioResult)
 		_, err := readerResult(context.Background())
 
@@ -384,7 +384,7 @@ func TestLocalEffectK(t *testing.T) {
 		transformK := LocalEffectK[string](transform)
 		outerEffect := transformK(innerEffect)
 
-		ioResult := Provide[OuterCtx, string](OuterCtx{Path: "test"})(outerEffect)
+		ioResult := Provide[string](OuterCtx{Path: "test"})(outerEffect)
 		readerResult := RunSync(ioResult)
 		_, err := readerResult(context.Background())
 
@@ -417,7 +417,7 @@ func TestLocalEffectK(t *testing.T) {
 		transform := LocalEffectK[string](loadConfigEffect)
 		appEffect := transform(configEffect)
 
-		ioResult := Provide[AppContext, string](AppContext{
+		ioResult := Provide[string](AppContext{
 			ConfigFile: "config.json",
 		})(appEffect)
 		readerResult := RunSync(ioResult)
@@ -456,7 +456,7 @@ func TestLocalEffectK(t *testing.T) {
 		level1Effect := transform12(level2Effect)
 
 		// Run with Level1 context
-		ioResult := Provide[Level1, string](Level1{A: "a"})(level1Effect)
+		ioResult := Provide[string](Level1{A: "a"})(level1Effect)
 		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
@@ -497,7 +497,7 @@ func TestLocalEffectK(t *testing.T) {
 		transform := LocalEffectK[string](transformWithContext)
 		appEffect := transform(dbEffect)
 
-		ioResult := Provide[AppConfig, string](AppConfig{
+		ioResult := Provide[string](AppConfig{
 			Environment: "prod",
 			DBHost:      "localhost",
 			DBPort:      5432,
@@ -534,14 +534,14 @@ func TestLocalEffectK(t *testing.T) {
 		outerEffect := transform(innerEffect)
 
 		// Test with invalid config
-		ioResult := Provide[RawConfig, string](RawConfig{APIKey: ""})(outerEffect)
+		ioResult := Provide[string](RawConfig{APIKey: ""})(outerEffect)
 		readerResult := RunSync(ioResult)
 		_, err := readerResult(context.Background())
 
 		assert.Error(t, err)
 
 		// Test with valid config
-		ioResult2 := Provide[RawConfig, string](RawConfig{APIKey: "valid-key"})(outerEffect)
+		ioResult2 := Provide[string](RawConfig{APIKey: "valid-key"})(outerEffect)
 		readerResult2 := RunSync(ioResult2)
 		result, err2 := readerResult2(context.Background())
 
@@ -569,7 +569,7 @@ func TestLocalEffectK(t *testing.T) {
 		})
 
 		// Use Local for second transformation (pure)
-		local12 := Local[Level1, Level2, string](func(l1 Level1) Level2 {
+		local12 := Local[string](func(l1 Level1) Level2 {
 			return Level2{Data: l1.Value}
 		})
 
@@ -578,7 +578,7 @@ func TestLocalEffectK(t *testing.T) {
 		effect1 := local12(effect2)
 
 		// Run
-		ioResult := Provide[Level1, string](Level1{Value: "test"})(effect1)
+		ioResult := Provide[string](Level1{Value: "test"})(effect1)
 		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
@@ -610,7 +610,7 @@ func TestLocalEffectK(t *testing.T) {
 		transform := LocalEffectK[int](complexTransform)
 		outerEffect := transform(innerEffect)
 
-		ioResult := Provide[OuterCtx, int](OuterCtx{Multiplier: 3})(outerEffect)
+		ioResult := Provide[int](OuterCtx{Multiplier: 3})(outerEffect)
 		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
@@ -618,3 +618,379 @@ func TestLocalEffectK(t *testing.T) {
 		assert.Equal(t, 60, result) // 3 * 10 * 2
 	})
 }
+
+func TestLocalReaderK(t *testing.T) {
+	t.Run("basic Reader transformation", func(t *testing.T) {
+		type SimpleConfig struct {
+			Port int
+		}
+
+		// Reader that transforms string path to SimpleConfig using runtime context
+		loadConfig := func(path string) reader.Reader[SimpleConfig] {
+			return func(ctx context.Context) SimpleConfig {
+				// Could extract values from runtime context here
+				return SimpleConfig{Port: 8080}
+			}
+		}
+
+		// Effect that uses the config
+		configEffect := Of[SimpleConfig]("connected")
+
+		// Transform using LocalReaderK
+		transform := LocalReaderK[string](loadConfig)
+		pathEffect := transform(configEffect)
+
+		// Run with path
+		ioResult := Provide[string]("config.json")(pathEffect)
+		readerResult := RunSync(ioResult)
+		result, err := readerResult(context.Background())
+
+		assert.NoError(t, err)
+		assert.Equal(t, "connected", result)
+	})
+
+	t.Run("extract config from runtime context", func(t *testing.T) {
+		type ctxKey string
+		const configKey ctxKey = "config"
+
+		type DetailedConfig struct {
+			Host string
+			Port int
+		}
+
+		// Reader that extracts config from runtime context
+		extractConfig := func(path string) reader.Reader[DetailedConfig] {
+			return func(ctx context.Context) DetailedConfig {
+				if cfg, ok := ctx.Value(configKey).(DetailedConfig); ok {
+					return cfg
+				}
+				// Default config if not in runtime context
+				return DetailedConfig{Host: "localhost", Port: 8080}
+			}
+		}
+
+		// Effect that uses the config
+		configEffect := Chain(func(cfg DetailedConfig) Effect[DetailedConfig, string] {
+			return Of[DetailedConfig](fmt.Sprintf("%s:%d", cfg.Host, cfg.Port))
+		})(readerreaderioresult.Ask[DetailedConfig]())
+
+		transform := LocalReaderK[string](extractConfig)
+		pathEffect := transform(configEffect)
+
+		// With config in runtime context
+		ctxWithConfig := context.WithValue(context.Background(), configKey, DetailedConfig{Host: "api.example.com", Port: 443})
+		ioResult := Provide[string]("ignored")(pathEffect)
+		readerResult := RunSync(ioResult)
+		result, err := readerResult(ctxWithConfig)
+
+		assert.NoError(t, err)
+		assert.Equal(t, "api.example.com:443", result)
+
+		// Without config in runtime context (uses default)
+		ioResult2 := Provide[string]("ignored")(pathEffect)
+		readerResult2 := RunSync(ioResult2)
+		result2, err2 := readerResult2(context.Background())
+
+		assert.NoError(t, err2)
+		assert.Equal(t, "localhost:8080", result2)
+	})
+
+	t.Run("runtime context-aware transformation", func(t *testing.T) {
+		type ctxKey string
+		const multiplierKey ctxKey = "multiplier"
+
+		// Reader that uses runtime context to compute context
+		computeValue := func(base int) reader.Reader[int] {
+			return func(ctx context.Context) int {
+				if mult, ok := ctx.Value(multiplierKey).(int); ok {
+					return base * mult
+				}
+				return base
+			}
+		}
+
+		// Effect that uses the computed value
+		valueEffect := Chain(func(val int) Effect[int, string] {
+			return Of[int](fmt.Sprintf("Value: %d", val))
+		})(readerreaderioresult.Ask[int]())
+
+		transform := LocalReaderK[string](computeValue)
+		baseEffect := transform(valueEffect)
+
+		// With multiplier in runtime context
+		ctxWithMult := context.WithValue(context.Background(), multiplierKey, 10)
+		ioResult := Provide[string](5)(baseEffect)
+		readerResult := RunSync(ioResult)
+		result, err := readerResult(ctxWithMult)
+
+		assert.NoError(t, err)
+		assert.Equal(t, "Value: 50", result)
+
+		// Without multiplier (uses base value)
+		ioResult2 := Provide[string](5)(baseEffect)
+		readerResult2 := RunSync(ioResult2)
+		result2, err2 := readerResult2(context.Background())
+
+		assert.NoError(t, err2)
+		assert.Equal(t, "Value: 5", result2)
+	})
+
+	t.Run("compose multiple LocalReaderK", func(t *testing.T) {
+		type ctxKey string
+		const prefixKey ctxKey = "prefix"
+
+		// First transformation: int -> string using runtime context
+		intToString := func(n int) reader.Reader[string] {
+			return func(ctx context.Context) string {
+				if prefix, ok := ctx.Value(prefixKey).(string); ok {
+					return fmt.Sprintf("%s-%d", prefix, n)
+				}
+				return fmt.Sprintf("%d", n)
+			}
+		}
+
+		// Second transformation: string -> SimpleConfig
+		type SimpleConfig struct {
+			Port int
+		}
+
+		stringToConfig := func(s string) reader.Reader[SimpleConfig] {
+			return func(ctx context.Context) SimpleConfig {
+				return SimpleConfig{Port: len(s) * 100}
+			}
+		}
+
+		// Effect that uses the config
+		configEffect := Chain(func(cfg SimpleConfig) Effect[SimpleConfig, string] {
+			return Of[SimpleConfig](fmt.Sprintf("Port: %d", cfg.Port))
+		})(readerreaderioresult.Ask[SimpleConfig]())
+
+		// Compose transformations
+		step1 := LocalReaderK[string](stringToConfig)
+		step2 := LocalReaderK[string](intToString)
+
+		effect1 := step1(configEffect)
+		effect2 := step2(effect1)
+
+		// With prefix in runtime context
+		ctxWithPrefix := context.WithValue(context.Background(), prefixKey, "test")
+		ioResult := Provide[string](42)(effect2)
+		readerResult := RunSync(ioResult)
+		result, err := readerResult(ctxWithPrefix)
+
+		assert.NoError(t, err)
+		// "test-42" has length 7, so port = 700
+		assert.Equal(t, "Port: 700", result)
+
+		// Without prefix
+		ioResult2 := Provide[string](42)(effect2)
+		readerResult2 := RunSync(ioResult2)
+		result2, err2 := readerResult2(context.Background())
+
+		assert.NoError(t, err2)
+		// "42" has length 2, so port = 200
+		assert.Equal(t, "Port: 200", result2)
+	})
+
+	t.Run("error propagation from Effect", func(t *testing.T) {
+		type SimpleConfig struct {
+			Port int
+		}
+
+		// Reader transformation (pure, cannot fail)
+		loadConfig := func(path string) reader.Reader[SimpleConfig] {
+			return func(ctx context.Context) SimpleConfig {
+				return SimpleConfig{Port: 8080}
+			}
+		}
+
+		// Effect that returns an error
+		expectedErr := assert.AnError
+		failingEffect := Fail[SimpleConfig, string](expectedErr)
+
+		transform := LocalReaderK[string](loadConfig)
+		pathEffect := transform(failingEffect)
+
+		ioResult := Provide[string]("config.json")(pathEffect)
+		readerResult := RunSync(ioResult)
+		_, err := readerResult(context.Background())
+
+		// Error from the Effect should propagate
+		assert.Error(t, err)
+		assert.Equal(t, expectedErr, err)
+	})
+
+	t.Run("real-world: environment selection based on runtime context", func(t *testing.T) {
+		type Environment string
+		const (
+			Dev  Environment = "dev"
+			Prod Environment = "prod"
+		)
+
+		type ctxKey string
+		const envKey ctxKey = "environment"
+
+		type EnvConfig struct {
+			Name string
+		}
+
+		type DetailedConfig struct {
+			Host string
+			Port int
+		}
+
+		// Reader that selects config based on runtime context environment
+		selectConfig := func(envName EnvConfig) reader.Reader[DetailedConfig] {
+			return func(ctx context.Context) DetailedConfig {
+				env := Dev
+				if e, ok := ctx.Value(envKey).(Environment); ok {
+					env = e
+				}
+
+				switch env {
+				case Prod:
+					return DetailedConfig{Host: "api.production.com", Port: 443}
+				default:
+					return DetailedConfig{Host: "localhost", Port: 8080}
+				}
+			}
+		}
+
+		// Effect that uses the selected config
+		configEffect := Chain(func(cfg DetailedConfig) Effect[DetailedConfig, string] {
+			return Of[DetailedConfig](fmt.Sprintf("Connecting to %s:%d", cfg.Host, cfg.Port))
+		})(readerreaderioresult.Ask[DetailedConfig]())
+
+		transform := LocalReaderK[string](selectConfig)
+		envEffect := transform(configEffect)
+
+		// Production environment
+		ctxProd := context.WithValue(context.Background(), envKey, Prod)
+		ioResult := Provide[string](EnvConfig{Name: "app"})(envEffect)
+		readerResult := RunSync(ioResult)
+		result, err := readerResult(ctxProd)
+
+		assert.NoError(t, err)
+		assert.Equal(t, "Connecting to api.production.com:443", result)
+
+		// Development environment (default)
+		ioResult2 := Provide[string](EnvConfig{Name: "app"})(envEffect)
+		readerResult2 := RunSync(ioResult2)
+		result2, err2 := readerResult2(context.Background())
+
+		assert.NoError(t, err2)
+		assert.Equal(t, "Connecting to localhost:8080", result2)
+	})
+
+	t.Run("composes with other Local functions", func(t *testing.T) {
+		type Level1 struct {
+			Value string
+		}
+		type Level2 struct {
+			Data string
+		}
+		type Level3 struct {
+			Info string
+		}
+
+		// Effect at deepest level
+		effect3 := Of[Level3]("result")
+
+		// Use LocalReaderK for first transformation (with runtime context access)
+		localReaderK23 := LocalReaderK[string](func(l2 Level2) reader.Reader[Level3] {
+			return func(ctx context.Context) Level3 {
+				return Level3{Info: l2.Data}
+			}
+		})
+
+		// Use Local for second transformation (pure)
+		local12 := Local[string](func(l1 Level1) Level2 {
+			return Level2{Data: l1.Value}
+		})
+
+		// Compose them
+		effect2 := localReaderK23(effect3)
+		effect1 := local12(effect2)
+
+		// Run
+		ioResult := Provide[string](Level1{Value: "test"})(effect1)
+		readerResult := RunSync(ioResult)
+		result, err := readerResult(context.Background())
+
+		assert.NoError(t, err)
+		assert.Equal(t, "result", result)
+	})
+}
+
+func TestAsk(t *testing.T) {
+	t.Run("returns context as value", func(t *testing.T) {
+		ctx := "my-context"
+		result, err := runEffect(Ask[string](), ctx)
+
+		assert.NoError(t, err)
+		assert.Equal(t, ctx, result)
+	})
+
+	t.Run("works with struct context", func(t *testing.T) {
+		type Config struct {
+			Host string
+			Port int
+		}
+
+		cfg := Config{Host: "localhost", Port: 8080}
+		result, err := runEffect(Ask[Config](), cfg)
+
+		assert.NoError(t, err)
+		assert.Equal(t, cfg, result)
+	})
+
+	t.Run("can be chained with Map to extract a field", func(t *testing.T) {
+		type Config struct {
+			Host string
+			Port int
+		}
+
+		hostEffect := Map[Config](func(cfg Config) string {
+			return cfg.Host
+		})(Ask[Config]())
+
+		result, err := runEffect(hostEffect, Config{Host: "example.com", Port: 443})
+
+		assert.NoError(t, err)
+		assert.Equal(t, "example.com", result)
+	})
+
+	t.Run("can be chained with Chain to produce a derived effect", func(t *testing.T) {
+		type Config struct {
+			APIKey string
+		}
+
+		derived := Chain(func(cfg Config) Effect[Config, string] {
+			if cfg.APIKey == "" {
+				return Fail[Config, string](assert.AnError)
+			}
+			return Of[Config]("authenticated: " + cfg.APIKey)
+		})(Ask[Config]())
+
+		// Valid key
+		result, err := runEffect(derived, Config{APIKey: "secret"})
+		assert.NoError(t, err)
+		assert.Equal(t, "authenticated: secret", result)
+
+		// Empty key
+		_, err = runEffect(derived, Config{APIKey: ""})
+		assert.Error(t, err)
+		assert.Equal(t, assert.AnError, err)
+	})
+
+	t.Run("is idempotent - multiple calls return same context", func(t *testing.T) {
+		ctx := TestContext{Value: "shared"}
+
+		r1, err1 := runEffect(Ask[TestContext](), ctx)
+		r2, err2 := runEffect(Ask[TestContext](), ctx)
+
+		assert.NoError(t, err1)
+		assert.NoError(t, err2)
+		assert.Equal(t, r1, r2)
+	})
+}

v2/effect/effect.go

@@ -1,51 +1,759 @@
+// 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 effect
 
 import (
+	thunk "github.com/IBM/fp-go/v2/context/readerioresult"
 	"github.com/IBM/fp-go/v2/context/readerreaderioresult"
 	"github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/internal/fromreader"
+	"github.com/IBM/fp-go/v2/io"
+	"github.com/IBM/fp-go/v2/reader"
+	"github.com/IBM/fp-go/v2/readerio"
 	"github.com/IBM/fp-go/v2/result"
 )
 
+// FromThunk lifts a Thunk (context-independent IO computation with error handling) into an Effect.
+// This allows you to integrate computations that don't need the effect's context type C
+// into effect chains. The Thunk will be executed with the runtime context when the effect runs.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect (not used by the thunk)
+//   - A: The type of the success value
+//
+// # Parameters
+//
+//   - f: A Thunk[A] that performs IO with error handling
+//
+// # Returns
+//
+//   - Effect[C, A]: An effect that ignores its context and executes the thunk
+//
+// # Example
+//
+//	thunk := func(ctx context.Context) io.IO[result.Result[int]] {
+//	    return func() result.Result[int] {
+//	        // Perform IO operation
+//	        return result.Of(42)
+//	    }
+//	}
+//
+//	eff := effect.FromThunk[MyContext](thunk)
+//	// eff can be used in any context but executes the thunk
+//
+//go:inline
+func FromThunk[C, A any](f Thunk[A]) Effect[C, A] {
+	return reader.Of[C](f)
+}
+
+//go:inline
+func FromResult[C, A any](r Result[A]) Effect[C, A] {
+	return readerreaderioresult.FromEither[C](r)
+}
+
+// Succeed creates a successful Effect that produces the given value.
+// This is the primary way to lift a pure value into the Effect context.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - A: The type of the success value
+//
+// # Parameters
+//
+//   - a: The value to wrap in a successful effect
+//
+// # Returns
+//
+//   - Effect[C, A]: An effect that always succeeds with the given value
+//
+// # Example
+//
+//	eff := effect.Succeed[MyContext](42)
+//	result, err := runEffect(eff, myContext)
+//	// result == 42, err == nil
 func Succeed[C, A any](a A) Effect[C, A] {
 	return readerreaderioresult.Of[C](a)
 }
 
+// Fail creates a failed Effect with the given error.
+// This is used to represent computations that have failed.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - A: The type of the success value (never produced)
+//
+// # Parameters
+//
+//   - err: The error that caused the failure
+//
+// # Returns
+//
+//   - Effect[C, A]: An effect that always fails with the given error
+//
+// # Example
+//
+//	eff := effect.Fail[MyContext, int](errors.New("failed"))
+//	_, err := runEffect(eff, myContext)
+//	// err == errors.New("failed")
 func Fail[C, A any](err error) Effect[C, A] {
 	return readerreaderioresult.Left[C, A](err)
 }
 
+// Of creates a successful Effect that produces the given value.
+// This is an alias for Succeed and follows the pointed functor convention.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - A: The type of the success value
+//
+// # Parameters
+//
+//   - a: The value to wrap in a successful effect
+//
+// # Returns
+//
+//   - Effect[C, A]: An effect that always succeeds with the given value
+//
+// # Example
+//
+//	eff := effect.Of[MyContext]("hello")
+//	result, err := runEffect(eff, myContext)
+//	// result == "hello", err == nil
 func Of[C, A any](a A) Effect[C, A] {
 	return readerreaderioresult.Of[C](a)
 }
 
+// Map transforms the success value of an Effect using the provided function.
+// If the effect fails, the error is propagated unchanged.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - A: The input value type
+//   - B: The output value type
+//
+// # Parameters
+//
+//   - f: The transformation function to apply to the success value
+//
+// # Returns
+//
+//   - Operator[C, A, B]: A function that transforms Effect[C, A] to Effect[C, B]
+//
+// # Example
+//
+//	eff := effect.Of[MyContext](42)
+//	mapped := effect.Map[MyContext](func(x int) string {
+//		return strconv.Itoa(x)
+//	})(eff)
+//	// mapped produces "42"
 func Map[C, A, B any](f func(A) B) Operator[C, A, B] {
 	return readerreaderioresult.Map[C](f)
 }
 
