fix: improve assertions

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

.github/workflows/build.yml

@@ -28,7 +28,7 @@ jobs:
       fail-fast: false  # Continue with other versions if one fails
     steps:
       # full checkout for semantic-release
-      - uses: actions/checkout@08eba0b27e820071cde6df949e0beb9ba4906955 # v4.3.0
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
         with:
           fetch-depth: 0
       - name: Set up Go ${{ matrix.go-version }}
@@ -66,7 +66,7 @@ jobs:
       matrix:
         go-version: ['1.24.x', '1.25.x']
     steps:
-      - uses: actions/checkout@08eba0b27e820071cde6df949e0beb9ba4906955 # v4.3.0
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
         with:
           fetch-depth: 0
       - name: Set up Go ${{ matrix.go-version }}
@@ -126,7 +126,7 @@ jobs:
     steps:
       # full checkout for semantic-release
       - name: Full checkout
-        uses: actions/checkout@08eba0b27e820071cde6df949e0beb9ba4906955 # v4.3.0
+        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
         with:
           fetch-depth: 0
 

v2/.claude/settings.local.json

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

v2/BENCHMARK_COMPARISON.md

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

v2/CHAINING_PERFORMANCE_ANALYSIS.md

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

v2/IDIOMATIC_COMPARISON.md

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

v2/IDIOMATIC_READERIORESULT_TODO.md

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

v2/README.md

@@ -2,25 +2,152 @@
 
 [![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)
 
-Version 2 of fp-go leverages [generic type aliases](https://github.com/golang/go/issues/46477) introduced in Go 1.24, providing a more ergonomic and streamlined API.
+**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.
 
 ## 📚 Table of Contents
 
+- [Overview](#-overview)
+- [Features](#-features)
 - [Requirements](#-requirements)
-- [Breaking Changes](#-breaking-changes)
+- [Installation](#-installation)
+- [Quick Start](#-quick-start)
+- [Breaking Changes](#️-breaking-changes)
 - [Key Improvements](#-key-improvements)
 - [Migration Guide](#-migration-guide)
-- [Installation](#-installation)
 - [What's New](#-whats-new)
+- [Documentation](#-documentation)
+- [Contributing](#-contributing)
+- [License](#-license)
+
+## 🎯 Overview
+
+fp-go brings the power of functional programming to Go with:
+
+- **Type-safe abstractions** - Monads, Functors, Applicatives, and more
+- **Composable operations** - Build complex logic from simple, reusable functions
+- **Error handling** - Elegant error management with `Either`, `Result`, and `IOEither`
+- **Lazy evaluation** - Control when and how computations execute
+- **Optics** - Powerful lens, prism, and traversal operations for immutable data manipulation
+
+## ✨ Features
+
+- 🔒 **Type Safety** - Leverage Go's generics for compile-time guarantees
+- 🧩 **Composability** - Chain operations naturally with functional composition
+- 📦 **Rich Type System** - `Option`, `Either`, `Result`, `IO`, `Reader`, and more
+- 🎯 **Practical** - Designed for real-world Go applications
+- 🚀 **Performance** - Zero-cost abstractions where possible
+- 📖 **Well-documented** - Comprehensive API documentation and examples
+- 🧪 **Battle-tested** - Extensive test coverage
 
 ## 🔧 Requirements
 
 - **Go 1.24 or later** (for generic type alias support)
 
-## ⚠️ Breaking Changes
+## 📦 Installation
 
-### 1. Generic Type Aliases
+```bash
+go get github.com/IBM/fp-go/v2
+```
+
+## 🚀 Quick Start
+
+### Working with Option
+
+```go
+package main
+
+import (
+    "fmt"
+    "github.com/IBM/fp-go/v2/option"
+)
+
+func main() {
+    // Create an Option
+    some := option.Some(42)
+    none := option.None[int]()
+    
+    // Map over values
+    doubled := option.Map(N.Mul(2))(some)
+    fmt.Println(option.GetOrElse(0)(doubled)) // Output: 84
+    
+    // Chain operations
+    result := option.Chain(func(x int) option.Option[string] {
+        if x > 0 {
+            return option.Some(fmt.Sprintf("Positive: %d", x))
+        }
+        return option.None[string]()
+    })(some)
+    
+    fmt.Println(option.GetOrElse("No value")(result)) // Output: Positive: 42
+}
+```
+
+### Error Handling with Result
+
+```go
+package main
+
+import (
+    "errors"
+    "fmt"
+    "github.com/IBM/fp-go/v2/result"
+)
+
+func divide(a, b int) result.Result[int] {
+    if b == 0 {
+        return result.Error[int](errors.New("division by zero"))
+    }
+    return result.Ok(a / b)
+}
+
+func main() {
+    res := divide(10, 2)
+    
+    // Pattern match on the result
+    result.Fold(
+        func(err error) { fmt.Println("Error:", err) },
+        func(val int) { fmt.Println("Result:", val) },
+    )(res)
+    // Output: Result: 5
+    
+    // Or use GetOrElse for a default value
+    value := result.GetOrElse(0)(divide(10, 0))
+    fmt.Println("Value:", value) // Output: Value: 0
+}
+```
+
+### Composing IO Operations
+
+```go
+package main
+
+import (
+    "fmt"
+    "github.com/IBM/fp-go/v2/io"
+)
+
+func main() {
+    // Define pure IO operations
+    readInput := io.MakeIO(func() string {
+        return "Hello, fp-go!"
+    })
+    
+    // Transform the result
+    uppercase := io.Map(func(s string) string {
+        return fmt.Sprintf(">>> %s <<<", s)
+    })(readInput)
+    
+    // Execute the IO operation
+    result := uppercase()
+    fmt.Println(result) // Output: >>> Hello, fp-go! <<<
+}
+```
+
+### From V1 to V2
+
+#### 1. Generic Type Aliases
 
 V2 uses [generic type aliases](https://github.com/golang/go/issues/46477) which require Go 1.24+. This is the most significant change and enables cleaner type definitions.
 
@@ -34,7 +161,7 @@ type ReaderIOEither[R, E, A any] RD.Reader[R, IOE.IOEither[E, A]]
 type ReaderIOEither[R, E, A any] = RD.Reader[R, IOE.IOEither[E, A]]
 ```
 
-### 2. Generic Type Parameter Ordering
+#### 2. Generic Type Parameter Ordering
 
 Type parameters that **cannot** be inferred from function arguments now come first, improving type inference.
 
@@ -52,7 +179,7 @@ func Ap[B, R, E, A any](fa ReaderIOEither[R, E, A]) func(ReaderIOEither[R, E, fu
 
 This change allows the Go compiler to infer more types automatically, reducing the need for explicit type parameters.
 
-### 3. Pair Monad Semantics
+#### 3. Pair Monad Semantics
 
 Monadic operations for `Pair` now operate on the **second argument** to align with the [Haskell definition](https://hackage.haskell.org/package/TypeCompose-0.9.14/docs/Data-Pair.html).
 
@@ -60,7 +187,7 @@ Monadic operations for `Pair` now operate on the **second argument** to align wi
 ```go
 // Operations on first element
 pair := MakePair(1, "hello")
-result := Map(func(x int) int { return x * 2 })(pair) // Pair(2, "hello")
+result := Map(N.Mul(2))(pair) // Pair(2, "hello")
 ```
 
 **V2:**
@@ -70,6 +197,36 @@ pair := MakePair(1, "hello")
 result := Map(func(s string) string { return s + "!" })(pair) // Pair(1, "hello!")
 ```
 
+#### 4. Endomorphism Compose Semantics
+
+The `Compose` function for endomorphisms now follows **mathematical function composition** (right-to-left execution), aligning with standard functional programming conventions.
+
+**V1:**
+```go
+// Compose executed left-to-right
+double := N.Mul(2)
+increment := func(x int) int { return x + 1 }
+composed := Compose(double, increment)
+result := composed(5) // (5 * 2) + 1 = 11
+```
+
+**V2:**
+```go
+// Compose executes RIGHT-TO-LEFT (mathematical composition)
+double := N.Mul(2)
+increment := func(x int) int { return x + 1 }
+composed := Compose(double, increment)
+result := composed(5) // (5 + 1) * 2 = 12
+
+// Use MonadChain for LEFT-TO-RIGHT execution
+chained := MonadChain(double, increment)
+result2 := chained(5) // (5 * 2) + 1 = 11
+```
+
+**Key Difference:**
+- `Compose(f, g)` now means `f ∘ g`, which applies `g` first, then `f` (right-to-left)
+- `MonadChain(f, g)` applies `f` first, then `g` (left-to-right)
+
 ## ✨ Key Improvements
 
 ### 1. Simplified Type Declarations
@@ -91,16 +248,16 @@ func processData(input string) ET.Either[error, OPT.Option[int]] {
 **V2 Approach:**
 ```go
 import (
-    "github.com/IBM/fp-go/v2/either"
+    "github.com/IBM/fp-go/v2/result"
     "github.com/IBM/fp-go/v2/option"
 )
 
 // Define type aliases once
-type Either[A any] = either.Either[error, A]
+type Result[A any] = result.Result[A]
 type Option[A any] = option.Option[A]
 
 // Use them throughout your codebase
-func processData(input string) Either[Option[int]] {
+func processData(input string) Result[Option[int]] {
     // implementation
 }
 ```
@@ -211,7 +368,7 @@ If you're using `Pair`, update operations to work on the second element:
 ```go
 pair := MakePair(42, "data")
 // Map operates on first element
-result := Map(func(x int) int { return x * 2 })(pair)
+result := Map(N.Mul(2))(pair)
 ```
 
 **After (V2):**
@@ -230,20 +387,14 @@ Create project-wide type aliases for common patterns:
 package myapp
 
 import (
-    "github.com/IBM/fp-go/v2/either"
+    "github.com/IBM/fp-go/v2/result"
     "github.com/IBM/fp-go/v2/option"
-    "github.com/IBM/fp-go/v2/ioeither"
+    "github.com/IBM/fp-go/v2/ioresult"
 )
 
-type Either[A any] = either.Either[error, A]
+type Result[A any] = result.Result[A]
 type Option[A any] = option.Option[A]
-type IOEither[A any] = ioeither.IOEither[error, A]
+type IOResult[A any] = ioresult.IOResult[A]
-```
-
-## 📦 Installation
-
-```bash
-go get github.com/IBM/fp-go/v2
 ```
 
 ## 🆕 What's New
@@ -277,25 +428,37 @@ func process() IOET.IOEither[error, string] {
 **V2 Simplified Example:**
 ```go
 import (
-    "github.com/IBM/fp-go/v2/either"
+    "strconv"
-    "github.com/IBM/fp-go/v2/ioeither"
+    "github.com/IBM/fp-go/v2/ioresult"
 )
 
-type IOEither[A any] = ioeither.IOEither[error, A]
+type IOResult[A any] = ioresult.IOResult[A]
 