+// Chain sequences two effects, where the second effect depends on the result of the first.
+// This is the monadic bind operation (flatMap) for effects.
+// If the first effect fails, the second is not executed.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effects
+//   - A: The input value type
+//   - B: The output value type
+//
+// # Parameters
+//
+//   - f: A function that takes the result of the first effect and returns a new effect
+//
+// # Returns
+//
+//   - Operator[C, A, B]: A function that transforms Effect[C, A] to Effect[C, B]
+//
+// # Example
+//
+//	eff := effect.Of[MyContext](42)
+//	chained := effect.Chain[MyContext](func(x int) Effect[MyContext, string] {
+//		return effect.Of[MyContext](strconv.Itoa(x * 2))
+//	})(eff)
+//	// chained produces "84"
+//
+//go:inline
 func Chain[C, A, B any](f Kleisli[C, A, B]) Operator[C, A, B] {
 	return readerreaderioresult.Chain(f)
 }
 
+//go:inline
+func ChainFirst[C, A, B any](f Kleisli[C, A, B]) Operator[C, A, A] {
+	return readerreaderioresult.ChainFirst(f)
+}
+
+// ChainFirstThunkK chains an effect with a function that returns a Thunk,
+// but discards the result and returns the original value.
+// This is useful for performing side effects (like logging or IO operations) that don't
+// need the effect's context, without changing the value flowing through the computation.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - A: The value type (preserved)
+//   - B: The type produced by the Thunk (discarded)
+//
+// # Parameters
+//
+//   - f: A function that takes A and returns Thunk[B] for side effects
+//
+// # Returns
+//
+//   - Operator[C, A, A]: A function that executes the Thunk but preserves the original value
+//
+// # Example
+//
+//	logToFile := func(n int) readerioresult.ReaderIOResult[any] {
+//	    return func(ctx context.Context) io.IO[result.Result[any]] {
+//	        return func() result.Result[any] {
+//	            // Perform IO operation that doesn't need effect context
+//	            fmt.Printf("Logging: %d\n", n)
+//	            return result.Of[any](nil)
+//	        }
+//	    }
+//	}
+//
+//	eff := effect.Of[MyContext](42)
+//	logged := effect.ChainFirstThunkK[MyContext](logToFile)(eff)
+//	// Prints "Logging: 42" but still produces 42
+//
+// # See Also
+//
+//   - ChainThunkK: Chains with a Thunk and uses its result
+//   - TapThunkK: Alias for ChainFirstThunkK
+//   - ChainFirstIOK: Similar but for IO operations
+//
+//go:inline
+func ChainFirstThunkK[C, A, B any](f thunk.Kleisli[A, B]) Operator[C, A, A] {
+	return fromreader.ChainFirstReaderK(
+		ChainFirst[C, A, B],
+		FromThunk[C, B],
+		f,
+	)
+}
+
+// TapThunkK is an alias for ChainFirstThunkK.
+// It chains an effect with a function that returns a Thunk for side effects,
+// but preserves the original value. This is useful for logging, debugging, or
+// performing IO operations that don't need the effect's context.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - A: The value type (preserved)
+//   - B: The type produced by the Thunk (discarded)
+//
+// # Parameters
+//
+//   - f: A function that takes A and returns Thunk[B] for side effects
+//
+// # Returns
+//
+//   - Operator[C, A, A]: A function that executes the Thunk but preserves the original value
+//
+// # Example
+//
+//	performSideEffect := func(n int) readerioresult.ReaderIOResult[any] {
+//	    return func(ctx context.Context) io.IO[result.Result[any]] {
+//	        return func() result.Result[any] {
+//	            // Perform context-independent IO operation
+//	            log.Printf("Processing value: %d", n)
+//	            return result.Of[any](nil)
+//	        }
+//	    }
+//	}
+//
+//	eff := effect.Of[MyContext](42)
+//	tapped := effect.TapThunkK[MyContext](performSideEffect)(eff)
+//	// Logs "Processing value: 42" but still produces 42
+//
+// # See Also
+//
+//   - ChainFirstThunkK: The underlying implementation
+//   - TapIOK: Similar but for IO operations
+//   - Tap: Similar but for full effects
+//
+//go:inline
+func TapThunkK[C, A, B any](f thunk.Kleisli[A, B]) Operator[C, A, A] {
+	return ChainFirstThunkK[C](f)
+}
+
+// ChainIOK chains an effect with a function that returns an IO action.
+// This is useful for integrating IO-based computations (synchronous side effects)
+// into effect chains. The IO action is automatically lifted into the Effect context.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - A: The input value type
+//   - B: The output value type
+//
+// # Parameters
+//
+//   - f: A function that takes A and returns IO[B]
+//
+// # Returns
+//
+//   - Operator[C, A, B]: A function that chains the IO-returning function with the effect
+//
+// # Example
+//
+//	performIO := func(n int) io.IO[string] {
+//	    return func() string {
+//	        // Perform synchronous side effect
+//	        return fmt.Sprintf("Value: %d", n)
+//	    }
+//	}
+//
+//	eff := effect.Of[MyContext](42)
+//	chained := effect.ChainIOK[MyContext](performIO)(eff)
+//	// chained produces "Value: 42"
+//
+//go:inline
+func ChainIOK[C, A, B any](f io.Kleisli[A, B]) Operator[C, A, B] {
+	return readerreaderioresult.ChainIOK[C](f)
+}
+
+// ChainFirstIOK chains an effect with a function that returns an IO action,
+// but discards the result and returns the original value.
+// This is useful for performing side effects (like logging) without changing the value.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - A: The value type (preserved)
+//   - B: The type produced by the IO action (discarded)
+//
+// # Parameters
+//
+//   - f: A function that takes A and returns IO[B] for side effects
+//
+// # Returns
+//
+//   - Operator[C, A, A]: A function that executes the IO action but preserves the original value
+//
+// # Example
+//
+//	logValue := func(n int) io.IO[any] {
+//	    return func() any {
+//	        fmt.Printf("Processing: %d\n", n)
+//	        return nil
+//	    }
+//	}
+//
+//	eff := effect.Of[MyContext](42)
+//	logged := effect.ChainFirstIOK[MyContext](logValue)(eff)
+//	// Prints "Processing: 42" but still produces 42
+//
+//go:inline
+func ChainFirstIOK[C, A, B any](f io.Kleisli[A, B]) Operator[C, A, A] {
+	return readerreaderioresult.ChainFirstIOK[C](f)
+}
+
+// TapIOK is an alias for ChainFirstIOK.
+// It chains an effect with a function that returns an IO action for side effects,
+// but preserves the original value. This is useful for logging, debugging, or
+// performing actions without changing the result.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - A: The value type (preserved)
+//   - B: The type produced by the IO action (discarded)
+//
+// # Parameters
+//
+//   - f: A function that takes A and returns IO[B] for side effects
+//
+// # Returns
+//
+//   - Operator[C, A, A]: A function that executes the IO action but preserves the original value
+//
+// # Example
+//
+//	logValue := func(n int) io.IO[any] {
+//	    return func() any {
+//	        fmt.Printf("Value: %d\n", n)
+//	        return nil
+//	    }
+//	}
+//
+//	eff := effect.Of[MyContext](42)
+//	tapped := effect.TapIOK[MyContext](logValue)(eff)
+//	// Prints "Value: 42" but still produces 42
+//
+//go:inline
+func TapIOK[C, A, B any](f io.Kleisli[A, B]) Operator[C, A, A] {
+	return readerreaderioresult.ChainFirstIOK[C](f)
+}
+
+// Ap applies a function wrapped in an Effect to a value wrapped in an Effect.
+// This is the applicative apply operation, useful for applying effects in parallel.
+//
+// # Type Parameters
+//
+//   - B: The output value type
+//   - C: The context type required by the effects
+//   - A: The input value type
+//
+// # Parameters
+//
+//   - fa: The effect containing the value to apply the function to
+//
+// # Returns
+//
+//   - Operator[C, func(A) B, B]: A function that applies the function effect to the value effect
+//
+// # Example
+//
+//	fnEff := effect.Of[MyContext](func(x int) int { return x * 2 })
+//	valEff := effect.Of[MyContext](21)
+//	result := effect.Ap[int](valEff)(fnEff)
+//	// result produces 42
 func Ap[B, C, A any](fa Effect[C, A]) Operator[C, func(A) B, B] {
 	return readerreaderioresult.Ap[B](fa)
 }
 
+// Suspend delays the evaluation of an effect until it is run.
+// This is useful for recursive effects or when you need lazy evaluation.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - A: The type of the success value
+//
+// # Parameters
+//
+//   - fa: A lazy computation that produces an effect
+//
+// # Returns
+//
+//   - Effect[C, A]: An effect that evaluates the lazy computation when run
+//
+// # Example
+//
+//	var recursiveEff func(int) Effect[MyContext, int]
+//	recursiveEff = func(n int) Effect[MyContext, int] {
+//		if n <= 0 {
+//			return effect.Of[MyContext](0)
+//		}
+//		return effect.Suspend(func() Effect[MyContext, int] {
+//			return effect.Map[MyContext](func(x int) int {
+//				return x + n
+//			})(recursiveEff(n - 1))
+//		})
+//	}
 func Suspend[C, A any](fa Lazy[Effect[C, A]]) Effect[C, A] {
 	return readerreaderioresult.Defer(fa)
 }
 
+// Tap executes a side effect for its effect, but returns the original value.
+// This is useful for logging, debugging, or performing actions without changing the result.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effects
+//   - A: The value type
+//   - ANY: The type produced by the side effect (ignored)
+//
+// # Parameters
+//
+//   - f: A function that performs a side effect based on the value
+//
+// # Returns
+//
+//   - Operator[C, A, A]: A function that executes the side effect but preserves the original value
+//
+// # Example
+//
+//	eff := effect.Of[MyContext](42)
+//	tapped := effect.Tap[MyContext](func(x int) Effect[MyContext, any] {
+//		fmt.Println("Value:", x)
+//		return effect.Of[MyContext, any](nil)
+//	})(eff)
+//	// Prints "Value: 42" but still produces 42
 func Tap[C, A, ANY any](f Kleisli[C, A, ANY]) Operator[C, A, A] {
 	return readerreaderioresult.Tap(f)
 }
 
+// Ternary creates a conditional effect based on a predicate.
+// If the predicate returns true, onTrue is executed; otherwise, onFalse is executed.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effects
+//   - A: The input value type
+//   - B: The output value type
+//
+// # Parameters
+//
+//   - pred: A predicate function to test the input value
+//   - onTrue: The effect to execute if the predicate is true
+//   - onFalse: The effect to execute if the predicate is false
+//
+// # Returns
+//
+//   - Kleisli[C, A, B]: A function that conditionally executes one of two effects
+//
+// # Example
+//
+//	kleisli := effect.Ternary(
+//		func(x int) bool { return x > 10 },
+//		func(x int) Effect[MyContext, string] {
+//			return effect.Of[MyContext]("large")
+//		},
+//		func(x int) Effect[MyContext, string] {
+//			return effect.Of[MyContext]("small")
+//		},
+//	)
+//	result := kleisli(15) // produces "large"
 func Ternary[C, A, B any](pred Predicate[A], onTrue, onFalse Kleisli[C, A, B]) Kleisli[C, A, B] {
 	return function.Ternary(pred, onTrue, onFalse)
 }
 
+// ChainResultK chains an effect with a function that returns a Result.
+// This is useful for integrating Result-based computations into effect chains.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - A: The input value type
+//   - B: The output value type
+//
+// # Parameters
+//
+//   - f: A function that takes A and returns Result[B]
+//
+// # Returns
+//
+//   - Operator[C, A, B]: A function that chains the Result-returning function with the effect
+//
+// # Example
+//
+//	parseIntResult := result.Eitherize1(strconv.Atoi)
+//	eff := effect.Of[MyContext]("42")
+//	chained := effect.ChainResultK[MyContext](parseIntResult)(eff)
+//	// chained produces 42 as an int
+//
+//go:inline
 func ChainResultK[C, A, B any](f result.Kleisli[A, B]) Operator[C, A, B] {
 	return readerreaderioresult.ChainResultK[C](f)
 }
 
+// ChainReaderK chains an effect with a function that returns a Reader.
+// This is useful for integrating Reader-based computations (pure context-dependent functions)
+// into effect chains. The Reader is automatically lifted into the Effect context.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - A: The input value type
+//   - B: The output value type
+//
+// # Parameters
+//
+//   - f: A function that takes A and returns Reader[C, B]
+//
+// # Returns
+//
+//   - Operator[C, A, B]: A function that chains the Reader-returning function with the effect
+//
+// # Example
+//
+//	type Config struct { Multiplier int }
+//
+//	getMultiplied := func(n int) reader.Reader[Config, int] {
+//	    return func(cfg Config) int {
+//	        return n * cfg.Multiplier
+//	    }
+//	}
+//
+//	eff := effect.Of[Config](5)
+//	chained := effect.ChainReaderK[Config](getMultiplied)(eff)
+//	// With Config{Multiplier: 3}, produces 15
+//
+//go:inline
+func ChainReaderK[C, A, B any](f reader.Kleisli[C, A, B]) Operator[C, A, B] {
+	return readerreaderioresult.ChainReaderK(f)
+}
+
+// ChainThunkK chains an effect with a function that returns a Thunk.
+// This is useful for integrating Thunk-based computations (context-independent IO with error handling)
+// into effect chains. The Thunk is automatically lifted into the Effect context.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - A: The input value type
+//   - B: The output value type
+//
+// # Parameters
+//
+//   - f: A function that takes A and returns Thunk[B] (readerioresult.Kleisli[A, B])
+//
+// # Returns
+//
+//   - Operator[C, A, B]: A function that chains the Thunk-returning function with the effect
+//
+// # Example
+//
+//	performIO := func(n int) readerioresult.ReaderIOResult[string] {
+//	    return func(ctx context.Context) io.IO[result.Result[string]] {
+//	        return func() result.Result[string] {
+//	            // Perform IO operation that doesn't need effect context
+//	            return result.Of(fmt.Sprintf("Processed: %d", n))
+//	        }
+//	    }
+//	}
+//
+//	eff := effect.Of[MyContext](42)
+//	chained := effect.ChainThunkK[MyContext](performIO)(eff)
+//	// chained produces "Processed: 42"
+//
+//go:inline
+func ChainThunkK[C, A, B any](f thunk.Kleisli[A, B]) Operator[C, A, B] {
+	return fromreader.ChainReaderK(
+		Chain[C, A, B],
+		FromThunk[C, B],
+		f,
+	)
+}
+
+// ChainReaderIOK chains an effect with a function that returns a ReaderIO.
+// This is useful for integrating ReaderIO-based computations (context-dependent IO operations)
+// into effect chains. The ReaderIO is automatically lifted into the Effect context.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - A: The input value type
+//   - B: The output value type
+//
+// # Parameters
+//
+//   - f: A function that takes A and returns ReaderIO[C, B]
+//
+// # Returns
+//
+//   - Operator[C, A, B]: A function that chains the ReaderIO-returning function with the effect
+//
+// # Example
+//
+//	type Config struct { LogPrefix string }
+//
+//	logAndDouble := func(n int) readerio.ReaderIO[Config, int] {
+//	    return func(cfg Config) io.IO[int] {
+//	        return func() int {
+//	            fmt.Printf("%s: %d\n", cfg.LogPrefix, n)
+//	            return n * 2
+//	        }
+//	    }
+//	}
+//
+//	eff := effect.Of[Config](21)
+//	chained := effect.ChainReaderIOK[Config](logAndDouble)(eff)
+//	// Logs "prefix: 21" and produces 42
+//
+//go:inline
+func ChainReaderIOK[C, A, B any](f readerio.Kleisli[C, A, B]) Operator[C, A, B] {
+	return readerreaderioresult.ChainReaderIOK(f)
+}
+
+// Read provides a context to an effect, partially applying it.
+// This converts an Effect[C, A] to a Thunk[A] by supplying the required context.
+//
+// # Type Parameters
+//
+//   - A: The type of the success value
+//   - C: The context type
+//
+// # Parameters
+//
+//   - c: The context to provide to the effect
+//
+// # Returns
+//
+//   - func(Effect[C, A]) Thunk[A]: A function that converts an effect to a thunk
+//
+// # Example
+//
+//	ctx := MyContext{Value: "test"}
+//	eff := effect.Of[MyContext](42)
+//	thunk := effect.Read[int](ctx)(eff)
+//	// thunk is now a Thunk[int] that can be run without context
+//
+//go:inline
 func Read[A, C any](c C) func(Effect[C, A]) Thunk[A] {
 	return readerreaderioresult.Read[A](c)
 }
+
+// Asks creates an Effect that projects a value from the context using a Reader function.
+// This is useful for extracting specific fields or computing derived values from the context.
+// It's essentially a lifted version of the Reader pattern into the Effect context.
+//
+// # Type Parameters
+//
+//   - C: The context type
+//   - A: The type of the projected value
+//
+// # Parameters
+//
+//   - r: A Reader function that extracts or computes a value from the context
+//
+// # Returns
+//
+//   - Effect[C, A]: An effect that succeeds with the projected value
+//
+// # Example
+//
+//	type Config struct {
+//		Host string
+//		Port int
+//	}
+//
+//	// Extract a specific field
+//	getHost := effect.Asks[Config](func(cfg Config) string {
+//		return cfg.Host
+//	})
+//
+//	// Compute a derived value
+//	getURL := effect.Asks[Config](func(cfg Config) string {
+//		return fmt.Sprintf("http://%s:%d", cfg.Host, cfg.Port)
+//	})
+//
+//	result, err := runEffect(getHost, Config{Host: "localhost", Port: 8080})
+//	// result == "localhost", err == nil
+//
+// # See Also
+//
+// See Also:
+//
+//   - Ask: Returns the entire context as the value
+//   - Map: Transforms the value after extraction
+//
+//go:inline
+func Asks[C, A any](r Reader[C, A]) Effect[C, A] {
+	return readerreaderioresult.Asks(r)
+}

v2/effect/effect_additional_test.go

@@ -0,0 +1,649 @@
+// 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 effect
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"strconv"
+	"testing"
+
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/io"
+	"github.com/IBM/fp-go/v2/reader"
+	"github.com/IBM/fp-go/v2/result"
+	"github.com/stretchr/testify/assert"
+)
+
+// TestSucceed tests the Succeed function
+func TestSucceed_Success(t *testing.T) {
+	t.Run("creates successful effect with int", func(t *testing.T) {
+		eff := Succeed[TestConfig](42)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Of(42), outcome)
+	})
+
+	t.Run("creates successful effect with string", func(t *testing.T) {
+		eff := Succeed[TestConfig]("hello")
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Of("hello"), outcome)
+	})
+
+	t.Run("creates successful effect with zero value", func(t *testing.T) {
+		eff := Succeed[TestConfig](0)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Of(0), outcome)
+	})
+}
+
+// TestFail tests the Fail function
+func TestFail_Failure(t *testing.T) {
+	t.Run("creates failed effect with error", func(t *testing.T) {
+		testErr := errors.New("test error")
+		eff := Fail[TestConfig, int](testErr)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Left[int](testErr), outcome)
+	})
+
+	t.Run("preserves error message", func(t *testing.T) {
+		testErr := errors.New("specific error message")
+		eff := Fail[TestConfig, string](testErr)
+		outcome := eff(testConfig)(context.Background())()
+		assert.True(t, result.IsLeft(outcome))
+		extractedErr := result.MonadFold(outcome,
+			F.Identity[error],
+			func(string) error { return nil },
+		)
+		assert.Equal(t, testErr, extractedErr)
+	})
+}
+
+// TestOf tests the Of function
+func TestOf_Success(t *testing.T) {
+	t.Run("creates successful effect with value", func(t *testing.T) {
+		eff := Of[TestConfig](100)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Of(100), outcome)
+	})
+
+	t.Run("is equivalent to Succeed", func(t *testing.T) {
+		value := "test"
+		eff1 := Of[TestConfig](value)
+		eff2 := Succeed[TestConfig](value)
+		outcome1 := eff1(testConfig)(context.Background())()
+		outcome2 := eff2(testConfig)(context.Background())()
+		assert.Equal(t, outcome1, outcome2)
+	})
+}
+
+// TestMap tests the Map function
+func TestMap_Success(t *testing.T) {
+	t.Run("transforms success value", func(t *testing.T) {
+		eff := F.Pipe1(
+			Of[TestConfig](42),
+			Map[TestConfig](func(x int) int { return x * 2 }),
+		)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Of(84), outcome)
+	})
+
+	t.Run("transforms type", func(t *testing.T) {
+		eff := F.Pipe1(
+			Of[TestConfig](42),
+			Map[TestConfig](func(x int) string { return strconv.Itoa(x) }),
+		)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Of("42"), outcome)
+	})
+
+	t.Run("chains multiple maps", func(t *testing.T) {
+		eff := F.Pipe2(
+			Of[TestConfig](10),
+			Map[TestConfig](func(x int) int { return x + 5 }),
+			Map[TestConfig](func(x int) int { return x * 2 }),
+		)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Of(30), outcome)
+	})
+}
+
+func TestMap_Failure(t *testing.T) {
+	t.Run("propagates error unchanged", func(t *testing.T) {
+		testErr := errors.New("test error")
+		eff := F.Pipe1(
+			Fail[TestConfig, int](testErr),
+			Map[TestConfig](func(x int) int { return x * 2 }),
+		)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Left[int](testErr), outcome)
+	})
+}
+
+// TestChain tests the Chain function
+func TestChain_Success(t *testing.T) {
+	t.Run("sequences two effects", func(t *testing.T) {
+		eff := F.Pipe1(
+			Of[TestConfig](42),
+			Chain(func(x int) Effect[TestConfig, string] {
+				return Of[TestConfig](strconv.Itoa(x))
+			}),
+		)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Of("42"), outcome)
+	})
+
+	t.Run("chains multiple effects", func(t *testing.T) {
+		eff := F.Pipe2(
+			Of[TestConfig](10),
+			Chain(func(x int) Effect[TestConfig, int] {
+				return Of[TestConfig](x + 5)
+			}),
+			Chain(func(x int) Effect[TestConfig, int] {
+				return Of[TestConfig](x * 2)
+			}),
+		)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Of(30), outcome)
+	})
+}
+
+func TestChain_Failure(t *testing.T) {
+	t.Run("propagates error from first effect", func(t *testing.T) {
+		testErr := errors.New("first error")
+		eff := F.Pipe1(
+			Fail[TestConfig, int](testErr),
+			Chain(func(x int) Effect[TestConfig, string] {
+				return Of[TestConfig]("should not execute")
+			}),
+		)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Left[string](testErr), outcome)
+	})
+
+	t.Run("propagates error from second effect", func(t *testing.T) {
+		testErr := errors.New("second error")
+		eff := F.Pipe1(
+			Of[TestConfig](42),
+			Chain(func(x int) Effect[TestConfig, string] {
+				return Fail[TestConfig, string](testErr)
+			}),
+		)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Left[string](testErr), outcome)
+	})
+}
+
+// TestChainIOK tests the ChainIOK function
+func TestChainIOK_Success(t *testing.T) {
+	t.Run("chains with IO action", func(t *testing.T) {
+		counter := 0
+		eff := F.Pipe1(
+			Of[TestConfig](42),
+			ChainIOK[TestConfig](func(x int) io.IO[string] {
+				return func() string {
+					counter++
+					return fmt.Sprintf("Value: %d", x)
+				}
+			}),
+		)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Of("Value: 42"), outcome)
+		assert.Equal(t, 1, counter)
+	})
+
+	t.Run("chains multiple IO actions", func(t *testing.T) {
+		log := []string{}
+		eff := F.Pipe2(
+			Of[TestConfig](10),
+			ChainIOK[TestConfig](func(x int) io.IO[int] {
+				return func() int {
+					log = append(log, "first")
+					return x + 5
+				}
+			}),
+			ChainIOK[TestConfig](func(x int) io.IO[int] {
+				return func() int {
+					log = append(log, "second")
+					return x * 2
+				}
+			}),
+		)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Of(30), outcome)
+		assert.Equal(t, []string{"first", "second"}, log)
+	})
+}
+
+func TestChainIOK_Failure(t *testing.T) {
+	t.Run("propagates error from previous effect", func(t *testing.T) {
+		testErr := errors.New("test error")
+		executed := false
+		eff := F.Pipe1(
+			Fail[TestConfig, int](testErr),
+			ChainIOK[TestConfig](func(x int) io.IO[string] {
+				return func() string {
+					executed = true
+					return "should not execute"
+				}
+			}),
+		)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Left[string](testErr), outcome)
+		assert.False(t, executed)
+	})
+}
+
+// TestChainFirstIOK tests the ChainFirstIOK function
+func TestChainFirstIOK_Success(t *testing.T) {
+	t.Run("executes IO but preserves value", func(t *testing.T) {
+		log := []string{}
+		eff := F.Pipe1(
+			Of[TestConfig](42),
+			ChainFirstIOK[TestConfig](func(x int) io.IO[any] {
+				return func() any {
+					log = append(log, fmt.Sprintf("logged: %d", x))
+					return nil
+				}
+			}),
+		)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Of(42), outcome)
+		assert.Equal(t, []string{"logged: 42"}, log)
+	})
+
+	t.Run("chains multiple side effects", func(t *testing.T) {
+		log := []string{}
+		eff := F.Pipe2(
+			Of[TestConfig](10),
+			ChainFirstIOK[TestConfig](func(x int) io.IO[any] {
+				return func() any {
+					log = append(log, "first")
+					return nil
+				}
+			}),
+			ChainFirstIOK[TestConfig](func(x int) io.IO[any] {
+				return func() any {
+					log = append(log, "second")
+					return nil
+				}
+			}),
+		)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Of(10), outcome)
+		assert.Equal(t, []string{"first", "second"}, log)
+	})
+}
+
+func TestChainFirstIOK_Failure(t *testing.T) {
+	t.Run("propagates error without executing IO", func(t *testing.T) {
+		testErr := errors.New("test error")
+		executed := false
+		eff := F.Pipe1(
+			Fail[TestConfig, int](testErr),
+			ChainFirstIOK[TestConfig](func(x int) io.IO[any] {
+				return func() any {
+					executed = true
+					return nil
+				}
+			}),
+		)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Left[int](testErr), outcome)
+		assert.False(t, executed)
+	})
+}
+
+// TestTapIOK tests the TapIOK function
+func TestTapIOK_Success(t *testing.T) {
+	t.Run("executes IO but preserves value", func(t *testing.T) {
+		log := []string{}
+		eff := F.Pipe1(
+			Of[TestConfig](42),
+			TapIOK[TestConfig](func(x int) io.IO[any] {
+				return func() any {
+					log = append(log, fmt.Sprintf("tapped: %d", x))
+					return nil
+				}
+			}),
+		)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Of(42), outcome)
+		assert.Equal(t, []string{"tapped: 42"}, log)
+	})
+
+	t.Run("is equivalent to ChainFirstIOK", func(t *testing.T) {
+		log1 := []string{}
+		log2 := []string{}
+
+		eff1 := F.Pipe1(
+			Of[TestConfig](10),
+			TapIOK[TestConfig](func(x int) io.IO[any] {
+				return func() any {
+					log1 = append(log1, "tap")
+					return nil
+				}
+			}),
+		)
+
+		eff2 := F.Pipe1(
+			Of[TestConfig](10),
+			ChainFirstIOK[TestConfig](func(x int) io.IO[any] {
+				return func() any {
+					log2 = append(log2, "tap")
+					return nil
+				}
+			}),
+		)
+
+		outcome1 := eff1(testConfig)(context.Background())()
+		outcome2 := eff2(testConfig)(context.Background())()
+
+		assert.Equal(t, outcome1, outcome2)
+		assert.Equal(t, log1, log2)
+	})
+}
+
+func TestTapIOK_Failure(t *testing.T) {
+	t.Run("propagates error without executing IO", func(t *testing.T) {
+		testErr := errors.New("test error")
+		executed := false
+		eff := F.Pipe1(
+			Fail[TestConfig, int](testErr),
+			TapIOK[TestConfig](func(x int) io.IO[any] {
+				return func() any {
+					executed = true
+					return nil
+				}
+			}),
+		)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Left[int](testErr), outcome)
+		assert.False(t, executed)
+	})
+}
+
+// TestChainResultK tests the ChainResultK function
+func TestChainResultK_Success(t *testing.T) {
+	t.Run("chains with Result-returning function", func(t *testing.T) {
+		parseIntResult := result.Eitherize1(strconv.Atoi)
+		eff := F.Pipe1(
+			Of[TestConfig]("42"),
+			ChainResultK[TestConfig](parseIntResult),
+		)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Of(42), outcome)
+	})
+
+	t.Run("chains multiple Result operations", func(t *testing.T) {
+		parseIntResult := result.Eitherize1(strconv.Atoi)
+		eff := F.Pipe2(
+			Of[TestConfig]("10"),
+			ChainResultK[TestConfig](parseIntResult),
+			ChainResultK[TestConfig](func(x int) result.Result[string] {
+				return result.Of(fmt.Sprintf("Value: %d", x*2))
+			}),
+		)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Of("Value: 20"), outcome)
+	})
+}
+
+func TestChainResultK_Failure(t *testing.T) {
+	t.Run("propagates error from previous effect", func(t *testing.T) {
+		testErr := errors.New("test error")
+		parseIntResult := result.Eitherize1(strconv.Atoi)
+		eff := F.Pipe1(
+			Fail[TestConfig, string](testErr),
+			ChainResultK[TestConfig](parseIntResult),
+		)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Left[int](testErr), outcome)
+	})
+
+	t.Run("propagates error from Result function", func(t *testing.T) {
+		parseIntResult := result.Eitherize1(strconv.Atoi)
+		eff := F.Pipe1(
+			Of[TestConfig]("not a number"),
+			ChainResultK[TestConfig](parseIntResult),
+		)
+		outcome := eff(testConfig)(context.Background())()
+		assert.True(t, result.IsLeft(outcome))
+	})
+}
+
+// TestAp tests the Ap function
+func TestAp_Success(t *testing.T) {
+	t.Run("applies function effect to value effect", func(t *testing.T) {
+		fnEff := Of[TestConfig](func(x int) int { return x * 2 })
+		valEff := Of[TestConfig](21)
+		eff := Ap[int](valEff)(fnEff)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Of(42), outcome)
+	})
+
+	t.Run("applies function with different types", func(t *testing.T) {
+		fnEff := Of[TestConfig](func(x int) string { return strconv.Itoa(x) })
+		valEff := Of[TestConfig](42)
+		eff := Ap[string](valEff)(fnEff)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Of("42"), outcome)
+	})
+}
+
+func TestAp_Failure(t *testing.T) {
+	t.Run("propagates error from function effect", func(t *testing.T) {
+		testErr := errors.New("function error")
+		fnEff := Fail[TestConfig, func(int) int](testErr)
+		valEff := Of[TestConfig](42)
+		eff := Ap[int](valEff)(fnEff)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Left[int](testErr), outcome)
+	})
+
+	t.Run("propagates error from value effect", func(t *testing.T) {
+		testErr := errors.New("value error")
+		fnEff := Of[TestConfig](func(x int) int { return x * 2 })
+		valEff := Fail[TestConfig, int](testErr)
+		eff := Ap[int](valEff)(fnEff)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Left[int](testErr), outcome)
+	})
+}
+
+// TestSuspend tests the Suspend function
+func TestSuspend_Success(t *testing.T) {
+	t.Run("delays evaluation of effect", func(t *testing.T) {
+		counter := 0
+		eff := Suspend(func() Effect[TestConfig, int] {
+			counter++
+			return Of[TestConfig](42)
+		})
+		assert.Equal(t, 0, counter, "should not evaluate immediately")
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, 1, counter, "should evaluate when run")
+		assert.Equal(t, result.Of(42), outcome)
+	})
+
+	t.Run("enables recursive effects", func(t *testing.T) {
+		var factorial func(int) Effect[TestConfig, int]
+		factorial = func(n int) Effect[TestConfig, int] {
+			if n <= 1 {
+				return Of[TestConfig](1)
+			}
+			return Suspend(func() Effect[TestConfig, int] {
+				return F.Pipe1(
+					factorial(n-1),
+					Map[TestConfig](func(x int) int { return x * n }),
+				)
+			})
+		}
+
+		outcome := factorial(5)(testConfig)(context.Background())()
+		assert.Equal(t, result.Of(120), outcome)
+	})
+}
+
+// TestTap tests the Tap function
+func TestTap_Success(t *testing.T) {
+	t.Run("executes side effect but preserves value", func(t *testing.T) {
+		log := []string{}
+		eff := F.Pipe1(
+			Of[TestConfig](42),
+			Tap(func(x int) Effect[TestConfig, any] {
+				log = append(log, fmt.Sprintf("tapped: %d", x))
+				return Of[TestConfig, any](nil)
+			}),
+		)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Of(42), outcome)
+		assert.Equal(t, []string{"tapped: 42"}, log)
+	})
+
+	t.Run("chains multiple taps", func(t *testing.T) {
+		log := []string{}
+		eff := F.Pipe2(
+			Of[TestConfig](10),
+			Tap(func(x int) Effect[TestConfig, any] {
+				log = append(log, "first")
+				return Of[TestConfig, any](nil)
+			}),
+			Tap(func(x int) Effect[TestConfig, any] {
+				log = append(log, "second")
+				return Of[TestConfig, any](nil)
+			}),
+		)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Of(10), outcome)
+		assert.Equal(t, []string{"first", "second"}, log)
+	})
+}
+
+func TestTap_Failure(t *testing.T) {
+	t.Run("propagates error without executing tap", func(t *testing.T) {
+		testErr := errors.New("test error")
+		executed := false
+		eff := F.Pipe1(
+			Fail[TestConfig, int](testErr),
+			Tap(func(x int) Effect[TestConfig, any] {
+				executed = true
+				return Of[TestConfig, any](nil)
+			}),
+		)
+		outcome := eff(testConfig)(context.Background())()
+		assert.Equal(t, result.Left[int](testErr), outcome)
+		assert.False(t, executed)
+	})
+}
+
+// TestTernary tests the Ternary function
+func TestTernary_Success(t *testing.T) {
+	t.Run("executes onTrue when predicate is true", func(t *testing.T) {
+		kleisli := Ternary(
+			func(x int) bool { return x > 10 },
+			func(x int) Effect[TestConfig, string] {
+				return Of[TestConfig]("large")
+			},
+			func(x int) Effect[TestConfig, string] {
+				return Of[TestConfig]("small")
+			},
+		)
+		outcome := kleisli(15)(testConfig)(context.Background())()
+		assert.Equal(t, result.Of("large"), outcome)
+	})
+
+	t.Run("executes onFalse when predicate is false", func(t *testing.T) {
+		kleisli := Ternary(
+			func(x int) bool { return x > 10 },
+			func(x int) Effect[TestConfig, string] {
+				return Of[TestConfig]("large")
+			},
+			func(x int) Effect[TestConfig, string] {
+				return Of[TestConfig]("small")
+			},
+		)
+		outcome := kleisli(5)(testConfig)(context.Background())()
+		assert.Equal(t, result.Of("small"), outcome)
+	})
+
+	t.Run("works with boundary value", func(t *testing.T) {
+		kleisli := Ternary(
+			func(x int) bool { return x >= 10 },
+			func(x int) Effect[TestConfig, string] {
+				return Of[TestConfig]("gte")
+			},
+			func(x int) Effect[TestConfig, string] {
+				return Of[TestConfig]("lt")
+			},
+		)
+		outcome := kleisli(10)(testConfig)(context.Background())()
+		assert.Equal(t, result.Of("gte"), outcome)
+	})
+}
+
+// TestRead tests the Read function
+func TestRead_Success(t *testing.T) {
+	t.Run("provides context to effect", func(t *testing.T) {
+		eff := Of[TestConfig](42)
+		thunk := Read[int](testConfig)(eff)
+		outcome := thunk(context.Background())()
+		assert.Equal(t, result.Of(42), outcome)
+	})
+
+	t.Run("converts effect to thunk", func(t *testing.T) {
+		eff := F.Pipe1(
+			Of[TestConfig](10),
+			Map[TestConfig](func(x int) int { return x * testConfig.Multiplier }),
+		)
+		thunk := Read[int](testConfig)(eff)
+		outcome := thunk(context.Background())()
+		assert.Equal(t, result.Of(30), outcome)
+	})
+
+	t.Run("works with different contexts", func(t *testing.T) {
+		cfg1 := TestConfig{Multiplier: 2, Prefix: "A", DatabaseURL: ""}
+		cfg2 := TestConfig{Multiplier: 5, Prefix: "B", DatabaseURL: ""}
+
+		// Create an effect that uses the context's Multiplier
+		eff := F.Pipe1(
+			Of[TestConfig](10),
+			ChainReaderK(func(x int) reader.Reader[TestConfig, int] {
+				return func(cfg TestConfig) int {
+					return x * cfg.Multiplier
+				}
+			}),
+		)
+
+		thunk1 := Read[int](cfg1)(eff)
+		thunk2 := Read[int](cfg2)(eff)
+
+		outcome1 := thunk1(context.Background())()
+		outcome2 := thunk2(context.Background())()
+
+		assert.Equal(t, result.Of(20), outcome1)
+		assert.Equal(t, result.Of(50), outcome2)
+	})
+}
+
+func TestRead_Failure(t *testing.T) {
+	t.Run("propagates error from effect", func(t *testing.T) {
+		testErr := errors.New("test error")
+		eff := Fail[TestConfig, int](testErr)
+		thunk := Read[int](testConfig)(eff)
+		outcome := thunk(context.Background())()
+		assert.Equal(t, result.Left[int](testErr), outcome)
+	})
+}

v2/effect/effect_missing_test.go

@@ -0,0 +1,213 @@
+// 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 effect
+
+import (
+	"context"
+	"errors"
+	"strconv"
+	"testing"
+
+	"github.com/IBM/fp-go/v2/result"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestRead(t *testing.T) {
+	t.Run("provides context to effect", func(t *testing.T) {
+		ctx := TestContext{Value: "test-context"}
+		eff := Of[TestContext](42)
+
+		thunk := Read[int](ctx)(eff)
+		ioResult := thunk(context.Background())
+		res := ioResult()
+
+		assert.True(t, result.IsRight(res))
+		value, err := result.Unwrap(res)
+		assert.NoError(t, err)
+		assert.Equal(t, 42, value)
+	})
+
+	t.Run("provides context to failing effect", func(t *testing.T) {
+		expectedErr := errors.New("read error")
+		ctx := TestContext{Value: "test"}
+		eff := Fail[TestContext, string](expectedErr)
+
+		thunk := Read[string](ctx)(eff)
+		ioResult := thunk(context.Background())
+		res := ioResult()
+
+		assert.True(t, result.IsLeft(res))
+		_, err := result.Unwrap(res)
+		assert.Error(t, err)
+		assert.Equal(t, expectedErr, err)
+	})
+
+	t.Run("provides context to chained effects", func(t *testing.T) {
+		ctx := TestContext{Value: "base"}
+		eff := Chain(func(x int) Effect[TestContext, string] {
+			return Of[TestContext](strconv.Itoa(x * 2))
+		})(Of[TestContext](21))
+
+		thunk := Read[string](ctx)(eff)
+		ioResult := thunk(context.Background())
+		res := ioResult()
+
+		assert.True(t, result.IsRight(res))
+		value, err := result.Unwrap(res)
+		assert.NoError(t, err)
+		assert.Equal(t, "42", value)
+	})
+
+	t.Run("works with different context types", func(t *testing.T) {
+		type CustomContext struct {
+			ID   int
+			Name string
+		}
+
+		ctx := CustomContext{ID: 100, Name: "custom"}
+		eff := Of[CustomContext]("result")
+
+		thunk := Read[string](ctx)(eff)
+		ioResult := thunk(context.Background())
+		res := ioResult()
+
+		assert.True(t, result.IsRight(res))
+		value, err := result.Unwrap(res)
+		assert.NoError(t, err)
+		assert.Equal(t, "result", value)
+	})
+
+	t.Run("can be composed with RunSync", func(t *testing.T) {
+		ctx := TestContext{Value: "test"}
+		eff := Of[TestContext](100)
+
+		thunk := Read[int](ctx)(eff)
+		readerResult := RunSync(thunk)
+		value, err := readerResult(context.Background())
+
+		assert.NoError(t, err)
+		assert.Equal(t, 100, value)
+	})
+}
+
+func TestChainResultK(t *testing.T) {
+	t.Run("chains successful Result function", func(t *testing.T) {
+		parseIntResult := result.Eitherize1(strconv.Atoi)
+		eff := Of[TestContext]("42")
+		chained := ChainResultK[TestContext](parseIntResult)(eff)
+
+		result, err := runEffect(chained, TestContext{Value: "test"})
+
+		assert.NoError(t, err)
+		assert.Equal(t, 42, result)
+	})
+
+	t.Run("chains failing Result function", func(t *testing.T) {
+		parseIntResult := result.Eitherize1(strconv.Atoi)
+		eff := Of[TestContext]("not-a-number")
+		chained := ChainResultK[TestContext](parseIntResult)(eff)
+
+		_, err := runEffect(chained, TestContext{Value: "test"})
+
+		assert.Error(t, err)
+		assert.Contains(t, err.Error(), "invalid syntax")
+	})
+
+	t.Run("propagates error from original effect", func(t *testing.T) {
+		expectedErr := errors.New("original error")
+		parseIntResult := result.Eitherize1(strconv.Atoi)
+		eff := Fail[TestContext, string](expectedErr)
+		chained := ChainResultK[TestContext](parseIntResult)(eff)
+
+		_, err := runEffect(chained, TestContext{Value: "test"})
+
+		assert.Error(t, err)
+		assert.Equal(t, expectedErr, err)
+	})
+
+	t.Run("chains multiple Result functions", func(t *testing.T) {
+		parseIntResult := result.Eitherize1(strconv.Atoi)
+		formatResult := func(x int) result.Result[string] {
+			return result.Of("value: " + strconv.Itoa(x))
+		}
+
+		eff := Of[TestContext]("42")
+		chained := ChainResultK[TestContext](formatResult)(
+			ChainResultK[TestContext](parseIntResult)(eff),
+		)
+
+		result, err := runEffect(chained, TestContext{Value: "test"})
+
+		assert.NoError(t, err)
+		assert.Equal(t, "value: 42", result)
+	})
+
+	t.Run("integrates with other effect operations", func(t *testing.T) {
+		parseIntResult := result.Eitherize1(strconv.Atoi)
+
+		eff := Map[TestContext](func(x int) string {
+			return "final: " + strconv.Itoa(x)
+		})(ChainResultK[TestContext](parseIntResult)(Of[TestContext]("100")))
+
+		result, err := runEffect(eff, TestContext{Value: "test"})
+
+		assert.NoError(t, err)
+		assert.Equal(t, "final: 100", result)
+	})
+
+	t.Run("works with custom Result functions", func(t *testing.T) {
+		validatePositive := func(x int) result.Result[int] {
+			if x > 0 {
+				return result.Of(x)
+			}
+			return result.Left[int](errors.New("must be positive"))
+		}
+
+		parseIntResult := result.Eitherize1(strconv.Atoi)
+
+		// Test with positive number
+		eff1 := ChainResultK[TestContext](validatePositive)(
+			ChainResultK[TestContext](parseIntResult)(Of[TestContext]("42")),
+		)
+		result1, err1 := runEffect(eff1, TestContext{Value: "test"})
+		assert.NoError(t, err1)
+		assert.Equal(t, 42, result1)
+
+		// Test with negative number
+		eff2 := ChainResultK[TestContext](validatePositive)(
+			ChainResultK[TestContext](parseIntResult)(Of[TestContext]("-5")),
+		)
+		_, err2 := runEffect(eff2, TestContext{Value: "test"})
+		assert.Error(t, err2)
+		assert.Contains(t, err2.Error(), "must be positive")
+	})
+
+	t.Run("preserves error context", func(t *testing.T) {
+		customError := errors.New("custom validation error")
+		validateFunc := func(s string) result.Result[string] {
+			if len(s) > 0 {
+				return result.Of(s)
+			}
+			return result.Left[string](customError)
+		}
+
+		eff := ChainResultK[TestContext](validateFunc)(Of[TestContext](""))
+		_, err := runEffect(eff, TestContext{Value: "test"})
+
+		assert.Error(t, err)
+		assert.Equal(t, customError, err)
+	})
+}