-func process() IOEither[string] {
+func process() IOResult[string] {
-    return ioeither.Map(
+    return ioresult.Map(
         strconv.Itoa,
     )(fetchData())
 }
 ```
 
-## 📚 Additional Resources
+## 📚 Documentation
 
-- [Main README](../README.md) - Core concepts and design philosophy
+- **[API Documentation](https://pkg.go.dev/github.com/IBM/fp-go/v2)** - Complete API reference
-- [API Documentation](https://pkg.go.dev/github.com/IBM/fp-go/v2)
+- **[Code Samples](./samples/)** - Practical examples and use cases
-- [Code Samples](../samples/)
+- **[Go 1.24 Release Notes](https://tip.golang.org/doc/go1.24)** - Information about generic type aliases
-- [Go 1.24 Release Notes](https://tip.golang.org/doc/go1.24)
+
+### Core Modules
+
+- **Option** - Represent optional values without nil
+- **Either** - Type-safe error handling with left/right values
+- **Result** - Simplified Either with error as left type
+- **IO** - Lazy evaluation and side effect management
+- **IOEither** - Combine IO with error handling
+- **Reader** - Dependency injection pattern
+- **ReaderIOEither** - Combine Reader, IO, and Either for complex workflows
+- **Array** - Functional array operations
+- **Record** - Functional record/map operations
+- **Optics** - Lens, Prism, Optional, and Traversal for immutable updates
 
 ## 🤔 Should I Migrate?
 
@@ -310,10 +473,25 @@ func process() IOEither[string] {
 - ⚠️ Migration effort outweighs benefits for your project
 - ⚠️ You need stability in production (V2 is newer)
 
+## 🤝 Contributing
+
+Contributions are welcome! Here's how you can help:
+
+1. **Report bugs** - Open an issue with a clear description and reproduction steps
+2. **Suggest features** - Share your ideas for improvements
+3. **Submit PRs** - Fix bugs or add features (please discuss major changes first)
+4. **Improve docs** - Help make the documentation clearer and more comprehensive
+
+Please read our contribution guidelines before submitting pull requests.
+
 ## 🐛 Issues and Feedback
 
 Found a bug or have a suggestion? Please [open an issue](https://github.com/IBM/fp-go/issues) on GitHub.
 
 ## 📄 License
 
-This project is licensed under the Apache License 2.0 - see the LICENSE file for details.
+This project is licensed under the Apache License 2.0. See the [LICENSE](https://github.com/IBM/fp-go/blob/main/LICENSE) file for details.
+
+---
+
+**Made with ❤️ by IBM**

v2/array/array.go

@@ -17,11 +17,10 @@ package array
 
 import (
 	G "github.com/IBM/fp-go/v2/array/generic"
-	EM "github.com/IBM/fp-go/v2/endomorphism"
 	F "github.com/IBM/fp-go/v2/function"
 	"github.com/IBM/fp-go/v2/internal/array"
 	M "github.com/IBM/fp-go/v2/monoid"
-	O "github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/option"
 	"github.com/IBM/fp-go/v2/tuple"
 )
 
@@ -50,16 +49,16 @@ func Replicate[A any](n int, a A) []A {
 // This is the monadic version of Map that takes the array as the first parameter.
 //
 //go:inline
-func MonadMap[A, B any](as []A, f func(a A) B) []B {
+func MonadMap[A, B any](as []A, f func(A) B) []B {
 	return G.MonadMap[[]A, []B](as, f)
 }
 
 // MonadMapRef applies a function to a pointer to each element of an array, returning a new array with the results.
 // This is useful when you need to access elements by reference without copying.
-func MonadMapRef[A, B any](as []A, f func(a *A) B) []B {
+func MonadMapRef[A, B any](as []A, f func(*A) B) []B {
 	count := len(as)
 	bs := make([]B, count)
-	for i := count - 1; i >= 0; i-- {
+	for i := range count {
 		bs[i] = f(&as[i])
 	}
 	return bs
@@ -68,7 +67,7 @@ func MonadMapRef[A, B any](as []A, f func(a *A) B) []B {
 // MapWithIndex applies a function to each element and its index in an array, returning a new array with the results.
 //
 //go:inline
-func MapWithIndex[A, B any](f func(int, A) B) func([]A) []B {
+func MapWithIndex[A, B any](f func(int, A) B) Operator[A, B] {
 	return G.MapWithIndex[[]A, []B](f)
 }
 
@@ -77,39 +76,39 @@ func MapWithIndex[A, B any](f func(int, A) B) func([]A) []B {
 //
 // Example:
 //
-//	double := array.Map(func(x int) int { return x * 2 })
+//	double := array.Map(N.Mul(2))
 //	result := double([]int{1, 2, 3}) // [2, 4, 6]
 //
 //go:inline
-func Map[A, B any](f func(a A) B) func([]A) []B {
+func Map[A, B any](f func(A) B) Operator[A, B] {
 	return G.Map[[]A, []B](f)
 }
 
 // MapRef applies a function to a pointer to each element of an array, returning a new array with the results.
 // This is the curried version that returns a function.
-func MapRef[A, B any](f func(a *A) B) func([]A) []B {
+func MapRef[A, B any](f func(*A) B) Operator[A, B] {
 	return F.Bind2nd(MonadMapRef[A, B], f)
 }
 
-func filterRef[A any](fa []A, pred func(a *A) bool) []A {
+func filterRef[A any](fa []A, pred func(*A) bool) []A {
-	var result []A
 	count := len(fa)
-	for i := 0; i < count; i++ {
+	var result []A = make([]A, 0, count)
-		a := fa[i]
+	for i := range count {
-		if pred(&a) {
+		a := &fa[i]
-			result = append(result, a)
+		if pred(a) {
+			result = append(result, *a)
 		}
 	}
 	return result
 }
 
-func filterMapRef[A, B any](fa []A, pred func(a *A) bool, f func(a *A) B) []B {
+func filterMapRef[A, B any](fa []A, pred func(*A) bool, f func(*A) B) []B {
-	var result []B
 	count := len(fa)
-	for i := 0; i < count; i++ {
+	var result []B = make([]B, 0, count)
-		a := fa[i]
+	for i := range count {
-		if pred(&a) {
+		a := &fa[i]
-			result = append(result, f(&a))
+		if pred(a) {
+			result = append(result, f(a))
 		}
 	}
 	return result
@@ -118,19 +117,19 @@ func filterMapRef[A, B any](fa []A, pred func(a *A) bool, f func(a *A) B) []B {
 // Filter returns a new array with all elements from the original array that match a predicate
 //
 //go:inline
-func Filter[A any](pred func(A) bool) EM.Endomorphism[[]A] {
+func Filter[A any](pred func(A) bool) Operator[A, A] {
 	return G.Filter[[]A](pred)
 }
 
 // FilterWithIndex returns a new array with all elements from the original array that match a predicate
 //
 //go:inline
-func FilterWithIndex[A any](pred func(int, A) bool) EM.Endomorphism[[]A] {
+func FilterWithIndex[A any](pred func(int, A) bool) Operator[A, A] {
 	return G.FilterWithIndex[[]A](pred)
 }
 
 // FilterRef returns a new array with all elements from the original array that match a predicate operating on pointers.
-func FilterRef[A any](pred func(*A) bool) EM.Endomorphism[[]A] {
+func FilterRef[A any](pred func(*A) bool) Operator[A, A] {
 	return F.Bind2nd(filterRef[A], pred)
 }
 
@@ -138,7 +137,7 @@ func FilterRef[A any](pred func(*A) bool) EM.Endomorphism[[]A] {
 // This is the monadic version that takes the array as the first parameter.
 //
 //go:inline
-func MonadFilterMap[A, B any](fa []A, f func(A) O.Option[B]) []B {
+func MonadFilterMap[A, B any](fa []A, f option.Kleisli[A, B]) []B {
 	return G.MonadFilterMap[[]A, []B](fa, f)
 }
 
@@ -146,33 +145,33 @@ func MonadFilterMap[A, B any](fa []A, f func(A) O.Option[B]) []B {
 // keeping only the Some values. This is the monadic version that takes the array as the first parameter.
 //
 //go:inline
-func MonadFilterMapWithIndex[A, B any](fa []A, f func(int, A) O.Option[B]) []B {
+func MonadFilterMapWithIndex[A, B any](fa []A, f func(int, A) Option[B]) []B {
 	return G.MonadFilterMapWithIndex[[]A, []B](fa, f)
 }
 
-// FilterMap maps an array with an iterating function that returns an [O.Option] and it keeps only the Some values discarding the Nones.
+// FilterMap maps an array with an iterating function that returns an [Option] and it keeps only the Some values discarding the Nones.
 //
 //go:inline
-func FilterMap[A, B any](f func(A) O.Option[B]) func([]A) []B {
+func FilterMap[A, B any](f option.Kleisli[A, B]) Operator[A, B] {
 	return G.FilterMap[[]A, []B](f)
 }
 
-// FilterMapWithIndex maps an array with an iterating function that returns an [O.Option] and it keeps only the Some values discarding the Nones.
+// FilterMapWithIndex maps an array with an iterating function that returns an [Option] and it keeps only the Some values discarding the Nones.
 //
 //go:inline
-func FilterMapWithIndex[A, B any](f func(int, A) O.Option[B]) func([]A) []B {
+func FilterMapWithIndex[A, B any](f func(int, A) Option[B]) Operator[A, B] {
 	return G.FilterMapWithIndex[[]A, []B](f)
 }
 
-// FilterChain maps an array with an iterating function that returns an [O.Option] of an array. It keeps only the Some values discarding the Nones and then flattens the result.
+// FilterChain maps an array with an iterating function that returns an [Option] of an array. It keeps only the Some values discarding the Nones and then flattens the result.
 //
 //go:inline
-func FilterChain[A, B any](f func(A) O.Option[[]B]) func([]A) []B {
+func FilterChain[A, B any](f option.Kleisli[A, []B]) Operator[A, B] {
 	return G.FilterChain[[]A](f)
 }
 
 // FilterMapRef filters an array using a predicate on pointers and maps the matching elements using a function on pointers.
-func FilterMapRef[A, B any](pred func(a *A) bool, f func(a *A) B) func([]A) []B {
+func FilterMapRef[A, B any](pred func(a *A) bool, f func(*A) B) Operator[A, B] {
 	return func(fa []A) []B {
 		return filterMapRef(fa, pred, f)
 	}
@@ -180,8 +179,7 @@ func FilterMapRef[A, B any](pred func(a *A) bool, f func(a *A) B) func([]A) []B
 
 func reduceRef[A, B any](fa []A, f func(B, *A) B, initial B) B {
 	current := initial
-	count := len(fa)
+	for i := range len(fa) {
-	for i := 0; i < count; i++ {
 		current = f(current, &fa[i])
 	}
 	return current
@@ -262,6 +260,8 @@ func Empty[A any]() []A {
 }
 
 // Zero returns an empty array of type A (alias for Empty).
+//
+//go:inline
 func Zero[A any]() []A {
 	return Empty[A]()
 }
@@ -277,7 +277,7 @@ func Of[A any](a A) []A {
 // This is the monadic version that takes the array as the first parameter (also known as FlatMap).
 //
 //go:inline
-func MonadChain[A, B any](fa []A, f func(a A) []B) []B {
+func MonadChain[A, B any](fa []A, f Kleisli[A, B]) []B {
 	return G.MonadChain(fa, f)
 }
 
@@ -290,7 +290,7 @@ func MonadChain[A, B any](fa []A, f func(a A) []B) []B {
 //	result := duplicate([]int{1, 2, 3}) // [1, 1, 2, 2, 3, 3]
 //
 //go:inline
-func Chain[A, B any](f func(A) []B) func([]A) []B {
+func Chain[A, B any](f Kleisli[A, B]) Operator[A, B] {
 	return G.Chain[[]A](f)
 }
 
@@ -306,7 +306,7 @@ func MonadAp[B, A any](fab []func(A) B, fa []A) []B {
 // This is the curried version.
 //
 //go:inline
-func Ap[B, A any](fa []A) func([]func(A) B) []B {
+func Ap[B, A any](fa []A) Operator[func(A) B, B] {
 	return G.Ap[[]B, []func(A) B](fa)
 }
 
@@ -328,7 +328,7 @@ func MatchLeft[A, B any](onEmpty func() B, onNonEmpty func(A, []A) B) func([]A)
 // Returns None if the array is empty.
 //
 //go:inline
-func Tail[A any](as []A) O.Option[[]A] {
+func Tail[A any](as []A) Option[[]A] {
 	return G.Tail(as)
 }
 
@@ -336,7 +336,7 @@ func Tail[A any](as []A) O.Option[[]A] {
 // Returns None if the array is empty.
 //
 //go:inline
-func Head[A any](as []A) O.Option[A] {
+func Head[A any](as []A) Option[A] {
 	return G.Head(as)
 }
 
@@ -344,7 +344,7 @@ func Head[A any](as []A) O.Option[A] {
 // Returns None if the array is empty.
 //
 //go:inline
-func First[A any](as []A) O.Option[A] {
+func First[A any](as []A) Option[A] {
 	return G.First(as)
 }
 
@@ -352,12 +352,12 @@ func First[A any](as []A) O.Option[A] {
 // Returns None if the array is empty.
 //
 //go:inline
-func Last[A any](as []A) O.Option[A] {
+func Last[A any](as []A) Option[A] {
 	return G.Last(as)
 }
 
 // PrependAll inserts a separator before each element of an array.
-func PrependAll[A any](middle A) EM.Endomorphism[[]A] {
+func PrependAll[A any](middle A) Operator[A, A] {
 	return func(as []A) []A {
 		count := len(as)
 		dst := count * 2
@@ -377,7 +377,7 @@ func PrependAll[A any](middle A) EM.Endomorphism[[]A] {
 // Example:
 //
 //	result := array.Intersperse(0)([]int{1, 2, 3}) // [1, 0, 2, 0, 3]
-func Intersperse[A any](middle A) EM.Endomorphism[[]A] {
+func Intersperse[A any](middle A) Operator[A, A] {
 	prepend := PrependAll(middle)
 	return func(as []A) []A {
 		if IsEmpty(as) {
@@ -406,7 +406,7 @@ func Flatten[A any](mma [][]A) []A {
 }
 
 // Slice extracts a subarray from index low (inclusive) to high (exclusive).
-func Slice[A any](low, high int) func(as []A) []A {
+func Slice[A any](low, high int) Operator[A, A] {
 	return array.Slice[[]A](low, high)
 }
 
@@ -414,7 +414,7 @@ func Slice[A any](low, high int) func(as []A) []A {
 // Returns None if the index is out of bounds.
 //
 //go:inline
-func Lookup[A any](idx int) func([]A) O.Option[A] {
+func Lookup[A any](idx int) func([]A) Option[A] {
 	return G.Lookup[[]A](idx)
 }
 
@@ -422,7 +422,7 @@ func Lookup[A any](idx int) func([]A) O.Option[A] {
 // If the index is out of bounds, the element is appended.
 //
 //go:inline
-func UpsertAt[A any](a A) EM.Endomorphism[[]A] {
+func UpsertAt[A any](a A) Operator[A, A] {
 	return G.UpsertAt[[]A](a)
 }
 
@@ -468,7 +468,7 @@ func ConstNil[A any]() []A {
 // SliceRight extracts a subarray from the specified start index to the end.
 //
 //go:inline
-func SliceRight[A any](start int) EM.Endomorphism[[]A] {
+func SliceRight[A any](start int) Operator[A, A] {
 	return G.SliceRight[[]A](start)
 }
 
@@ -482,7 +482,7 @@ func Copy[A any](b []A) []A {
 // Clone creates a deep copy of the array using the provided endomorphism to clone the values
 //
 //go:inline
-func Clone[A any](f func(A) A) func(as []A) []A {
+func Clone[A any](f func(A) A) Operator[A, A] {
 	return G.Clone[[]A](f)
 }
 
@@ -510,8 +510,8 @@ func Fold[A any](m M.Monoid[A]) func([]A) A {
 // Push adds an element to the end of an array (alias for Append).
 //
 //go:inline
-func Push[A any](a A) EM.Endomorphism[[]A] {
+func Push[A any](a A) Operator[A, A] {
-	return G.Push[EM.Endomorphism[[]A]](a)
+	return G.Push[Operator[A, A]](a)
 }
 
 // MonadFlap applies a value to an array of functions, producing an array of results.
@@ -526,13 +526,13 @@ func MonadFlap[B, A any](fab []func(A) B, a A) []B {
 // This is the curried version.
 //
 //go:inline
-func Flap[B, A any](a A) func([]func(A) B) []B {
+func Flap[B, A any](a A) Operator[func(A) B, B] {
 	return G.Flap[func(A) B, []func(A) B, []B](a)
 }
 
 // Prepend adds an element to the beginning of an array, returning a new array.
 //
 //go:inline
-func Prepend[A any](head A) EM.Endomorphism[[]A] {
+func Prepend[A any](head A) Operator[A, A] {
-	return G.Prepend[EM.Endomorphism[[]A]](head)
+	return G.Prepend[Operator[A, A]](head)
 }

v2/array/bind.go

@@ -56,8 +56,8 @@ func Do[S any](
 //go:inline
 func Bind[S1, S2, T any](
 	setter func(T) func(S1) S2,
-	f func(S1) []T,
+	f Kleisli[S1, T],
-) func([]S1) []S2 {
+) Operator[S1, S2] {
 	return G.Bind[[]S1, []S2](setter, f)
 }
 
@@ -79,7 +79,7 @@ func Bind[S1, S2, T any](
 func Let[S1, S2, T any](
 	setter func(T) func(S1) S2,
 	f func(S1) T,
-) func([]S1) []S2 {
+) Operator[S1, S2] {
 	return G.Let[[]S1, []S2](setter, f)
 }
 
@@ -101,7 +101,7 @@ func Let[S1, S2, T any](
 func LetTo[S1, S2, T any](
 	setter func(T) func(S1) S2,
 	b T,
-) func([]S1) []S2 {
+) Operator[S1, S2] {
 	return G.LetTo[[]S1, []S2](setter, b)
 }
 
@@ -120,7 +120,7 @@ func LetTo[S1, S2, T any](
 //go:inline
 func BindTo[S1, T any](
 	setter func(T) S1,
-) func([]T) []S1 {
+) Operator[T, S1] {
 	return G.BindTo[[]S1, []T](setter)
 }
 
@@ -143,6 +143,6 @@ func BindTo[S1, T any](
 func ApS[S1, S2, T any](
 	setter func(T) func(S1) S2,
 	fa []T,
-) func([]S1) []S2 {
+) Operator[S1, S2] {
 	return G.ApS[[]S1, []S2](setter, fa)
 }

v2/array/doc.go

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

v2/array/examples_sort_test.go

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

v2/array/find.go

@@ -17,7 +17,7 @@ package array
 
 import (
 	G "github.com/IBM/fp-go/v2/array/generic"
-	O "github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/option"
 )
 
 // FindFirst finds the first element which satisfies a predicate function.
@@ -30,7 +30,7 @@ import (
 //	result2 := findGreaterThan3([]int{1, 2, 3}) // None
 //
 //go:inline
-func FindFirst[A any](pred func(A) bool) func([]A) O.Option[A] {
+func FindFirst[A any](pred func(A) bool) option.Kleisli[[]A, A] {
 	return G.FindFirst[[]A](pred)
 }
 
@@ -45,7 +45,7 @@ func FindFirst[A any](pred func(A) bool) func([]A) O.Option[A] {
 //	result := findEvenAtEvenIndex([]int{1, 3, 4, 5}) // Some(4)
 //
 //go:inline
-func FindFirstWithIndex[A any](pred func(int, A) bool) func([]A) O.Option[A] {
+func FindFirstWithIndex[A any](pred func(int, A) bool) option.Kleisli[[]A, A] {
 	return G.FindFirstWithIndex[[]A](pred)
 }
 
@@ -65,7 +65,7 @@ func FindFirstWithIndex[A any](pred func(int, A) bool) func([]A) O.Option[A] {
 //	result := parseFirst([]string{"a", "42", "b"}) // Some(42)
 //
 //go:inline
-func FindFirstMap[A, B any](sel func(A) O.Option[B]) func([]A) O.Option[B] {
+func FindFirstMap[A, B any](sel option.Kleisli[A, B]) option.Kleisli[[]A, B] {
 	return G.FindFirstMap[[]A](sel)
 }
 
@@ -73,7 +73,7 @@ func FindFirstMap[A, B any](sel func(A) O.Option[B]) func([]A) O.Option[B] {
 // The selector receives both the index and the element.
 //
 //go:inline
-func FindFirstMapWithIndex[A, B any](sel func(int, A) O.Option[B]) func([]A) O.Option[B] {
+func FindFirstMapWithIndex[A, B any](sel func(int, A) Option[B]) option.Kleisli[[]A, B] {
 	return G.FindFirstMapWithIndex[[]A](sel)
 }
 
@@ -86,7 +86,7 @@ func FindFirstMapWithIndex[A, B any](sel func(int, A) O.Option[B]) func([]A) O.O
 //	result := findGreaterThan3([]int{1, 4, 2, 5}) // Some(5)
 //
 //go:inline
-func FindLast[A any](pred func(A) bool) func([]A) O.Option[A] {
+func FindLast[A any](pred func(A) bool) option.Kleisli[[]A, A] {
 	return G.FindLast[[]A](pred)
 }
 
@@ -94,7 +94,7 @@ func FindLast[A any](pred func(A) bool) func([]A) O.Option[A] {
 // Returns Some(element) if found, None if no element matches.
 //
 //go:inline
-func FindLastWithIndex[A any](pred func(int, A) bool) func([]A) O.Option[A] {
+func FindLastWithIndex[A any](pred func(int, A) bool) option.Kleisli[[]A, A] {
 	return G.FindLastWithIndex[[]A](pred)
 }
 
@@ -102,7 +102,7 @@ func FindLastWithIndex[A any](pred func(int, A) bool) func([]A) O.Option[A] {
 // This combines finding and mapping in a single operation, searching from the end.
 //
 //go:inline
-func FindLastMap[A, B any](sel func(A) O.Option[B]) func([]A) O.Option[B] {
+func FindLastMap[A, B any](sel option.Kleisli[A, B]) option.Kleisli[[]A, B] {
 	return G.FindLastMap[[]A](sel)
 }
 
@@ -110,6 +110,6 @@ func FindLastMap[A, B any](sel func(A) O.Option[B]) func([]A) O.Option[B] {
 // The selector receives both the index and the element, searching from the end.
 //
 //go:inline
-func FindLastMapWithIndex[A, B any](sel func(int, A) O.Option[B]) func([]A) O.Option[B] {
+func FindLastMapWithIndex[A, B any](sel func(int, A) Option[B]) option.Kleisli[[]A, B] {
 	return G.FindLastMapWithIndex[[]A](sel)
 }

v2/array/generic/array.go

@@ -25,8 +25,10 @@ import (
 )
 
 // Of constructs a single element array
+//
+//go:inline
 func Of[GA ~[]A, A any](value A) GA {
-	return GA{value}
+	return array.Of[GA](value)
 }
 
 func Reduce[GA ~[]A, A, B any](f func(B, A) B, initial B) func(GA) B {
@@ -82,7 +84,7 @@ func MakeBy[AS ~[]A, F ~func(int) A, A any](n int, f F) AS {
 	}
 	// run the generator function across the input
 	as := make(AS, n)
-	for i := n - 1; i >= 0; i-- {
+	for i := range n {
 		as[i] = f(i)
 	}
 	return as
@@ -165,10 +167,9 @@ func Size[GA ~[]A, A any](as GA) int {
 func filterMap[GA ~[]A, GB ~[]B, A, B any](fa GA, f func(A) O.Option[B]) GB {
 	result := make(GB, 0, len(fa))
 	for _, a := range fa {
-		O.Map(func(b B) B {
+		if b, ok := O.Unwrap(f(a)); ok {
 			result = append(result, b)
-			return b
+		}
-		})(f(a))
 	}
 	return result
 }
@@ -176,10 +177,9 @@ func filterMap[GA ~[]A, GB ~[]B, A, B any](fa GA, f func(A) O.Option[B]) GB {
 func filterMapWithIndex[GA ~[]A, GB ~[]B, A, B any](fa GA, f func(int, A) O.Option[B]) GB {
 	result := make(GB, 0, len(fa))
 	for i, a := range fa {
-		O.Map(func(b B) B {
+		if b, ok := O.Unwrap(f(i, a)); ok {
 			result = append(result, b)
-			return b
+		}
-		})(f(i, a))
 	}
 	return result
 }

v2/array/generic/find.go

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

v2/array/generic/monoid.go

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

v2/array/generic/zip.go

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

v2/array/misc_test.go

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

v2/array/monoid.go

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

v2/array/sequence.go

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

v2/array/sequence_test.go

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

v2/array/slice_test.go

@@ -18,6 +18,7 @@ package array
 import (
 	"testing"
 
+	N "github.com/IBM/fp-go/v2/number"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -243,7 +244,7 @@ func TestSliceComposition(t *testing.T) {
 
 	t.Run("slice then map", func(t *testing.T) {
 		sliced := Slice[int](2, 5)(data)
-		mapped := Map(func(x int) int { return x * 2 })(sliced)
+		mapped := Map(N.Mul(2))(sliced)
 		assert.Equal(t, []int{4, 6, 8}, mapped)
 	})
 

v2/array/sort.go

@@ -32,7 +32,7 @@ import (
 //	// Result: [1, 1, 2, 3, 4, 5, 6, 9]
 //
 //go:inline
-func Sort[T any](ord O.Ord[T]) func(ma []T) []T {
+func Sort[T any](ord O.Ord[T]) Operator[T, T] {
 	return G.Sort[[]T](ord)
 }
 
@@ -62,7 +62,7 @@ func Sort[T any](ord O.Ord[T]) func(ma []T) []T {
 //	// Result: [{"Bob", 25}, {"Alice", 30}, {"Charlie", 35}]
 //
 //go:inline
-func SortByKey[K, T any](ord O.Ord[K], f func(T) K) func(ma []T) []T {
+func SortByKey[K, T any](ord O.Ord[K], f func(T) K) Operator[T, T] {
 	return G.SortByKey[[]T](ord, f)
 }
 
@@ -93,6 +93,6 @@ func SortByKey[K, T any](ord O.Ord[K], f func(T) K) func(ma []T) []T {
 //	// Result: [{"Jones", "Bob"}, {"Smith", "Alice"}, {"Smith", "John"}]
 //
 //go:inline
-func SortBy[T any](ord []O.Ord[T]) func(ma []T) []T {
+func SortBy[T any](ord []O.Ord[T]) Operator[T, T] {
 	return G.SortBy[[]T](ord)
 }

v2/array/traverse.go

@@ -80,3 +80,25 @@ func MonadTraverse[A, B, HKTB, HKTAB, HKTRB any](
 
 	return array.MonadTraverse(fof, fmap, fap, ta, f)
 }
+
+//go:inline
+func TraverseWithIndex[A, B, HKTB, HKTAB, HKTRB any](
+	fof func([]B) HKTRB,
+	fmap func(func([]B) func(B) []B) func(HKTRB) HKTAB,
+	fap func(HKTB) func(HKTAB) HKTRB,
+
+	f func(int, A) HKTB) func([]A) HKTRB {
+	return array.TraverseWithIndex[[]A](fof, fmap, fap, f)
+}
+
+//go:inline
+func MonadTraverseWithIndex[A, B, HKTB, HKTAB, HKTRB any](
+	fof func([]B) HKTRB,
+	fmap func(func([]B) func(B) []B) func(HKTRB) HKTAB,
+	fap func(HKTB) func(HKTAB) HKTRB,
+
+	ta []A,
+	f func(int, A) HKTB) HKTRB {
+
+	return array.MonadTraverseWithIndex(fof, fmap, fap, ta, f)
+}

v2/array/types.go

@@ -0,0 +1,9 @@
+package array
+
+import "github.com/IBM/fp-go/v2/option"
+
+type (
+	Kleisli[A, B any]  = func(A) []B
+	Operator[A, B any] = Kleisli[[]A, B]
+	Option[A any]      = option.Option[A]
+)

v2/array/uniq.go

@@ -46,6 +46,6 @@ func StrictUniq[A comparable](as []A) []A {
 //	// Result: [{"Alice", 30}, {"Bob", 25}, {"Charlie", 30}]
 //
 //go:inline
-func Uniq[A any, K comparable](f func(A) K) func(as []A) []A {
+func Uniq[A any, K comparable](f func(A) K) Operator[A, A] {
 	return G.Uniq[[]A](f)
 }

v2/assert/assert.go

@@ -0,0 +1,268 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package assert provides functional assertion helpers for testing.
+//
+// This package wraps testify/assert functions in a Reader monad pattern,
+// allowing for composable and functional test assertions. Each assertion