v2/effect/eitherize.go

@@ -0,0 +1,208 @@
+// 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 effect
+
+import (
+	"context"
+
+	"github.com/IBM/fp-go/v2/context/readerreaderioresult"
+)
+
+// Eitherize converts a function that returns a value and error into an Effect.
+//
+// This function takes a function that accepts a context C and context.Context,
+// returning a value T and an error, and converts it into an Effect[C, T].
+// The error is automatically converted into a failure, while successful
+// values become successes.
+//
+// This is particularly useful for integrating standard Go error-handling patterns into
+// the effect system. It is especially helpful for adapting interface member functions
+// that accept a context. When you have an interface method with signature
+// (receiver, context.Context) (T, error), you can use Eitherize to convert it into
+// an Effect where the receiver becomes the context C.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - T: The success value type
+//
+// # Parameters
+//
+//   - f: A function that takes C and context.Context and returns (T, error)
+//
+// # Returns
+//
+//   - Effect[C, T]: An effect that depends on C, performs IO, and produces T
+//
+// # Example Usage
+//
+//	type AppConfig struct {
+//	    DatabaseURL string
+//	}
+//
+//	// A function using standard Go error handling
+//	func fetchUser(cfg AppConfig, ctx context.Context) (*User, error) {
+//	    // Implementation that may return an error
+//	    return &User{ID: 1, Name: "Alice"}, nil
+//	}
+//
+//	// Convert to Effect
+//	fetchUserEffect := effect.Eitherize(fetchUser)
+//
+//	// Use in functional composition
+//	pipeline := F.Pipe1(
+//	    fetchUserEffect,
+//	    effect.Map[AppConfig](func(u *User) string { return u.Name }),
+//	)
+//
+//	// Execute with config
+//	cfg := AppConfig{DatabaseURL: "postgres://localhost"}
+//	result, err := effect.RunSync(effect.Provide[*User](cfg)(pipeline))(context.Background())
+//
+// # Adapting Interface Methods
+//
+// Eitherize is particularly useful for adapting interface member functions:
+//
+//	type UserRepository interface {
+//	    GetUser(ctx context.Context, id int) (*User, error)
+//	}
+//
+//	type UserRepo struct {
+//	    db *sql.DB
+//	}
+//
+//	func (r *UserRepo) GetUser(ctx context.Context, id int) (*User, error) {
+//	    // Implementation
+//	    return &User{ID: id}, nil
+//	}
+//
+//	// Adapt the method by binding the first parameter (receiver)
+//	repo := &UserRepo{db: db}
+//	getUserEffect := effect.Eitherize(func(id int, ctx context.Context) (*User, error) {
+//	    return repo.GetUser(ctx, id)
+//	})
+//
+//	// Now getUserEffect has type: Effect[int, *User]
+//	// The receiver (repo) is captured in the closure
+//	// The id becomes the context C
+//
+// # See Also
+//
+//   - Eitherize1: For functions that take an additional parameter
+//   - readerreaderioresult.Eitherize: The underlying implementation
+//
+//go:inline
+func Eitherize[C, T any](f func(C, context.Context) (T, error)) Effect[C, T] {
+	return readerreaderioresult.Eitherize(f)
+}
+
+// Eitherize1 converts a function that takes an additional parameter and returns a value
+// and error into a Kleisli arrow.
+//
+// This function takes a function that accepts a context C, context.Context, and
+// an additional parameter A, returning a value T and an error, and converts it into a
+// Kleisli arrow (A -> Effect[C, T]). The error is automatically converted into a failure,
+// while successful values become successes.
+//
+// This is useful for creating composable operations that depend on context and
+// an input value, following standard Go error-handling patterns. It is especially helpful
+// for adapting interface member functions that accept a context and additional parameters.
+// When you have an interface method with signature (receiver, context.Context, A) (T, error),
+// you can use Eitherize1 to convert it into a Kleisli arrow where the receiver becomes
+// the context C and A becomes the input parameter.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - A: The input parameter type
+//   - T: The success value type
+//
+// # Parameters
+//
+//   - f: A function that takes C, context.Context, and A, returning (T, error)
+//
+// # Returns
+//
+//   - Kleisli[C, A, T]: A function from A to Effect[C, T]
+//
+// # Example Usage
+//
+//	type AppConfig struct {
+//	    DatabaseURL string
+//	}
+//
+//	// A function using standard Go error handling
+//	func fetchUserByID(cfg AppConfig, ctx context.Context, id int) (*User, error) {
+//	    // Implementation that may return an error
+//	    return &User{ID: id, Name: "Alice"}, nil
+//	}
+//
+//	// Convert to Kleisli arrow
+//	fetchUserKleisli := effect.Eitherize1(fetchUserByID)
+//
+//	// Use in functional composition with Chain
+//	pipeline := F.Pipe1(
+//	    effect.Succeed[AppConfig](123),
+//	    effect.Chain[AppConfig](fetchUserKleisli),
+//	)
+//
+//	// Execute with config
+//	cfg := AppConfig{DatabaseURL: "postgres://localhost"}
+//	result, err := effect.RunSync(effect.Provide[*User](cfg)(pipeline))(context.Background())
+//
+// # Adapting Interface Methods
+//
+// Eitherize1 is particularly useful for adapting interface member functions with parameters:
+//
+//	type UserRepository interface {
+//	    GetUserByID(ctx context.Context, id int) (*User, error)
+//	    UpdateUser(ctx context.Context, user *User) error
+//	}
+//
+//	type UserRepo struct {
+//	    db *sql.DB
+//	}
+//
+//	func (r *UserRepo) GetUserByID(ctx context.Context, id int) (*User, error) {
+//	    // Implementation
+//	    return &User{ID: id}, nil
+//	}
+//
+//	// Adapt the method - receiver becomes C, id becomes A
+//	repo := &UserRepo{db: db}
+//	getUserKleisli := effect.Eitherize1(func(r *UserRepo, ctx context.Context, id int) (*User, error) {
+//	    return r.GetUserByID(ctx, id)
+//	})
+//
+//	// Now getUserKleisli has type: Kleisli[*UserRepo, int, *User]
+//	// Which is: func(int) Effect[*UserRepo, *User]
+//	// Use it in composition:
+//	pipeline := F.Pipe1(
+//	    effect.Succeed[*UserRepo](123),
+//	    effect.Chain[*UserRepo](getUserKleisli),
+//	)
+//	result, err := effect.RunSync(effect.Provide[*User](repo)(pipeline))(context.Background())
+//
+// # See Also
+//
+//   - Eitherize: For functions without an additional parameter
+//   - Chain: For composing Kleisli arrows
+//   - readerreaderioresult.Eitherize1: The underlying implementation
+//
+//go:inline
+func Eitherize1[C, A, T any](f func(C, context.Context, A) (T, error)) Kleisli[C, A, T] {
+	return readerreaderioresult.Eitherize1(f)
+}

v2/effect/eitherize_test.go

@@ -0,0 +1,507 @@
+// 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 effect
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"strconv"
+	"testing"
+
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/stretchr/testify/assert"
+)
+
+// TestEitherize_Success tests successful conversion with Eitherize
+func TestEitherize_Success(t *testing.T) {
+	t.Run("converts successful function to Effect", func(t *testing.T) {
+		// Arrange
+		successFunc := func(cfg TestConfig, ctx context.Context) (string, error) {
+			return cfg.Prefix + "-success", nil
+		}
+		eff := Eitherize(successFunc)
+
+		// Act
+		result, err := runEffect(eff, testConfig)
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, "LOG-success", result)
+	})
+
+	t.Run("preserves context values", func(t *testing.T) {
+		// Arrange
+		type ctxKey string
+		key := ctxKey("testKey")
+		expectedValue := "contextValue"
+
+		contextFunc := func(cfg TestConfig, ctx context.Context) (string, error) {
+			value := ctx.Value(key)
+			if value == nil {
+				return "", errors.New("context value not found")
+			}
+			return value.(string), nil
+		}
+		eff := Eitherize(contextFunc)
+
+		// Act
+		ioResult := Provide[string](testConfig)(eff)
+		readerResult := RunSync(ioResult)
+		ctx := context.WithValue(context.Background(), key, expectedValue)
+		result, err := readerResult(ctx)
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, expectedValue, result)
+	})
+
+	t.Run("works with different types", func(t *testing.T) {
+		// Arrange
+		intFunc := func(cfg TestConfig, ctx context.Context) (int, error) {
+			return cfg.Multiplier, nil
+		}
+		eff := Eitherize(intFunc)
+
+		// Act
+		result, err := runEffect(eff, testConfig)
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, 3, result)
+	})
+}
+
+// TestEitherize_Failure tests error handling with Eitherize
+func TestEitherize_Failure(t *testing.T) {
+	t.Run("converts error to failure", func(t *testing.T) {
+		// Arrange
+		expectedErr := errors.New("operation failed")
+		failFunc := func(cfg TestConfig, ctx context.Context) (string, error) {
+			return "", expectedErr
+		}
+		eff := Eitherize(failFunc)
+
+		// Act
+		_, err := runEffect(eff, testConfig)
+
+		// Assert
+		assert.Error(t, err)
+		assert.Equal(t, expectedErr, err)
+	})
+
+	t.Run("preserves error message", func(t *testing.T) {
+		// Arrange
+		expectedErr := fmt.Errorf("validation error: field is required")
+		failFunc := func(cfg TestConfig, ctx context.Context) (int, error) {
+			return 0, expectedErr
+		}
+		eff := Eitherize(failFunc)
+
+		// Act
+		_, err := runEffect(eff, testConfig)
+
+		// Assert
+		assert.Error(t, err)
+		assert.Equal(t, expectedErr, err)
+	})
+}
+
+// TestEitherize_EdgeCases tests edge cases for Eitherize
+func TestEitherize_EdgeCases(t *testing.T) {
+	t.Run("handles nil context", func(t *testing.T) {
+		// Arrange
+		nilCtxFunc := func(cfg TestConfig, ctx context.Context) (string, error) {
+			if ctx == nil {
+				return "nil-context", nil
+			}
+			return "non-nil-context", nil
+		}
+		eff := Eitherize(nilCtxFunc)
+
+		// Act
+		ioResult := Provide[string](testConfig)(eff)
+		readerResult := RunSync(ioResult)
+		result, err := readerResult(nil)
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, "nil-context", result)
+	})
+
+	t.Run("handles zero value config", func(t *testing.T) {
+		// Arrange
+		zeroFunc := func(cfg TestConfig, ctx context.Context) (string, error) {
+			return cfg.Prefix, nil
+		}
+		eff := Eitherize(zeroFunc)
+
+		// Act
+		result, err := runEffect(eff, TestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, "", result)
+	})
+
+	t.Run("handles pointer types", func(t *testing.T) {
+		// Arrange
+		type User struct {
+			Name string
+		}
+		ptrFunc := func(cfg TestConfig, ctx context.Context) (*User, error) {
+			return &User{Name: cfg.Prefix}, nil
+		}
+		eff := Eitherize(ptrFunc)
+
+		// Act
+		result, err := runEffect(eff, testConfig)
+
+		// Assert
+		assert.NoError(t, err)
+		assert.NotNil(t, result)
+		assert.Equal(t, "LOG", result.Name)
+	})
+}
+
+// TestEitherize_Integration tests integration with other operations
+func TestEitherize_Integration(t *testing.T) {
+	t.Run("composes with Map", func(t *testing.T) {
+		// Arrange
+		baseFunc := func(cfg TestConfig, ctx context.Context) (int, error) {
+			return cfg.Multiplier, nil
+		}
+		eff := Eitherize(baseFunc)
+
+		// Act
+		pipeline := F.Pipe1(
+			eff,
+			Map[TestConfig](func(n int) string { return strconv.Itoa(n) }),
+		)
+		result, err := runEffect(pipeline, testConfig)
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, "3", result)
+	})
+
+	t.Run("composes with Chain", func(t *testing.T) {
+		// Arrange
+		firstFunc := func(cfg TestConfig, ctx context.Context) (int, error) {
+			return cfg.Multiplier, nil
+		}
+		secondFunc := func(n int) Effect[TestConfig, string] {
+			return Succeed[TestConfig](fmt.Sprintf("value: %d", n))
+		}
+
+		// Act
+		pipeline := F.Pipe1(
+			Eitherize(firstFunc),
+			Chain[TestConfig](secondFunc),
+		)
+		result, err := runEffect(pipeline, testConfig)
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, "value: 3", result)
+	})
+}
+
+// TestEitherize1_Success tests successful conversion with Eitherize1
+func TestEitherize1_Success(t *testing.T) {
+	t.Run("converts successful function to Kleisli", func(t *testing.T) {
+		// Arrange
+		multiplyFunc := func(cfg TestConfig, ctx context.Context, n int) (int, error) {
+			return n * cfg.Multiplier, nil
+		}
+		kleisli := Eitherize1(multiplyFunc)
+
+		// Act
+		eff := kleisli(10)
+		result, err := runEffect(eff, testConfig)
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, 30, result)
+	})
+
+	t.Run("works with string input", func(t *testing.T) {
+		// Arrange
+		concatFunc := func(cfg TestConfig, ctx context.Context, s string) (string, error) {
+			return cfg.Prefix + "-" + s, nil
+		}
+		kleisli := Eitherize1(concatFunc)
+
+		// Act
+		eff := kleisli("input")
+		result, err := runEffect(eff, testConfig)
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, "LOG-input", result)
+	})
+
+	t.Run("preserves context in Kleisli", func(t *testing.T) {
+		// Arrange
+		type ctxKey string
+		key := ctxKey("factor")
+
+		scaleFunc := func(cfg TestConfig, ctx context.Context, n int) (int, error) {
+			factor := ctx.Value(key)
+			if factor == nil {
+				return n * cfg.Multiplier, nil
+			}
+			return n * factor.(int), nil
+		}
+		kleisli := Eitherize1(scaleFunc)
+
+		// Act
+		eff := kleisli(5)
+		ioResult := Provide[int](testConfig)(eff)
+		readerResult := RunSync(ioResult)
+		ctx := context.WithValue(context.Background(), key, 7)
+		result, err := readerResult(ctx)
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, 35, result)
+	})
+}
+
+// TestEitherize1_Failure tests error handling with Eitherize1
+func TestEitherize1_Failure(t *testing.T) {
+	t.Run("converts error to failure in Kleisli", func(t *testing.T) {
+		// Arrange
+		expectedErr := errors.New("division by zero")
+		divideFunc := func(cfg TestConfig, ctx context.Context, n int) (int, error) {
+			if n == 0 {
+				return 0, expectedErr
+			}
+			return 100 / n, nil
+		}
+		kleisli := Eitherize1(divideFunc)
+
+		// Act
+		eff := kleisli(0)
+		_, err := runEffect(eff, testConfig)
+
+		// Assert
+		assert.Error(t, err)
+		assert.Equal(t, expectedErr, err)
+	})
+
+	t.Run("preserves error context", func(t *testing.T) {
+		// Arrange
+		validateFunc := func(cfg TestConfig, ctx context.Context, s string) (string, error) {
+			if len(s) > 10 {
+				return "", fmt.Errorf("string too long: %d > 10", len(s))
+			}
+			return s, nil
+		}
+		kleisli := Eitherize1(validateFunc)
+
+		// Act
+		eff := kleisli("this-string-is-too-long")
+		_, err := runEffect(eff, testConfig)
+
+		// Assert
+		assert.Error(t, err)
+		assert.Contains(t, err.Error(), "string too long")
+	})
+}
+
+// TestEitherize1_EdgeCases tests edge cases for Eitherize1
+func TestEitherize1_EdgeCases(t *testing.T) {
+	t.Run("handles zero value input", func(t *testing.T) {
+		// Arrange
+		zeroFunc := func(cfg TestConfig, ctx context.Context, n int) (int, error) {
+			return n, nil
+		}
+		kleisli := Eitherize1(zeroFunc)
+
+		// Act
+		eff := kleisli(0)
+		result, err := runEffect(eff, testConfig)
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, 0, result)
+	})
+
+	t.Run("handles pointer input", func(t *testing.T) {
+		// Arrange
+		type Input struct {
+			Value int
+		}
+		ptrFunc := func(cfg TestConfig, ctx context.Context, in *Input) (int, error) {
+			if in == nil {
+				return 0, errors.New("nil input")
+			}
+			return in.Value * cfg.Multiplier, nil
+		}
+		kleisli := Eitherize1(ptrFunc)
+
+		// Act
+		eff := kleisli(&Input{Value: 7})
+		result, err := runEffect(eff, testConfig)
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, 21, result)
+	})
+
+	t.Run("handles nil pointer input", func(t *testing.T) {
+		// Arrange
+		type Input struct {
+			Value int
+		}
+		ptrFunc := func(cfg TestConfig, ctx context.Context, in *Input) (int, error) {
+			if in == nil {
+				return 0, errors.New("nil input")
+			}
+			return in.Value, nil
+		}
+		kleisli := Eitherize1(ptrFunc)
+
+		// Act
+		eff := kleisli((*Input)(nil))
+		_, err := runEffect(eff, testConfig)
+
+		// Assert
+		assert.Error(t, err)
+		assert.Contains(t, err.Error(), "nil input")
+	})
+}
+
+// TestEitherize1_Integration tests integration with other operations
+func TestEitherize1_Integration(t *testing.T) {
+	t.Run("composes with Chain", func(t *testing.T) {
+		// Arrange
+		parseFunc := func(cfg TestConfig, ctx context.Context, s string) (int, error) {
+			return strconv.Atoi(s)
+		}
+		doubleFunc := func(n int) Effect[TestConfig, int] {
+			return Succeed[TestConfig](n * 2)
+		}
+
+		parseKleisli := Eitherize1(parseFunc)
+
+		// Act
+		pipeline := F.Pipe2(
+			Succeed[TestConfig]("42"),
+			Chain[TestConfig](parseKleisli),
+			Chain[TestConfig](doubleFunc),
+		)
+		result, err := runEffect(pipeline, testConfig)
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, 84, result)
+	})
+
+	t.Run("handles error in chain", func(t *testing.T) {
+		// Arrange
+		parseFunc := func(cfg TestConfig, ctx context.Context, s string) (int, error) {
+			return strconv.Atoi(s)
+		}
+		parseKleisli := Eitherize1(parseFunc)
+
+		// Act
+		pipeline := F.Pipe1(
+			Succeed[TestConfig]("not-a-number"),
+			Chain[TestConfig](parseKleisli),
+		)
+		_, err := runEffect(pipeline, testConfig)
+
+		// Assert
+		assert.Error(t, err)
+	})
+
+	t.Run("composes multiple Kleisli arrows", func(t *testing.T) {
+		// Arrange
+		parseFunc := func(cfg TestConfig, ctx context.Context, s string) (int, error) {
+			return strconv.Atoi(s)
+		}
+		formatFunc := func(cfg TestConfig, ctx context.Context, n int) (string, error) {
+			return fmt.Sprintf("%s-%d", cfg.Prefix, n), nil
+		}
+
+		parseKleisli := Eitherize1(parseFunc)
+		formatKleisli := Eitherize1(formatFunc)
+
+		// Act
+		pipeline := F.Pipe2(
+			Succeed[TestConfig]("123"),
+			Chain[TestConfig](parseKleisli),
+			Chain[TestConfig](formatKleisli),
+		)
+		result, err := runEffect(pipeline, testConfig)
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, "LOG-123", result)
+	})
+}
+
+// TestEitherize_TypeSafety tests type safety across different scenarios
+func TestEitherize_TypeSafety(t *testing.T) {
+	t.Run("Eitherize with complex types", func(t *testing.T) {
+		// Arrange
+		type ComplexResult struct {
+			Data  map[string]int
+			Count int
+		}
+
+		complexFunc := func(cfg TestConfig, ctx context.Context) (ComplexResult, error) {
+			return ComplexResult{
+				Data:  map[string]int{cfg.Prefix: cfg.Multiplier},
+				Count: cfg.Multiplier,
+			}, nil
+		}
+		eff := Eitherize(complexFunc)
+
+		// Act
+		result, err := runEffect(eff, testConfig)
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, 3, result.Data["LOG"])
+		assert.Equal(t, 3, result.Count)
+	})
+
+	t.Run("Eitherize1 with different input and output types", func(t *testing.T) {
+		// Arrange
+		type Input struct {
+			ID int
+		}
+		type Output struct {
+			Name string
+		}
+
+		convertFunc := func(cfg TestConfig, ctx context.Context, in Input) (Output, error) {
+			return Output{Name: fmt.Sprintf("%s-%d", cfg.Prefix, in.ID)}, nil
+		}
+		kleisli := Eitherize1(convertFunc)
+
+		// Act
+		eff := kleisli(Input{ID: 99})
+		result, err := runEffect(eff, testConfig)
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, "LOG-99", result.Name)
+	})
+}