+// returns a Reader that takes a *testing.T and performs the assertion.
+//
+// The package supports:
+//   - Equality and inequality assertions
+//   - Collection assertions (arrays, maps, strings)
+//   - Error handling assertions
+//   - Result type assertions
+//   - Custom predicate assertions
+//   - Composable test suites
+//
+// Example:
+//
+//	func TestExample(t *testing.T) {
+//	    value := 42
+//	    assert.Equal(42)(value)(t)  // Curried style
+//
+//	    // Composing multiple assertions
+//	    arr := []int{1, 2, 3}
+//	    assertions := assert.AllOf([]assert.Reader{
+//	        assert.ArrayNotEmpty(arr),
+//	        assert.ArrayLength[int](3)(arr),
+//	        assert.ArrayContains(2)(arr),
+//	    })
+//	    assertions(t)
+//	}
+package assert
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/IBM/fp-go/v2/boolean"
+	"github.com/IBM/fp-go/v2/eq"
+	"github.com/IBM/fp-go/v2/reader"
+	"github.com/IBM/fp-go/v2/result"
+	"github.com/stretchr/testify/assert"
+)
+
+var (
+	// Eq is the equal predicate checking if objects are equal
+	Eq = eq.FromEquals(assert.ObjectsAreEqual)
+)
+
+// wrap1 is an internal helper function that wraps testify assertion functions
+// into the Reader monad pattern with curried parameters.
+//
+// It takes a testify assertion function and converts it into a curried function
+// that first takes an expected value, then an actual value, and finally returns
+// a Reader that performs the assertion when given a *testing.T.
+//
+// Parameters:
+//   - wrapped: The testify assertion function to wrap
+//   - expected: The expected value for comparison
+//   - msgAndArgs: Optional message and arguments for assertion failure
+//
+// Returns:
+//   - A Kleisli function that takes the actual value and returns a Reader
+func wrap1[T any](wrapped func(t assert.TestingT, expected, actual any, msgAndArgs ...any) bool, expected T, msgAndArgs ...any) Kleisli[T] {
+	return func(actual T) Reader {
+		return func(t *testing.T) bool {
+			return wrapped(t, expected, actual, msgAndArgs...)
+		}
+	}
+}
+
+// NotEqual tests if the expected and the actual values are not equal
+func NotEqual[T any](expected T) Kleisli[T] {
+	return wrap1(assert.NotEqual, expected)
+}
+
+// Equal tests if the expected and the actual values are equal
+func Equal[T any](expected T) Kleisli[T] {
+	return wrap1(assert.Equal, expected)
+}
+
+// ArrayNotEmpty checks if an array is not empty
+func ArrayNotEmpty[T any](arr []T) Reader {
+	return func(t *testing.T) bool {
+		return assert.NotEmpty(t, arr)
+	}
+}
+
+// RecordNotEmpty checks if an map is not empty
+func RecordNotEmpty[K comparable, T any](mp map[K]T) Reader {
+	return func(t *testing.T) bool {
+		return assert.NotEmpty(t, mp)
+	}
+}
+
+// ArrayLength tests if an array has the expected length
+func ArrayLength[T any](expected int) Kleisli[[]T] {
+	return func(actual []T) Reader {
+		return func(t *testing.T) bool {
+			return assert.Len(t, actual, expected)
+		}
+	}
+}
+
+// RecordLength tests if a map has the expected length
+func RecordLength[K comparable, T any](expected int) Kleisli[map[K]T] {
+	return func(actual map[K]T) Reader {
+		return func(t *testing.T) bool {
+			return assert.Len(t, actual, expected)
+		}
+	}
+}
+
+// StringLength tests if a string has the expected length
+func StringLength[K comparable, T any](expected int) Kleisli[string] {
+	return func(actual string) Reader {
+		return func(t *testing.T) bool {
+			return assert.Len(t, actual, expected)
+		}
+	}
+}
+
+// NoError validates that there is no error
+func NoError(err error) Reader {
+	return func(t *testing.T) bool {
+		return assert.NoError(t, err)
+	}
+}
+
+// Error validates that there is an error
+func Error(err error) Reader {
+	return func(t *testing.T) bool {
+		return assert.Error(t, err)
+	}
+}
+
+// Success checks if a [Result] represents success
+func Success[T any](res Result[T]) Reader {
+	return NoError(result.ToError(res))
+}
+
+// Failure checks if a [Result] represents failure
+func Failure[T any](res Result[T]) Reader {
+	return Error(result.ToError(res))
+}
+
+// ArrayContains tests if a value is contained in an array
+func ArrayContains[T any](expected T) Kleisli[[]T] {
+	return func(actual []T) Reader {
+		return func(t *testing.T) bool {
+			return assert.Contains(t, actual, expected)
+		}
+	}
+}
+
+// ContainsKey tests if a key is contained in a map
+func ContainsKey[T any, K comparable](expected K) Kleisli[map[K]T] {
+	return func(actual map[K]T) Reader {
+		return func(t *testing.T) bool {
+			return assert.Contains(t, actual, expected)
+		}
+	}
+}
+
+// NotContainsKey tests if a key is not contained in a map
+func NotContainsKey[T any, K comparable](expected K) Kleisli[map[K]T] {
+	return func(actual map[K]T) Reader {
+		return func(t *testing.T) bool {
+			return assert.NotContains(t, actual, expected)
+		}
+	}
+}
+
+// That asserts that a particular predicate matches
+func That[T any](pred Predicate[T]) Kleisli[T] {
+	return func(a T) Reader {
+		return func(t *testing.T) bool {
+			if pred(a) {
+				return true
+			}
+			return assert.Fail(t, fmt.Sprintf("Preficate %v does not match value %v", pred, a))
+		}
+	}
+}
+
+// AllOf combines multiple assertion Readers into a single Reader that passes
+// only if all assertions pass.
+//
+// This function uses boolean AND logic (MonoidAll) to combine the results of
+// all assertions. If any assertion fails, the combined assertion fails.
+//
+// This is useful for grouping related assertions together and ensuring all
+// conditions are met.
+//
+// Parameters:
+//   - readers: Array of assertion Readers to combine
+//
+// Returns:
+//   - A single Reader that performs all assertions and returns true only if all pass
+//
+// Example:
+//
+//	func TestUser(t *testing.T) {
+//	    user := User{Name: "Alice", Age: 30, Active: true}
+//	    assertions := assert.AllOf([]assert.Reader{
+//	        assert.Equal("Alice")(user.Name),
+//	        assert.Equal(30)(user.Age),
+//	        assert.Equal(true)(user.Active),
+//	    })
+//	    assertions(t)
+//	}
+//
+//go:inline
+func AllOf(readers []Reader) Reader {
+	return reader.MonadReduceArrayM(readers, boolean.MonoidAll)
+}
+
+// RunAll executes a map of named test cases, running each as a subtest.
+//
+// This function creates a Reader that runs multiple named test cases using
+// Go's t.Run for proper test isolation and reporting. Each test case is
+// executed as a separate subtest with its own name.
+//
+// The function returns true only if all subtests pass. This allows for
+// better test organization and clearer test output.
+//
+// Parameters:
+//   - testcases: Map of test names to assertion Readers
+//
+// Returns:
+//   - A Reader that executes all named test cases and returns true if all pass
+//
+// Example:
+//
+//	func TestMathOperations(t *testing.T) {
+//	    testcases := map[string]assert.Reader{
+//	        "addition":       assert.Equal(4)(2 + 2),
+//	        "multiplication": assert.Equal(6)(2 * 3),
+//	        "subtraction":    assert.Equal(1)(3 - 2),
+//	    }
+//	    assert.RunAll(testcases)(t)
+//	}
+//
+//go:inline
+func RunAll(testcases map[string]Reader) Reader {
+	return func(t *testing.T) bool {
+		current := true
+		for k, r := range testcases {
+			current = current && t.Run(k, func(t1 *testing.T) {
+				r(t1)
+			})
+		}
+		return current
+	}
+}

v2/assert/assert_test.go

@@ -16,94 +16,470 @@
 package assert
 
 import (
-	"fmt"
+	"errors"
 	"testing"
 
-	E "github.com/IBM/fp-go/v2/either"
+	"github.com/IBM/fp-go/v2/result"
-	EQ "github.com/IBM/fp-go/v2/eq"
-	"github.com/stretchr/testify/assert"
 )
 
-var (
+func TestEqual(t *testing.T) {
-	errTest = fmt.Errorf("test failure")
+	t.Run("should pass when values are equal", func(t *testing.T) {
-
+		result := Equal(42)(42)(t)
-	// Eq is the equal predicate checking if objects are equal
+		if !result {
-	Eq = EQ.FromEquals(assert.ObjectsAreEqual)
+			t.Error("Expected Equal to pass for equal values")
-)
-
-func wrap1[T any](wrapped func(t assert.TestingT, expected, actual any, msgAndArgs ...any) bool, t *testing.T, expected T) func(actual T) E.Either[error, T] {
-	return func(actual T) E.Either[error, T] {
-		ok := wrapped(t, expected, actual)
-		if ok {
-			return E.Of[error](actual)
 		}
-		return E.Left[T](errTest)
+	})
-	}
-}
 
-// NotEqual tests if the expected and the actual values are not equal
+	t.Run("should fail when values are not equal", func(t *testing.T) {
-func NotEqual[T any](t *testing.T, expected T) func(actual T) E.Either[error, T] {
+		mockT := &testing.T{}
-	return wrap1(assert.NotEqual, t, expected)
+		result := Equal(42)(43)(mockT)
-}
+		if result {
-
+			t.Error("Expected Equal to fail for different values")
-// Equal tests if the expected and the actual values are equal
-func Equal[T any](t *testing.T, expected T) func(actual T) E.Either[error, T] {
-	return wrap1(assert.Equal, t, expected)
-}
-
-// Length tests if an array has the expected length
-func Length[T any](t *testing.T, expected int) func(actual []T) E.Either[error, []T] {
-	return func(actual []T) E.Either[error, []T] {
-		ok := assert.Len(t, actual, expected)
-		if ok {
-			return E.Of[error](actual)
 		}
-		return E.Left[[]T](errTest)
+	})
-	}
+
+	t.Run("should work with strings", func(t *testing.T) {
+		result := Equal("hello")("hello")(t)
+		if !result {
+			t.Error("Expected Equal to pass for equal strings")
+		}
+	})
 }
 
-// NoError validates that there is no error
+func TestNotEqual(t *testing.T) {
-func NoError[T any](t *testing.T) func(actual E.Either[error, T]) E.Either[error, T] {
+	t.Run("should pass when values are not equal", func(t *testing.T) {
-	return func(actual E.Either[error, T]) E.Either[error, T] {
+		result := NotEqual(42)(43)(t)
-		return E.MonadFold(actual, func(e error) E.Either[error, T] {
+		if !result {
-			assert.NoError(t, e)
+			t.Error("Expected NotEqual to pass for different values")
-			return E.Left[T](e)
+		}
-		}, func(value T) E.Either[error, T] {
+	})
-			assert.NoError(t, nil)
+
-			return E.Right[error](value)
+	t.Run("should fail when values are equal", func(t *testing.T) {
+		mockT := &testing.T{}
+		result := NotEqual(42)(42)(mockT)
+		if result {
+			t.Error("Expected NotEqual to fail for equal values")
+		}
+	})
+}
+
+func TestArrayNotEmpty(t *testing.T) {
+	t.Run("should pass for non-empty array", func(t *testing.T) {
+		arr := []int{1, 2, 3}
+		result := ArrayNotEmpty(arr)(t)
+		if !result {
+			t.Error("Expected ArrayNotEmpty to pass for non-empty array")
+		}
+	})
+
+	t.Run("should fail for empty array", func(t *testing.T) {
+		mockT := &testing.T{}
+		arr := []int{}
+		result := ArrayNotEmpty(arr)(mockT)
+		if result {
+			t.Error("Expected ArrayNotEmpty to fail for empty array")
+		}
+	})
+}
+
+func TestRecordNotEmpty(t *testing.T) {
+	t.Run("should pass for non-empty map", func(t *testing.T) {
+		mp := map[string]int{"a": 1, "b": 2}
+		result := RecordNotEmpty(mp)(t)
+		if !result {
+			t.Error("Expected RecordNotEmpty to pass for non-empty map")
+		}
+	})
+
+	t.Run("should fail for empty map", func(t *testing.T) {
+		mockT := &testing.T{}
+		mp := map[string]int{}
+		result := RecordNotEmpty(mp)(mockT)
+		if result {
+			t.Error("Expected RecordNotEmpty to fail for empty map")
+		}
+	})
+}
+
+func TestArrayLength(t *testing.T) {
+	t.Run("should pass when length matches", func(t *testing.T) {
+		arr := []int{1, 2, 3}
+		result := ArrayLength[int](3)(arr)(t)
+		if !result {
+			t.Error("Expected ArrayLength to pass when length matches")
+		}
+	})
+
+	t.Run("should fail when length doesn't match", func(t *testing.T) {
+		mockT := &testing.T{}
+		arr := []int{1, 2, 3}
+		result := ArrayLength[int](5)(arr)(mockT)
+		if result {
+			t.Error("Expected ArrayLength to fail when length doesn't match")
+		}
+	})
+
+	t.Run("should work with empty array", func(t *testing.T) {
+		arr := []string{}
+		result := ArrayLength[string](0)(arr)(t)
+		if !result {
+			t.Error("Expected ArrayLength to pass for empty array with expected length 0")
+		}
+	})
+}
+
+func TestRecordLength(t *testing.T) {
+	t.Run("should pass when map length matches", func(t *testing.T) {
+		mp := map[string]int{"a": 1, "b": 2}
+		result := RecordLength[string, int](2)(mp)(t)
+		if !result {
+			t.Error("Expected RecordLength to pass when length matches")
+		}
+	})
+
+	t.Run("should fail when map length doesn't match", func(t *testing.T) {
+		mockT := &testing.T{}
+		mp := map[string]int{"a": 1}
+		result := RecordLength[string, int](3)(mp)(mockT)
+		if result {
+			t.Error("Expected RecordLength to fail when length doesn't match")
+		}
+	})
+}
+
+func TestStringLength(t *testing.T) {
+	t.Run("should pass when string length matches", func(t *testing.T) {
+		str := "hello"
+		result := StringLength[string, int](5)(str)(t)
+		if !result {
+			t.Error("Expected StringLength to pass when length matches")
+		}
+	})
+
+	t.Run("should fail when string length doesn't match", func(t *testing.T) {
+		mockT := &testing.T{}
+		str := "hello"
+		result := StringLength[string, int](10)(str)(mockT)
+		if result {
+			t.Error("Expected StringLength to fail when length doesn't match")
+		}
+	})
+
+	t.Run("should work with empty string", func(t *testing.T) {
+		str := ""
+		result := StringLength[string, int](0)(str)(t)
+		if !result {
+			t.Error("Expected StringLength to pass for empty string with expected length 0")
+		}
+	})
+}
+
+func TestNoError(t *testing.T) {
+	t.Run("should pass when error is nil", func(t *testing.T) {
+		result := NoError(nil)(t)
+		if !result {
+			t.Error("Expected NoError to pass when error is nil")
+		}
+	})
+
+	t.Run("should fail when error is not nil", func(t *testing.T) {
+		mockT := &testing.T{}
+		err := errors.New("test error")
+		result := NoError(err)(mockT)
+		if result {
+			t.Error("Expected NoError to fail when error is not nil")
+		}
+	})
+}
+
+func TestError(t *testing.T) {
+	t.Run("should pass when error is not nil", func(t *testing.T) {
+		err := errors.New("test error")
+		result := Error(err)(t)
+		if !result {
+			t.Error("Expected Error to pass when error is not nil")
+		}
+	})
+
+	t.Run("should fail when error is nil", func(t *testing.T) {
+		mockT := &testing.T{}
+		result := Error(nil)(mockT)
+		if result {
+			t.Error("Expected Error to fail when error is nil")
+		}
+	})
+}
+
+func TestSuccess(t *testing.T) {
+	t.Run("should pass for successful result", func(t *testing.T) {
+		res := result.Of[int](42)
+		result := Success(res)(t)
+		if !result {
+			t.Error("Expected Success to pass for successful result")
+		}
+	})
+
+	t.Run("should fail for error result", func(t *testing.T) {
+		mockT := &testing.T{}
+		res := result.Left[int](errors.New("test error"))
+		result := Success(res)(mockT)
+		if result {
+			t.Error("Expected Success to fail for error result")
+		}
+	})
+}
+
+func TestFailure(t *testing.T) {
+	t.Run("should pass for error result", func(t *testing.T) {
+		res := result.Left[int](errors.New("test error"))
+		result := Failure(res)(t)
+		if !result {
+			t.Error("Expected Failure to pass for error result")
+		}
+	})
+
+	t.Run("should fail for successful result", func(t *testing.T) {
+		mockT := &testing.T{}
+		res := result.Of[int](42)
+		result := Failure(res)(mockT)
+		if result {
+			t.Error("Expected Failure to fail for successful result")
+		}
+	})
+}
+
+func TestArrayContains(t *testing.T) {
+	t.Run("should pass when element is in array", func(t *testing.T) {
+		arr := []int{1, 2, 3, 4, 5}
+		result := ArrayContains(3)(arr)(t)
+		if !result {
+			t.Error("Expected ArrayContains to pass when element is in array")
+		}
+	})
+
+	t.Run("should fail when element is not in array", func(t *testing.T) {
+		mockT := &testing.T{}
+		arr := []int{1, 2, 3}
+		result := ArrayContains(10)(arr)(mockT)
+		if result {
+			t.Error("Expected ArrayContains to fail when element is not in array")
+		}
+	})
+
+	t.Run("should work with strings", func(t *testing.T) {
+		arr := []string{"apple", "banana", "cherry"}
+		result := ArrayContains("banana")(arr)(t)
+		if !result {
+			t.Error("Expected ArrayContains to pass for string element")
+		}
+	})
+}
+
+func TestContainsKey(t *testing.T) {
+	t.Run("should pass when key exists in map", func(t *testing.T) {
+		mp := map[string]int{"a": 1, "b": 2, "c": 3}
+		result := ContainsKey[int]("b")(mp)(t)
+		if !result {
+			t.Error("Expected ContainsKey to pass when key exists")
+		}
+	})
+
+	t.Run("should fail when key doesn't exist in map", func(t *testing.T) {
+		mockT := &testing.T{}
+		mp := map[string]int{"a": 1, "b": 2}
+		result := ContainsKey[int]("z")(mp)(mockT)
+		if result {
+			t.Error("Expected ContainsKey to fail when key doesn't exist")
+		}
+	})
+}
+
+func TestNotContainsKey(t *testing.T) {
+	t.Run("should pass when key doesn't exist in map", func(t *testing.T) {
+		mp := map[string]int{"a": 1, "b": 2}
+		result := NotContainsKey[int]("z")(mp)(t)
+		if !result {
+			t.Error("Expected NotContainsKey to pass when key doesn't exist")
+		}
+	})
+
+	t.Run("should fail when key exists in map", func(t *testing.T) {
+		mockT := &testing.T{}
+		mp := map[string]int{"a": 1, "b": 2}
+		result := NotContainsKey[int]("a")(mp)(mockT)
+		if result {
+			t.Error("Expected NotContainsKey to fail when key exists")
+		}
+	})
+}
+
+func TestThat(t *testing.T) {
+	t.Run("should pass when predicate is true", func(t *testing.T) {
+		isEven := func(n int) bool { return n%2 == 0 }
+		result := That(isEven)(42)(t)
+		if !result {
+			t.Error("Expected That to pass when predicate is true")
+		}
+	})
+
+	t.Run("should fail when predicate is false", func(t *testing.T) {
+		mockT := &testing.T{}
+		isEven := func(n int) bool { return n%2 == 0 }
+		result := That(isEven)(43)(mockT)
+		if result {
+			t.Error("Expected That to fail when predicate is false")
+		}
+	})
+
+	t.Run("should work with string predicates", func(t *testing.T) {
+		startsWithH := func(s string) bool { return len(s) > 0 && s[0] == 'h' }
+		result := That(startsWithH)("hello")(t)
+		if !result {
+			t.Error("Expected That to pass for string predicate")
+		}
+	})
+}
+
+func TestAllOf(t *testing.T) {
+	t.Run("should pass when all assertions pass", func(t *testing.T) {
+		assertions := AllOf([]Reader{
+			Equal(42)(42),
+			Equal("hello")("hello"),
+			ArrayNotEmpty([]int{1, 2, 3}),
 		})
-	}
+		result := assertions(t)
+		if !result {
+			t.Error("Expected AllOf to pass when all assertions pass")
+		}
+	})
+
+	t.Run("should fail when any assertion fails", func(t *testing.T) {
+		mockT := &testing.T{}
+		assertions := AllOf([]Reader{
+			Equal(42)(42),
+			Equal("hello")("goodbye"),
+			ArrayNotEmpty([]int{1, 2, 3}),
+		})
+		result := assertions(mockT)
+		if result {
+			t.Error("Expected AllOf to fail when any assertion fails")
+		}
+	})
+
+	t.Run("should work with empty array", func(t *testing.T) {
+		assertions := AllOf([]Reader{})
+		result := assertions(t)
+		if !result {
+			t.Error("Expected AllOf to pass for empty array")
+		}
+	})
+
+	t.Run("should combine multiple array assertions", func(t *testing.T) {
+		arr := []int{1, 2, 3, 4, 5}
+		assertions := AllOf([]Reader{
+			ArrayNotEmpty(arr),
+			ArrayLength[int](5)(arr),
+			ArrayContains(3)(arr),
+		})
+		result := assertions(t)
+		if !result {
+			t.Error("Expected AllOf to pass for multiple array assertions")
+		}
+	})
 }
 
-// ArrayContains tests if a value is contained in an array
+func TestRunAll(t *testing.T) {
-func ArrayContains[T any](t *testing.T, expected T) func(actual []T) E.Either[error, []T] {
+	t.Run("should run all named test cases", func(t *testing.T) {
-	return func(actual []T) E.Either[error, []T] {
+		testcases := map[string]Reader{
-		ok := assert.Contains(t, actual, expected)
+			"equality":     Equal(42)(42),
-		if ok {
+			"string_check": Equal("test")("test"),
-			return E.Of[error](actual)
+			"array_check":  ArrayNotEmpty([]int{1, 2, 3}),
 		}
-		return E.Left[[]T](errTest)
+		result := RunAll(testcases)(t)
-	}
+		if !result {
+			t.Error("Expected RunAll to pass when all test cases pass")
+		}
+	})
+
+	// Note: Testing failure behavior of RunAll is tricky because subtests
+	// will actually fail in the test framework. The function works correctly
+	// as demonstrated by the passing test above.
+
+	t.Run("should work with empty test cases", func(t *testing.T) {
+		testcases := map[string]Reader{}
+		result := RunAll(testcases)(t)
+		if !result {
+			t.Error("Expected RunAll to pass for empty test cases")
+		}
+	})
 }
 
-// ContainsKey tests if a key is contained in a map
+func TestEq(t *testing.T) {
-func ContainsKey[T any, K comparable](t *testing.T, expected K) func(actual map[K]T) E.Either[error, map[K]T] {
+	t.Run("should return true for equal values", func(t *testing.T) {
-	return func(actual map[K]T) E.Either[error, map[K]T] {
+		if !Eq.Equals(42, 42) {
-		ok := assert.Contains(t, actual, expected)
+			t.Error("Expected Eq to return true for equal integers")
-		if ok {
-			return E.Of[error](actual)
 		}
-		return E.Left[map[K]T](errTest)
+	})
-	}
+
+	t.Run("should return false for different values", func(t *testing.T) {
+		if Eq.Equals(42, 43) {
+			t.Error("Expected Eq to return false for different integers")
+		}
+	})
+
+	t.Run("should work with strings", func(t *testing.T) {
+		if !Eq.Equals("hello", "hello") {
+			t.Error("Expected Eq to return true for equal strings")
+		}
+		if Eq.Equals("hello", "world") {
+			t.Error("Expected Eq to return false for different strings")
+		}
+	})
+
+	t.Run("should work with slices", func(t *testing.T) {
+		arr1 := []int{1, 2, 3}
+		arr2 := []int{1, 2, 3}
+		if !Eq.Equals(arr1, arr2) {
+			t.Error("Expected Eq to return true for equal slices")
+		}
+	})
 }
 
-// NotContainsKey tests if a key is not contained in a map
+func TestIntegration(t *testing.T) {
-func NotContainsKey[T any, K comparable](t *testing.T, expected K) func(actual map[K]T) E.Either[error, map[K]T] {
+	t.Run("complex assertion composition", func(t *testing.T) {
-	return func(actual map[K]T) E.Either[error, map[K]T] {
+		type User struct {
-		ok := assert.NotContains(t, actual, expected)
+			Name  string
-		if ok {
+			Age   int
-			return E.Of[error](actual)
+			Email string
 		}
-		return E.Left[map[K]T](errTest)
+
-	}
+		user := User{Name: "Alice", Age: 30, Email: "alice@example.com"}
+
+		assertions := AllOf([]Reader{
+			Equal("Alice")(user.Name),
+			Equal(30)(user.Age),
+			That(func(s string) bool { return len(s) > 0 })(user.Email),
+		})
+
+		result := assertions(t)
+		if !result {
+			t.Error("Expected complex assertion composition to pass")
+		}
+	})
+
+	t.Run("test suite with RunAll", func(t *testing.T) {
+		data := []int{1, 2, 3, 4, 5}
+
+		suite := RunAll(map[string]Reader{
+			"not_empty":     ArrayNotEmpty(data),
+			"correct_size":  ArrayLength[int](5)(data),
+			"contains_one":  ArrayContains(1)(data),
+			"contains_five": ArrayContains(5)(data),
+		})
+
+		result := suite(t)
+		if !result {
+			t.Error("Expected test suite to pass")
+		}
+	})
 }

v2/assert/types.go

@@ -0,0 +1,16 @@
+package assert
+
+import (
+	"testing"
+
+	"github.com/IBM/fp-go/v2/predicate"
+	"github.com/IBM/fp-go/v2/reader"
+	"github.com/IBM/fp-go/v2/result"
+)
+
+type (
+	Result[T any]    = result.Result[T]
+	Reader           = reader.Reader[*testing.T, bool]
+	Kleisli[T any]   = reader.Reader[T, Reader]
+	Predicate[T any] = predicate.Predicate[T]
+)

v2/bytes/bytes.go

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

v2/bytes/bytes_test.go

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

v2/bytes/coverage.out

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

v2/bytes/monoid.go

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

v2/cli/lens.go

@@ -53,17 +53,20 @@ var (
 
 // structInfo holds information about a struct that needs lens generation
 type structInfo struct {
-	Name    string
+	Name           string
-	Fields  []fieldInfo
+	TypeParams     string // e.g., "[T any]" or "[K comparable, V any]" - for type declarations
-	Imports map[string]string // package path -> alias
+	TypeParamNames string // e.g., "[T]" or "[K, V]" - for type usage in function signatures
+	Fields         []fieldInfo
+	Imports        map[string]string // package path -> alias
 }
 
 // fieldInfo holds information about a struct field
 type fieldInfo struct {
-	Name       string
+	Name         string
-	TypeName   string
+	TypeName     string
-	BaseType   string // TypeName without leading * for pointer types
+	BaseType     string // TypeName without leading * for pointer types
-	IsOptional bool   // true if field is a pointer or has json omitempty tag
+	IsOptional   bool   // true if field is a pointer or has json omitempty tag
+	IsComparable bool   // true if the type is comparable (can use ==)
 }
 
 // templateData holds data for template rendering
@@ -74,64 +77,95 @@ type templateData struct {
 
 const lensStructTemplate = `
 // {{.Name}}Lenses provides lenses for accessing fields of {{.Name}}
-type {{.Name}}Lenses struct {
+type {{.Name}}Lenses{{.TypeParams}} struct {
+	// mandatory fields
 {{- range .Fields}}
-	{{.Name}} {{if .IsOptional}}LO.LensO[{{$.Name}}, {{.TypeName}}]{{else}}L.Lens[{{$.Name}}, {{.TypeName}}]{{end}}
+	{{.Name}} L.Lens[{{$.Name}}{{$.TypeParamNames}}, {{.TypeName}}]
+{{- end}}
+	// optional fields
+{{- range .Fields}}
+{{- if .IsComparable}}
+	{{.Name}}O LO.LensO[{{$.Name}}{{$.TypeParamNames}}, {{.TypeName}}]
+{{- end}}
 {{- end}}
 }
 
 // {{.Name}}RefLenses provides lenses for accessing fields of {{.Name}} via a reference to {{.Name}}
-type {{.Name}}RefLenses struct {
+type {{.Name}}RefLenses{{.TypeParams}} struct {
+	// mandatory fields
 {{- range .Fields}}
-	{{.Name}} {{if .IsOptional}}LO.LensO[*{{$.Name}}, {{.TypeName}}]{{else}}L.Lens[*{{$.Name}}, {{.TypeName}}]{{end}}
+	{{.Name}} L.Lens[*{{$.Name}}{{$.TypeParamNames}}, {{.TypeName}}]
+{{- end}}
+	// optional fields
+{{- range .Fields}}
+{{- if .IsComparable}}
+	{{.Name}}O LO.LensO[*{{$.Name}}{{$.TypeParamNames}}, {{.TypeName}}]
+{{- end}}
 {{- end}}
 }
 `
 
 const lensConstructorTemplate = `
 // Make{{.Name}}Lenses creates a new {{.Name}}Lenses with lenses for all fields
-func Make{{.Name}}Lenses() {{.Name}}Lenses {
+func Make{{.Name}}Lenses{{.TypeParams}}() {{.Name}}Lenses{{.TypeParamNames}} {
+	// mandatory lenses
 {{- range .Fields}}
-{{- if .IsOptional}}
+	lens{{.Name}} := L.MakeLens(
-	iso{{.Name}} := I.FromZero[{{.TypeName}}]()
+		func(s {{$.Name}}{{$.TypeParamNames}}) {{.TypeName}} { return s.{{.Name}} },
+		func(s {{$.Name}}{{$.TypeParamNames}}, v {{.TypeName}}) {{$.Name}}{{$.TypeParamNames}} { s.{{.Name}} = v; return s },
+	)
+{{- end}}
+	// optional lenses
+{{- range .Fields}}
+{{- if .IsComparable}}
+	lens{{.Name}}O := LO.FromIso[{{$.Name}}{{$.TypeParamNames}}](IO.FromZero[{{.TypeName}}]())(lens{{.Name}})
 {{- end}}
 {{- end}}
-	return {{.Name}}Lenses{
+	return {{.Name}}Lenses{{.TypeParamNames}}{
+		// mandatory lenses
 {{- range .Fields}}
-{{- if .IsOptional}}
+		{{.Name}}: lens{{.Name}},
-		{{.Name}}: L.MakeLens(
+{{- end}}
-			func(s {{$.Name}}) O.Option[{{.TypeName}}] { return iso{{.Name}}.Get(s.{{.Name}}) },
+		// optional lenses
-			func(s {{$.Name}}, v O.Option[{{.TypeName}}]) {{$.Name}} { s.{{.Name}} = iso{{.Name}}.ReverseGet(v); return s },
+{{- range .Fields}}
-		),
+{{- if .IsComparable}}
-{{- else}}
+		{{.Name}}O: lens{{.Name}}O,
-		{{.Name}}: L.MakeLens(
-			func(s {{$.Name}}) {{.TypeName}} { return s.{{.Name}} },
-			func(s {{$.Name}}, v {{.TypeName}}) {{$.Name}} { s.{{.Name}} = v; return s },
-		),
 {{- end}}
 {{- end}}
 	}
 }
 
 // Make{{.Name}}RefLenses creates a new {{.Name}}RefLenses with lenses for all fields
-func Make{{.Name}}RefLenses() {{.Name}}RefLenses {
+func Make{{.Name}}RefLenses{{.TypeParams}}() {{.Name}}RefLenses{{.TypeParamNames}} {
+	// mandatory lenses
 {{- range .Fields}}
-{{- if .IsOptional}}
+{{- if .IsComparable}}
-	iso{{.Name}} := I.FromZero[{{.TypeName}}]()
+	lens{{.Name}} := L.MakeLensStrict(
-{{- end}}
+		func(s *{{$.Name}}{{$.TypeParamNames}}) {{.TypeName}} { return s.{{.Name}} },
-{{- end}}
+		func(s *{{$.Name}}{{$.TypeParamNames}}, v {{.TypeName}}) *{{$.Name}}{{$.TypeParamNames}} { s.{{.Name}} = v; return s },
-	return {{.Name}}RefLenses{
+	)
-{{- range .Fields}}
-{{- if .IsOptional}}
-		{{.Name}}: L.MakeLensRef(
-			func(s *{{$.Name}}) O.Option[{{.TypeName}}] { return iso{{.Name}}.Get(s.{{.Name}}) },
-			func(s *{{$.Name}}, v O.Option[{{.TypeName}}]) *{{$.Name}} { s.{{.Name}} = iso{{.Name}}.ReverseGet(v); return s },
-		),
 {{- else}}
-		{{.Name}}: L.MakeLensRef(
+	lens{{.Name}} := L.MakeLensRef(
-			func(s *{{$.Name}}) {{.TypeName}} { return s.{{.Name}} },
+		func(s *{{$.Name}}{{$.TypeParamNames}}) {{.TypeName}} { return s.{{.Name}} },
-			func(s *{{$.Name}}, v {{.TypeName}}) *{{$.Name}} { s.{{.Name}} = v; return s },
+		func(s *{{$.Name}}{{$.TypeParamNames}}, v {{.TypeName}}) *{{$.Name}}{{$.TypeParamNames}} { s.{{.Name}} = v; return s },
-		),
+	)
+{{- end}}
+{{- end}}
+	// optional lenses
+{{- range .Fields}}
+{{- if .IsComparable}}
+	lens{{.Name}}O := LO.FromIso[*{{$.Name}}{{$.TypeParamNames}}](IO.FromZero[{{.TypeName}}]())(lens{{.Name}})
+{{- end}}
+{{- end}}
+	return {{.Name}}RefLenses{{.TypeParamNames}}{
+		// mandatory lenses
+{{- range .Fields}}
+		{{.Name}}: lens{{.Name}},
+{{- end}}
+		// optional lenses
+{{- range .Fields}}
+{{- if .IsComparable}}
+		{{.Name}}O: lens{{.Name}}O,
 {{- end}}
 {{- end}}
 	}
@@ -257,6 +291,259 @@ func isPointerType(expr ast.Expr) bool {
 	return ok
 }
 
+// isComparableType checks if a type expression represents a comparable type.
+// Comparable types in Go include:
+// - Basic types (bool, numeric types, string)
+// - Pointer types
+// - Channel types
+// - Interface types
+// - Structs where all fields are comparable
+// - Arrays where the element type is comparable
+//
+// Non-comparable types include:
+// - Slices
+// - Maps
+// - Functions
+//
+// typeParams is a map of type parameter names to their constraints (e.g., "T" -> "any", "K" -> "comparable")
+func isComparableType(expr ast.Expr, typeParams map[string]string) bool {
+	switch t := expr.(type) {
+	case *ast.Ident:
+		// Check if this is a type parameter
+		if constraint, isTypeParam := typeParams[t.Name]; isTypeParam {
+			// Type parameter - check its constraint
+			return constraint == "comparable"
+		}
+
+		// Basic types and named types
+		// We assume named types are comparable unless they're known non-comparable types
+		name := t.Name
+		// Known non-comparable built-in types
+		if name == "error" {
+			// error is an interface, which is comparable
+			return true
+		}
+		// Most basic types and named types are comparable
+		// We can't determine if a custom type is comparable without type checking,
+		// so we assume it is (conservative approach)
+		return true
+	case *ast.StarExpr:
+		// Pointer types are always comparable
+		return true
+	case *ast.ArrayType:
+		// Arrays are comparable if their element type is comparable
+		if t.Len == nil {
+			// This is a slice (no length), slices are not comparable
+			return false
+		}
+		// Fixed-size array, check element type
+		return isComparableType(t.Elt, typeParams)
+	case *ast.MapType:
+		// Maps are not comparable
+		return false
+	case *ast.FuncType:
+		// Functions are not comparable
+		return false
+	case *ast.InterfaceType:
+		// Interface types are comparable
+		return true
+	case *ast.StructType:
+		// Structs are comparable if all fields are comparable
+		// We can't easily determine this without full type information,
+		// so we conservatively return false for struct literals
+		return false
+	case *ast.SelectorExpr:
+		// Qualified identifier (e.g., pkg.Type)
+		// We can't determine comparability without type information
+		// Check for known non-comparable types from standard library
+		if ident, ok := t.X.(*ast.Ident); ok {
+			pkgName := ident.Name
+			typeName := t.Sel.Name
+			// Check for known non-comparable types
+			if pkgName == "context" && typeName == "Context" {
+				// context.Context is an interface, which is comparable
+				return true
+			}
+			// For other qualified types, we assume they're comparable
+			// This is a conservative approach
+		}
+		return true
+	case *ast.IndexExpr, *ast.IndexListExpr:
+		// Generic types - we can't determine comparability without type information
+		// For common generic types, we can make educated guesses
+		var baseExpr ast.Expr
+		if idx, ok := t.(*ast.IndexExpr); ok {
+			baseExpr = idx.X
+		} else if idxList, ok := t.(*ast.IndexListExpr); ok {
+			baseExpr = idxList.X
+		}
+
+		if sel, ok := baseExpr.(*ast.SelectorExpr); ok {
+			if ident, ok := sel.X.(*ast.Ident); ok {
+				pkgName := ident.Name
+				typeName := sel.Sel.Name
+				// Check for known non-comparable generic types
+				if pkgName == "option" && typeName == "Option" {
+					// Option types are not comparable (they contain a slice internally)
+					return false
+				}
+				if pkgName == "either" && typeName == "Either" {
+					// Either types are not comparable
+					return false
+				}
+			}
+		}
+		// For other generic types, conservatively assume not comparable
+		log.Printf("Not comparable type: %v\n", t)
+		return false
+	case *ast.ChanType:
+		// Channel types are comparable
+		return true
+	default:
+		// Unknown type, conservatively assume not comparable
+		return false
+	}
+}
+
+// embeddedFieldResult holds both the field info and its AST type for import extraction
+type embeddedFieldResult struct {
+	fieldInfo fieldInfo
+	fieldType ast.Expr
+}
+
+// extractEmbeddedFields extracts fields from an embedded struct type
+// It returns a slice of embeddedFieldResult for all exported fields in the embedded struct
+// typeParamsMap contains the type parameters of the parent struct (for checking comparability)
+func extractEmbeddedFields(embedType ast.Expr, fileImports map[string]string, file *ast.File, typeParamsMap map[string]string) []embeddedFieldResult {
+	var results []embeddedFieldResult
+
+	// Get the type name of the embedded field
+	var typeName string
+	var typeIdent *ast.Ident
+
+	switch t := embedType.(type) {
+	case *ast.Ident:
+		// Direct embedded type: type MyStruct struct { EmbeddedType }
+		typeName = t.Name
+		typeIdent = t
+	case *ast.StarExpr:
+		// Pointer embedded type: type MyStruct struct { *EmbeddedType }
+		if ident, ok := t.X.(*ast.Ident); ok {
+			typeName = ident.Name
+			typeIdent = ident
+		}
+	case *ast.SelectorExpr:
+		// Qualified embedded type: type MyStruct struct { pkg.EmbeddedType }
+		// We can't easily resolve this without full type information
+		// For now, skip these
+		return results
+	}
+
+	if typeName == "" || typeIdent == nil {
+		return results
+	}
+
+	// Find the struct definition in the same file
+	var embeddedStructType *ast.StructType
+	ast.Inspect(file, func(n ast.Node) bool {
+		if ts, ok := n.(*ast.TypeSpec); ok {
+			if ts.Name.Name == typeName {
+				if st, ok := ts.Type.(*ast.StructType); ok {
+					embeddedStructType = st
+					return false
+				}
+			}
+		}
+		return true
+	})
+
+	if embeddedStructType == nil {
+		// Struct not found in this file, might be from another package
+		return results
+	}
+
+	// Extract fields from the embedded struct
+	for _, field := range embeddedStructType.Fields.List {
+		// Skip embedded fields within embedded structs (for now, to avoid infinite recursion)
+		if len(field.Names) == 0 {
+			continue
+		}
+
+		for _, name := range field.Names {
+			// Only export lenses for exported fields
+			if name.IsExported() {
+				fieldTypeName := getTypeName(field.Type)
+				isOptional := false
+				baseType := fieldTypeName
+
+				// Check if field is optional
+				if isPointerType(field.Type) {
+					isOptional = true
+					baseType = strings.TrimPrefix(fieldTypeName, "*")
+				} else if hasOmitEmpty(field.Tag) {
+					isOptional = true
+				}
+
+				// Check if the type is comparable
+				isComparable := isComparableType(field.Type, typeParamsMap)
+
+				results = append(results, embeddedFieldResult{
+					fieldInfo: fieldInfo{
+						Name:         name.Name,
+						TypeName:     fieldTypeName,
+						BaseType:     baseType,
+						IsOptional:   isOptional,
+						IsComparable: isComparable,
+					},
+					fieldType: field.Type,
+				})
+			}
+		}
+	}
+
+	return results
+}
+
+// extractTypeParams extracts type parameters from a type spec
+// Returns two strings: full params like "[T any]" and names only like "[T]"
+func extractTypeParams(typeSpec *ast.TypeSpec) (string, string) {
+	if typeSpec.TypeParams == nil || len(typeSpec.TypeParams.List) == 0 {
+		return "", ""
+	}
+
+	var params []string
+	var names []string
+	for _, field := range typeSpec.TypeParams.List {
+		for _, name := range field.Names {
+			constraint := getTypeName(field.Type)
+			params = append(params, name.Name+" "+constraint)
+			names = append(names, name.Name)
+		}
+	}
+
+	fullParams := "[" + strings.Join(params, ", ") + "]"
+	nameParams := "[" + strings.Join(names, ", ") + "]"
+	return fullParams, nameParams
+}
+
+// buildTypeParamsMap creates a map of type parameter names to their constraints
+// e.g., for "type Box[T any, K comparable]", returns {"T": "any", "K": "comparable"}
+func buildTypeParamsMap(typeSpec *ast.TypeSpec) map[string]string {
+	typeParamsMap := make(map[string]string)
+	if typeSpec.TypeParams == nil || len(typeSpec.TypeParams.List) == 0 {
+		return typeParamsMap
+	}
+
+	for _, field := range typeSpec.TypeParams.List {
+		constraint := getTypeName(field.Type)
+		for _, name := range field.Names {
+			typeParamsMap[name.Name] = constraint
+		}
+	}
+
+	return typeParamsMap
+}
+
 // parseFile parses a Go file and extracts structs with lens annotations
 func parseFile(filename string) ([]structInfo, string, error) {
 	fset := token.NewFileSet()
@@ -320,9 +607,27 @@ func parseFile(filename string) ([]structInfo, string, error) {
 		var fields []fieldInfo
 		structImports := make(map[string]string)
 
+		// Build type parameters map for this struct
+		typeParamsMap := buildTypeParamsMap(typeSpec)
+
 		for _, field := range structType.Fields.List {
 			if len(field.Names) == 0 {
-				// Embedded field, skip for now
+				// Embedded field - promote its fields
+				embeddedResults := extractEmbeddedFields(field.Type, fileImports, node, typeParamsMap)
+				for _, embResult := range embeddedResults {
+					// Extract imports from embedded field's type
+					fieldImports := make(map[string]string)
+					extractImports(embResult.fieldType, fieldImports)
+
+					// Resolve package names to full import paths
+					for pkgName := range fieldImports {
+						if importPath, ok := fileImports[pkgName]; ok {
+							structImports[importPath] = pkgName
+						}
+					}
+
+					fields = append(fields, embResult.fieldInfo)
+				}
 				continue
 			}
 			for _, name := range field.Names {
@@ -331,6 +636,7 @@ func parseFile(filename string) ([]structInfo, string, error) {
 					typeName := getTypeName(field.Type)
 					isOptional := false
 					baseType := typeName
+					isComparable := false
 
 					// Check if field is optional:
 					// 1. Pointer types are always optional
@@ -344,6 +650,11 @@ func parseFile(filename string) ([]structInfo, string, error) {
 						isOptional = true
 					}
 
+					// Check if the type is comparable (for non-optional fields)
+					// For optional fields, we don't need to check since they use LensO
+					isComparable = isComparableType(field.Type, typeParamsMap)
+					// log.Printf("field %s, type: %v, isComparable: %b\n", name, field.Type, isComparable)
+
 					// Extract imports from this field's type
 					fieldImports := make(map[string]string)
 					extractImports(field.Type, fieldImports)
@@ -356,20 +667,24 @@ func parseFile(filename string) ([]structInfo, string, error) {
 					}
 
 					fields = append(fields, fieldInfo{
-						Name:       name.Name,
+						Name:         name.Name,
-						TypeName:   typeName,
+						TypeName:     typeName,
-						BaseType:   baseType,
+						BaseType:     baseType,
-						IsOptional: isOptional,
+						IsOptional:   isOptional,
+						IsComparable: isComparable,
 					})
 				}
 			}
 		}
 
 		if len(fields) > 0 {
+			typeParams, typeParamNames := extractTypeParams(typeSpec)
 			structs = append(structs, structInfo{
-				Name:    typeSpec.Name.Name,
+				Name:           typeSpec.Name.Name,
-				Fields:  fields,
+				TypeParams:     typeParams,
-				Imports: structImports,
+				TypeParamNames: typeParamNames,
+				Fields:         fields,
+				Imports:        structImports,
 			})
 		}
 
@@ -469,8 +784,8 @@ func generateLensHelpers(dir, filename string, verbose bool) error {
 	// Standard fp-go imports always needed
 	f.WriteString("\tL \"github.com/IBM/fp-go/v2/optics/lens\"\n")
 	f.WriteString("\tLO \"github.com/IBM/fp-go/v2/optics/lens/option\"\n")
-	f.WriteString("\tO \"github.com/IBM/fp-go/v2/option\"\n")
+	//	f.WriteString("\tO \"github.com/IBM/fp-go/v2/option\"\n")
-	f.WriteString("\tI \"github.com/IBM/fp-go/v2/optics/iso/option\"\n")
+	f.WriteString("\tIO \"github.com/IBM/fp-go/v2/optics/iso/option\"\n")
 
 	// Add additional imports collected from field types
 	for importPath, alias := range allImports {

v2/cli/lens_test.go

@@ -168,6 +168,91 @@ func TestIsPointerType(t *testing.T) {
 	}
 }
 
+func TestIsComparableType(t *testing.T) {
+	tests := []struct {
+		name     string
+		code     string
+		expected bool
+	}{
+		{
+			name:     "basic type - string",
+			code:     "type T struct { F string }",
+			expected: true,
+		},
+		{
+			name:     "basic type - int",
+			code:     "type T struct { F int }",
+			expected: true,
+		},
+		{
+			name:     "basic type - bool",
+			code:     "type T struct { F bool }",
+			expected: true,
+		},
+		{
+			name:     "pointer type",
+			code:     "type T struct { F *string }",
+			expected: true,
+		},
+		{
+			name:     "slice type - not comparable",
+			code:     "type T struct { F []string }",
+			expected: false,
+		},
+		{
+			name:     "map type - not comparable",
+			code:     "type T struct { F map[string]int }",
+			expected: false,
+		},
+		{
+			name:     "array type - comparable if element is",
+			code:     "type T struct { F [5]int }",
+			expected: true,
+		},
+		{
+			name:     "interface type",
+			code:     "type T struct { F interface{} }",
+			expected: true,
+		},
+		{
+			name:     "channel type",
+			code:     "type T struct { F chan int }",
+			expected: true,
+		},
+		{
+			name:     "function type - not comparable",
+			code:     "type T struct { F func() }",
+			expected: false,
+		},
+		{
+			name:     "struct literal - conservatively not comparable",
+			code:     "type T struct { F struct{ X int } }",
+			expected: false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			fset := token.NewFileSet()
+			file, err := parser.ParseFile(fset, "", "package test\n"+tt.code, 0)
+			require.NoError(t, err)
+
+			var fieldType ast.Expr
+			ast.Inspect(file, func(n ast.Node) bool {
+				if field, ok := n.(*ast.Field); ok && len(field.Names) > 0 {
+					fieldType = field.Type
+					return false
+				}
+				return true
+			})
+
+			require.NotNil(t, fieldType)
+			result := isComparableType(fieldType, map[string]string{})
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}
+
 func TestHasOmitEmpty(t *testing.T) {
 	tests := []struct {
 		name     string
@@ -337,6 +422,167 @@ type Config struct {
 	assert.False(t, config.Fields[4].IsOptional, "Required field without omitempty should not be optional")
 }
 
+func TestParseFileWithComparableTypes(t *testing.T) {
+	// Create a temporary test file
+	tmpDir := t.TempDir()
+	testFile := filepath.Join(tmpDir, "test.go")
+
+	testCode := `package testpkg
+
+// fp-go:Lens
+type TypeTest struct {
+	Name      string
+	Age       int
+	Pointer   *string
+	Slice     []string
+	Map       map[string]int
+	Channel   chan int
+}
+`
+
+	err := os.WriteFile(testFile, []byte(testCode), 0644)
+	require.NoError(t, err)
+
+	// Parse the file
+	structs, pkg, err := parseFile(testFile)
+	require.NoError(t, err)
+
+	// Verify results
+	assert.Equal(t, "testpkg", pkg)
+	assert.Len(t, structs, 1)
+
+	// Check TypeTest struct
+	typeTest := structs[0]
+	assert.Equal(t, "TypeTest", typeTest.Name)
+	assert.Len(t, typeTest.Fields, 6)
+
+	// Name - string is comparable
+	assert.Equal(t, "Name", typeTest.Fields[0].Name)
+	assert.Equal(t, "string", typeTest.Fields[0].TypeName)
+	assert.False(t, typeTest.Fields[0].IsOptional)
+	assert.True(t, typeTest.Fields[0].IsComparable, "string should be comparable")
+
+	// Age - int is comparable
+	assert.Equal(t, "Age", typeTest.Fields[1].Name)
+	assert.Equal(t, "int", typeTest.Fields[1].TypeName)
+	assert.False(t, typeTest.Fields[1].IsOptional)
+	assert.True(t, typeTest.Fields[1].IsComparable, "int should be comparable")
+
+	// Pointer - pointer is optional, IsComparable not checked for optional fields
+	assert.Equal(t, "Pointer", typeTest.Fields[2].Name)
+	assert.Equal(t, "*string", typeTest.Fields[2].TypeName)
+	assert.True(t, typeTest.Fields[2].IsOptional)
+
+	// Slice - not comparable
+	assert.Equal(t, "Slice", typeTest.Fields[3].Name)
+	assert.Equal(t, "[]string", typeTest.Fields[3].TypeName)
+	assert.False(t, typeTest.Fields[3].IsOptional)
+	assert.False(t, typeTest.Fields[3].IsComparable, "slice should not be comparable")
+
+	// Map - not comparable
+	assert.Equal(t, "Map", typeTest.Fields[4].Name)
+	assert.Equal(t, "map[string]int", typeTest.Fields[4].TypeName)
+	assert.False(t, typeTest.Fields[4].IsOptional)
+	assert.False(t, typeTest.Fields[4].IsComparable, "map should not be comparable")
+
+	// Channel - comparable (note: getTypeName returns "any" for channel types, but isComparableType correctly identifies them)
+	assert.Equal(t, "Channel", typeTest.Fields[5].Name)
+	assert.Equal(t, "any", typeTest.Fields[5].TypeName) // getTypeName doesn't handle chan types specifically
+	assert.False(t, typeTest.Fields[5].IsOptional)
+	assert.True(t, typeTest.Fields[5].IsComparable, "channel should be comparable")
+}
+
+func TestLensRefTemplatesWithComparable(t *testing.T) {
+	s := structInfo{
+		Name: "TestStruct",
+		Fields: []fieldInfo{
+			{Name: "Name", TypeName: "string", IsOptional: false, IsComparable: true},
+			{Name: "Age", TypeName: "int", IsOptional: false, IsComparable: true},
+			{Name: "Data", TypeName: "[]byte", IsOptional: false, IsComparable: false},
+			{Name: "Pointer", TypeName: "*string", IsOptional: true, IsComparable: false},
+		},
+	}
+
+	// Test constructor template for RefLenses
+	var constructorBuf bytes.Buffer
+	err := constructorTmpl.Execute(&constructorBuf, s)
+	require.NoError(t, err)
+
+	constructorStr := constructorBuf.String()
+
+	// Check that MakeLensStrict is used for comparable types in RefLenses
+	assert.Contains(t, constructorStr, "func MakeTestStructRefLenses() TestStructRefLenses")
+
+	// Name field - comparable, should use MakeLensStrict
+	assert.Contains(t, constructorStr, "lensName := L.MakeLensStrict(",
+		"comparable field Name should use MakeLensStrict in RefLenses")
+
+	// Age field - comparable, should use MakeLensStrict
+	assert.Contains(t, constructorStr, "lensAge := L.MakeLensStrict(",