v2/effect/filter.go

@@ -0,0 +1,296 @@
+// 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 effect
+
+import (
+	"github.com/IBM/fp-go/v2/context/readerreaderioresult"
+	"github.com/IBM/fp-go/v2/option"
+)
+
+// Filter lifts a filtering operation on a higher-kinded type into an Effect operator.
+// This is a generic function that works with any filterable data structure by taking
+// a filter function and returning an operator that can be used in effect chains.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - HKTA: The higher-kinded type being filtered (e.g., []A, Seq[A])
+//   - A: The element type being filtered
+//
+// # Parameters
+//
+//   - filter: A function that takes a predicate and returns an endomorphism on HKTA
+//
+// # Returns
+//
+//   - func(Predicate[A]) Operator[C, HKTA, HKTA]: A function that takes a predicate
+//     and returns an operator that filters effects containing HKTA values
+//
+// # Example Usage
+//
+//	import A "github.com/IBM/fp-go/v2/array"
+//
+//	// Create a custom filter operator for arrays
+//	filterOp := Filter[MyContext](A.Filter[int])
+//	isEven := func(n int) bool { return n%2 == 0 }
+//
+//	pipeline := F.Pipe2(
+//	    Succeed[MyContext]([]int{1, 2, 3, 4, 5}),
+//	    filterOp(isEven),
+//	    Map[MyContext](func(arr []int) int { return len(arr) }),
+//	)
+//	// Result: Effect that produces 2 (count of even numbers)
+//
+// # See Also
+//
+//   - FilterArray: Specialized version for array filtering
+//   - FilterIter: Specialized version for iterator filtering
+//   - FilterMap: For filtering and mapping simultaneously
+//
+//go:inline
+func Filter[C, HKTA, A any](
+	filter func(Predicate[A]) Endomorphism[HKTA],
+) func(Predicate[A]) Operator[C, HKTA, HKTA] {
+	return readerreaderioresult.Filter[C](filter)
+}
+
+// FilterArray creates an operator that filters array elements within an Effect based on a predicate.
+// Elements that satisfy the predicate are kept, while others are removed.
+// This is a specialized version of Filter for arrays.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - A: The element type in the array
+//
+// # Parameters
+//
+//   - p: A predicate function that tests each element
+//
+// # Returns
+//
+//   - Operator[C, []A, []A]: An operator that filters array elements in an effect
+//
+// # Example Usage
+//
+//	isPositive := func(n int) bool { return n > 0 }
+//	filterPositive := FilterArray[MyContext](isPositive)
+//
+//	pipeline := F.Pipe1(
+//	    Succeed[MyContext]([]int{-2, -1, 0, 1, 2, 3}),
+//	    filterPositive,
+//	)
+//	// Result: Effect that produces []int{1, 2, 3}
+//
+// # See Also
+//
+//   - Filter: Generic version for any filterable type
+//   - FilterIter: For filtering iterators
+//   - FilterMapArray: For filtering and mapping arrays simultaneously
+//
+//go:inline
+func FilterArray[C, A any](p Predicate[A]) Operator[C, []A, []A] {
+	return readerreaderioresult.FilterArray[C](p)
+}
+
+// FilterIter creates an operator that filters iterator elements within an Effect based on a predicate.
+// Elements that satisfy the predicate are kept in the resulting iterator, while others are removed.
+// This is a specialized version of Filter for iterators (Seq).
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - A: The element type in the iterator
+//
+// # Parameters
+//
+//   - p: A predicate function that tests each element
+//
+// # Returns
+//
+//   - Operator[C, Seq[A], Seq[A]]: An operator that filters iterator elements in an effect
+//
+// # Example Usage
+//
+//	isEven := func(n int) bool { return n%2 == 0 }
+//	filterEven := FilterIter[MyContext](isEven)
+//
+//	pipeline := F.Pipe1(
+//	    Succeed[MyContext](slices.Values([]int{1, 2, 3, 4, 5, 6})),
+//	    filterEven,
+//	)
+//	// Result: Effect that produces an iterator over [2, 4, 6]
+//
+// # See Also
+//
+//   - Filter: Generic version for any filterable type
+//   - FilterArray: For filtering arrays
+//   - FilterMapIter: For filtering and mapping iterators simultaneously
+//
+//go:inline
+func FilterIter[C, A any](p Predicate[A]) Operator[C, Seq[A], Seq[A]] {
+	return readerreaderioresult.FilterIter[C](p)
+}
+
+// FilterMap lifts a filter-map operation on a higher-kinded type into an Effect operator.
+// This combines filtering and mapping in a single operation: elements are transformed
+// using a function that returns Option, and only Some values are kept in the result.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - HKTA: The input higher-kinded type (e.g., []A, Seq[A])
+//   - HKTB: The output higher-kinded type (e.g., []B, Seq[B])
+//   - A: The input element type
+//   - B: The output element type
+//
+// # Parameters
+//
+//   - filter: A function that takes an option.Kleisli and returns a transformation from HKTA to HKTB
+//
+// # Returns
+//
+//   - func(option.Kleisli[A, B]) Operator[C, HKTA, HKTB]: A function that takes a Kleisli arrow
+//     and returns an operator that filter-maps effects
+//
+// # Example Usage
+//
+//	import A "github.com/IBM/fp-go/v2/array"
+//	import O "github.com/IBM/fp-go/v2/option"
+//
+//	// Parse and filter positive integers
+//	parsePositive := func(s string) O.Option[int] {
+//	    var n int
+//	    if _, err := fmt.Sscanf(s, "%d", &n); err == nil && n > 0 {
+//	        return O.Some(n)
+//	    }
+//	    return O.None[int]()
+//	}
+//
+//	filterMapOp := FilterMap[MyContext](A.FilterMap[string, int])
+//	pipeline := F.Pipe1(
+//	    Succeed[MyContext]([]string{"1", "-2", "3", "invalid", "5"}),
+//	    filterMapOp(parsePositive),
+//	)
+//	// Result: Effect that produces []int{1, 3, 5}
+//
+// # See Also
+//
+//   - FilterMapArray: Specialized version for arrays
+//   - FilterMapIter: Specialized version for iterators
+//   - Filter: For filtering without transformation
+//
+//go:inline
+func FilterMap[C, HKTA, HKTB, A, B any](
+	filter func(option.Kleisli[A, B]) Reader[HKTA, HKTB],
+) func(option.Kleisli[A, B]) Operator[C, HKTA, HKTB] {
+	return readerreaderioresult.FilterMap[C](filter)
+}
+
+// FilterMapArray creates an operator that filters and maps array elements within an Effect.
+// Each element is transformed using a function that returns Option[B]. Elements that
+// produce Some(b) are kept in the result array, while None values are filtered out.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - A: The input element type
+//   - B: The output element type
+//
+// # Parameters
+//
+//   - p: A Kleisli arrow from A to Option[B] that transforms and filters elements
+//
+// # Returns
+//
+//   - Operator[C, []A, []B]: An operator that filter-maps array elements in an effect
+//
+// # Example Usage
+//
+//	import O "github.com/IBM/fp-go/v2/option"
+//
+//	// Double even numbers, filter out odd numbers
+//	doubleEven := func(n int) O.Option[int] {
+//	    if n%2 == 0 {
+//	        return O.Some(n * 2)
+//	    }
+//	    return O.None[int]()
+//	}
+//
+//	pipeline := F.Pipe1(
+//	    Succeed[MyContext]([]int{1, 2, 3, 4, 5}),
+//	    FilterMapArray[MyContext](doubleEven),
+//	)
+//	// Result: Effect that produces []int{4, 8}
+//
+// # See Also
+//
+//   - FilterMap: Generic version for any filterable type
+//   - FilterMapIter: For filter-mapping iterators
+//   - FilterArray: For filtering without transformation
+//
+//go:inline
+func FilterMapArray[C, A, B any](p option.Kleisli[A, B]) Operator[C, []A, []B] {
+	return readerreaderioresult.FilterMapArray[C](p)
+}
+
+// FilterMapIter creates an operator that filters and maps iterator elements within an Effect.
+// Each element is transformed using a function that returns Option[B]. Elements that
+// produce Some(b) are kept in the resulting iterator, while None values are filtered out.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - A: The input element type
+//   - B: The output element type
+//
+// # Parameters
+//
+//   - p: A Kleisli arrow from A to Option[B] that transforms and filters elements
+//
+// # Returns
+//
+//   - Operator[C, Seq[A], Seq[B]]: An operator that filter-maps iterator elements in an effect
+//
+// # Example Usage
+//
+//	import O "github.com/IBM/fp-go/v2/option"
+//
+//	// Parse strings to integers, keeping only valid ones
+//	parseInt := func(s string) O.Option[int] {
+//	    var n int
+//	    if _, err := fmt.Sscanf(s, "%d", &n); err == nil {
+//	        return O.Some(n)
+//	    }
+//	    return O.None[int]()
+//	}
+//
+//	pipeline := F.Pipe1(
+//	    Succeed[MyContext](slices.Values([]string{"1", "2", "invalid", "3"})),
+//	    FilterMapIter[MyContext](parseInt),
+//	)
+//	// Result: Effect that produces an iterator over [1, 2, 3]
+//
+// # See Also
+//
+//   - FilterMap: Generic version for any filterable type
+//   - FilterMapArray: For filter-mapping arrays
+//   - FilterIter: For filtering without transformation
+//
+//go:inline
+func FilterMapIter[C, A, B any](p option.Kleisli[A, B]) Operator[C, Seq[A], Seq[B]] {
+	return readerreaderioresult.FilterMapIter[C](p)
+}