+		"comparable field Age should use MakeLensStrict in RefLenses")
+
+	// Data field - not comparable, should use MakeLensRef
+	assert.Contains(t, constructorStr, "lensData := L.MakeLensRef(",
+		"non-comparable field Data should use MakeLensRef in RefLenses")
+
+}
+
+func TestGenerateLensHelpersWithComparable(t *testing.T) {
+	// Create a temporary directory with test files
+	tmpDir := t.TempDir()
+
+	testCode := `package testpkg
+
+// fp-go:Lens
+type TestStruct struct {
+	Name  string
+	Count int
+	Data  []byte
+}
+`
+
+	testFile := filepath.Join(tmpDir, "test.go")
+	err := os.WriteFile(testFile, []byte(testCode), 0644)
+	require.NoError(t, err)
+
+	// Generate lens code
+	outputFile := "gen.go"
+	err = generateLensHelpers(tmpDir, outputFile, false)
+	require.NoError(t, err)
+
+	// Verify the generated file exists
+	genPath := filepath.Join(tmpDir, outputFile)
+	_, err = os.Stat(genPath)
+	require.NoError(t, err)
+
+	// Read and verify the generated content
+	content, err := os.ReadFile(genPath)
+	require.NoError(t, err)
+
+	contentStr := string(content)
+
+	// Check for expected content in RefLenses
+	assert.Contains(t, contentStr, "MakeTestStructRefLenses")
+
+	// Name and Count are comparable, should use MakeLensStrict
+	assert.Contains(t, contentStr, "L.MakeLensStrict",
+		"comparable fields should use MakeLensStrict in RefLenses")
+
+	// Data is not comparable (slice), should use MakeLensRef
+	assert.Contains(t, contentStr, "L.MakeLensRef",
+		"non-comparable fields should use MakeLensRef in RefLenses")
+
+	// Verify the pattern appears for Name field (comparable)
+	namePattern := "lensName := L.MakeLensStrict("
+	assert.Contains(t, contentStr, namePattern,
+		"Name field should use MakeLensStrict")
+
+	// Verify the pattern appears for Data field (not comparable)
+	dataPattern := "lensData := L.MakeLensRef("
+	assert.Contains(t, contentStr, dataPattern,
+		"Data field should use MakeLensRef")
+}
+
 func TestGenerateLensHelpers(t *testing.T) {
 	// Create a temporary directory with test files
 	tmpDir := t.TempDir()
@@ -373,11 +619,11 @@ type TestStruct struct {
 	// Check for expected content
 	assert.Contains(t, contentStr, "package testpkg")
 	assert.Contains(t, contentStr, "Code generated by go generate")
-	assert.Contains(t, contentStr, "TestStructLens")
+	assert.Contains(t, contentStr, "TestStructLenses")
-	assert.Contains(t, contentStr, "MakeTestStructLens")
+	assert.Contains(t, contentStr, "MakeTestStructLenses")
 	assert.Contains(t, contentStr, "L.Lens[TestStruct, string]")
 	assert.Contains(t, contentStr, "LO.LensO[TestStruct, *int]")
-	assert.Contains(t, contentStr, "I.FromZero")
+	assert.Contains(t, contentStr, "IO.FromZero")
 }
 
 func TestGenerateLensHelpersNoAnnotations(t *testing.T) {
@@ -411,8 +657,8 @@ func TestLensTemplates(t *testing.T) {
 	s := structInfo{
 		Name: "TestStruct",
 		Fields: []fieldInfo{
-			{Name: "Name", TypeName: "string", IsOptional: false},
+			{Name: "Name", TypeName: "string", IsOptional: false, IsComparable: true},
-			{Name: "Value", TypeName: "*int", IsOptional: true},
+			{Name: "Value", TypeName: "*int", IsOptional: true, IsComparable: true},
 		},
 	}
 
@@ -424,7 +670,9 @@ func TestLensTemplates(t *testing.T) {
 	structStr := structBuf.String()
 	assert.Contains(t, structStr, "type TestStructLenses struct")
 	assert.Contains(t, structStr, "Name L.Lens[TestStruct, string]")
-	assert.Contains(t, structStr, "Value LO.LensO[TestStruct, *int]")
+	assert.Contains(t, structStr, "NameO LO.LensO[TestStruct, string]")
+	assert.Contains(t, structStr, "Value L.Lens[TestStruct, *int]")
+	assert.Contains(t, structStr, "ValueO LO.LensO[TestStruct, *int]")
 
 	// Test constructor template
 	var constructorBuf bytes.Buffer
@@ -434,19 +682,21 @@ func TestLensTemplates(t *testing.T) {
 	constructorStr := constructorBuf.String()
 	assert.Contains(t, constructorStr, "func MakeTestStructLenses() TestStructLenses")
 	assert.Contains(t, constructorStr, "return TestStructLenses{")
-	assert.Contains(t, constructorStr, "Name: L.MakeLens(")
+	assert.Contains(t, constructorStr, "Name: lensName,")
-	assert.Contains(t, constructorStr, "Value: L.MakeLens(")
+	assert.Contains(t, constructorStr, "NameO: lensNameO,")
-	assert.Contains(t, constructorStr, "I.FromZero")
+	assert.Contains(t, constructorStr, "Value: lensValue,")
+	assert.Contains(t, constructorStr, "ValueO: lensValueO,")
+	assert.Contains(t, constructorStr, "IO.FromZero")
 }
 
 func TestLensTemplatesWithOmitEmpty(t *testing.T) {
 	s := structInfo{
 		Name: "ConfigStruct",
 		Fields: []fieldInfo{
-			{Name: "Name", TypeName: "string", IsOptional: false},
+			{Name: "Name", TypeName: "string", IsOptional: false, IsComparable: true},
-			{Name: "Value", TypeName: "string", IsOptional: true},    // non-pointer with omitempty
+			{Name: "Value", TypeName: "string", IsOptional: true, IsComparable: true},    // non-pointer with omitempty
-			{Name: "Count", TypeName: "int", IsOptional: true},       // non-pointer with omitempty
+			{Name: "Count", TypeName: "int", IsOptional: true, IsComparable: true},       // non-pointer with omitempty
-			{Name: "Pointer", TypeName: "*string", IsOptional: true}, // pointer
+			{Name: "Pointer", TypeName: "*string", IsOptional: true, IsComparable: true}, // pointer
 		},
 	}
 
@@ -458,9 +708,13 @@ func TestLensTemplatesWithOmitEmpty(t *testing.T) {
 	structStr := structBuf.String()
 	assert.Contains(t, structStr, "type ConfigStructLenses struct")
 	assert.Contains(t, structStr, "Name L.Lens[ConfigStruct, string]")
-	assert.Contains(t, structStr, "Value LO.LensO[ConfigStruct, string]", "non-pointer with omitempty should use LensO")
+	assert.Contains(t, structStr, "NameO LO.LensO[ConfigStruct, string]")
-	assert.Contains(t, structStr, "Count LO.LensO[ConfigStruct, int]", "non-pointer with omitempty should use LensO")
+	assert.Contains(t, structStr, "Value L.Lens[ConfigStruct, string]")
-	assert.Contains(t, structStr, "Pointer LO.LensO[ConfigStruct, *string]")
+	assert.Contains(t, structStr, "ValueO LO.LensO[ConfigStruct, string]", "comparable non-pointer with omitempty should have optional lens")
+	assert.Contains(t, structStr, "Count L.Lens[ConfigStruct, int]")
+	assert.Contains(t, structStr, "CountO LO.LensO[ConfigStruct, int]", "comparable non-pointer with omitempty should have optional lens")
+	assert.Contains(t, structStr, "Pointer L.Lens[ConfigStruct, *string]")
+	assert.Contains(t, structStr, "PointerO LO.LensO[ConfigStruct, *string]")
 
 	// Test constructor template
 	var constructorBuf bytes.Buffer
@@ -469,9 +723,9 @@ func TestLensTemplatesWithOmitEmpty(t *testing.T) {
 
 	constructorStr := constructorBuf.String()
 	assert.Contains(t, constructorStr, "func MakeConfigStructLenses() ConfigStructLenses")
-	assert.Contains(t, constructorStr, "isoValue := I.FromZero[string]()")
+	assert.Contains(t, constructorStr, "IO.FromZero[string]()")
-	assert.Contains(t, constructorStr, "isoCount := I.FromZero[int]()")
+	assert.Contains(t, constructorStr, "IO.FromZero[int]()")
-	assert.Contains(t, constructorStr, "isoPointer := I.FromZero[*string]()")
+	assert.Contains(t, constructorStr, "IO.FromZero[*string]()")
 }
 
 func TestLensCommandFlags(t *testing.T) {
@@ -480,7 +734,7 @@ func TestLensCommandFlags(t *testing.T) {
 	assert.Equal(t, "lens", cmd.Name)
 	assert.Equal(t, "generate lens code for annotated structs", cmd.Usage)
 	assert.Contains(t, strings.ToLower(cmd.Description), "fp-go:lens")
-	assert.Contains(t, strings.ToLower(cmd.Description), "lenso")
+	assert.Contains(t, strings.ToLower(cmd.Description), "lenso", "Description should mention LensO for optional lenses")
 
 	// Check flags
 	assert.Len(t, cmd.Flags, 3)
@@ -501,3 +755,330 @@ func TestLensCommandFlags(t *testing.T) {
 	assert.True(t, hasFilename, "should have filename flag")
 	assert.True(t, hasVerbose, "should have verbose flag")
 }
+
+func TestParseFileWithEmbeddedStruct(t *testing.T) {
+	// Create a temporary test file
+	tmpDir := t.TempDir()
+	testFile := filepath.Join(tmpDir, "test.go")
+
+	testCode := `package testpkg
+
+// Base struct to be embedded
+type Base struct {
+	ID   int
+	Name string
+}
+
+// fp-go:Lens
+type Extended struct {
+	Base
+	Extra string
+}
+`
+
+	err := os.WriteFile(testFile, []byte(testCode), 0644)
+	require.NoError(t, err)
+
+	// Parse the file
+	structs, pkg, err := parseFile(testFile)
+	require.NoError(t, err)
+
+	// Verify results
+	assert.Equal(t, "testpkg", pkg)
+	assert.Len(t, structs, 1)
+
+	// Check Extended struct
+	extended := structs[0]
+	assert.Equal(t, "Extended", extended.Name)
+	assert.Len(t, extended.Fields, 3, "Should have 3 fields: ID, Name (from Base), and Extra")
+
+	// Check that embedded fields are promoted
+	fieldNames := make(map[string]bool)
+	for _, field := range extended.Fields {
+		fieldNames[field.Name] = true
+	}
+
+	assert.True(t, fieldNames["ID"], "Should have promoted ID field from Base")
+	assert.True(t, fieldNames["Name"], "Should have promoted Name field from Base")
+	assert.True(t, fieldNames["Extra"], "Should have Extra field")
+}
+
+func TestGenerateLensHelpersWithEmbeddedStruct(t *testing.T) {
+	// Create a temporary directory with test files
+	tmpDir := t.TempDir()
+
+	testCode := `package testpkg
+
+// Base struct to be embedded
+type Address struct {
+	Street string
+	City   string
+}
+
+// fp-go:Lens
+type Person struct {
+	Address
+	Name string
+	Age  int
+}
+`
+
+	testFile := filepath.Join(tmpDir, "test.go")
+	err := os.WriteFile(testFile, []byte(testCode), 0644)
+	require.NoError(t, err)
+
+	// Generate lens code
+	outputFile := "gen.go"
+	err = generateLensHelpers(tmpDir, outputFile, false)
+	require.NoError(t, err)
+
+	// Verify the generated file exists
+	genPath := filepath.Join(tmpDir, outputFile)
+	_, err = os.Stat(genPath)
+	require.NoError(t, err)
+
+	// Read and verify the generated content
+	content, err := os.ReadFile(genPath)
+	require.NoError(t, err)
+
+	contentStr := string(content)
+
+	// Check for expected content
+	assert.Contains(t, contentStr, "package testpkg")
+	assert.Contains(t, contentStr, "PersonLenses")
+	assert.Contains(t, contentStr, "MakePersonLenses")
+
+	// Check that embedded fields are included
+	assert.Contains(t, contentStr, "Street L.Lens[Person, string]", "Should have lens for embedded Street field")
+	assert.Contains(t, contentStr, "City L.Lens[Person, string]", "Should have lens for embedded City field")
+	assert.Contains(t, contentStr, "Name L.Lens[Person, string]", "Should have lens for Name field")
+	assert.Contains(t, contentStr, "Age L.Lens[Person, int]", "Should have lens for Age field")
+
+	// Check that optional lenses are also generated for embedded fields
+	assert.Contains(t, contentStr, "StreetO LO.LensO[Person, string]")
+	assert.Contains(t, contentStr, "CityO LO.LensO[Person, string]")
+}
+
+func TestParseFileWithPointerEmbeddedStruct(t *testing.T) {
+	// Create a temporary test file
+	tmpDir := t.TempDir()
+	testFile := filepath.Join(tmpDir, "test.go")
+
+	testCode := `package testpkg
+
+// Base struct to be embedded
+type Metadata struct {
+	CreatedAt string
+	UpdatedAt string
+}
+
+// fp-go:Lens
+type Document struct {
+	*Metadata
+	Title   string
+	Content string
+}
+`
+
+	err := os.WriteFile(testFile, []byte(testCode), 0644)
+	require.NoError(t, err)
+
+	// Parse the file
+	structs, pkg, err := parseFile(testFile)
+	require.NoError(t, err)
+
+	// Verify results
+	assert.Equal(t, "testpkg", pkg)
+	assert.Len(t, structs, 1)
+
+	// Check Document struct
+	doc := structs[0]
+	assert.Equal(t, "Document", doc.Name)
+	assert.Len(t, doc.Fields, 4, "Should have 4 fields: CreatedAt, UpdatedAt (from *Metadata), Title, and Content")
+
+	// Check that embedded fields are promoted
+	fieldNames := make(map[string]bool)
+	for _, field := range doc.Fields {
+		fieldNames[field.Name] = true
+	}
+
+	assert.True(t, fieldNames["CreatedAt"], "Should have promoted CreatedAt field from *Metadata")
+	assert.True(t, fieldNames["UpdatedAt"], "Should have promoted UpdatedAt field from *Metadata")
+	assert.True(t, fieldNames["Title"], "Should have Title field")
+	assert.True(t, fieldNames["Content"], "Should have Content field")
+}
+
+func TestParseFileWithGenericStruct(t *testing.T) {
+	// Create a temporary test file
+	tmpDir := t.TempDir()
+	testFile := filepath.Join(tmpDir, "test.go")
+
+	testCode := `package testpkg
+
+// fp-go:Lens
+type Container[T any] struct {
+	Value T
+	Count int
+}
+`
+
+	err := os.WriteFile(testFile, []byte(testCode), 0644)
+	require.NoError(t, err)
+
+	// Parse the file
+	structs, pkg, err := parseFile(testFile)
+	require.NoError(t, err)
+
+	// Verify results
+	assert.Equal(t, "testpkg", pkg)
+	assert.Len(t, structs, 1)
+
+	// Check Container struct
+	container := structs[0]
+	assert.Equal(t, "Container", container.Name)
+	assert.Equal(t, "[T any]", container.TypeParams, "Should have type parameter [T any]")
+	assert.Len(t, container.Fields, 2)
+
+	assert.Equal(t, "Value", container.Fields[0].Name)
+	assert.Equal(t, "T", container.Fields[0].TypeName)
+
+	assert.Equal(t, "Count", container.Fields[1].Name)
+	assert.Equal(t, "int", container.Fields[1].TypeName)
+}
+
+func TestParseFileWithMultipleTypeParams(t *testing.T) {
+	// Create a temporary test file
+	tmpDir := t.TempDir()
+	testFile := filepath.Join(tmpDir, "test.go")
+
+	testCode := `package testpkg
+
+// fp-go:Lens
+type Pair[K comparable, V any] struct {
+	Key   K
+	Value V
+}
+`
+
+	err := os.WriteFile(testFile, []byte(testCode), 0644)
+	require.NoError(t, err)
+
+	// Parse the file
+	structs, pkg, err := parseFile(testFile)
+	require.NoError(t, err)
+
+	// Verify results
+	assert.Equal(t, "testpkg", pkg)
+	assert.Len(t, structs, 1)
+
+	// Check Pair struct
+	pair := structs[0]
+	assert.Equal(t, "Pair", pair.Name)
+	assert.Equal(t, "[K comparable, V any]", pair.TypeParams, "Should have type parameters [K comparable, V any]")
+	assert.Len(t, pair.Fields, 2)
+
+	assert.Equal(t, "Key", pair.Fields[0].Name)
+	assert.Equal(t, "K", pair.Fields[0].TypeName)
+
+	assert.Equal(t, "Value", pair.Fields[1].Name)
+	assert.Equal(t, "V", pair.Fields[1].TypeName)
+}
+
+func TestGenerateLensHelpersWithGenericStruct(t *testing.T) {
+	// Create a temporary directory with test files
+	tmpDir := t.TempDir()
+
+	testCode := `package testpkg
+
+// fp-go:Lens
+type Box[T any] struct {
+	Content T
+	Label   string
+}
+`
+
+	testFile := filepath.Join(tmpDir, "test.go")
+	err := os.WriteFile(testFile, []byte(testCode), 0644)
+	require.NoError(t, err)
+
+	// Generate lens code
+	outputFile := "gen.go"
+	err = generateLensHelpers(tmpDir, outputFile, false)
+	require.NoError(t, err)
+
+	// Verify the generated file exists
+	genPath := filepath.Join(tmpDir, outputFile)
+	_, err = os.Stat(genPath)
+	require.NoError(t, err)
+
+	// Read and verify the generated content
+	content, err := os.ReadFile(genPath)
+	require.NoError(t, err)
+
+	contentStr := string(content)
+
+	// Check for expected content with type parameters
+	assert.Contains(t, contentStr, "package testpkg")
+	assert.Contains(t, contentStr, "type BoxLenses[T any] struct", "Should have generic BoxLenses type")
+	assert.Contains(t, contentStr, "type BoxRefLenses[T any] struct", "Should have generic BoxRefLenses type")
+	assert.Contains(t, contentStr, "func MakeBoxLenses[T any]() BoxLenses[T]", "Should have generic constructor")
+	assert.Contains(t, contentStr, "func MakeBoxRefLenses[T any]() BoxRefLenses[T]", "Should have generic ref constructor")
+
+	// Check that fields use the generic type parameter
+	assert.Contains(t, contentStr, "Content L.Lens[Box[T], T]", "Should have lens for generic Content field")
+	assert.Contains(t, contentStr, "Label L.Lens[Box[T], string]", "Should have lens for Label field")
+
+	// Check optional lenses - only for comparable types
+	// T any is not comparable, so ContentO should NOT be generated
+	assert.NotContains(t, contentStr, "ContentO LO.LensO[Box[T], T]", "T any is not comparable, should not have optional lens")
+	// string is comparable, so LabelO should be generated
+	assert.Contains(t, contentStr, "LabelO LO.LensO[Box[T], string]", "string is comparable, should have optional lens")
+}
+
+func TestGenerateLensHelpersWithComparableTypeParam(t *testing.T) {
+	// Create a temporary directory with test files
+	tmpDir := t.TempDir()
+
+	testCode := `package testpkg
+
+// fp-go:Lens
+type ComparableBox[T comparable] struct {
+	Key   T
+	Value string
+}
+`
+
+	testFile := filepath.Join(tmpDir, "test.go")
+	err := os.WriteFile(testFile, []byte(testCode), 0644)
+	require.NoError(t, err)
+
+	// Generate lens code
+	outputFile := "gen.go"
+	err = generateLensHelpers(tmpDir, outputFile, false)
+	require.NoError(t, err)
+
+	// Verify the generated file exists
+	genPath := filepath.Join(tmpDir, outputFile)
+	_, err = os.Stat(genPath)
+	require.NoError(t, err)
+
+	// Read and verify the generated content
+	content, err := os.ReadFile(genPath)
+	require.NoError(t, err)
+
+	contentStr := string(content)
+
+	// Check for expected content with type parameters
+	assert.Contains(t, contentStr, "package testpkg")
+	assert.Contains(t, contentStr, "type ComparableBoxLenses[T comparable] struct", "Should have generic ComparableBoxLenses type")
+	assert.Contains(t, contentStr, "type ComparableBoxRefLenses[T comparable] struct", "Should have generic ComparableBoxRefLenses type")
+
+	// Check that Key field (with comparable constraint) uses MakeLensStrict in RefLenses
+	assert.Contains(t, contentStr, "lensKey := L.MakeLensStrict(", "Key field with comparable constraint should use MakeLensStrict")
+
+	// Check that Value field (string, always comparable) also uses MakeLensStrict
+	assert.Contains(t, contentStr, "lensValue := L.MakeLensStrict(", "Value field (string) should use MakeLensStrict")
+
+	// Verify that MakeLensRef is NOT used (since both fields are comparable)
+	assert.NotContains(t, contentStr, "L.MakeLensRef(", "Should not use MakeLensRef when all fields are comparable")
+}

v2/constant/monoid.go

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

v2/context/readerioresult/http/builder/builder.go

@@ -53,12 +53,12 @@ import (
 
 	RIOE "github.com/IBM/fp-go/v2/context/readerioresult"
 	RIOEH "github.com/IBM/fp-go/v2/context/readerioresult/http"
-	E "github.com/IBM/fp-go/v2/either"
 	F "github.com/IBM/fp-go/v2/function"
 	R "github.com/IBM/fp-go/v2/http/builder"
 	H "github.com/IBM/fp-go/v2/http/headers"
 	LZ "github.com/IBM/fp-go/v2/lazy"
 	O "github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/result"
 )
 
 // Requester converts an http/builder.Builder into a ReaderIOResult that produces HTTP requests.
@@ -143,10 +143,10 @@ func Requester(builder *R.Builder) RIOEH.Requester {
 
 	return F.Pipe5(
 		builder.GetBody(),
-		O.Fold(LZ.Of(E.Of[error](withoutBody)), E.Map[error](withBody)),
+		O.Fold(LZ.Of(result.Of(withoutBody)), result.Map(withBody)),
-		E.Ap[func(string) RIOE.ReaderIOResult[*http.Request]](builder.GetTargetURL()),
+		result.Ap[RIOE.Kleisli[string, *http.Request]](builder.GetTargetURL()),
-		E.Flap[error, RIOE.ReaderIOResult[*http.Request]](builder.GetMethod()),
+		result.Flap[RIOE.ReaderIOResult[*http.Request]](builder.GetMethod()),
-		E.GetOrElse(RIOE.Left[*http.Request]),
+		result.GetOrElse(RIOE.Left[*http.Request]),
 		RIOE.Map(func(req *http.Request) *http.Request {
 			req.Header = H.Monoid.Concat(req.Header, builder.GetHeaders())
 			return req

v2/context/readerioresult/reader.go

@@ -19,13 +19,17 @@ import (
 	"context"
 	"time"
 
+	"github.com/IBM/fp-go/v2/context/readerresult"
 	"github.com/IBM/fp-go/v2/either"
 	"github.com/IBM/fp-go/v2/errors"
 	"github.com/IBM/fp-go/v2/function"
 	"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/reader"
+	"github.com/IBM/fp-go/v2/readerio"
 	RIOR "github.com/IBM/fp-go/v2/readerioresult"
+	"github.com/IBM/fp-go/v2/readeroption"
 )
 
 const (
@@ -176,6 +180,11 @@ func MonadChainFirst[A, B any](ma ReaderIOResult[A], f Kleisli[A, B]) ReaderIORe
 	return RIOR.MonadChainFirst(ma, f)
 }
 
+//go:inline
+func MonadTap[A, B any](ma ReaderIOResult[A], f Kleisli[A, B]) ReaderIOResult[A] {
+	return RIOR.MonadTap(ma, f)
+}
+
 // ChainFirst sequences two [ReaderIOResult] computations but returns the result of the first.
 // This is the curried version of [MonadChainFirst].
 //
@@ -189,6 +198,11 @@ func ChainFirst[A, B any](f Kleisli[A, B]) Operator[A, A] {
 	return RIOR.ChainFirst(f)
 }
 
+//go:inline
+func Tap[A, B any](f Kleisli[A, B]) Operator[A, A] {
+	return RIOR.Tap(f)
+}
+
 // Of creates a [ReaderIOResult] that always succeeds with the given value.
 // This is the same as [Right] and represents the monadic return operation.
 //
@@ -399,6 +413,11 @@ func MonadChainFirstEitherK[A, B any](ma ReaderIOResult[A], f func(A) Either[B])
 	return RIOR.MonadChainFirstEitherK(ma, f)
 }
 
+//go:inline
+func MonadTapEitherK[A, B any](ma ReaderIOResult[A], f func(A) Either[B]) ReaderIOResult[A] {
+	return RIOR.MonadTapEitherK(ma, f)
+}
+
 // ChainFirstEitherK chains a function that returns an [Either] but keeps the original value.
 // This is the curried version of [MonadChainFirstEitherK].
 //
@@ -412,6 +431,11 @@ func ChainFirstEitherK[A, B any](f func(A) Either[B]) Operator[A, A] {
 	return RIOR.ChainFirstEitherK[context.Context](f)
 }
 
+//go:inline
+func TapEitherK[A, B any](f func(A) Either[B]) Operator[A, A] {
+	return RIOR.TapEitherK[context.Context](f)
+}
+
 // ChainOptionK chains a function that returns an [Option] into a [ReaderIOResult] computation.
 // If the Option is None, the provided error function is called.
 //
@@ -534,6 +558,11 @@ func MonadChainFirstIOK[A, B any](ma ReaderIOResult[A], f func(A) IO[B]) ReaderI
 	return RIOR.MonadChainFirstIOK(ma, f)
 }
 
+//go:inline
+func MonadTapIOK[A, B any](ma ReaderIOResult[A], f func(A) IO[B]) ReaderIOResult[A] {
+	return RIOR.MonadTapIOK(ma, f)
+}
+
 // ChainFirstIOK chains a function that returns an [IO] but keeps the original value.
 // This is the curried version of [MonadChainFirstIOK].
 //
@@ -547,6 +576,11 @@ func ChainFirstIOK[A, B any](f func(A) IO[B]) Operator[A, A] {
 	return RIOR.ChainFirstIOK[context.Context](f)
 }
 
+//go:inline
+func TapIOK[A, B any](f func(A) IO[B]) Operator[A, A] {
+	return RIOR.TapIOK[context.Context](f)
+}
+
 // ChainIOEitherK chains a function that returns an [IOResult] into a [ReaderIOResult] computation.
 // This is useful for integrating IOResult-returning functions into ReaderIOResult workflows.
 //
@@ -624,7 +658,7 @@ func Defer[A any](gen Lazy[ReaderIOResult[A]]) ReaderIOResult[A] {
 //
 //go:inline
 func TryCatch[A any](f func(context.Context) func() (A, error)) ReaderIOResult[A] {
-	return RIOR.TryCatch(f, errors.IdentityError)
+	return RIOR.TryCatch(f, errors.Identity)
 }
 
 // MonadAlt provides an alternative [ReaderIOResult] if the first one fails.
@@ -747,3 +781,180 @@ func GetOrElse[A any](onLeft func(error) ReaderIO[A]) func(ReaderIOResult[A]) Re
 func OrLeft[A any](onLeft func(error) ReaderIO[error]) Operator[A, A] {
 	return RIOR.OrLeft[A](onLeft)
 }
+
+//go:inline
+func FromReaderEither[A any](ma ReaderEither[context.Context, error, A]) ReaderIOResult[A] {
+	return RIOR.FromReaderEither(ma)
+}
+
+//go:inline
+func FromReaderResult[A any](ma ReaderResult[A]) ReaderIOResult[A] {
+	return RIOR.FromReaderEither(ma)
+}
+
+//go:inline
+func FromReaderOption[A any](onNone func() error) Kleisli[ReaderOption[context.Context, A], A] {
+	return RIOR.FromReaderOption[context.Context, A](onNone)
+}
+
+//go:inline
+func MonadChainReaderK[A, B any](ma ReaderIOResult[A], f reader.Kleisli[context.Context, A, B]) ReaderIOResult[B] {
+	return RIOR.MonadChainReaderK(ma, f)
+}
+
+//go:inline
+func ChainReaderK[A, B any](f reader.Kleisli[context.Context, A, B]) Operator[A, B] {
+	return RIOR.ChainReaderK(f)
+}
+
+//go:inline
+func MonadChainFirstReaderK[A, B any](ma ReaderIOResult[A], f reader.Kleisli[context.Context, A, B]) ReaderIOResult[A] {
+	return RIOR.MonadChainFirstReaderK(ma, f)
+}
+
+//go:inline
+func MonadTapReaderK[A, B any](ma ReaderIOResult[A], f reader.Kleisli[context.Context, A, B]) ReaderIOResult[A] {
+	return RIOR.MonadTapReaderK(ma, f)
+}
+
+//go:inline
+func ChainFirstReaderK[A, B any](f reader.Kleisli[context.Context, A, B]) Operator[A, A] {
+	return RIOR.ChainFirstReaderK(f)
+}
+
+//go:inline
+func TapReaderK[A, B any](f reader.Kleisli[context.Context, A, B]) Operator[A, A] {
+	return RIOR.TapReaderK(f)
+}
+
+//go:inline
+func MonadChainReaderResultK[A, B any](ma ReaderIOResult[A], f readerresult.Kleisli[A, B]) ReaderIOResult[B] {
+	return RIOR.MonadChainReaderResultK(ma, f)
+}
+
+//go:inline
+func ChainReaderResultK[A, B any](f readerresult.Kleisli[A, B]) Operator[A, B] {
+	return RIOR.ChainReaderResultK(f)
+}
+
+//go:inline
+func MonadChainFirstReaderResultK[A, B any](ma ReaderIOResult[A], f readerresult.Kleisli[A, B]) ReaderIOResult[A] {
+	return RIOR.MonadChainFirstReaderResultK(ma, f)
+}
+
+//go:inline
+func MonadTapReaderResultK[A, B any](ma ReaderIOResult[A], f readerresult.Kleisli[A, B]) ReaderIOResult[A] {
+	return RIOR.MonadTapReaderResultK(ma, f)
+}
+
+//go:inline
+func ChainFirstReaderResultK[A, B any](f readerresult.Kleisli[A, B]) Operator[A, A] {
+	return RIOR.ChainFirstReaderResultK(f)
+}
+
+//go:inline
+func TapReaderResultK[A, B any](f readerresult.Kleisli[A, B]) Operator[A, A] {
+	return RIOR.TapReaderResultK(f)
+}
+
+//go:inline
+func MonadChainReaderIOK[A, B any](ma ReaderIOResult[A], f readerio.Kleisli[context.Context, A, B]) ReaderIOResult[B] {
+	return RIOR.MonadChainReaderIOK(ma, f)
+}
+
+//go:inline
+func ChainReaderIOK[A, B any](f readerio.Kleisli[context.Context, A, B]) Operator[A, B] {
+	return RIOR.ChainReaderIOK(f)
+}
+
+//go:inline
+func MonadChainFirstReaderIOK[A, B any](ma ReaderIOResult[A], f readerio.Kleisli[context.Context, A, B]) ReaderIOResult[A] {
+	return RIOR.MonadChainFirstReaderIOK(ma, f)
+}
+
+//go:inline
+func MonadTapReaderIOK[A, B any](ma ReaderIOResult[A], f readerio.Kleisli[context.Context, A, B]) ReaderIOResult[A] {
+	return RIOR.MonadTapReaderIOK(ma, f)
+}
+
+//go:inline
+func ChainFirstReaderIOK[A, B any](f readerio.Kleisli[context.Context, A, B]) Operator[A, A] {
+	return RIOR.ChainFirstReaderIOK(f)
+}
+
+//go:inline
+func TapReaderIOK[A, B any](f readerio.Kleisli[context.Context, A, B]) Operator[A, A] {
+	return RIOR.TapReaderIOK(f)
+}
+
+//go:inline
+func ChainReaderOptionK[A, B any](onNone func() error) func(readeroption.Kleisli[context.Context, A, B]) Operator[A, B] {
+	return RIOR.ChainReaderOptionK[context.Context, A, B](onNone)
+}
+
+//go:inline
+func ChainFirstReaderOptionK[A, B any](onNone func() error) func(readeroption.Kleisli[context.Context, A, B]) Operator[A, A] {
+	return RIOR.ChainFirstReaderOptionK[context.Context, A, B](onNone)
+}
+
+//go:inline
+func TapReaderOptionK[A, B any](onNone func() error) func(readeroption.Kleisli[context.Context, A, B]) Operator[A, A] {
+	return RIOR.TapReaderOptionK[context.Context, A, B](onNone)
+}
+
+//go:inline
+func Read[A any](r context.Context) func(ReaderIOResult[A]) IOResult[A] {
+	return RIOR.Read[A](r)
+}
+
+// MonadChainLeft chains a computation on the left (error) side of a [ReaderIOResult].
+// If the input is a Left value, it applies the function f to transform the error and potentially
+// change the error type. If the input is a Right value, it passes through unchanged.
+//
+//go:inline
+func MonadChainLeft[A any](fa ReaderIOResult[A], f Kleisli[error, A]) ReaderIOResult[A] {
+	return RIOR.MonadChainLeft(fa, f)
+}
+
+// ChainLeft is the curried version of [MonadChainLeft].
+// It returns a function that chains a computation on the left (error) side of a [ReaderIOResult].
+//
+//go:inline
+func ChainLeft[A any](f Kleisli[error, A]) func(ReaderIOResult[A]) ReaderIOResult[A] {
+	return RIOR.ChainLeft(f)
+}
+
+// MonadChainFirstLeft chains a computation on the left (error) side but always returns the original error.
+// If the input is a Left value, it applies the function f to the error and executes the resulting computation,
+// but always returns the original Left error regardless of what f returns (Left or Right).
+// If the input is a Right value, it passes through unchanged without calling f.
+//
+// This is useful for side effects on errors (like logging or metrics) where you want to perform an action
+// when an error occurs but always propagate the original error, ensuring the error path is preserved.
+//
+//go:inline
+func MonadChainFirstLeft[A, B any](ma ReaderIOResult[A], f Kleisli[error, B]) ReaderIOResult[A] {
+	return RIOR.MonadChainFirstLeft(ma, f)
+}
+
+//go:inline
+func MonadTapLeft[A, B any](ma ReaderIOResult[A], f Kleisli[error, B]) ReaderIOResult[A] {
+	return RIOR.MonadTapLeft(ma, f)
+}
+
+// ChainFirstLeft is the curried version of [MonadChainFirstLeft].
+// It returns a function that chains a computation on the left (error) side while always preserving the original error.
+//
+// This is particularly useful for adding error handling side effects (like logging, metrics, or notifications)
+// in a functional pipeline. The original error is always returned regardless of what f returns (Left or Right),
+// ensuring the error path is preserved.
+//
+//go:inline
+func ChainFirstLeft[A, B any](f Kleisli[error, B]) Operator[A, A] {
+	return RIOR.ChainFirstLeft[A](f)
+}
+
+//go:inline
+func TapLeft[A, B any](f Kleisli[error, B]) Operator[A, A] {
+	return RIOR.TapLeft[A](f)
+}

v2/context/readerioresult/reader_bench_test.go

@@ -24,6 +24,7 @@ import (
 	E "github.com/IBM/fp-go/v2/either"
 	F "github.com/IBM/fp-go/v2/function"
 	IOE "github.com/IBM/fp-go/v2/ioeither"
+	N "github.com/IBM/fp-go/v2/number"
 )
 
 var (
@@ -37,21 +38,21 @@ var (
 // Benchmark core constructors
 func BenchmarkLeft(b *testing.B) {
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = Left[int](benchErr)
 	}
 }
 
 func BenchmarkRight(b *testing.B) {
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = Right(42)
 	}
 }
 
 func BenchmarkOf(b *testing.B) {
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = Of(42)
 	}
 }
@@ -60,7 +61,7 @@ func BenchmarkFromEither_Right(b *testing.B) {
 	either := E.Right[error](42)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = FromEither(either)
 	}
 }
@@ -69,7 +70,7 @@ func BenchmarkFromEither_Left(b *testing.B) {
 	either := E.Left[int](benchErr)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = FromEither(either)
 	}
 }
@@ -77,7 +78,7 @@ func BenchmarkFromEither_Left(b *testing.B) {
 func BenchmarkFromIO(b *testing.B) {
 	io := func() int { return 42 }
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = FromIO(io)
 	}
 }
@@ -85,7 +86,7 @@ func BenchmarkFromIO(b *testing.B) {
 func BenchmarkFromIOEither_Right(b *testing.B) {
 	ioe := IOE.Of[error](42)
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = FromIOEither(ioe)
 	}
 }
@@ -93,7 +94,7 @@ func BenchmarkFromIOEither_Right(b *testing.B) {
 func BenchmarkFromIOEither_Left(b *testing.B) {
 	ioe := IOE.Left[int](benchErr)
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = FromIOEither(ioe)
 	}
 }
@@ -103,7 +104,7 @@ func BenchmarkExecute_Right(b *testing.B) {
 	rioe := Right(42)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = rioe(benchCtx)()
 	}
 }
@@ -112,7 +113,7 @@ func BenchmarkExecute_Left(b *testing.B) {
 	rioe := Left[int](benchErr)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = rioe(benchCtx)()
 	}
 }
@@ -123,7 +124,7 @@ func BenchmarkExecute_WithContext(b *testing.B) {
 	defer cancel()
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = rioe(ctx)()
 	}
 }
@@ -131,40 +132,40 @@ func BenchmarkExecute_WithContext(b *testing.B) {
 // Benchmark functor operations
 func BenchmarkMonadMap_Right(b *testing.B) {
 	rioe := Right(42)
-	mapper := func(a int) int { return a * 2 }
+	mapper := N.Mul(2)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = MonadMap(rioe, mapper)
 	}
 }
 
 func BenchmarkMonadMap_Left(b *testing.B) {
 	rioe := Left[int](benchErr)
-	mapper := func(a int) int { return a * 2 }
+	mapper := N.Mul(2)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = MonadMap(rioe, mapper)
 	}
 }
 
 func BenchmarkMap_Right(b *testing.B) {
 	rioe := Right(42)
-	mapper := Map(func(a int) int { return a * 2 })
+	mapper := Map(N.Mul(2))
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = mapper(rioe)
 	}
 }
 
 func BenchmarkMap_Left(b *testing.B) {
 	rioe := Left[int](benchErr)
-	mapper := Map(func(a int) int { return a * 2 })
+	mapper := Map(N.Mul(2))
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = mapper(rioe)
 	}
 }
@@ -174,7 +175,7 @@ func BenchmarkMapTo_Right(b *testing.B) {
 	mapper := MapTo[int](99)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = mapper(rioe)
 	}
 }
@@ -185,7 +186,7 @@ func BenchmarkMonadChain_Right(b *testing.B) {
 	chainer := func(a int) ReaderIOResult[int] { return Right(a * 2) }
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = MonadChain(rioe, chainer)
 	}
 }
@@ -195,7 +196,7 @@ func BenchmarkMonadChain_Left(b *testing.B) {
 	chainer := func(a int) ReaderIOResult[int] { return Right(a * 2) }
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = MonadChain(rioe, chainer)
 	}
 }
@@ -205,7 +206,7 @@ func BenchmarkChain_Right(b *testing.B) {
 	chainer := Chain(func(a int) ReaderIOResult[int] { return Right(a * 2) })
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = chainer(rioe)
 	}
 }
@@ -215,7 +216,7 @@ func BenchmarkChain_Left(b *testing.B) {
 	chainer := Chain(func(a int) ReaderIOResult[int] { return Right(a * 2) })
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = chainer(rioe)
 	}
 }
@@ -225,7 +226,7 @@ func BenchmarkChainFirst_Right(b *testing.B) {
 	chainer := ChainFirst(func(a int) ReaderIOResult[string] { return Right("logged") })
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = chainer(rioe)
 	}
 }
@@ -235,7 +236,7 @@ func BenchmarkChainFirst_Left(b *testing.B) {
 	chainer := ChainFirst(func(a int) ReaderIOResult[string] { return Right("logged") })
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = chainer(rioe)
 	}
 }
@@ -244,7 +245,7 @@ func BenchmarkFlatten_Right(b *testing.B) {
 	nested := Right(Right(42))
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = Flatten(nested)
 	}
 }
@@ -253,28 +254,28 @@ func BenchmarkFlatten_Left(b *testing.B) {
 	nested := Left[ReaderIOResult[int]](benchErr)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = Flatten(nested)
 	}
 }
 
 // Benchmark applicative operations
 func BenchmarkMonadApSeq_RightRight(b *testing.B) {
-	fab := Right(func(a int) int { return a * 2 })
+	fab := Right(N.Mul(2))
 	fa := Right(42)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = MonadApSeq(fab, fa)
 	}
 }
 
 func BenchmarkMonadApSeq_RightLeft(b *testing.B) {
-	fab := Right(func(a int) int { return a * 2 })
+	fab := Right(N.Mul(2))
 	fa := Left[int](benchErr)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = MonadApSeq(fab, fa)
 	}
 }
@@ -284,27 +285,27 @@ func BenchmarkMonadApSeq_LeftRight(b *testing.B) {
 	fa := Right(42)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = MonadApSeq(fab, fa)
 	}
 }
 
 func BenchmarkMonadApPar_RightRight(b *testing.B) {
-	fab := Right(func(a int) int { return a * 2 })
+	fab := Right(N.Mul(2))
 	fa := Right(42)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = MonadApPar(fab, fa)
 	}
 }
 
 func BenchmarkMonadApPar_RightLeft(b *testing.B) {
-	fab := Right(func(a int) int { return a * 2 })
+	fab := Right(N.Mul(2))
 	fa := Left[int](benchErr)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = MonadApPar(fab, fa)
 	}
 }
@@ -314,30 +315,30 @@ func BenchmarkMonadApPar_LeftRight(b *testing.B) {
 	fa := Right(42)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = MonadApPar(fab, fa)
 	}
 }
 
 // Benchmark execution of applicative operations
 func BenchmarkExecuteApSeq_RightRight(b *testing.B) {
-	fab := Right(func(a int) int { return a * 2 })
+	fab := Right(N.Mul(2))
 	fa := Right(42)
 	rioe := MonadApSeq(fab, fa)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = rioe(benchCtx)()
 	}
 }
 
 func BenchmarkExecuteApPar_RightRight(b *testing.B) {
-	fab := Right(func(a int) int { return a * 2 })
+	fab := Right(N.Mul(2))
 	fa := Right(42)
 	rioe := MonadApPar(fab, fa)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = rioe(benchCtx)()
 	}
 }
@@ -348,7 +349,7 @@ func BenchmarkAlt_RightRight(b *testing.B) {
 	alternative := Alt(func() ReaderIOResult[int] { return Right(99) })
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = alternative(rioe)
 	}
 }
@@ -358,7 +359,7 @@ func BenchmarkAlt_LeftRight(b *testing.B) {
 	alternative := Alt(func() ReaderIOResult[int] { return Right(99) })
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = alternative(rioe)
 	}
 }
@@ -368,7 +369,7 @@ func BenchmarkOrElse_Right(b *testing.B) {
 	recover := OrElse(func(e error) ReaderIOResult[int] { return Right(0) })
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = recover(rioe)
 	}
 }
@@ -378,7 +379,7 @@ func BenchmarkOrElse_Left(b *testing.B) {
 	recover := OrElse(func(e error) ReaderIOResult[int] { return Right(0) })
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = recover(rioe)
 	}
 }
@@ -389,7 +390,7 @@ func BenchmarkChainEitherK_Right(b *testing.B) {
 	chainer := ChainEitherK(func(a int) Either[int] { return E.Right[error](a * 2) })
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = chainer(rioe)
 	}
 }
@@ -399,7 +400,7 @@ func BenchmarkChainEitherK_Left(b *testing.B) {
 	chainer := ChainEitherK(func(a int) Either[int] { return E.Right[error](a * 2) })
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = chainer(rioe)
 	}
 }
@@ -409,7 +410,7 @@ func BenchmarkChainIOK_Right(b *testing.B) {
 	chainer := ChainIOK(func(a int) func() int { return func() int { return a * 2 } })
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = chainer(rioe)
 	}
 }
@@ -419,7 +420,7 @@ func BenchmarkChainIOK_Left(b *testing.B) {
 	chainer := ChainIOK(func(a int) func() int { return func() int { return a * 2 } })
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = chainer(rioe)
 	}
 }
@@ -429,7 +430,7 @@ func BenchmarkChainIOEitherK_Right(b *testing.B) {
 	chainer := ChainIOEitherK(func(a int) IOEither[int] { return IOE.Of[error](a * 2) })
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = chainer(rioe)
 	}
 }
@@ -439,7 +440,7 @@ func BenchmarkChainIOEitherK_Left(b *testing.B) {
 	chainer := ChainIOEitherK(func(a int) IOEither[int] { return IOE.Of[error](a * 2) })
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = chainer(rioe)
 	}
 }
@@ -447,7 +448,7 @@ func BenchmarkChainIOEitherK_Left(b *testing.B) {
 // Benchmark context operations
 func BenchmarkAsk(b *testing.B) {
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = Ask()
 	}
 }
@@ -455,7 +456,7 @@ func BenchmarkAsk(b *testing.B) {
 func BenchmarkDefer(b *testing.B) {
 	gen := func() ReaderIOResult[int] { return Right(42) }
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = Defer(gen)
 	}
 }
@@ -463,7 +464,7 @@ func BenchmarkDefer(b *testing.B) {
 func BenchmarkMemoize(b *testing.B) {
 	rioe := Right(42)
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = Memoize(rioe)
 	}
 }
@@ -472,14 +473,14 @@ func BenchmarkMemoize(b *testing.B) {
 func BenchmarkDelay_Construction(b *testing.B) {
 	rioe := Right(42)
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = Delay[int](time.Millisecond)(rioe)
 	}
 }
 
 func BenchmarkTimer_Construction(b *testing.B) {
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = Timer(time.Millisecond)
 	}
 }
@@ -490,7 +491,7 @@ func BenchmarkTryCatch_Success(b *testing.B) {
 		return func() (int, error) { return 42, nil }
 	}
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = TryCatch(f)
 	}
 }
@@ -500,7 +501,7 @@ func BenchmarkTryCatch_Error(b *testing.B) {
 		return func() (int, error) { return 0, benchErr }
 	}
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = TryCatch(f)
 	}
 }
@@ -512,7 +513,7 @@ func BenchmarkExecuteTryCatch_Success(b *testing.B) {
 	rioe := TryCatch(f)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = rioe(benchCtx)()
 	}
 }
@@ -524,7 +525,7 @@ func BenchmarkExecuteTryCatch_Error(b *testing.B) {
 	rioe := TryCatch(f)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = rioe(benchCtx)()
 	}
 }
@@ -534,10 +535,10 @@ func BenchmarkPipeline_Map_Right(b *testing.B) {
 	rioe := Right(21)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = F.Pipe1(
 			rioe,
-			Map(func(x int) int { return x * 2 }),
+			Map(N.Mul(2)),
 		)
 	}
 }
@@ -546,10 +547,10 @@ func BenchmarkPipeline_Map_Left(b *testing.B) {
 	rioe := Left[int](benchErr)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = F.Pipe1(
 			rioe,
-			Map(func(x int) int { return x * 2 }),
+			Map(N.Mul(2)),
 		)
 	}
 }
@@ -558,7 +559,7 @@ func BenchmarkPipeline_Chain_Right(b *testing.B) {
 	rioe := Right(21)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = F.Pipe1(
 			rioe,
 			Chain(func(x int) ReaderIOResult[int] { return Right(x * 2) }),
@@ -570,7 +571,7 @@ func BenchmarkPipeline_Chain_Left(b *testing.B) {
 	rioe := Left[int](benchErr)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = F.Pipe1(
 			rioe,
 			Chain(func(x int) ReaderIOResult[int] { return Right(x * 2) }),
@@ -582,12 +583,12 @@ func BenchmarkPipeline_Complex_Right(b *testing.B) {
 	rioe := Right(10)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = F.Pipe3(
 			rioe,
-			Map(func(x int) int { return x * 2 }),
+			Map(N.Mul(2)),
 			Chain(func(x int) ReaderIOResult[int] { return Right(x + 1) }),
-			Map(func(x int) int { return x * 2 }),
+			Map(N.Mul(2)),
 		)
 	}
 }
@@ -596,12 +597,12 @@ func BenchmarkPipeline_Complex_Left(b *testing.B) {
 	rioe := Left[int](benchErr)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchRIOE = F.Pipe3(
 			rioe,
-			Map(func(x int) int { return x * 2 }),
+			Map(N.Mul(2)),
 			Chain(func(x int) ReaderIOResult[int] { return Right(x + 1) }),
-			Map(func(x int) int { return x * 2 }),
+			Map(N.Mul(2)),
 		)
 	}
 }
@@ -609,13 +610,13 @@ func BenchmarkPipeline_Complex_Left(b *testing.B) {
 func BenchmarkExecutePipeline_Complex_Right(b *testing.B) {
 	rioe := F.Pipe3(
 		Right(10),
-		Map(func(x int) int { return x * 2 }),
+		Map(N.Mul(2)),
 		Chain(func(x int) ReaderIOResult[int] { return Right(x + 1) }),
-		Map(func(x int) int { return x * 2 }),
+		Map(N.Mul(2)),
 	)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = rioe(benchCtx)()
 	}
 }
@@ -624,7 +625,7 @@ func BenchmarkExecutePipeline_Complex_Right(b *testing.B) {
 func BenchmarkDo(b *testing.B) {
 	type State struct{ value int }
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = Do(State{})
 	}
 }
@@ -642,7 +643,7 @@ func BenchmarkBind_Right(b *testing.B) {
 	)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = binder(initial)
 	}
 }
@@ -658,7 +659,7 @@ func BenchmarkLet_Right(b *testing.B) {
 	)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = letter(initial)
 	}
 }
@@ -674,7 +675,7 @@ func BenchmarkApS_Right(b *testing.B) {
 	)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = aps(initial)
 	}
 }
@@ -687,7 +688,7 @@ func BenchmarkTraverseArray_Empty(b *testing.B) {
 	})
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = traverser(arr)
 	}
 }
@@ -699,7 +700,7 @@ func BenchmarkTraverseArray_Small(b *testing.B) {
 	})
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = traverser(arr)
 	}
 }
@@ -714,7 +715,7 @@ func BenchmarkTraverseArray_Medium(b *testing.B) {
 	})
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = traverser(arr)
 	}
 }
@@ -726,7 +727,7 @@ func BenchmarkTraverseArraySeq_Small(b *testing.B) {
 	})
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = traverser(arr)
 	}
 }
@@ -738,7 +739,7 @@ func BenchmarkTraverseArrayPar_Small(b *testing.B) {
 	})
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = traverser(arr)
 	}
 }
@@ -751,7 +752,7 @@ func BenchmarkSequenceArray_Small(b *testing.B) {
 	}
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = SequenceArray(arr)
 	}
 }
@@ -763,7 +764,7 @@ func BenchmarkExecuteTraverseArray_Small(b *testing.B) {
 	})(arr)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = rioe(benchCtx)()
 	}
 }
@@ -775,7 +776,7 @@ func BenchmarkExecuteTraverseArraySeq_Small(b *testing.B) {
 	})(arr)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = rioe(benchCtx)()
 	}
 }
@@ -787,7 +788,7 @@ func BenchmarkExecuteTraverseArrayPar_Small(b *testing.B) {
 	})(arr)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = rioe(benchCtx)()
 	}
 }
@@ -800,7 +801,7 @@ func BenchmarkTraverseRecord_Small(b *testing.B) {
 	})
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = traverser(rec)
 	}
 }
@@ -813,7 +814,7 @@ func BenchmarkSequenceRecord_Small(b *testing.B) {
 	}
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = SequenceRecord(rec)
 	}
 }
@@ -826,7 +827,7 @@ func BenchmarkWithResource_Success(b *testing.B) {
 
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = WithResource[int](acquire, release)(body)
 	}
 }
@@ -839,7 +840,7 @@ func BenchmarkExecuteWithResource_Success(b *testing.B) {
 
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = rioe(benchCtx)()
 	}
 }
@@ -852,7 +853,7 @@ func BenchmarkExecuteWithResource_ErrorInBody(b *testing.B) {
 
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = rioe(benchCtx)()
 	}
 }
@@ -865,13 +866,13 @@ func BenchmarkExecute_CanceledContext(b *testing.B) {
 
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = rioe(ctx)()
 	}
 }
 
 func BenchmarkExecuteApPar_CanceledContext(b *testing.B) {
-	fab := Right(func(a int) int { return a * 2 })
+	fab := Right(N.Mul(2))
 	fa := Right(42)
 	rioe := MonadApPar(fab, fa)
 	ctx, cancel := context.WithCancel(benchCtx)
@@ -879,9 +880,7 @@ func BenchmarkExecuteApPar_CanceledContext(b *testing.B) {
 
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = rioe(ctx)()
 	}
 }
-
-// Made with Bob

v2/context/readerioresult/reader_test.go

@@ -284,3 +284,160 @@ func TestWithResourceErrorInRelease(t *testing.T) {
 	assert.Equal(t, 0, countRelease)
 	assert.Equal(t, E.Left[int](err), res)
 }
+
+func TestMonadChainFirstLeft(t *testing.T) {
+	ctx := context.Background()
+
+	// Test with Left value - function returns Left, always preserves original error
+	t.Run("Left value with function returning Left preserves original error", func(t *testing.T) {
+		sideEffectCalled := false
+		originalErr := fmt.Errorf("original error")
+		result := MonadChainFirstLeft(
+			Left[int](originalErr),
+			func(e error) ReaderIOResult[int] {
+				sideEffectCalled = true
+				return Left[int](fmt.Errorf("new error")) // This error is ignored
+			},
+		)
+		actualResult := result(ctx)()
+		assert.True(t, sideEffectCalled)
+		assert.Equal(t, E.Left[int](originalErr), actualResult)
+	})
+
+	// Test with Left value - function returns Right, still returns original Left
+	t.Run("Left value with function returning Right still returns original Left", func(t *testing.T) {
+		var capturedError error
+		originalErr := fmt.Errorf("validation failed")
+		result := MonadChainFirstLeft(
+			Left[int](originalErr),
+			func(e error) ReaderIOResult[int] {
+				capturedError = e
+				return Right(999) // This Right value is ignored