v2/effect/filter_test.go

@@ -0,0 +1,653 @@
+// 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 effect
+
+import (
+	"errors"
+	"fmt"
+	"slices"
+	"testing"
+
+	A "github.com/IBM/fp-go/v2/array"
+	F "github.com/IBM/fp-go/v2/function"
+	N "github.com/IBM/fp-go/v2/number"
+	O "github.com/IBM/fp-go/v2/option"
+	"github.com/stretchr/testify/assert"
+)
+
+type FilterTestConfig struct {
+	MaxValue int
+	MinValue int
+}
+
+// Helper to collect iterator results from an effect
+func collectSeqEffect[C, A any](eff Effect[C, Seq[A]], cfg C) []A {
+	result, err := runEffect(eff, cfg)
+	if err != nil {
+		return nil
+	}
+	return slices.Collect(result)
+}
+
+func TestFilterArray_Success(t *testing.T) {
+	t.Run("filters array keeping matching elements", func(t *testing.T) {
+		// Arrange
+		isPositive := N.MoreThan(0)
+		filterOp := FilterArray[FilterTestConfig](isPositive)
+		input := Succeed[FilterTestConfig]([]int{1, -2, 3, -4, 5})
+
+		// Act
+		result, err := runEffect(filterOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{1, 3, 5}, result)
+	})
+
+	t.Run("returns empty array when no elements match", func(t *testing.T) {
+		// Arrange
+		isNegative := N.LessThan(0)
+		filterOp := FilterArray[FilterTestConfig](isNegative)
+		input := Succeed[FilterTestConfig]([]int{1, 2, 3})
+
+		// Act
+		result, err := runEffect(filterOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{}, result)
+	})
+
+	t.Run("returns all elements when all match", func(t *testing.T) {
+		// Arrange
+		alwaysTrue := func(n int) bool { return true }
+		filterOp := FilterArray[FilterTestConfig](alwaysTrue)
+		input := Succeed[FilterTestConfig]([]int{1, 2, 3})
+
+		// Act
+		result, err := runEffect(filterOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{1, 2, 3}, result)
+	})
+}
+
+func TestFilterIter_Success(t *testing.T) {
+	t.Run("filters iterator keeping matching elements", func(t *testing.T) {
+		// Arrange
+		isEven := func(n int) bool { return n%2 == 0 }
+		filterOp := FilterIter[FilterTestConfig](isEven)
+		input := Succeed[FilterTestConfig](slices.Values([]int{1, 2, 3, 4, 5, 6}))
+
+		// Act
+		collected := collectSeqEffect(filterOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.Equal(t, []int{2, 4, 6}, collected)
+	})
+
+	t.Run("returns empty iterator when no elements match", func(t *testing.T) {
+		// Arrange
+		isNegative := N.LessThan(0)
+		filterOp := FilterIter[FilterTestConfig](isNegative)
+		input := Succeed[FilterTestConfig](slices.Values([]int{1, 2, 3}))
+
+		// Act
+		collected := collectSeqEffect(filterOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.Empty(t, collected)
+	})
+}
+
+func TestFilterArray_WithContext(t *testing.T) {
+	t.Run("uses context for filtering", func(t *testing.T) {
+		// Arrange
+		cfg := FilterTestConfig{MaxValue: 100, MinValue: 0}
+		inRange := func(n int) bool { return n >= cfg.MinValue && n <= cfg.MaxValue }
+		filterOp := FilterArray[FilterTestConfig](inRange)
+		input := Succeed[FilterTestConfig]([]int{-10, 50, 150, 75})
+
+		// Act
+		result, err := runEffect(filterOp(input), cfg)
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{50, 75}, result)
+	})
+}
+
+func TestFilterArray_EdgeCases(t *testing.T) {
+	t.Run("handles empty array", func(t *testing.T) {
+		// Arrange
+		isPositive := N.MoreThan(0)
+		filterOp := FilterArray[FilterTestConfig](isPositive)
+		input := Succeed[FilterTestConfig]([]int{})
+
+		// Act
+		result, err := runEffect(filterOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{}, result)
+	})
+
+	t.Run("preserves error from input", func(t *testing.T) {
+		// Arrange
+		isPositive := N.MoreThan(0)
+		filterOp := FilterArray[FilterTestConfig](isPositive)
+		inputErr := errors.New("input error")
+		input := Fail[FilterTestConfig, []int](inputErr)
+
+		// Act
+		_, err := runEffect(filterOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.Error(t, err)
+		assert.Equal(t, inputErr, err)
+	})
+}
+
+func TestFilterIter_EdgeCases(t *testing.T) {
+	t.Run("handles empty iterator", func(t *testing.T) {
+		// Arrange
+		isPositive := N.MoreThan(0)
+		filterOp := FilterIter[FilterTestConfig](isPositive)
+		input := Succeed[FilterTestConfig](slices.Values([]int{}))
+
+		// Act
+		collected := collectSeqEffect(filterOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.Empty(t, collected)
+	})
+
+	t.Run("preserves error from input", func(t *testing.T) {
+		// Arrange
+		isPositive := N.MoreThan(0)
+		filterOp := FilterIter[FilterTestConfig](isPositive)
+		inputErr := errors.New("input error")
+		input := Fail[FilterTestConfig, Seq[int]](inputErr)
+
+		// Act
+		_, err := runEffect(filterOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.Error(t, err)
+		assert.Equal(t, inputErr, err)
+	})
+}
+
+func TestFilter_GenericFilter(t *testing.T) {
+	t.Run("works with custom filter function", func(t *testing.T) {
+		// Arrange
+		customFilter := func(p Predicate[int]) Endomorphism[[]int] {
+			return A.Filter(p)
+		}
+		filterOp := Filter[FilterTestConfig](customFilter)
+		isEven := func(n int) bool { return n%2 == 0 }
+		input := Succeed[FilterTestConfig]([]int{1, 2, 3, 4, 5})
+
+		// Act
+		result, err := runEffect(filterOp(isEven)(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{2, 4}, result)
+	})
+}
+
+func TestFilterMapArray_Success(t *testing.T) {
+	t.Run("filters and maps array elements", func(t *testing.T) {
+		// Arrange
+		parsePositive := func(n int) O.Option[string] {
+			if n > 0 {
+				return O.Some(fmt.Sprintf("positive:%d", n))
+			}
+			return O.None[string]()
+		}
+		filterMapOp := FilterMapArray[FilterTestConfig](parsePositive)
+		input := Succeed[FilterTestConfig]([]int{-1, 2, -3, 4, 5})
+
+		// Act
+		result, err := runEffect(filterMapOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []string{"positive:2", "positive:4", "positive:5"}, result)
+	})
+
+	t.Run("returns empty when no elements match", func(t *testing.T) {
+		// Arrange
+		neverMatch := func(n int) O.Option[int] {
+			return O.None[int]()
+		}
+		filterMapOp := FilterMapArray[FilterTestConfig](neverMatch)
+		input := Succeed[FilterTestConfig]([]int{1, 2, 3})
+
+		// Act
+		result, err := runEffect(filterMapOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{}, result)
+	})
+
+	t.Run("maps all elements when all match", func(t *testing.T) {
+		// Arrange
+		double := func(n int) O.Option[int] {
+			return O.Some(n * 2)
+		}
+		filterMapOp := FilterMapArray[FilterTestConfig](double)
+		input := Succeed[FilterTestConfig]([]int{1, 2, 3})
+
+		// Act
+		result, err := runEffect(filterMapOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{2, 4, 6}, result)
+	})
+}
+
+func TestFilterMapIter_Success(t *testing.T) {
+	t.Run("filters and maps iterator elements", func(t *testing.T) {
+		// Arrange
+		doubleEven := func(n int) O.Option[int] {
+			if n%2 == 0 {
+				return O.Some(n * 2)
+			}
+			return O.None[int]()
+		}
+		filterMapOp := FilterMapIter[FilterTestConfig](doubleEven)
+		input := Succeed[FilterTestConfig](slices.Values([]int{1, 2, 3, 4, 5}))
+
+		// Act
+		collected := collectSeqEffect(filterMapOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.Equal(t, []int{4, 8}, collected)
+	})
+}
+
+func TestFilterMapArray_TypeConversion(t *testing.T) {
+	t.Run("converts int to string", func(t *testing.T) {
+		// Arrange
+		intToString := func(n int) O.Option[string] {
+			if n > 0 {
+				return O.Some(fmt.Sprintf("%d", n))
+			}
+			return O.None[string]()
+		}
+		filterMapOp := FilterMapArray[FilterTestConfig](intToString)
+		input := Succeed[FilterTestConfig]([]int{-1, 2, -3, 4})
+
+		// Act
+		result, err := runEffect(filterMapOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []string{"2", "4"}, result)
+	})
+
+	t.Run("converts string to int", func(t *testing.T) {
+		// Arrange
+		parseEven := func(s string) O.Option[int] {
+			var n int
+			if _, err := fmt.Sscanf(s, "%d", &n); err == nil && n%2 == 0 {
+				return O.Some(n)
+			}
+			return O.None[int]()
+		}
+		filterMapOp := FilterMapArray[FilterTestConfig](parseEven)
+		input := Succeed[FilterTestConfig]([]string{"1", "2", "3", "4", "invalid"})
+
+		// Act
+		result, err := runEffect(filterMapOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{2, 4}, result)
+	})
+}
+
+func TestFilterMapArray_EdgeCases(t *testing.T) {
+	t.Run("handles empty array", func(t *testing.T) {
+		// Arrange
+		double := func(n int) O.Option[int] {
+			return O.Some(n * 2)
+		}
+		filterMapOp := FilterMapArray[FilterTestConfig](double)
+		input := Succeed[FilterTestConfig]([]int{})
+
+		// Act
+		result, err := runEffect(filterMapOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{}, result)
+	})
+
+	t.Run("preserves error from input", func(t *testing.T) {
+		// Arrange
+		double := func(n int) O.Option[int] {
+			return O.Some(n * 2)
+		}
+		filterMapOp := FilterMapArray[FilterTestConfig](double)
+		inputErr := errors.New("input error")
+		input := Fail[FilterTestConfig, []int](inputErr)
+
+		// Act
+		_, err := runEffect(filterMapOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.Error(t, err)
+		assert.Equal(t, inputErr, err)
+	})
+}
+
+func TestFilterMapIter_EdgeCases(t *testing.T) {
+	t.Run("handles empty iterator", func(t *testing.T) {
+		// Arrange
+		double := func(n int) O.Option[int] {
+			return O.Some(n * 2)
+		}
+		filterMapOp := FilterMapIter[FilterTestConfig](double)
+		input := Succeed[FilterTestConfig](slices.Values([]int{}))
+
+		// Act
+		collected := collectSeqEffect(filterMapOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.Empty(t, collected)
+	})
+}
+
+func TestFilterMap_GenericFilterMap(t *testing.T) {
+	t.Run("works with custom filterMap function", func(t *testing.T) {
+		// Arrange
+		customFilterMap := func(f O.Kleisli[int, string]) Reader[[]int, []string] {
+			return A.FilterMap(f)
+		}
+		filterMapOp := FilterMap[FilterTestConfig](customFilterMap)
+		intToString := func(n int) O.Option[string] {
+			if n > 0 {
+				return O.Some(fmt.Sprintf("%d", n))
+			}
+			return O.None[string]()
+		}
+		input := Succeed[FilterTestConfig]([]int{-1, 2, -3, 4})
+
+		// Act
+		result, err := runEffect(filterMapOp(intToString)(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []string{"2", "4"}, result)
+	})
+}
+
+func TestFilter_Composition(t *testing.T) {
+	t.Run("chains multiple filters", func(t *testing.T) {
+		// Arrange
+		isPositive := N.MoreThan(0)
+		isEven := func(n int) bool { return n%2 == 0 }
+		filterPositive := FilterArray[FilterTestConfig](isPositive)
+		filterEven := FilterArray[FilterTestConfig](isEven)
+		input := Succeed[FilterTestConfig]([]int{-2, -1, 0, 1, 2, 3, 4, 5, 6})
+
+		// Act
+		result, err := runEffect(filterEven(filterPositive(input)), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{2, 4, 6}, result)
+	})
+
+	t.Run("chains filter and filterMap", func(t *testing.T) {
+		// Arrange
+		isPositive := N.MoreThan(0)
+		doubleEven := func(n int) O.Option[int] {
+			if n%2 == 0 {
+				return O.Some(n * 2)
+			}
+			return O.None[int]()
+		}
+		filterOp := FilterArray[FilterTestConfig](isPositive)
+		filterMapOp := FilterMapArray[FilterTestConfig](doubleEven)
+		input := Succeed[FilterTestConfig]([]int{-2, 1, 2, 3, 4, 5})
+
+		// Act
+		result, err := runEffect(filterMapOp(filterOp(input)), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{4, 8}, result)
+	})
+}
+
+func TestFilter_WithComplexTypes(t *testing.T) {
+	type User struct {
+		Name string
+		Age  int
+	}
+
+	t.Run("filters structs", func(t *testing.T) {
+		// Arrange
+		isAdult := func(u User) bool { return u.Age >= 18 }
+		filterOp := FilterArray[FilterTestConfig](isAdult)
+		users := []User{
+			{Name: "Alice", Age: 25},
+			{Name: "Bob", Age: 16},
+			{Name: "Charlie", Age: 30},
+		}
+		input := Succeed[FilterTestConfig](users)
+
+		// Act
+		result, err := runEffect(filterOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		expected := []User{
+			{Name: "Alice", Age: 25},
+			{Name: "Charlie", Age: 30},
+		}
+		assert.Equal(t, expected, result)
+	})
+
+	t.Run("filterMaps structs to different type", func(t *testing.T) {
+		// Arrange
+		extractAdultName := func(u User) O.Option[string] {
+			if u.Age >= 18 {
+				return O.Some(u.Name)
+			}
+			return O.None[string]()
+		}
+		filterMapOp := FilterMapArray[FilterTestConfig](extractAdultName)
+		users := []User{
+			{Name: "Alice", Age: 25},
+			{Name: "Bob", Age: 16},
+			{Name: "Charlie", Age: 30},
+		}
+		input := Succeed[FilterTestConfig](users)
+
+		// Act
+		result, err := runEffect(filterMapOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []string{"Alice", "Charlie"}, result)
+	})
+}
+
+func TestFilter_BoundaryConditions(t *testing.T) {
+	t.Run("filters with boundary predicate", func(t *testing.T) {
+		// Arrange
+		inRange := func(n int) bool { return n >= 0 && n <= 100 }
+		filterOp := FilterArray[FilterTestConfig](inRange)
+		input := Succeed[FilterTestConfig]([]int{-1, 0, 50, 100, 101})
+
+		// Act
+		result, err := runEffect(filterOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{0, 50, 100}, result)
+	})
+
+	t.Run("filterMap with boundary conditions", func(t *testing.T) {
+		// Arrange
+		clampToRange := func(n int) O.Option[int] {
+			if n >= 0 && n <= 100 {
+				return O.Some(n)
+			}
+			return O.None[int]()
+		}
+		filterMapOp := FilterMapArray[FilterTestConfig](clampToRange)
+		input := Succeed[FilterTestConfig]([]int{-1, 0, 50, 100, 101})
+
+		// Act
+		result, err := runEffect(filterMapOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{0, 50, 100}, result)
+	})
+}
+
+func TestFilter_WithIterators(t *testing.T) {
+	t.Run("filters large iterator efficiently", func(t *testing.T) {
+		// Arrange
+		isEven := func(n int) bool { return n%2 == 0 }
+		filterOp := FilterIter[FilterTestConfig](isEven)
+
+		// Create iterator for range 0-99
+		makeSeq := func(yield func(int) bool) {
+			for i := range 100 {
+				if !yield(i) {
+					return
+				}
+			}
+		}
+		input := Succeed[FilterTestConfig](Seq[int](makeSeq))
+
+		// Act
+		collected := collectSeqEffect(filterOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.Equal(t, 50, len(collected))
+		assert.Equal(t, 0, collected[0])
+		assert.Equal(t, 98, collected[49])
+	})
+
+	t.Run("filterMap with iterator", func(t *testing.T) {
+		// Arrange
+		squareEven := func(n int) O.Option[int] {
+			if n%2 == 0 {
+				return O.Some(n * n)
+			}
+			return O.None[int]()
+		}
+		filterMapOp := FilterMapIter[FilterTestConfig](squareEven)
+		input := Succeed[FilterTestConfig](slices.Values([]int{1, 2, 3, 4, 5}))
+
+		// Act
+		collected := collectSeqEffect(filterMapOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.Equal(t, []int{4, 16}, collected)
+	})
+}
+
+func TestFilter_ErrorPropagation(t *testing.T) {
+	t.Run("filter propagates Left through chain", func(t *testing.T) {
+		// Arrange
+		isPositive := N.MoreThan(0)
+		filterOp := FilterArray[FilterTestConfig](isPositive)
+		originalErr := errors.New("original error")
+
+		// Create an effect that fails
+		failedEffect := F.Pipe1(
+			Succeed[FilterTestConfig]([]int{1, 2, 3}),
+			Chain(func([]int) Effect[FilterTestConfig, []int] {
+				return Fail[FilterTestConfig, []int](originalErr)
+			}),
+		)
+
+		// Act
+		_, err := runEffect(filterOp(failedEffect), FilterTestConfig{})
+
+		// Assert
+		assert.Error(t, err)
+		assert.Equal(t, originalErr, err)
+	})
+
+	t.Run("filterMap propagates Left through chain", func(t *testing.T) {
+		// Arrange
+		double := func(n int) O.Option[int] {
+			return O.Some(n * 2)
+		}
+		filterMapOp := FilterMapArray[FilterTestConfig](double)
+		originalErr := errors.New("original error")
+
+		// Create an effect that fails
+		failedEffect := F.Pipe1(
+			Succeed[FilterTestConfig]([]int{1, 2, 3}),
+			Chain(func([]int) Effect[FilterTestConfig, []int] {
+				return Fail[FilterTestConfig, []int](originalErr)
+			}),
+		)
+
+		// Act
+		_, err := runEffect(filterMapOp(failedEffect), FilterTestConfig{})
+
+		// Assert
+		assert.Error(t, err)
+		assert.Equal(t, originalErr, err)
+	})
+}
+
+func TestFilter_Integration(t *testing.T) {
+	t.Run("complex filtering pipeline", func(t *testing.T) {
+		// Arrange: Filter positive numbers, then double evens, then filter > 5
+		isPositive := N.MoreThan(0)
+		doubleEven := func(n int) O.Option[int] {
+			if n%2 == 0 {
+				return O.Some(n * 2)
+			}
+			return O.None[int]()
+		}
+		isGreaterThan5 := N.MoreThan(5)
+
+		pipeline := F.Pipe3(
+			Succeed[FilterTestConfig]([]int{-2, -1, 0, 1, 2, 3, 4, 5, 6}),
+			FilterArray[FilterTestConfig](isPositive),
+			FilterMapArray[FilterTestConfig](doubleEven),
+			FilterArray[FilterTestConfig](isGreaterThan5),
+		)
+
+		// Act
+		result, err := runEffect(pipeline, FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		// Positive: [1,2,3,4,5,6] -> DoubleEven: [4,8,12] -> >5: [8,12]
+		assert.Equal(t, []int{8, 12}, result)
+	})
+}
+
+

v2/effect/monoid.go

@@ -1,3 +1,18 @@
+// 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 effect
 
 import (
@@ -5,10 +20,66 @@ import (
 	"github.com/IBM/fp-go/v2/monoid"
 )
 
+// ApplicativeMonoid creates a monoid for effects using applicative semantics.
+// This combines effects by running both and combining their results using the provided monoid.
+// If either effect fails, the combined effect fails.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effects
+//   - A: The value type that has a monoid instance
+//
+// # Parameters
+//
+//   - m: The monoid instance for combining values of type A
+//
+// # Returns
+//
+//   - Monoid[Effect[C, A]]: A monoid for combining effects
+//
+// # Example
+//
+//	stringMonoid := monoid.MakeMonoid(
+//		func(a, b string) string { return a + b },
+//		"",
+//	)
+//	effectMonoid := effect.ApplicativeMonoid[MyContext](stringMonoid)
+//	eff1 := effect.Of[MyContext]("Hello")
+//	eff2 := effect.Of[MyContext](" World")
+//	combined := effectMonoid.Concat(eff1, eff2)
+//	// combined produces "Hello World"
 func ApplicativeMonoid[C, A any](m monoid.Monoid[A]) Monoid[Effect[C, A]] {
 	return readerreaderioresult.ApplicativeMonoid[C](m)
 }
 
+// AlternativeMonoid creates a monoid for effects using alternative semantics.
+// This tries the first effect, and if it fails, tries the second effect.
+// If both succeed, their results are combined using the provided monoid.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effects
+//   - A: The value type that has a monoid instance
+//
+// # Parameters
+//
+//   - m: The monoid instance for combining values of type A
+//
+// # Returns
+//
+//   - Monoid[Effect[C, A]]: A monoid for combining effects with fallback behavior
+//
+// # Example
+//
+//	stringMonoid := monoid.MakeMonoid(
+//		func(a, b string) string { return a + b },
+//		"",
+//	)
+//	effectMonoid := effect.AlternativeMonoid[MyContext](stringMonoid)
+//	eff1 := effect.Fail[MyContext, string](errors.New("failed"))
+//	eff2 := effect.Of[MyContext]("fallback")
+//	combined := effectMonoid.Concat(eff1, eff2)
+//	// combined produces "fallback" (first failed, so second is used)
 func AlternativeMonoid[C, A any](m monoid.Monoid[A]) Monoid[Effect[C, A]] {
 	return readerreaderioresult.AlternativeMonoid[C](m)
 }

v2/effect/profunctor.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 effect
+
+import (
+	F "github.com/IBM/fp-go/v2/function"
+)
+
+// Promap is the profunctor map operation that transforms both the input and output of an Effect.
+// It applies f to the input context (contravariantly) and g to the output value (covariantly).
+//
+// See: https://github.com/fantasyland/fantasy-land?tab=readme-ov-file#profunctor
+//
+// This operation allows you to:
+//   - Modify the context before passing it to the Effect (via f)
+//   - Transform the success value after the computation completes (via g)
+//
+// Promap is particularly useful for adapting effects to work with different context types
+// while simultaneously transforming their output values.
+//
+// # Type Parameters
+//
+//   - E: The original context type expected by the Effect
+//   - A: The original success type produced by the Effect
+//   - D: The new input context type
+//   - B: The new output success type
+//
+// # Parameters
+//
+//   - f: Function to transform the input context from D to E (contravariant)
+//   - g: Function to transform the output success value from A to B (covariant)
+//
+// # Returns
+//
+//   - A Kleisli arrow that takes an Effect[E, A] and returns a function from D to B
+//
+// # Example Usage
+//
+//	type AppConfig struct {
+//	    DatabaseURL string
+//	    APIKey      string
+//	}
+//
+//	type DBConfig struct {
+//	    URL string
+//	}
+//
+//	// Effect that uses DBConfig and returns an int
+//	getUserCount := func(cfg DBConfig) effect.Effect[context.Context, int] {
+//	    return effect.Succeed[context.Context](42)
+//	}
+//
+//	// Transform AppConfig to DBConfig
+//	extractDBConfig := func(app AppConfig) DBConfig {
+//	    return DBConfig{URL: app.DatabaseURL}
+//	}
+//
+//	// Transform int to string
+//	formatCount := func(count int) string {
+//	    return fmt.Sprintf("Users: %d", count)
+//	}
+//
+//	// Adapt the effect to work with AppConfig and return string
+//	adapted := effect.Promap(extractDBConfig, formatCount)(getUserCount)
+//	result := adapted(AppConfig{DatabaseURL: "localhost:5432", APIKey: "secret"})
+//
+//go:inline
+func Promap[E, A, D, B any](f Reader[D, E], g Reader[A, B]) Kleisli[D, Effect[E, A], B] {
+	return F.Flow2(
+		Local[A](f),
+		Map[D](g),
+	)
+}