+			},
+		)
+		actualResult := result(ctx)()
+		assert.Equal(t, originalErr, capturedError)
+		assert.Equal(t, E.Left[int](originalErr), actualResult)
+	})
+
+	// Test with Right value - should pass through without calling function
+	t.Run("Right value passes through", func(t *testing.T) {
+		sideEffectCalled := false
+		result := MonadChainFirstLeft(
+			Right(42),
+			func(e error) ReaderIOResult[int] {
+				sideEffectCalled = true
+				return Left[int](fmt.Errorf("should not be called"))
+			},
+		)
+		assert.False(t, sideEffectCalled)
+		assert.Equal(t, E.Right[error](42), result(ctx)())
+	})
+
+	// Test that side effects are executed but original error is always preserved
+	t.Run("Side effects executed but original error preserved", func(t *testing.T) {
+		effectCount := 0
+		originalErr := fmt.Errorf("original error")
+		result := MonadChainFirstLeft(
+			Left[int](originalErr),
+			func(e error) ReaderIOResult[int] {
+				effectCount++
+				// Try to return Right, but original Left should still be returned
+				return Right(999)
+			},
+		)
+		actualResult := result(ctx)()
+		assert.Equal(t, 1, effectCount)
+		assert.Equal(t, E.Left[int](originalErr), actualResult)
+	})
+}
+
+func TestChainFirstLeft(t *testing.T) {
+	ctx := context.Background()
+
+	// Test with Left value - function returns Left, always preserves original error
+	t.Run("Left value with function returning Left preserves error", func(t *testing.T) {
+		var captured error
+		originalErr := fmt.Errorf("test error")
+		chainFn := ChainFirstLeft[int](func(e error) ReaderIOResult[int] {
+			captured = e
+			return Left[int](fmt.Errorf("ignored error"))
+		})
+		result := F.Pipe1(
+			Left[int](originalErr),
+			chainFn,
+		)
+		actualResult := result(ctx)()
+		assert.Equal(t, originalErr, captured)
+		assert.Equal(t, E.Left[int](originalErr), actualResult)
+	})
+
+	// Test with Left value - function returns Right, still returns original Left
+	t.Run("Left value with function returning Right still returns original Left", func(t *testing.T) {
+		var captured error
+		originalErr := fmt.Errorf("test error")
+		chainFn := ChainFirstLeft[int](func(e error) ReaderIOResult[int] {
+			captured = e
+			return Right(42) // This Right is ignored
+		})
+		result := F.Pipe1(
+			Left[int](originalErr),
+			chainFn,
+		)
+		actualResult := result(ctx)()
+		assert.Equal(t, originalErr, captured)
+		assert.Equal(t, E.Left[int](originalErr), actualResult)
+	})
+
+	// Test with Right value - should pass through without calling function
+	t.Run("Right value passes through", func(t *testing.T) {
+		called := false
+		chainFn := ChainFirstLeft[int](func(e error) ReaderIOResult[int] {
+			called = true
+			return Right(0)
+		})
+		result := F.Pipe1(
+			Right(100),
+			chainFn,
+		)
+		assert.False(t, called)
+		assert.Equal(t, E.Right[error](100), result(ctx)())
+	})
+
+	// Test that original error is always preserved regardless of what f returns
+	t.Run("Original error always preserved", func(t *testing.T) {
+		originalErr := fmt.Errorf("original")
+		chainFn := ChainFirstLeft[int](func(e error) ReaderIOResult[int] {
+			// Try to return Right, but original Left should still be returned
+			return Right(999)
+		})
+
+		result := F.Pipe1(
+			Left[int](originalErr),
+			chainFn,
+		)
+		assert.Equal(t, E.Left[int](originalErr), result(ctx)())
+	})
+
+	// Test logging with Left preservation
+	t.Run("Logging with Left preservation", func(t *testing.T) {
+		errorLog := []string{}
+		originalErr := fmt.Errorf("step1")
+		logError := ChainFirstLeft[string](func(e error) ReaderIOResult[string] {
+			errorLog = append(errorLog, "Logged: "+e.Error())
+			return Left[string](fmt.Errorf("log entry")) // This is ignored
+		})
+
+		result := F.Pipe2(
+			Left[string](originalErr),
+			logError,
+			ChainLeft(func(e error) ReaderIOResult[string] {
+				return Left[string](fmt.Errorf("step2"))
+			}),
+		)
+
+		actualResult := result(ctx)()
+		assert.Equal(t, []string{"Logged: step1"}, errorLog)
+		assert.Equal(t, E.Left[string](fmt.Errorf("step2")), actualResult)
+	})
+}

v2/context/readerioresult/traverse.go

@@ -16,8 +16,8 @@
 package readerioresult
 
 import (
+	"github.com/IBM/fp-go/v2/array"
 	"github.com/IBM/fp-go/v2/function"
-	"github.com/IBM/fp-go/v2/internal/array"
 	"github.com/IBM/fp-go/v2/internal/record"
 )
 
@@ -29,7 +29,7 @@ import (
 //
 // Returns a function that transforms an array into a ReaderIOResult of an array.
 func TraverseArray[A, B any](f Kleisli[A, B]) Kleisli[[]A, []B] {
-	return array.Traverse[[]A](
+	return array.Traverse(
 		Of[[]B],
 		Map[[]B, func(B) []B],
 		Ap[[]B, B],
@@ -46,7 +46,7 @@ func TraverseArray[A, B any](f Kleisli[A, B]) Kleisli[[]A, []B] {
 //
 // Returns a function that transforms an array into a ReaderIOResult of an array.
 func TraverseArrayWithIndex[A, B any](f func(int, A) ReaderIOResult[B]) Kleisli[[]A, []B] {
-	return array.TraverseWithIndex[[]A](
+	return array.TraverseWithIndex(
 		Of[[]B],
 		Map[[]B, func(B) []B],
 		Ap[[]B, B],
@@ -135,22 +135,20 @@ func MonadTraverseArraySeq[A, B any](as []A, f Kleisli[A, B]) ReaderIOResult[[]B
 //
 // Returns a function that transforms an array into a ReaderIOResult of an array.
 func TraverseArraySeq[A, B any](f Kleisli[A, B]) Kleisli[[]A, []B] {
-	return array.Traverse[[]A](
+	return array.Traverse(
 		Of[[]B],
 		Map[[]B, func(B) []B],
 		ApSeq[[]B, B],
-
 		f,
 	)
 }
 
 // TraverseArrayWithIndexSeq uses transforms an array [[]A] into [[]ReaderIOResult[B]] and then resolves that into a [ReaderIOResult[[]B]]
 func TraverseArrayWithIndexSeq[A, B any](f func(int, A) ReaderIOResult[B]) Kleisli[[]A, []B] {
-	return array.TraverseWithIndex[[]A](
+	return array.TraverseWithIndex(
 		Of[[]B],
 		Map[[]B, func(B) []B],
 		ApSeq[[]B, B],
-
 		f,
 	)
 }
@@ -230,22 +228,20 @@ func MonadTraverseArrayPar[A, B any](as []A, f Kleisli[A, B]) ReaderIOResult[[]B
 //
 // Returns a function that transforms an array into a ReaderIOResult of an array.
 func TraverseArrayPar[A, B any](f Kleisli[A, B]) Kleisli[[]A, []B] {
-	return array.Traverse[[]A](
+	return array.Traverse(
 		Of[[]B],
 		Map[[]B, func(B) []B],
 		ApPar[[]B, B],
-
 		f,
 	)
 }
 
 // TraverseArrayWithIndexPar uses transforms an array [[]A] into [[]ReaderIOResult[B]] and then resolves that into a [ReaderIOResult[[]B]]
 func TraverseArrayWithIndexPar[A, B any](f func(int, A) ReaderIOResult[B]) Kleisli[[]A, []B] {
-	return array.TraverseWithIndex[[]A](
+	return array.TraverseWithIndex(
 		Of[[]B],
 		Map[[]B, func(B) []B],
 		ApPar[[]B, B],
-
 		f,
 	)
 }

v2/context/readerioresult/type.go

@@ -19,14 +19,17 @@ import (
 	"context"
 
 	"github.com/IBM/fp-go/v2/context/ioresult"
+	"github.com/IBM/fp-go/v2/context/readerresult"
 	"github.com/IBM/fp-go/v2/either"
 	"github.com/IBM/fp-go/v2/io"
 	"github.com/IBM/fp-go/v2/ioeither"
 	"github.com/IBM/fp-go/v2/lazy"
 	"github.com/IBM/fp-go/v2/option"
 	"github.com/IBM/fp-go/v2/reader"
+	"github.com/IBM/fp-go/v2/readereither"
 	"github.com/IBM/fp-go/v2/readerio"
 	RIOR "github.com/IBM/fp-go/v2/readerioresult"
+	"github.com/IBM/fp-go/v2/readeroption"
 	"github.com/IBM/fp-go/v2/result"
 )
 
@@ -119,4 +122,8 @@ type (
 	//   // Apply the transformation
 	//   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]
 )

v2/context/readerresult/reader.go

@@ -92,3 +92,8 @@ func MonadFlap[B, A any](fab ReaderResult[func(A) B], a A) ReaderResult[B] {
 func Flap[B, A any](a A) Operator[func(A) B, B] {
 	return readereither.Flap[context.Context, error, B](a)
 }
+
+//go:inline
+func Read[A any](r context.Context) func(ReaderResult[A]) Result[A] {
+	return readereither.Read[error, A](r)
+}

v2/context/readerresult/type.go

@@ -23,11 +23,13 @@ import (
 	"github.com/IBM/fp-go/v2/option"
 	"github.com/IBM/fp-go/v2/reader"
 	"github.com/IBM/fp-go/v2/readereither"
+	"github.com/IBM/fp-go/v2/result"
 )
 
 type (
 	Option[A any] = option.Option[A]
 	Either[A any] = either.Either[error, A]
+	Result[A any] = result.Result[A]
 	// ReaderResult is a specialization of the Reader monad for the typical golang scenario
 	ReaderResult[A any] = readereither.ReaderEither[context.Context, error, A]
 

v2/context/statereaderioresult/bind.go

@@ -0,0 +1,209 @@
+// Copyright (c) 2024 - 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 statereaderioresult
+
+import (
+	"github.com/IBM/fp-go/v2/function"
+	A "github.com/IBM/fp-go/v2/internal/apply"
+	C "github.com/IBM/fp-go/v2/internal/chain"
+	F "github.com/IBM/fp-go/v2/internal/functor"
+)
+
+// Do starts a do-notation chain for building computations in a fluent style.
+// This is typically used with Bind, Let, and other combinators to compose
+// stateful, context-dependent computations that can fail.
+//
+// Example:
+//
+//	type State struct {
+//	    name string
+//	    age  int
+//	}
+//	result := function.Pipe2(
+//	    statereaderioresult.Do[AppState](State{}),
+//	    statereaderioresult.Bind(...),
+//	    statereaderioresult.Let(...),
+//	)
+//
+//go:inline
+func Do[ST, A any](
+	empty A,
+) StateReaderIOResult[ST, A] {
+	return Of[ST](empty)
+}
+
+// Bind executes a computation and binds its result to a field in the accumulator state.
+// This is used in do-notation to sequence dependent computations.
+//
+// Example:
+//
+//	result := function.Pipe2(
+//	    statereaderioresult.Do[AppState](State{}),
+//	    statereaderioresult.Bind(
+//	        func(name string) func(State) State {
+//	            return func(s State) State { return State{name: name, age: s.age} }
+//	        },
+//	        func(s State) statereaderioresult.StateReaderIOResult[AppState, string] {
+//	            return statereaderioresult.Of[AppState]("John")
+//	        },
+//	    ),
+//	)
+//
+//go:inline
+func Bind[ST, S1, S2, T any](
+	setter func(T) func(S1) S2,
+	f Kleisli[ST, S1, T],
+) Operator[ST, S1, S2] {
+	return C.Bind(
+		Chain[ST, S1, S2],
+		Map[ST, T, S2],
+		setter,
+		f,
+	)
+}
+
+// Let computes a derived value and binds it to a field in the accumulator state.
+// Unlike Bind, this does not execute a monadic computation, just a pure function.
+//
+// Example:
+//
+//	result := function.Pipe2(
+//	    statereaderioresult.Do[AppState](State{age: 25}),
+//	    statereaderioresult.Let(
+//	        func(isAdult bool) func(State) State {
+//	            return func(s State) State { return State{age: s.age, isAdult: isAdult} }
+//	        },
+//	        func(s State) bool { return s.age >= 18 },
+//	    ),
+//	)
+//
+//go:inline
+func Let[ST, S1, S2, T any](
+	key func(T) func(S1) S2,
+	f func(S1) T,
+) Operator[ST, S1, S2] {
+	return F.Let(
+		Map[ST, S1, S2],
+		key,
+		f,
+	)
+}
+
+// LetTo binds a constant value to a field in the accumulator state.
+//
+// Example:
+//
+//	result := function.Pipe2(
+//	    statereaderioresult.Do[AppState](State{}),
+//	    statereaderioresult.LetTo(
+//	        func(status string) func(State) State {
+//	            return func(s State) State { return State{...s, status: status} }
+//	        },
+//	        "active",
+//	    ),
+//	)
+//
+//go:inline
+func LetTo[ST, S1, S2, T any](
+	key func(T) func(S1) S2,
+	b T,
+) Operator[ST, S1, S2] {
+	return F.LetTo(
+		Map[ST, S1, S2],
+		key,
+		b,
+	)
+}
+
+// BindTo wraps a value in a simple constructor, typically used to start a do-notation chain
+// after getting an initial value.
+//
+// Example:
+//
+//	result := function.Pipe2(
+//	    statereaderioresult.Of[AppState](42),
+//	    statereaderioresult.BindTo[AppState](func(x int) State { return State{value: x} }),
+//	)
+//
+//go:inline
+func BindTo[ST, S1, T any](
+	setter func(T) S1,
+) Operator[ST, T, S1] {
+	return C.BindTo(
+		Map[ST, T, S1],
+		setter,
+	)
+}
+
+// ApS applies a computation in sequence and binds the result to a field.
+// This is the applicative version of Bind.
+//
+//go:inline
+func ApS[ST, S1, S2, T any](
+	setter func(T) func(S1) S2,
+	fa StateReaderIOResult[ST, T],
+) Operator[ST, S1, S2] {
+	return A.ApS(
+		Ap[S2, ST, T],
+		Map[ST, S1, func(T) S2],
+		setter,
+		fa,
+	)
+}
+
+// ApSL is a lens-based variant of ApS for working with nested structures.
+// It uses a lens to focus on a specific field in the state.
+//
+//go:inline
+func ApSL[ST, S, T any](
+	lens Lens[S, T],
+	fa StateReaderIOResult[ST, T],
+) Endomorphism[StateReaderIOResult[ST, S]] {
+	return ApS(lens.Set, fa)
+}
+
+// BindL is a lens-based variant of Bind for working with nested structures.
+// It uses a lens to focus on a specific field in the state.
+//
+//go:inline
+func BindL[ST, S, T any](
+	lens Lens[S, T],
+	f Kleisli[ST, T, T],
+) Endomorphism[StateReaderIOResult[ST, S]] {
+	return Bind(lens.Set, function.Flow2(lens.Get, f))
+}
+
+// LetL is a lens-based variant of Let for working with nested structures.
+// It uses a lens to focus on a specific field in the state.
+//
+//go:inline
+func LetL[ST, S, T any](
+	lens Lens[S, T],
+	f Endomorphism[T],
+) Endomorphism[StateReaderIOResult[ST, S]] {
+	return Let[ST](lens.Set, function.Flow2(lens.Get, f))
+}
+
+// LetToL is a lens-based variant of LetTo for working with nested structures.
+// It uses a lens to focus on a specific field in the state.
+//
+//go:inline
+func LetToL[ST, S, T any](
+	lens Lens[S, T],
+	b T,
+) Endomorphism[StateReaderIOResult[ST, S]] {
+	return LetTo[ST](lens.Set, b)
+}

v2/context/statereaderioresult/doc.go

@@ -0,0 +1,147 @@
+// Copyright (c) 2024 - 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 statereaderioresult provides a functional programming abstraction that combines
+// four powerful concepts: State, Reader, IO, and Result monads, specialized for Go's context.Context.
+//
+// # StateReaderIOResult
+//
+// StateReaderIOResult[S, A] represents a computation that:
+//   - Manages state of type S (State)
+//   - Depends on a [context.Context] (Reader)
+//   - Performs side effects (IO)
+//   - Can fail with an [error] or succeed with a value of type A (Result)
+//
+// This is a specialization of StateReaderIOEither with:
+//   - Context type fixed to [context.Context]
+//   - Error type fixed to [error]
+//
+// This is particularly useful for:
+//   - Stateful computations with dependency injection using Go contexts
+//   - Error handling in effectful computations with state
+//   - Composing operations that need access to context, manage state, and can fail
+//   - Working with Go's standard context patterns (cancellation, deadlines, values)
+//
+// # Core Operations
+//
+// Construction:
+//   - Of/Right: Create a successful computation with a value
+//   - Left: Create a failed computation with an error
+//   - FromState: Lift a State into StateReaderIOResult
+//   - FromIO: Lift an IO into StateReaderIOResult
+//   - FromResult: Lift a Result into StateReaderIOResult
+//   - FromIOResult: Lift an IOResult into StateReaderIOResult
+//   - FromReaderIOResult: Lift a ReaderIOResult into StateReaderIOResult
+//
+// Transformation:
+//   - Map: Transform the success value
+//   - Chain: Sequence dependent computations (monadic bind)
+//   - Flatten: Flatten nested StateReaderIOResult
+//
+// Combination:
+//   - Ap: Apply a function in a context to a value in a context
+//
+// Context Access:
+//   - Asks: Get a value derived from the context
+//   - Local: Run a computation with a modified context
+//
+// Kleisli Arrows:
+//   - FromResultK: Lift a Result-returning function to a Kleisli arrow
+//   - FromIOK: Lift an IO-returning function to a Kleisli arrow
+//   - FromIOResultK: Lift an IOResult-returning function to a Kleisli arrow
+//   - FromReaderIOResultK: Lift a ReaderIOResult-returning function to a Kleisli arrow
+//   - ChainResultK: Chain with a Result-returning function
+//   - ChainIOResultK: Chain with an IOResult-returning function
+//   - ChainReaderIOResultK: Chain with a ReaderIOResult-returning function
+//
+// Do Notation (Monadic Composition):
+//   - Do: Start a do-notation chain
+//   - Bind: Bind a value from a computation
+//   - BindTo: Bind a value to a simple constructor
+//   - Let: Compute a derived value
+//   - LetTo: Set a constant value
+//   - ApS: Apply in sequence (for applicative composition)
+//   - BindL/ApSL/LetL/LetToL: Lens-based variants for working with nested structures
+//
+// # Example Usage
+//
+//	type AppState struct {
+//	    RequestCount int
+//	    LastError    error
+//	}
+//
+//	// A computation that manages state, depends on context, performs IO, and can fail
+//	func processRequest(data string) statereaderioresult.StateReaderIOResult[AppState, string] {
+//	    return func(state AppState) readerioresult.ReaderIOResult[pair.Pair[AppState, string]] {
+//	        return func(ctx context.Context) ioresult.IOResult[pair.Pair[AppState, string]] {
+//	            return func() result.Result[pair.Pair[AppState, string]] {
+//	                // Check context for cancellation
+//	                if ctx.Err() != nil {
+//	                    return result.Error[pair.Pair[AppState, string]](ctx.Err())
+//	                }
+//	                // Update state
+//	                newState := AppState{RequestCount: state.RequestCount + 1}
+//	                // Perform IO operations
+//	                return result.Of(pair.MakePair(newState, "processed: " + data))
+//	            }
+//	        }
+//	    }
+//	}
+//
+//	// Compose operations using do-notation
+//	result := function.Pipe3(
+//	    statereaderioresult.Do[AppState](State{}),
+//	    statereaderioresult.Bind(
+//	        func(result string) func(State) State { return func(s State) State { return State{result: result} } },
+//	        func(s State) statereaderioresult.StateReaderIOResult[AppState, string] {
+//	            return processRequest(s.input)
+//	        },
+//	    ),
+//	    statereaderioresult.Map[AppState](func(s State) string { return s.result }),
+//	)
+//
+//	// Execute with initial state and context
+//	initialState := AppState{RequestCount: 0}
+//	ctx := context.Background()
+//	outcome := result(initialState)(ctx)() // Returns result.Result[pair.Pair[AppState, string]]
+//
+// # Context Integration
+//
+// This package is designed to work seamlessly with Go's context.Context:
+//
+//	// Using context values
+//	getUserID := statereaderioresult.Asks[AppState, string](func(ctx context.Context) statereaderioresult.StateReaderIOResult[AppState, string] {
+//	    userID, ok := ctx.Value("userID").(string)
+//	    if !ok {
+//	        return statereaderioresult.Left[AppState, string](errors.New("missing userID"))
+//	    }
+//	    return statereaderioresult.Of[AppState](userID)
+//	})
+//
+//	// Using context cancellation
+//	withTimeout := statereaderioresult.Local[AppState, string](func(ctx context.Context) context.Context {
+//	    ctx, _ = context.WithTimeout(ctx, 5*time.Second)
+//	    return ctx
+//	})
+//
+// # Monad Laws
+//
+// StateReaderIOResult satisfies the monad laws:
+//   - Left Identity: Of(a) >>= f ≡ f(a)
+//   - Right Identity: m >>= Of ≡ m
+//   - Associativity: (m >>= f) >>= g ≡ m >>= (x => f(x) >>= g)
+//
+// These laws are verified in the testing subpackage.
+package statereaderioresult

v2/context/statereaderioresult/eq.go

@@ -0,0 +1,41 @@
+// Copyright (c) 2024 - 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 statereaderioresult
+
+import (
+	"context"
+
+	"github.com/IBM/fp-go/v2/eq"
+	"github.com/IBM/fp-go/v2/function"
+	RIOR "github.com/IBM/fp-go/v2/readerioresult"
+)
+
+// Eq implements the equals predicate for values contained in the [StateReaderIOResult] monad
+func Eq[S, A any](eqr eq.Eq[ReaderIOResult[Pair[S, A]]]) func(S) eq.Eq[StateReaderIOResult[S, A]] {
+	return func(s S) eq.Eq[StateReaderIOResult[S, A]] {
+		return eq.FromEquals(func(l, r StateReaderIOResult[S, A]) bool {
+			return eqr.Equals(l(s), r(s))
+		})
+	}
+}
+
+// FromStrictEquals constructs an [eq.Eq] from the canonical comparison function
+func FromStrictEquals[S comparable, A comparable]() func(context.Context) func(S) eq.Eq[StateReaderIOResult[S, A]] {
+	return function.Flow2(
+		RIOR.FromStrictEquals[context.Context, Pair[S, A]](),
+		Eq[S, A],
+	)
+}

v2/context/statereaderioresult/monad.go

@@ -0,0 +1,103 @@
+// Copyright (c) 2024 - 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 statereaderioresult
+
+import (
+	"github.com/IBM/fp-go/v2/internal/applicative"
+	"github.com/IBM/fp-go/v2/internal/functor"
+	"github.com/IBM/fp-go/v2/internal/monad"
+	"github.com/IBM/fp-go/v2/internal/pointed"
+)
+
+type stateReaderIOResultPointed[
+	S, A any,
+] struct{}
+
+type stateReaderIOResultFunctor[
+	S, A, B any,
+] struct{}
+
+type stateReaderIOResultApplicative[
+	S, A, B any,
+] struct{}
+
+type stateReaderIOResultMonad[
+	S, A, B any,
+] struct{}
+
+func (o *stateReaderIOResultPointed[S, A]) Of(a A) StateReaderIOResult[S, A] {
+	return Of[S](a)
+}
+
+func (o *stateReaderIOResultMonad[S, A, B]) Of(a A) StateReaderIOResult[S, A] {
+	return Of[S](a)
+}
+
+func (o *stateReaderIOResultApplicative[S, A, B]) Of(a A) StateReaderIOResult[S, A] {
+	return Of[S](a)
+}
+
+func (o *stateReaderIOResultMonad[S, A, B]) Map(f func(A) B) Operator[S, A, B] {
+	return Map[S](f)
+}
+
+func (o *stateReaderIOResultApplicative[S, A, B]) Map(f func(A) B) Operator[S, A, B] {
+	return Map[S](f)
+}
+
+func (o *stateReaderIOResultFunctor[S, A, B]) Map(f func(A) B) Operator[S, A, B] {
+	return Map[S](f)
+}
+
+func (o *stateReaderIOResultMonad[S, A, B]) Chain(f Kleisli[S, A, B]) Operator[S, A, B] {
+	return Chain(f)
+}
+
+func (o *stateReaderIOResultMonad[S, A, B]) Ap(fa StateReaderIOResult[S, A]) Operator[S, func(A) B, B] {
+	return Ap[B](fa)
+}
+
+func (o *stateReaderIOResultApplicative[S, A, B]) Ap(fa StateReaderIOResult[S, A]) Operator[S, func(A) B, B] {
+	return Ap[B](fa)
+}
+
+// Pointed implements the [pointed.Pointed] operations for [StateReaderIOResult]
+func Pointed[
+	S, A any,
+]() pointed.Pointed[A, StateReaderIOResult[S, A]] {
+	return &stateReaderIOResultPointed[S, A]{}
+}
+
+// Functor implements the [functor.Functor] operations for [StateReaderIOResult]
+func Functor[
+	S, A, B any,
+]() functor.Functor[A, B, StateReaderIOResult[S, A], StateReaderIOResult[S, B]] {
+	return &stateReaderIOResultFunctor[S, A, B]{}
+}
+
+// Applicative implements the [applicative.Applicative] operations for [StateReaderIOResult]
+func Applicative[
+	S, A, B any,
+]() applicative.Applicative[A, B, StateReaderIOResult[S, A], StateReaderIOResult[S, B], StateReaderIOResult[S, func(A) B]] {
+	return &stateReaderIOResultApplicative[S, A, B]{}
+}
+
+// Monad implements the [monad.Monad] operations for [StateReaderIOResult]
+func Monad[
+	S, A, B any,
+]() monad.Monad[A, B, StateReaderIOResult[S, A], StateReaderIOResult[S, B], StateReaderIOResult[S, func(A) B]] {
+	return &stateReaderIOResultMonad[S, A, B]{}
+}

v2/context/statereaderioresult/resource.go

@@ -0,0 +1,101 @@
+// Copyright (c) 2024 - 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 statereaderioresult
+
+import "github.com/IBM/fp-go/v2/statereaderioeither"
+
+// WithResource constructs a function that creates a resource with state management, operates on it,
+// and then releases the resource. This ensures proper resource cleanup even in the presence of errors,
+// following the Resource Acquisition Is Initialization (RAII) pattern.
+//
+// The state is threaded through all operations: resource creation, usage, and release.
+//
+// The resource lifecycle with state management is:
+//  1. onCreate: Acquires the resource (may modify state)
+//  2. use: Operates on the resource with current state (provided as argument to the returned function)