v2/effect/profunctor_test.go

@@ -0,0 +1,373 @@
+// 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 effect
+
+import (
+	"context"
+	"fmt"
+	"strconv"
+	"testing"
+
+	R "github.com/IBM/fp-go/v2/result"
+	"github.com/stretchr/testify/assert"
+)
+
+// Test types for profunctor tests
+type AppConfig struct {
+	DatabaseURL string
+	APIKey      string
+	Port        int
+}
+
+type DBConfig struct {
+	URL string
+}
+
+type ServerConfig struct {
+	Host string
+	Port int
+}
+
+// TestPromapBasic tests basic Promap functionality
+func TestPromapBasic(t *testing.T) {
+	t.Run("transform both context and output", func(t *testing.T) {
+		// Effect that uses DBConfig and returns an int
+		getUserCount := Succeed[DBConfig](42)
+
+		// Transform AppConfig to DBConfig
+		extractDBConfig := func(app AppConfig) DBConfig {
+			return DBConfig{URL: app.DatabaseURL}
+		}
+
+		// Transform int to string
+		formatCount := func(count int) string {
+			return fmt.Sprintf("Users: %d", count)
+		}
+
+		// Adapt the effect to work with AppConfig and return string
+		adapted := Promap(extractDBConfig, formatCount)(getUserCount)
+		result := adapted(AppConfig{
+			DatabaseURL: "localhost:5432",
+			APIKey:      "secret",
+			Port:        8080,
+		})(context.Background())()
+
+		assert.Equal(t, R.Of("Users: 42"), result)
+	})
+
+	t.Run("identity transformations", func(t *testing.T) {
+		// Effect that returns a value
+		getValue := Succeed[DBConfig](100)
+
+		// Identity transformations
+		identity := func(x DBConfig) DBConfig { return x }
+		identityInt := func(x int) int { return x }
+
+		// Apply identity transformations
+		adapted := Promap(identity, identityInt)(getValue)
+		result := adapted(DBConfig{URL: "localhost"})(context.Background())()
+
+		assert.Equal(t, R.Of(100), result)
+	})
+}
+
+// TestPromapComposition tests that Promap composes correctly
+func TestPromapComposition(t *testing.T) {
+	t.Run("compose multiple transformations", func(t *testing.T) {
+		// Effect that uses ServerConfig and returns the port
+		getPort := Map[ServerConfig](func(cfg ServerConfig) int {
+			return cfg.Port
+		})(Ask[ServerConfig]())
+
+		// First transformation: AppConfig -> ServerConfig
+		extractServerConfig := func(app AppConfig) ServerConfig {
+			return ServerConfig{Host: "localhost", Port: app.Port}
+		}
+
+		// Second transformation: int -> string
+		formatPort := func(port int) string {
+			return fmt.Sprintf(":%d", port)
+		}
+
+		// Apply transformations
+		adapted := Promap(extractServerConfig, formatPort)(getPort)
+		result := adapted(AppConfig{
+			DatabaseURL: "db.example.com",
+			APIKey:      "key123",
+			Port:        9000,
+		})(context.Background())()
+
+		assert.Equal(t, R.Of(":9000"), result)
+	})
+}
+
+// TestPromapWithErrors tests Promap with effects that can fail
+func TestPromapWithErrors(t *testing.T) {
+	t.Run("propagates errors correctly", func(t *testing.T) {
+		// Effect that fails
+		failingEffect := Fail[DBConfig, int](fmt.Errorf("database connection failed"))
+
+		// Transformations
+		extractDBConfig := func(app AppConfig) DBConfig {
+			return DBConfig{URL: app.DatabaseURL}
+		}
+		formatCount := func(count int) string {
+			return fmt.Sprintf("Count: %d", count)
+		}
+
+		// Apply transformations
+		adapted := Promap(extractDBConfig, formatCount)(failingEffect)
+		result := adapted(AppConfig{DatabaseURL: "localhost"})(context.Background())()
+
+		assert.True(t, R.IsLeft(result))
+		err := R.MonadFold(result,
+			func(e error) error { return e },
+			func(string) error { return nil },
+		)
+		assert.Error(t, err)
+		assert.Contains(t, err.Error(), "database connection failed")
+	})
+
+	t.Run("output transformation not applied on error", func(t *testing.T) {
+		callCount := 0
+
+		// Effect that fails
+		failingEffect := Fail[DBConfig, int](fmt.Errorf("error"))
+
+		// Transformation that counts calls
+		countingTransform := func(x int) string {
+			callCount++
+			return strconv.Itoa(x)
+		}
+
+		// Apply transformations
+		adapted := Promap(
+			func(app AppConfig) DBConfig { return DBConfig{URL: app.DatabaseURL} },
+			countingTransform,
+		)(failingEffect)
+		result := adapted(AppConfig{DatabaseURL: "localhost"})(context.Background())()
+
+		assert.True(t, R.IsLeft(result))
+		assert.Equal(t, 0, callCount, "output transformation should not be called on error")
+	})
+}
+
+// TestPromapWithComplexTypes tests Promap with more complex type transformations
+func TestPromapWithComplexTypes(t *testing.T) {
+	t.Run("transform struct to different struct", func(t *testing.T) {
+		type User struct {
+			ID   int
+			Name string
+		}
+
+		type UserDTO struct {
+			UserID   int
+			FullName string
+		}
+
+		// Effect that uses User and returns a string
+		getUserInfo := Map[User](func(user User) string {
+			return fmt.Sprintf("User %s (ID: %d)", user.Name, user.ID)
+		})(Ask[User]())
+
+		// Transform UserDTO to User
+		dtoToUser := func(dto UserDTO) User {
+			return User{ID: dto.UserID, Name: dto.FullName}
+		}
+
+		// Transform string to uppercase
+		toUpper := func(s string) string {
+			return fmt.Sprintf("INFO: %s", s)
+		}
+
+		// Apply transformations
+		adapted := Promap(dtoToUser, toUpper)(getUserInfo)
+		result := adapted(UserDTO{UserID: 123, FullName: "Alice"})(context.Background())()
+
+		assert.Equal(t, R.Of("INFO: User Alice (ID: 123)"), result)
+	})
+}
+
+// TestPromapChaining tests chaining multiple Promap operations
+func TestPromapChaining(t *testing.T) {
+	t.Run("chain multiple Promap operations", func(t *testing.T) {
+		// Base effect that doubles the input
+		baseEffect := Map[int](func(x int) int {
+			return x * 2
+		})(Ask[int]())
+
+		// First Promap: string -> int, int -> string
+		step1 := Promap(
+			func(s string) int {
+				n, _ := strconv.Atoi(s)
+				return n
+			},
+			strconv.Itoa,
+		)(baseEffect)
+
+		// Second Promap: float64 -> string, string -> float64
+		step2 := Promap(
+			func(f float64) string {
+				return fmt.Sprintf("%.0f", f)
+			},
+			func(s string) float64 {
+				f, _ := strconv.ParseFloat(s, 64)
+				return f
+			},
+		)(step1)
+
+		result := step2(21.0)(context.Background())()
+
+		assert.Equal(t, R.Of(42.0), result)
+	})
+}
+
+// TestPromapEdgeCases tests edge cases
+func TestPromapEdgeCases(t *testing.T) {
+	t.Run("zero values", func(t *testing.T) {
+		effect := Map[int](func(x int) int {
+			return x
+		})(Ask[int]())
+
+		adapted := Promap(
+			func(s string) int { return 0 },
+			func(x int) string { return "" },
+		)(effect)
+
+		result := adapted("anything")(context.Background())()
+
+		assert.Equal(t, R.Of(""), result)
+	})
+
+	t.Run("nil context handling", func(t *testing.T) {
+		effect := Succeed[int]("success")
+
+		adapted := Promap(
+			func(s string) int { return 42 },
+			func(s string) string { return s + "!" },
+		)(effect)
+
+		// Using background context instead of nil
+		result := adapted("test")(context.Background())()
+
+		assert.Equal(t, R.Of("success!"), result)
+	})
+}
+
+// TestPromapIntegration tests integration with other effect operations
+func TestPromapIntegration(t *testing.T) {
+	t.Run("Promap with Map", func(t *testing.T) {
+		// Base effect that adds 10
+		baseEffect := Map[int](func(x int) int {
+			return x + 10
+		})(Ask[int]())
+
+		// Apply Promap
+		promapped := Promap(
+			func(s string) int {
+				n, _ := strconv.Atoi(s)
+				return n
+			},
+			func(x int) int { return x * 2 },
+		)(baseEffect)
+
+		// Apply Map on top
+		mapped := Map[string](func(x int) string {
+			return fmt.Sprintf("Result: %d", x)
+		})(promapped)
+
+		result := mapped("5")(context.Background())()
+
+		assert.Equal(t, R.Of("Result: 30"), result)
+	})
+
+	t.Run("Promap with Chain", func(t *testing.T) {
+		// Base effect
+		baseEffect := Ask[int]()
+
+		// Apply Promap
+		promapped := Promap(
+			func(s string) int {
+				n, _ := strconv.Atoi(s)
+				return n
+			},
+			func(x int) int { return x * 2 },
+		)(baseEffect)
+
+		// Chain with another effect
+		chained := Chain(func(x int) Effect[string, string] {
+			return Succeed[string](fmt.Sprintf("Value: %d", x))
+		})(promapped)
+
+		result := chained("10")(context.Background())()
+
+		assert.Equal(t, R.Of("Value: 20"), result)
+	})
+}
+
+// BenchmarkPromap benchmarks the Promap operation
+func BenchmarkPromap(b *testing.B) {
+	effect := Map[int](func(x int) int {
+		return x * 2
+	})(Ask[int]())
+
+	adapted := Promap(
+		func(s string) int {
+			n, _ := strconv.Atoi(s)
+			return n
+		},
+		strconv.Itoa,
+	)(effect)
+
+	ctx := context.Background()
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_ = adapted("42")(ctx)()
+	}
+}
+
+// BenchmarkPromapChained benchmarks chained Promap operations
+func BenchmarkPromapChained(b *testing.B) {
+	baseEffect := Map[int](func(x int) int {
+		return x * 2
+	})(Ask[int]())
+
+	step1 := Promap(
+		func(s string) int {
+			n, _ := strconv.Atoi(s)
+			return n
+		},
+		strconv.Itoa,
+	)(baseEffect)
+
+	step2 := Promap(
+		func(f float64) string {
+			return fmt.Sprintf("%.0f", f)
+		},
+		func(s string) float64 {
+			f, _ := strconv.ParseFloat(s, 64)
+			return f
+		},
+	)(step1)
+
+	ctx := context.Background()
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_ = step2(21.0)(ctx)()
+	}
+}

v2/effect/retry.go

@@ -1,3 +1,18 @@
+// 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 effect
 
 import (
@@ -5,6 +20,39 @@ import (
 	"github.com/IBM/fp-go/v2/retry"
 )
 
+// Retrying executes an effect with retry logic based on a policy and check predicate.
+// The effect is retried according to the policy until either:
+//   - The effect succeeds and the check predicate returns false
+//   - The retry policy is exhausted
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - A: The type of the success value
+//
+// # Parameters
+//
+//   - policy: The retry policy defining retry limits and delays
+//   - action: An effectful computation that receives retry status and produces a value
+//   - check: A predicate that determines if the result should trigger a retry
+//
+// # Returns
+//
+//   - Effect[C, A]: An effect that retries according to the policy
+//
+// # Example
+//
+//	policy := retry.LimitRetries(3)
+//	eff := effect.Retrying[MyContext, string](
+//		policy,
+//		func(status retry.RetryStatus) Effect[MyContext, string] {
+//			return fetchData() // may fail
+//		},
+//		func(result Result[string]) bool {
+//			return result.IsLeft() // retry on error
+//		},
+//	)
+//	// Retries up to 3 times if fetchData fails
 func Retrying[C, A any](
 	policy retry.RetryPolicy,
 	action Kleisli[C, retry.RetryStatus, A],

v2/effect/run.go

@@ -1,3 +1,18 @@
+// 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 effect
 
 import (
@@ -8,10 +23,64 @@ import (
 	"github.com/IBM/fp-go/v2/result"
 )
 
-func Provide[C, A any](c C) func(Effect[C, A]) ReaderIOResult[A] {
+// Provide supplies a context to an effect, converting it to a Thunk.
+// This is the first step in running an effect - it eliminates the context dependency
+// by providing the required context value.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - A: The type of the success value
+//
+// # Parameters
+//
+//   - c: The context value to provide to the effect
+//
+// # Returns
+//
+//   - func(Effect[C, A]) ReaderIOResult[A]: A function that converts an effect to a thunk
+//
+// # Example
+//
+//	ctx := MyContext{APIKey: "secret"}
+//	eff := effect.Of[MyContext](42)
+//	thunk := effect.Provide[MyContext, int](ctx)(eff)
+//	// thunk is now a ReaderIOResult[int] that can be run
+func Provide[A, C any](c C) func(Effect[C, A]) ReaderIOResult[A] {
 	return readerreaderioresult.Read[A](c)
 }
 
+// RunSync executes a Thunk synchronously, converting it to a standard Go function.
+// This is the final step in running an effect - it executes the IO operations
+// and returns the result as a standard (value, error) tuple.
+//
+// # Type Parameters
+//
+//   - A: The type of the success value
+//
+// # Parameters
+//
+//   - fa: The thunk to execute
+//
+// # Returns
+//
+//   - readerresult.ReaderResult[A]: A function that takes a context.Context and returns (A, error)
+//
+// # Example
+//
+//	ctx := MyContext{APIKey: "secret"}
+//	eff := effect.Of[MyContext](42)
+//	thunk := effect.Provide[MyContext, int](ctx)(eff)
+//	readerResult := effect.RunSync(thunk)
+//	value, err := readerResult(context.Background())
+//	// value == 42, err == nil
+//
+// # Complete Example
+//
+//	// Typical usage pattern:
+//	result, err := effect.RunSync(
+//		effect.Provide[MyContext, string](myContext)(myEffect),
+//	)(context.Background())
 func RunSync[A any](fa ReaderIOResult[A]) readerresult.ReaderResult[A] {
 	return func(ctx context.Context) (A, error) {
 		return result.Unwrap(fa(ctx)())

v2/effect/run_test.go

@@ -28,7 +28,7 @@ func TestProvide(t *testing.T) {
 		ctx := TestContext{Value: "test-value"}
 		eff := Of[TestContext]("result")
 
-		ioResult := Provide[TestContext, string](ctx)(eff)
+		ioResult := Provide[string](ctx)(eff)
 		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
@@ -45,7 +45,7 @@ func TestProvide(t *testing.T) {
 		cfg := Config{Host: "localhost", Port: 8080}
 		eff := Of[Config]("connected")
 
-		ioResult := Provide[Config, string](cfg)(eff)
+		ioResult := Provide[string](cfg)(eff)
 		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
@@ -58,7 +58,7 @@ func TestProvide(t *testing.T) {
 		ctx := TestContext{Value: "test"}
 		eff := Fail[TestContext, string](expectedErr)
 
-		ioResult := Provide[TestContext, string](ctx)(eff)
+		ioResult := Provide[string](ctx)(eff)
 		readerResult := RunSync(ioResult)
 		_, err := readerResult(context.Background())
 
@@ -74,7 +74,7 @@ func TestProvide(t *testing.T) {
 		ctx := SimpleContext{ID: 42}
 		eff := Of[SimpleContext](100)
 
-		ioResult := Provide[SimpleContext, int](ctx)(eff)
+		ioResult := Provide[int](ctx)(eff)
 		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
@@ -89,7 +89,7 @@ func TestProvide(t *testing.T) {
 			return Of[TestContext]("result")
 		})(Of[TestContext](42))
 
-		ioResult := Provide[TestContext, string](ctx)(eff)
+		ioResult := Provide[string](ctx)(eff)
 		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
@@ -104,7 +104,7 @@ func TestProvide(t *testing.T) {
 			return "mapped"
 		})(Of[TestContext](42))
 
-		ioResult := Provide[TestContext, string](ctx)(eff)
+		ioResult := Provide[string](ctx)(eff)
 		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
@@ -118,7 +118,7 @@ func TestRunSync(t *testing.T) {
 		ctx := TestContext{Value: "test"}
 		eff := Of[TestContext](42)
 
-		ioResult := Provide[TestContext, int](ctx)(eff)
+		ioResult := Provide[int](ctx)(eff)
 		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
@@ -130,7 +130,7 @@ func TestRunSync(t *testing.T) {
 		ctx := TestContext{Value: "test"}
 		eff := Of[TestContext]("hello")
 
-		ioResult := Provide[TestContext, string](ctx)(eff)
+		ioResult := Provide[string](ctx)(eff)
 		readerResult := RunSync(ioResult)
 
 		bgCtx := context.Background()
@@ -145,7 +145,7 @@ func TestRunSync(t *testing.T) {
 		ctx := TestContext{Value: "test"}
 		eff := Fail[TestContext, int](expectedErr)
 
-		ioResult := Provide[TestContext, int](ctx)(eff)
+		ioResult := Provide[int](ctx)(eff)
 		readerResult := RunSync(ioResult)
 		_, err := readerResult(context.Background())
 
@@ -162,7 +162,7 @@ func TestRunSync(t *testing.T) {
 			return Of[TestContext](x + 10)
 		})(Of[TestContext](5)))
 
-		ioResult := Provide[TestContext, int](ctx)(eff)
+		ioResult := Provide[int](ctx)(eff)
 		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
@@ -174,7 +174,7 @@ func TestRunSync(t *testing.T) {
 		ctx := TestContext{Value: "test"}
 		eff := Of[TestContext](42)
 
-		ioResult := Provide[TestContext, int](ctx)(eff)
+		ioResult := Provide[int](ctx)(eff)
 		readerResult := RunSync(ioResult)
 
 		// Run multiple times
@@ -200,7 +200,7 @@ func TestRunSync(t *testing.T) {
 		user := User{Name: "Alice", Age: 30}
 		eff := Of[TestContext](user)
 
-		ioResult := Provide[TestContext, User](ctx)(eff)
+		ioResult := Provide[User](ctx)(eff)
 		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
@@ -222,7 +222,7 @@ func TestProvideAndRunSyncIntegration(t *testing.T) {
 		eff := Of[AppConfig]("API call successful")
 
 		// Provide config and run
-		result, err := RunSync(Provide[AppConfig, string](cfg)(eff))(context.Background())
+		result, err := RunSync(Provide[string](cfg)(eff))(context.Background())
 
 		assert.NoError(t, err)
 		assert.Equal(t, "API call successful", result)
@@ -238,7 +238,7 @@ func TestProvideAndRunSyncIntegration(t *testing.T) {
 
 		eff := Fail[AppConfig, string](expectedErr)
 
-		_, err := RunSync(Provide[AppConfig, string](cfg)(eff))(context.Background())
+		_, err := RunSync(Provide[string](cfg)(eff))(context.Background())
 
 		assert.Error(t, err)
 		assert.Equal(t, expectedErr, err)
@@ -253,7 +253,7 @@ func TestProvideAndRunSyncIntegration(t *testing.T) {
 			return Of[TestContext](x * 2)
 		})(Of[TestContext](21)))
 
-		result, err := RunSync(Provide[TestContext, string](ctx)(eff))(context.Background())
+		result, err := RunSync(Provide[string](ctx)(eff))(context.Background())
 
 		assert.NoError(t, err)
 		assert.Equal(t, "final", result)
@@ -281,7 +281,7 @@ func TestProvideAndRunSyncIntegration(t *testing.T) {
 			return State{X: x}
 		})(Of[TestContext](10)))
 
-		result, err := RunSync(Provide[TestContext, State](ctx)(eff))(context.Background())
+		result, err := RunSync(Provide[State](ctx)(eff))(context.Background())
 
 		assert.NoError(t, err)
 		assert.Equal(t, 10, result.X)
@@ -300,11 +300,11 @@ func TestProvideAndRunSyncIntegration(t *testing.T) {
 		innerEff := Of[InnerCtx]("inner result")
 
 		// Transform context
-		transformedEff := Local[OuterCtx, InnerCtx, string](func(outer OuterCtx) InnerCtx {
+		transformedEff := Local[string](func(outer OuterCtx) InnerCtx {
 			return InnerCtx{Data: outer.Value + "-transformed"}
 		})(innerEff)
 
-		result, err := RunSync(Provide[OuterCtx, string](outerCtx)(transformedEff))(context.Background())
+		result, err := RunSync(Provide[string](outerCtx)(transformedEff))(context.Background())
 
 		assert.NoError(t, err)
 		assert.Equal(t, "inner result", result)
@@ -318,7 +318,7 @@ func TestProvideAndRunSyncIntegration(t *testing.T) {
 			return Of[TestContext](x * 2)
 		})(input)
 
-		result, err := RunSync(Provide[TestContext, []int](ctx)(eff))(context.Background())
+		result, err := RunSync(Provide[[]int](ctx)(eff))(context.Background())
 
 		assert.NoError(t, err)
 		assert.Equal(t, []int{2, 4, 6, 8, 10}, result)

v2/effect/traverse.go

@@ -1,7 +1,55 @@
+// 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 effect
 
 import "github.com/IBM/fp-go/v2/context/readerreaderioresult"
 
+// TraverseArray applies an effectful function to each element of an array,
+// collecting the results into a new array. If any effect fails, the entire
+// traversal fails and returns the first error encountered.
+//
+// This is useful for performing effectful operations on collections while
+// maintaining the sequential order of results.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effects
+//   - A: The input element type
+//   - B: The output element type
+//
+// # Parameters
+//
+//   - f: An effectful function to apply to each element
+//
+// # Returns
+//
+//   - Kleisli[C, []A, []B]: A function that transforms an array of A to an effect producing an array of B
+//
+// # Example
+//
+//	parseIntEff := func(s string) Effect[MyContext, int] {
+//		val, err := strconv.Atoi(s)
+//		if err != nil {
+//			return effect.Fail[MyContext, int](err)
+//		}
+//		return effect.Of[MyContext](val)
+//	}
+//	input := []string{"1", "2", "3"}
+//	eff := effect.TraverseArray[MyContext](parseIntEff)(input)
+//	// eff produces []int{1, 2, 3}
 func TraverseArray[C, A, B any](f Kleisli[C, A, B]) Kleisli[C, []A, []B] {
 	return readerreaderioresult.TraverseArray(f)
 }

v2/effect/types.go

@@ -1,12 +1,29 @@
+// 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 effect
 
 import (
 	"github.com/IBM/fp-go/v2/context/readerioresult"
 	"github.com/IBM/fp-go/v2/context/readerreaderioresult"
 	"github.com/IBM/fp-go/v2/either"
+	"github.com/IBM/fp-go/v2/endomorphism"
 	"github.com/IBM/fp-go/v2/io"
 	"github.com/IBM/fp-go/v2/ioeither"
 	"github.com/IBM/fp-go/v2/ioresult"
+	"github.com/IBM/fp-go/v2/iterator/iter"
 	"github.com/IBM/fp-go/v2/lazy"
 	"github.com/IBM/fp-go/v2/monoid"
 	"github.com/IBM/fp-go/v2/optics/lens"
@@ -17,21 +34,71 @@ import (
 )
 
 type (
-	Either[E, A any]      = either.Either[E, A]
-	Reader[R, A any]      = reader.Reader[R, A]
-	ReaderIO[R, A any]    = readerio.ReaderIO[R, A]
-	IO[A any]             = io.IO[A]
-	IOEither[E, A any]    = ioeither.IOEither[E, A]
-	Lazy[A any]           = lazy.Lazy[A]
-	IOResult[A any]       = ioresult.IOResult[A]
-	ReaderIOResult[A any] = readerioresult.ReaderIOResult[A]
-	Monoid[A any]         = monoid.Monoid[A]
-	Effect[C, A any]      = readerreaderioresult.ReaderReaderIOResult[C, A]
-	Thunk[A any]          = ReaderIOResult[A]
-	Predicate[A any]      = predicate.Predicate[A]
-	Result[A any]         = result.Result[A]
-	Lens[S, T any]        = lens.Lens[S, T]
+	// Either represents a value that can be either a Left (error) or Right (success).
+	Either[E, A any] = either.Either[E, A]
 
-	Kleisli[C, A, B any]  = readerreaderioresult.Kleisli[C, A, B]
+	// Reader represents a computation that depends on a context R and produces a value A.
+	Reader[R, A any] = reader.Reader[R, A]
+
+	// ReaderIO represents a computation that depends on a context R and produces an IO action returning A.
+	ReaderIO[R, A any] = readerio.ReaderIO[R, A]
+
+	// IO represents a synchronous side effect that produces a value A.
+	IO[A any] = io.IO[A]
+
+	// IOEither represents a synchronous side effect that can fail with error E or succeed with value A.
+	IOEither[E, A any] = ioeither.IOEither[E, A]
+
+	// Lazy represents a lazily evaluated computation that produces a value A.
+	Lazy[A any] = lazy.Lazy[A]
+
+	// IOResult represents a synchronous side effect that can fail with an error or succeed with value A.
+	IOResult[A any] = ioresult.IOResult[A]
+
+	// ReaderIOResult represents a computation that depends on context and performs IO with error handling.
+	ReaderIOResult[A any] = readerioresult.ReaderIOResult[A]
+
+	// Monoid represents an algebraic structure with an associative binary operation and an identity element.
+	Monoid[A any] = monoid.Monoid[A]
+
+	// Effect represents an effectful computation that:
+	//   - Requires a context of type C
+	//   - Can perform I/O operations
+	//   - Can fail with an error
+	//   - Produces a value of type A on success
+	//
+	// This is the core type of the effect package, providing a complete effect system
+	// for managing dependencies, errors, and side effects in a composable way.
+	Effect[C, A any] = readerreaderioresult.ReaderReaderIOResult[C, A]
+
+	// Thunk represents a computation that performs IO with error handling but doesn't require context.
+	// It's equivalent to ReaderIOResult and is used as an intermediate step when providing context to an Effect.
+	Thunk[A any] = ReaderIOResult[A]
+
+	// Predicate represents a function that tests a value of type A and returns a boolean.
+	Predicate[A any] = predicate.Predicate[A]
+
+	// Result represents a computation result that can be either an error (Left) or a success value (Right).
+	Result[A any] = result.Result[A]
+
+	// Lens represents an optic for focusing on a field T within a structure S.
+	Lens[S, T any] = lens.Lens[S, T]
+
+	// Kleisli represents a function from A to Effect[C, B], enabling monadic composition.
+	// It's the fundamental building block for chaining effectful computations.
+	Kleisli[C, A, B any] = readerreaderioresult.Kleisli[C, A, B]
+
+	// Operator represents a function that transforms Effect[C, A] to Effect[C, B].
+	// It's used for lifting operations over effects.
 	Operator[C, A, B any] = readerreaderioresult.Operator[C, A, B]
+
+	// Endomorphism represents a function from type A to type A.
+	// It's an alias for endomorphism.Endomorphism[A].
+	Endomorphism[A any] = endomorphism.Endomorphism[A]
+
+	// Seq is an iterator over sequences of individual values.
+	// When called as seq(yield), seq calls yield(v) for each value v in the sequence,
+	// stopping early if yield returns false.
+	// See the [iter] package documentation for more details.
+	Seq[A any] = iter.Seq[A]
 )

v2/either/array.go

@@ -16,6 +16,9 @@
 package either
 
 import (
+	"iter"
+	"slices"
+
 	F "github.com/IBM/fp-go/v2/function"
 	RA "github.com/IBM/fp-go/v2/internal/array"
 )
@@ -178,3 +181,92 @@ func CompactArrayG[A1 ~[]Either[E, A], A2 ~[]A, E, A any](fa A1) A2 {
 func CompactArray[E, A any](fa []Either[E, A]) []A {
 	return CompactArrayG[[]Either[E, A], []A](fa)
 }
+
+// TraverseSeq transforms an iterator by applying a function that returns an Either to each element.
+// If any element produces a Left, the entire result is that Left (short-circuits).
+// Otherwise, returns Right containing an iterator of all Right values.
+//
+// The function eagerly evaluates all elements in the input iterator to detect any Left values,
+// then returns an iterator over the collected Right values. This is necessary because Either
+// represents computations that can fail, and we need to know if any element failed before
+// producing the result iterator.
+//
+// # Type Parameters
+//
+//   - E: The error type for Left values
+//   - A: The input element type
+//   - B: The output element type
+//
+// # Parameters
+//
+//   - f: A function that transforms each element into an Either
+//
+// # Returns
+//
+//   - A function that takes an iterator of A and returns Either containing an iterator of B
+//
+// # Example Usage
+//
+//	parse := func(s string) either.Either[error, int] {
+//	    v, err := strconv.Atoi(s)
+//	    return either.FromError(v, err)
+//	}
+//	input := slices.Values([]string{"1", "2", "3"})
+//	result := either.TraverseSeq(parse)(input)
+//	// result is Right(iterator over [1, 2, 3])
+//
+// # See Also
+//
+//   - TraverseArray: For slice-based traversal
+//   - SequenceSeq: For sequencing iterators of Either values
+func TraverseSeq[E, A, B any](f Kleisli[E, A, B]) Kleisli[E, iter.Seq[A], iter.Seq[B]] {
+	return func(ga iter.Seq[A]) Either[E, iter.Seq[B]] {
+		var bs []B
+		for a := range ga {
+			b := f(a)
+			if b.isLeft {
+				return Left[iter.Seq[B]](b.l)
+			}
+			bs = append(bs, b.r)
+		}
+		return Of[E](slices.Values(bs))
+	}
+}
+
+// SequenceSeq converts an iterator of Either into an Either of iterator.
+// If any element is Left, returns that Left (short-circuits).
+// Otherwise, returns Right containing an iterator of all the Right values.
+//
+// This function eagerly evaluates all Either values in the input iterator to detect
+// any Left values, then returns an iterator over the collected Right values.
+//
+// # Type Parameters
+//
+//   - E: The error type for Left values
+//   - A: The value type for Right values
+//
+// # Parameters
+//
+//   - ma: An iterator of Either values
+//
+// # Returns
+//
+//   - Either containing an iterator of Right values, or the first Left encountered
+//
+// # Example Usage
+//
+//	eithers := slices.Values([]either.Either[error, int]{
+//	    either.Right[error](1),
+//	    either.Right[error](2),
+//	    either.Right[error](3),
+//	})
+//	result := either.SequenceSeq(eithers)
+//	// result is Right(iterator over [1, 2, 3])
+//
+// # See Also
+//
+//   - SequenceArray: For slice-based sequencing
+//   - TraverseSeq: For transforming and sequencing in one step
+func SequenceSeq[E, A any](ma iter.Seq[Either[E, A]]) Either[E, iter.Seq[A]] {
+	return TraverseSeq(F.Identity[Either[E, A]])(ma)
+}

v2/either/array_test.go

@@ -1,27 +1,28 @@
 package either
 
 import (
+	"errors"
 	"fmt"
+	"iter"
+	"slices"
+	"strconv"
 	"testing"
 
-	A "github.com/IBM/fp-go/v2/array"
+	F "github.com/IBM/fp-go/v2/function"
 	TST "github.com/IBM/fp-go/v2/internal/testing"
 	"github.com/stretchr/testify/assert"
 )
 
 func TestCompactArray(t *testing.T) {
-	ar := A.From(
+	ar := []Either[string, string]{
 		Of[string]("ok"),
 		Left[string]("err"),
 		Of[string]("ok"),
-	)
-
-	res := CompactArray(ar)
-	assert.Equal(t, 2, len(res))
+	}
+	assert.Equal(t, 2, len(CompactArray(ar)))
 }
 
 func TestSequenceArray(t *testing.T) {
-
 	s := TST.SequenceArrayTest(
 		FromStrictEquals[error, bool](),
 		Pointed[error, string](),
@@ -29,14 +30,12 @@ func TestSequenceArray(t *testing.T) {
 		Functor[error, []string, bool](),
 		SequenceArray[error, string],
 	)
-
-	for i := 0; i < 10; i++ {
+	for i := range 10 {
 		t.Run(fmt.Sprintf("TestSequenceArray %d", i), s(i))
 	}
 }
 
 func TestSequenceArrayError(t *testing.T) {
-
 	s := TST.SequenceArrayErrorTest(
 		FromStrictEquals[error, bool](),
 		Left[string, error],
@@ -46,6 +45,243 @@ func TestSequenceArrayError(t *testing.T) {
 		Functor[error, []string, bool](),
 		SequenceArray[error, string],
 	)
-	// run across four bits
 	s(4)(t)
 }
+
+func TestTraverseSeq_Success(t *testing.T) {
+	parse := func(s string) Either[error, int] {
+		v, err := strconv.Atoi(s)
+		return TryCatchError(v, err)
+	}
+
+	collectInts := func(result Either[error, iter.Seq[int]]) []int {
+		return F.Pipe1(result, Fold(
+			func(e error) []int { t.Fatal(e); return nil },
+			slices.Collect[int],
+		))
+	}
+
+	t.Run("transforms all elements successfully", func(t *testing.T) {
+		result := TraverseSeq(parse)(slices.Values([]string{"1", "2", "3"}))
+		assert.Equal(t, []int{1, 2, 3}, collectInts(result))
+	})
+
+	t.Run("works with empty iterator", func(t *testing.T) {
+		result := TraverseSeq(parse)(slices.Values([]string{}))
+		assert.Empty(t, collectInts(result))
+	})
+
+	t.Run("works with single element", func(t *testing.T) {
+		result := TraverseSeq(parse)(slices.Values([]string{"42"}))
+		assert.Equal(t, []int{42}, collectInts(result))
+	})
+
+	t.Run("preserves order of elements", func(t *testing.T) {
+		result := TraverseSeq(parse)(slices.Values([]string{"10", "20", "30", "40", "50"}))
+		assert.Equal(t, []int{10, 20, 30, 40, 50}, collectInts(result))
+	})
+}
+
+func TestTraverseSeq_Failure(t *testing.T) {
+	parse := func(s string) Either[error, int] {
+		v, err := strconv.Atoi(s)
+		return TryCatchError(v, err)
+	}
+
+	extractErr := func(result Either[error, iter.Seq[int]]) error {
+		return F.Pipe1(result, Fold(
+			F.Identity[error],
+			func(_ iter.Seq[int]) error { t.Fatal("expected Left but got Right"); return nil },
+		))
+	}
+
+	t.Run("short-circuits on first Left", func(t *testing.T) {
+		err := extractErr(TraverseSeq(parse)(slices.Values([]string{"1", "invalid", "3"})))
+		assert.Error(t, err)
+		assert.Contains(t, err.Error(), "invalid syntax")
+	})
+
+	t.Run("returns first error when multiple failures exist", func(t *testing.T) {
+		err := extractErr(TraverseSeq(parse)(slices.Values([]string{"1", "bad1", "bad2"})))
+		assert.Error(t, err)
+		assert.Contains(t, err.Error(), "bad1")
+	})
+
+	t.Run("handles custom error types", func(t *testing.T) {
+		customErr := errors.New("custom validation error")
+		validate := func(n int) Either[error, int] {
+			if n == 2 {
+				return Left[int](customErr)
+			}
+			return Right[error](n * 10)
+		}
+		err := extractErr(TraverseSeq(validate)(slices.Values([]int{1, 2, 3})))
+		assert.Equal(t, customErr, err)
+	})
+}
+
+func TestTraverseSeq_EdgeCases(t *testing.T) {
+	t.Run("handles complex transformations", func(t *testing.T) {
+		type User struct {
+			ID   int
+			Name string
+		}
+
+		transform := func(id int) Either[error, User] {
+			return Right[error](User{ID: id, Name: fmt.Sprintf("User%d", id)})
+		}
+
+		result := TraverseSeq(transform)(slices.Values([]int{1, 2, 3}))
+		collected := F.Pipe1(result, Fold(
+			func(e error) []User { t.Fatal(e); return nil },
+			slices.Collect[User],
+		))
+
+		assert.Equal(t, []User{
+			{ID: 1, Name: "User1"},
+			{ID: 2, Name: "User2"},
+			{ID: 3, Name: "User3"},
+		}, collected)
+	})
+
+	t.Run("works with identity transformation", func(t *testing.T) {
+		input := slices.Values([]Either[error, int]{
+			Right[error](1),
+			Right[error](2),
+			Right[error](3),
+		})
+
+		result := TraverseSeq(F.Identity[Either[error, int]])(input)
+		collected := F.Pipe1(result, Fold(
+			func(e error) []int { t.Fatal(e); return nil },
+			slices.Collect[int],
+		))
+
+		assert.Equal(t, []int{1, 2, 3}, collected)
+	})
+}
+
+func TestSequenceSeq_Success(t *testing.T) {
+	collectInts := func(result Either[error, iter.Seq[int]]) []int {
+		return F.Pipe1(result, Fold(
+			func(e error) []int { t.Fatal(e); return nil },
+			slices.Collect[int],
+		))
+	}
+
+	t.Run("sequences multiple Right values", func(t *testing.T) {
+		input := slices.Values([]Either[error, int]{Right[error](1), Right[error](2), Right[error](3)})
+		assert.Equal(t, []int{1, 2, 3}, collectInts(SequenceSeq(input)))
+	})
+
+	t.Run("works with empty iterator", func(t *testing.T) {
+		input := slices.Values([]Either[error, string]{})
+		result := F.Pipe1(SequenceSeq(input), Fold(
+			func(e error) []string { t.Fatal(e); return nil },
+			slices.Collect[string],
+		))
+		assert.Empty(t, result)
+	})
+
+	t.Run("works with single Right value", func(t *testing.T) {
+		input := slices.Values([]Either[error, string]{Right[error]("hello")})
+		result := F.Pipe1(SequenceSeq(input), Fold(
+			func(e error) []string { t.Fatal(e); return nil },
+			slices.Collect[string],
+		))
+		assert.Equal(t, []string{"hello"}, result)
+	})
+
+	t.Run("preserves order of results", func(t *testing.T) {
+		input := slices.Values([]Either[error, int]{
+			Right[error](5), Right[error](4), Right[error](3), Right[error](2), Right[error](1),
+		})
+		assert.Equal(t, []int{5, 4, 3, 2, 1}, collectInts(SequenceSeq(input)))
+	})
+
+	t.Run("works with complex types", func(t *testing.T) {
+		type Item struct {
+			Value int
+			Label string
+		}
+
+		input := slices.Values([]Either[error, Item]{
+			Right[error](Item{Value: 1, Label: "first"}),
+			Right[error](Item{Value: 2, Label: "second"}),
+			Right[error](Item{Value: 3, Label: "third"}),
+		})
+
+		collected := F.Pipe1(SequenceSeq(input), Fold(
+			func(e error) []Item { t.Fatal(e); return nil },
+			slices.Collect[Item],
+		))
+
+		assert.Equal(t, []Item{
+			{Value: 1, Label: "first"},
+			{Value: 2, Label: "second"},
+			{Value: 3, Label: "third"},
+		}, collected)
+	})
+}
+
+func TestSequenceSeq_Failure(t *testing.T) {
+	extractErr := func(result Either[error, iter.Seq[int]]) error {
+		return F.Pipe1(result, Fold(
+			F.Identity[error],
+			func(_ iter.Seq[int]) error { t.Fatal("expected Left but got Right"); return nil },
+		))
+	}
+
+	t.Run("short-circuits on first Left", func(t *testing.T) {
+		testErr := errors.New("test error")
+		input := slices.Values([]Either[error, int]{Right[error](1), Left[int](testErr), Right[error](3)})
+		assert.Equal(t, testErr, extractErr(SequenceSeq(input)))
+	})
+
+	t.Run("returns first error when multiple Left values exist", func(t *testing.T) {
+		err1 := errors.New("error 1")
+		err2 := errors.New("error 2")
+		input := slices.Values([]Either[error, int]{Right[error](1), Left[int](err1), Left[int](err2)})
+		assert.Equal(t, err1, extractErr(SequenceSeq(input)))
+	})
+
+	t.Run("handles Left at the beginning", func(t *testing.T) {
+		testErr := errors.New("first error")
+		input := slices.Values([]Either[error, int]{Left[int](testErr), Right[error](2), Right[error](3)})
+		assert.Equal(t, testErr, extractErr(SequenceSeq(input)))
+	})
+
+	t.Run("handles Left at the end", func(t *testing.T) {
+		testErr := errors.New("last error")
+		input := slices.Values([]Either[error, int]{Right[error](1), Right[error](2), Left[int](testErr)})
+		assert.Equal(t, testErr, extractErr(SequenceSeq(input)))
+	})
+}
+
+func TestSequenceSeq_Integration(t *testing.T) {
+	t.Run("integrates with TraverseSeq", func(t *testing.T) {
+		parse := func(s string) Either[error, int] {
+			v, err := strconv.Atoi(s)
+			return TryCatchError(v, err)
+		}
+		result := TraverseSeq(parse)(slices.Values([]string{"1", "2", "3"}))
+		assert.True(t, IsRight(result))
+	})
+
+	t.Run("SequenceSeq is equivalent to TraverseSeq with Identity", func(t *testing.T) {
+		mkInput := func() []Either[error, int] {
+			return []Either[error, int]{Right[error](10), Right[error](20), Right[error](30)}
+		}
+
+		collected1 := F.Pipe1(SequenceSeq(slices.Values(mkInput())), Fold(
+			func(e error) []int { t.Fatal(e); return nil },
+			slices.Collect[int],
+		))
+		collected2 := F.Pipe1(TraverseSeq(F.Identity[Either[error, int]])(slices.Values(mkInput())), Fold(
+			func(e error) []int { t.Fatal(e); return nil },
+			slices.Collect[int],
+		))
+
+		assert.Equal(t, collected1, collected2)
+	})
+}

v2/either/either_test.go

@@ -379,7 +379,7 @@ func TestMonadChainLeft(t *testing.T) {
 func TestChainLeft(t *testing.T) {
 	t.Run("Curried function transforms Left value", func(t *testing.T) {
 		// Create a reusable error handler
-		handleNotFound := ChainLeft[error, string](func(err error) Either[string, int] {
+		handleNotFound := ChainLeft(func(err error) Either[string, int] {
 			if err.Error() == "not found" {
 				return Right[string](0)
 			}
@@ -391,7 +391,7 @@ func TestChainLeft(t *testing.T) {
 	})
 
 	t.Run("Curried function with Right value", func(t *testing.T) {
-		handler := ChainLeft[error, string](func(err error) Either[string, int] {
+		handler := ChainLeft(func(err error) Either[string, int] {
 			return Left[int]("should not be called")
 		})
 
@@ -401,7 +401,7 @@ func TestChainLeft(t *testing.T) {
 
 	t.Run("Use in pipeline with Pipe", func(t *testing.T) {
 		// Create error transformer
-		toStringError := ChainLeft[int, string](func(code int) Either[string, string] {
+		toStringError := ChainLeft(func(code int) Either[string, string] {
 			return Left[string](fmt.Sprintf("Error: %d", code))
 		})
 
@@ -414,12 +414,12 @@ func TestChainLeft(t *testing.T) {
 
 	t.Run("Compose multiple ChainLeft operations", func(t *testing.T) {
 		// First handler: convert error to string
-		handler1 := ChainLeft[error, string](func(err error) Either[string, int] {
+		handler1 := ChainLeft(func(err error) Either[string, int] {
 			return Left[int](err.Error())
 		})
 
 		// Second handler: add prefix to string error
-		handler2 := ChainLeft[string, string](func(s string) Either[string, int] {
+		handler2 := ChainLeft(func(s string) Either[string, int] {
 			return Left[int]("Handled: " + s)
 		})
 

v2/either/types.go

@@ -55,5 +55,7 @@ type (
 	// It's commonly used for filtering and conditional operations.
 	Predicate[A any] = predicate.Predicate[A]
 
+	// Pair represents a tuple of two values of types L and R.
+	// It's commonly used to return multiple values from functions or to group related data.
 	Pair[L, R any] = pair.Pair[L, R]
 )

v2/endomorphism/doc.go

@@ -40,7 +40,7 @@
 //	increment := N.Add(1)
 //
 //	// Compose them (RIGHT-TO-LEFT execution)
-//	composed := endomorphism.Compose(double, increment)
+//	composed := endomorphism.MonadCompose(double, increment)
 //	result := composed(5) // increment(5) then double: (5 + 1) * 2 = 12
 //
 //	// Chain them (LEFT-TO-RIGHT execution)
@@ -61,11 +61,11 @@
 //	monoid := endomorphism.Monoid[int]()
 //
 //	// Combine multiple endomorphisms (RIGHT-TO-LEFT execution)
-//	combined := M.ConcatAll(monoid)(
+//	combined := M.ConcatAll(monoid)([]endomorphism.Endomorphism[int]{
 //		N.Mul(2),  // applied third
 //		N.Add(1),  // applied second
 //		N.Mul(3),  // applied first
-//	)
+//	})
 //	result := combined(5) // (5 * 3) = 15, (15 + 1) = 16, (16 * 2) = 32
 //
 // # Monad Operations
@@ -87,7 +87,7 @@
 //	increment := N.Add(1)
 //
 //	// Compose: RIGHT-TO-LEFT (mathematical composition)
-//	composed := endomorphism.Compose(double, increment)
+//	composed := endomorphism.MonadCompose(double, increment)
 //	result1 := composed(5) // increment(5) * 2 = (5 + 1) * 2 = 12
 //
 //	// MonadChain: LEFT-TO-RIGHT (sequential application)