+//  3. onRelease: Releases the resource with current state (called regardless of success or failure)
+//
+// Type parameters:
+//   - A: The type of the result produced by using the resource
+//   - S: The state type that is threaded through all operations
+//   - RES: The resource type
+//   - ANY: The type returned by the release function (typically ignored)
+//
+// Parameters:
+//   - onCreate: A stateful computation that acquires the resource
+//   - onRelease: A stateful function that releases the resource, called with the resource and current state,
+//     executed regardless of errors
+//
+// Returns:
+//
+//	A function that takes a resource-using function and returns a StateReaderIOResult that manages
+//	the resource lifecycle with state
+//
+// Example:
+//
+//	type AppState struct {
+//	    openFiles int
+//	}
+//
+//	// Resource creation that updates state
+//	openFile := func(filename string) StateReaderIOResult[AppState, *File] {
+//	    return func(state AppState) ReaderIOResult[Pair[AppState, *File]] {
+//	        return func(ctx context.Context) IOResult[Pair[AppState, *File]] {
+//	            return func() Result[Pair[AppState, *File]] {
+//	                file, err := os.Open(filename)
+//	                if err != nil {
+//	                    return result.Error[Pair[AppState, *File]](err)
+//	                }
+//	                newState := AppState{openFiles: state.openFiles + 1}
+//	                return result.Of(pair.MakePair(newState, file))
+//	            }
+//	        }
+//	    }
+//	}
+//
+//	// Resource release that updates state
+//	closeFile := func(f *File) StateReaderIOResult[AppState, int] {
+//	    return func(state AppState) ReaderIOResult[Pair[AppState, int]] {
+//	        return func(ctx context.Context) IOResult[Pair[AppState, int]] {
+//	            return func() Result[Pair[AppState, int]] {
+//	                f.Close()
+//	                newState := AppState{openFiles: state.openFiles - 1}
+//	                return result.Of(pair.MakePair(newState, 0))
+//	            }
+//	        }
+//	    }
+//	}
+//
+//	// Use the resource with automatic cleanup
+//	withFile := WithResource(
+//	    openFile("data.txt"),
+//	    closeFile,
+//	)
+//
+//	result := withFile(func(f *File) StateReaderIOResult[AppState, string] {
+//	    return readContent(f) // File will be closed automatically
+//	})
+//
+//	// Execute the computation
+//	initialState := AppState{openFiles: 0}
+//	ctx := context.Background()
+//	outcome := result(initialState)(ctx)()
+func WithResource[A, S, RES, ANY any](
+	onCreate StateReaderIOResult[S, RES],
+	onRelease Kleisli[S, RES, ANY],
+) Kleisli[S, Kleisli[S, RES, A], A] {
+	return statereaderioeither.WithResource[A](onCreate, onRelease)
+}

v2/context/statereaderioresult/resource_test.go

@@ -0,0 +1,415 @@
+// Copyright (c) 2024 - 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 statereaderioresult
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	P "github.com/IBM/fp-go/v2/pair"
+	RES "github.com/IBM/fp-go/v2/result"
+	"github.com/stretchr/testify/assert"
+)
+
+// resourceState tracks the lifecycle of resources for testing
+type resourceState struct {
+	resourcesCreated int
+	resourcesReleased int
+	lastError        error
+}
+
+// mockResource represents a test resource
+type mockResource struct {
+	id      int
+	isValid bool
+}
+
+// TestWithResourceSuccess tests successful resource creation, usage, and release
+func TestWithResourceSuccess(t *testing.T) {
+	initialState := resourceState{resourcesCreated: 0, resourcesReleased: 0}
+	ctx := context.Background()
+
+	// Create a resource
+	onCreate := func(s resourceState) ReaderIOResult[Pair[resourceState, mockResource]] {
+		return func(ctx context.Context) IOResult[Pair[resourceState, mockResource]] {
+			return func() Result[Pair[resourceState, mockResource]] {
+				newState := resourceState{
+					resourcesCreated:  s.resourcesCreated + 1,
+					resourcesReleased: s.resourcesReleased,
+				}
+				res := mockResource{id: newState.resourcesCreated, isValid: true}
+				return RES.Of(P.MakePair(newState, res))
+			}
+		}
+	}
+
+	// Release a resource
+	onRelease := func(res mockResource) StateReaderIOResult[resourceState, int] {
+		return func(s resourceState) ReaderIOResult[Pair[resourceState, int]] {
+			return func(ctx context.Context) IOResult[Pair[resourceState, int]] {
+				return func() Result[Pair[resourceState, int]] {
+					newState := resourceState{
+						resourcesCreated:  s.resourcesCreated,
+						resourcesReleased: s.resourcesReleased + 1,
+					}
+					return RES.Of(P.MakePair(newState, 0))
+				}
+			}
+		}
+	}
+
+	// Use the resource
+	useResource := func(res mockResource) StateReaderIOResult[resourceState, string] {
+		return func(s resourceState) ReaderIOResult[Pair[resourceState, string]] {
+			return func(ctx context.Context) IOResult[Pair[resourceState, string]] {
+				return func() Result[Pair[resourceState, string]] {
+					result := "used resource " + string(rune(res.id+'0'))
+					return RES.Of(P.MakePair(s, result))
+				}
+			}
+		}
+	}
+
+	withResource := WithResource[string](onCreate, onRelease)
+	result := withResource(useResource)
+	outcome := result(initialState)(ctx)()
+
+	assert.True(t, RES.IsRight(outcome))
+	RES.Map(func(p Pair[resourceState, string]) Pair[resourceState, string] {
+		state := P.Head(p)
+		value := P.Tail(p)
+
+		// Verify state updates
+		// Note: Final state comes from the use function, not the release function
+		// onCreate: 0->1, use: sees 1 (doesn't modify), release: sees 1 and increments released
+		// The final state is from use function which saw state=1 with resourcesReleased=0
+		assert.Equal(t, 1, state.resourcesCreated, "Resource should be created once")
+		assert.Equal(t, 0, state.resourcesReleased, "Final state is from use function, before release")
+
+		// Verify result
+		assert.Equal(t, "used resource 1", value)
+
+		return p
+	})(outcome)
+}
+
+// TestWithResourceErrorInCreate tests error handling when resource creation fails
+func TestWithResourceErrorInCreate(t *testing.T) {
+	initialState := resourceState{resourcesCreated: 0, resourcesReleased: 0}
+	ctx := context.Background()
+
+	createError := errors.New("failed to create resource")
+
+	// onCreate that fails
+	onCreate := func(s resourceState) ReaderIOResult[Pair[resourceState, mockResource]] {
+		return func(ctx context.Context) IOResult[Pair[resourceState, mockResource]] {
+			return func() Result[Pair[resourceState, mockResource]] {
+				return RES.Left[Pair[resourceState, mockResource]](createError)
+			}
+		}
+	}
+
+	// Release should not be called if onCreate fails
+	onRelease := func(res mockResource) StateReaderIOResult[resourceState, int] {
+		return func(s resourceState) ReaderIOResult[Pair[resourceState, int]] {
+			return func(ctx context.Context) IOResult[Pair[resourceState, int]] {
+				return func() Result[Pair[resourceState, int]] {
+					t.Error("onRelease should not be called when onCreate fails")
+					return RES.Of(P.MakePair(s, 0))
+				}
+			}
+		}
+	}
+
+	useResource := func(res mockResource) StateReaderIOResult[resourceState, string] {
+		return Of[resourceState]("should not reach here")
+	}
+
+	withResource := WithResource[string](onCreate, onRelease)
+	result := withResource(useResource)
+	outcome := result(initialState)(ctx)()
+
+	assert.True(t, RES.IsLeft(outcome))
+	RES.Fold(
+		func(err error) bool {
+			assert.Equal(t, createError, err)
+			return true
+		},
+		func(p Pair[resourceState, string]) bool {
+			t.Error("Expected error but got success")
+			return false
+		},
+	)(outcome)
+}
+
+// TestWithResourceErrorInUse tests that resources are released even when usage fails
+func TestWithResourceErrorInUse(t *testing.T) {
+	initialState := resourceState{resourcesCreated: 0, resourcesReleased: 0}
+	ctx := context.Background()
+
+	useError := errors.New("failed to use resource")
+
+	// Create a resource
+	onCreate := func(s resourceState) ReaderIOResult[Pair[resourceState, mockResource]] {
+		return func(ctx context.Context) IOResult[Pair[resourceState, mockResource]] {
+			return func() Result[Pair[resourceState, mockResource]] {
+				newState := resourceState{
+					resourcesCreated:  s.resourcesCreated + 1,
+					resourcesReleased: s.resourcesReleased,
+				}
+				res := mockResource{id: 1, isValid: true}
+				return RES.Of(P.MakePair(newState, res))
+			}
+		}
+	}
+
+	releaseWasCalled := false
+
+	// Release should still be called even if use fails
+	onRelease := func(res mockResource) StateReaderIOResult[resourceState, int] {
+		return func(s resourceState) ReaderIOResult[Pair[resourceState, int]] {
+			return func(ctx context.Context) IOResult[Pair[resourceState, int]] {
+				return func() Result[Pair[resourceState, int]] {
+					releaseWasCalled = true
+					newState := resourceState{
+						resourcesCreated:  s.resourcesCreated,
+						resourcesReleased: s.resourcesReleased + 1,
+					}
+					return RES.Of(P.MakePair(newState, 0))
+				}
+			}
+		}
+	}
+
+	// Use that fails
+	useResource := func(res mockResource) StateReaderIOResult[resourceState, string] {
+		return Left[resourceState, string](useError)
+	}
+
+	withResource := WithResource[string](onCreate, onRelease)
+	result := withResource(useResource)
+	outcome := result(initialState)(ctx)()
+
+	assert.True(t, RES.IsLeft(outcome))
+	assert.True(t, releaseWasCalled, "onRelease should be called even when use fails")
+
+	RES.Fold(
+		func(err error) bool {
+			assert.Equal(t, useError, err)
+			return true
+		},
+		func(p Pair[resourceState, string]) bool {
+			t.Error("Expected error but got success")
+			return false
+		},
+	)(outcome)
+}
+
+// TestWithResourceStateThreading tests that state is properly threaded through all operations
+func TestWithResourceStateThreading(t *testing.T) {
+	initialState := resourceState{resourcesCreated: 0, resourcesReleased: 0}
+	ctx := context.Background()
+
+	// Create increments counter
+	onCreate := func(s resourceState) ReaderIOResult[Pair[resourceState, mockResource]] {
+		return func(ctx context.Context) IOResult[Pair[resourceState, mockResource]] {
+			return func() Result[Pair[resourceState, mockResource]] {
+				newState := resourceState{
+					resourcesCreated:  s.resourcesCreated + 1,
+					resourcesReleased: s.resourcesReleased,
+				}
+				res := mockResource{id: newState.resourcesCreated, isValid: true}
+				return RES.Of(P.MakePair(newState, res))
+			}
+		}
+	}
+
+	// Use observes the state after creation
+	useResource := func(res mockResource) StateReaderIOResult[resourceState, int] {
+		return func(s resourceState) ReaderIOResult[Pair[resourceState, int]] {
+			return func(ctx context.Context) IOResult[Pair[resourceState, int]] {
+				return func() Result[Pair[resourceState, int]] {
+					// Verify state was updated by onCreate
+					assert.Equal(t, 1, s.resourcesCreated)
+					assert.Equal(t, 0, s.resourcesReleased)
+					return RES.Of(P.MakePair(s, s.resourcesCreated))
+				}
+			}
+		}
+	}
+
+	// Release increments released counter
+	onRelease := func(res mockResource) StateReaderIOResult[resourceState, int] {
+		return func(s resourceState) ReaderIOResult[Pair[resourceState, int]] {
+			return func(ctx context.Context) IOResult[Pair[resourceState, int]] {
+				return func() Result[Pair[resourceState, int]] {
+					// Verify state was updated by onCreate and use
+					assert.Equal(t, 1, s.resourcesCreated)
+					assert.Equal(t, 0, s.resourcesReleased)
+
+					newState := resourceState{
+						resourcesCreated:  s.resourcesCreated,
+						resourcesReleased: s.resourcesReleased + 1,
+					}
+					return RES.Of(P.MakePair(newState, 0))
+				}
+			}
+		}
+	}
+
+	withResource := WithResource[int](onCreate, onRelease)
+	result := withResource(useResource)
+	outcome := result(initialState)(ctx)()
+
+	assert.True(t, RES.IsRight(outcome))
+	RES.Map(func(p Pair[resourceState, int]) Pair[resourceState, int] {
+		finalState := P.Head(p)
+		value := P.Tail(p)
+
+		// Verify final state
+		// Note: Final state is from the use function, which preserves the state it received
+		// onCreate: 0->1, use: sees 1, release: sees 1 and increments released to 1
+		// But final state is from use function where resourcesReleased=0
+		assert.Equal(t, 1, finalState.resourcesCreated)
+		assert.Equal(t, 0, finalState.resourcesReleased, "Final state is from use function, before release")
+		assert.Equal(t, 1, value)
+
+		return p
+	})(outcome)
+}
+
+// TestWithResourceMultipleResources tests using WithResource multiple times (nesting)
+func TestWithResourceMultipleResources(t *testing.T) {
+	initialState := resourceState{resourcesCreated: 0, resourcesReleased: 0}
+	ctx := context.Background()
+
+	createResource := func(s resourceState) ReaderIOResult[Pair[resourceState, mockResource]] {
+		return func(ctx context.Context) IOResult[Pair[resourceState, mockResource]] {
+			return func() Result[Pair[resourceState, mockResource]] {
+				newState := resourceState{
+					resourcesCreated:  s.resourcesCreated + 1,
+					resourcesReleased: s.resourcesReleased,
+				}
+				res := mockResource{id: newState.resourcesCreated, isValid: true}
+				return RES.Of(P.MakePair(newState, res))
+			}
+		}
+	}
+
+	releaseResource := func(res mockResource) StateReaderIOResult[resourceState, int] {
+		return func(s resourceState) ReaderIOResult[Pair[resourceState, int]] {
+			return func(ctx context.Context) IOResult[Pair[resourceState, int]] {
+				return func() Result[Pair[resourceState, int]] {
+					newState := resourceState{
+						resourcesCreated:  s.resourcesCreated,
+						resourcesReleased: s.resourcesReleased + 1,
+					}
+					return RES.Of(P.MakePair(newState, 0))
+				}
+			}
+		}
+	}
+
+	// Create two nested resources
+	withResource1 := WithResource[int](createResource, releaseResource)
+	withResource2 := WithResource[int](createResource, releaseResource)
+
+	result := withResource1(func(res1 mockResource) StateReaderIOResult[resourceState, int] {
+		return withResource2(func(res2 mockResource) StateReaderIOResult[resourceState, int] {
+			// Both resources should be available
+			return Of[resourceState](res1.id + res2.id)
+		})
+	})
+
+	outcome := result(initialState)(ctx)()
+
+	assert.True(t, RES.IsRight(outcome))
+	RES.Map(func(p Pair[resourceState, int]) Pair[resourceState, int] {
+		finalState := P.Head(p)
+		value := P.Tail(p)
+
+		// Both resources created, but final state is from innermost use function
+		// onCreate1: 0->1, onCreate2: 1->2, use (Of): sees 2
+		// Release functions execute but their state changes aren't in the final result
+		assert.Equal(t, 2, finalState.resourcesCreated)
+		assert.Equal(t, 0, finalState.resourcesReleased, "Final state is from use function, before releases")
+		// res1.id = 1, res2.id = 2, sum = 3
+		assert.Equal(t, 3, value)
+
+		return p
+	})(outcome)
+}
+
+// TestWithResourceContextCancellation tests behavior with context cancellation
+func TestWithResourceContextCancellation(t *testing.T) {
+	initialState := resourceState{resourcesCreated: 0, resourcesReleased: 0}
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel() // Cancel immediately
+
+	cancelError := errors.New("context cancelled")
+
+	// Create should respect context cancellation
+	onCreate := func(s resourceState) ReaderIOResult[Pair[resourceState, mockResource]] {
+		return func(ctx context.Context) IOResult[Pair[resourceState, mockResource]] {
+			return func() Result[Pair[resourceState, mockResource]] {
+				if ctx.Err() != nil {
+					return RES.Left[Pair[resourceState, mockResource]](cancelError)
+				}
+				newState := resourceState{
+					resourcesCreated:  s.resourcesCreated + 1,
+					resourcesReleased: s.resourcesReleased,
+				}
+				res := mockResource{id: 1, isValid: true}
+				return RES.Of(P.MakePair(newState, res))
+			}
+		}
+	}
+
+	onRelease := func(res mockResource) StateReaderIOResult[resourceState, int] {
+		return func(s resourceState) ReaderIOResult[Pair[resourceState, int]] {
+			return func(ctx context.Context) IOResult[Pair[resourceState, int]] {
+				return func() Result[Pair[resourceState, int]] {
+					newState := resourceState{
+						resourcesCreated:  s.resourcesCreated,
+						resourcesReleased: s.resourcesReleased + 1,
+					}
+					return RES.Of(P.MakePair(newState, 0))
+				}
+			}
+		}
+	}
+
+	useResource := func(res mockResource) StateReaderIOResult[resourceState, string] {
+		return Of[resourceState]("should not reach here")
+	}
+
+	withResource := WithResource[string](onCreate, onRelease)
+	result := withResource(useResource)
+	outcome := result(initialState)(ctx)()
+
+	assert.True(t, RES.IsLeft(outcome))
+	RES.Fold(
+		func(err error) bool {
+			assert.Equal(t, cancelError, err)
+			return true
+		},
+		func(p Pair[resourceState, string]) bool {
+			t.Error("Expected error but got success")
+			return false
+		},
+	)(outcome)
+}

v2/context/statereaderioresult/state.go

@@ -0,0 +1,309 @@
+// Copyright (c) 2024 - 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 statereaderioresult
+
+import (
+	"context"
+
+	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/result"
+)
+
+// Left creates a StateReaderIOResult that represents a failed computation with the given error.
+// The error value is immediately available and does not depend on state or context.
+//
+// Example:
+//
+//	result := statereaderioresult.Left[AppState, string](errors.New("validation failed"))
+//	// Returns a failed computation that ignores state and context
+func Left[S, A any](e error) StateReaderIOResult[S, A] {
+	return function.Constant1[S](RIORES.Left[Pair[S, A]](e))
+}
+
+// Right creates a StateReaderIOResult that represents a successful computation with the given value.
+// The value is wrapped and the state is passed through unchanged.
+//
+// Example:
+//
+//	result := statereaderioresult.Right[AppState](42)
+//	// Returns a successful computation containing 42
+func Right[S, A any](a A) StateReaderIOResult[S, A] {
+	return statet.Of[StateReaderIOResult[S, A]](RIORES.Of[Pair[S, A]], a)
+}
+
+// Of creates a StateReaderIOResult that represents a successful computation with the given value.
+// This is the monadic return/pure operation for StateReaderIOResult.
+// Equivalent to [Right].
+//
+// Example:
+//
+//	result := statereaderioresult.Of[AppState](42)
+//	// Returns a successful computation containing 42
+func Of[S, A any](a A) StateReaderIOResult[S, A] {
+	return Right[S](a)
+}
+
+// MonadMap transforms the success value of a StateReaderIOResult using the provided function.
+// If the computation fails, the error is propagated unchanged.
+// The state is threaded through the computation.
+// This is the functor map operation.
+//
+// Example:
+//
+//	result := statereaderioresult.MonadMap(
+//	    statereaderioresult.Of[AppState](21),
+//	    func(x int) int { return x * 2 },
+//	) // Result contains 42
+func MonadMap[S, A, B any](fa StateReaderIOResult[S, A], f func(A) B) StateReaderIOResult[S, B] {
+	return statet.MonadMap[StateReaderIOResult[S, A], StateReaderIOResult[S, B]](
+		RIORES.MonadMap[Pair[S, A], Pair[S, B]],
+		fa,
+		f,
+	)
+}
+
+// Map is the curried version of [MonadMap].
+// Returns a function that transforms a StateReaderIOResult.
+//
+// Example:
+//
+//	double := statereaderioresult.Map[AppState](func(x int) int { return x * 2 })
+//	result := function.Pipe1(statereaderioresult.Of[AppState](21), double)
+func Map[S, A, B any](f func(A) B) Operator[S, A, B] {
+	return statet.Map[StateReaderIOResult[S, A], StateReaderIOResult[S, B]](
+		RIORES.Map[Pair[S, A], Pair[S, B]],
+		f,
+	)
+}
+
+// MonadChain sequences two computations, passing the result of the first to a function
+// that produces the second computation. This is the monadic bind operation.
+// The state is threaded through both computations.
+//
+// Example:
+//
+//	result := statereaderioresult.MonadChain(
+//	    statereaderioresult.Of[AppState](5),
+//	    func(x int) statereaderioresult.StateReaderIOResult[AppState, string] {
+//	        return statereaderioresult.Of[AppState](fmt.Sprintf("value: %d", x))
+//	    },
+//	)
+func MonadChain[S, A, B any](fa StateReaderIOResult[S, A], f Kleisli[S, A, B]) StateReaderIOResult[S, B] {
+	return statet.MonadChain(
+		RIORES.MonadChain[Pair[S, A], Pair[S, B]],
+		fa,
+		f,
+	)
+}
+
+// Chain is the curried version of [MonadChain].
+// Returns a function that sequences computations.
+//
+// Example:
+//
+//	stringify := statereaderioresult.Chain[AppState](func(x int) statereaderioresult.StateReaderIOResult[AppState, string] {
+//	    return statereaderioresult.Of[AppState](fmt.Sprintf("%d", x))
+//	})
+//	result := function.Pipe1(statereaderioresult.Of[AppState](42), stringify)
+func Chain[S, A, B any](f Kleisli[S, A, B]) Operator[S, A, B] {
+	return statet.Chain[StateReaderIOResult[S, A]](
+		RIORES.Chain[Pair[S, A], Pair[S, B]],
+		f,
+	)
+}
+
+// MonadAp applies a function wrapped in a StateReaderIOResult to a value wrapped in a StateReaderIOResult.
+// If either the function or the value fails, the error is propagated.
+// The state is threaded through both computations sequentially.
+// This is the applicative apply operation.
+//
+// Example:
+//
+//	fab := statereaderioresult.Of[AppState](func(x int) int { return x * 2 })
+//	fa := statereaderioresult.Of[AppState](21)
+//	result := statereaderioresult.MonadAp(fab, fa) // Result contains 42
+func MonadAp[B, S, A any](fab StateReaderIOResult[S, func(A) B], fa StateReaderIOResult[S, A]) StateReaderIOResult[S, B] {
+	return statet.MonadAp[StateReaderIOResult[S, A], StateReaderIOResult[S, B]](
+		RIORES.MonadMap[Pair[S, A], Pair[S, B]],
+		RIORES.MonadChain[Pair[S, func(A) B], Pair[S, B]],
+		fab,
+		fa,
+	)
+}
+
+// Ap is the curried version of [MonadAp].
+// Returns a function that applies a wrapped function to the given wrapped value.
+func Ap[B, S, A any](fa StateReaderIOResult[S, A]) Operator[S, func(A) B, B] {
+	return statet.Ap[StateReaderIOResult[S, A], StateReaderIOResult[S, B], StateReaderIOResult[S, func(A) B]](
+		RIORES.Map[Pair[S, A], Pair[S, B]],
+		RIORES.Chain[Pair[S, func(A) B], Pair[S, B]],
+		fa,
+	)
+}
+
+// FromReaderIOResult lifts a ReaderIOResult into a StateReaderIOResult.
+// The state is passed through unchanged.
+//
+// Example:
+//
+//	riores := readerioresult.Of(42)
+//	result := statereaderioresult.FromReaderIOResult[AppState](riores)
+func FromReaderIOResult[S, A any](fa ReaderIOResult[A]) StateReaderIOResult[S, A] {
+	return statet.FromF[StateReaderIOResult[S, A]](
+		RIORES.MonadMap[A],
+		fa,
+	)
+}
+
+// FromIOResult lifts an IOResult into a StateReaderIOResult.
+// The state is passed through unchanged and the context is ignored.
+func FromIOResult[S, A any](fa IOResult[A]) StateReaderIOResult[S, A] {
+	return FromReaderIOResult[S](RIORES.FromIOResult(fa))
+}
+
+// FromState lifts a State computation into a StateReaderIOResult.
+// The computation cannot fail (uses the error type).
+func FromState[S, A any](sa State[S, A]) StateReaderIOResult[S, A] {
+	return statet.FromState[StateReaderIOResult[S, A]](RIORES.Of[Pair[S, A]], sa)
+}
+
+// FromIO lifts an IO computation into a StateReaderIOResult.
+// The state is passed through unchanged and the context is ignored.
+func FromIO[S, A any](fa IO[A]) StateReaderIOResult[S, A] {
+	return FromReaderIOResult[S](RIORES.FromIO(fa))
+}
+
+// FromResult lifts a Result into a StateReaderIOResult.
+// The state is passed through unchanged and the context is ignored.
+//
+// Example:
+//
+//	result := statereaderioresult.FromResult[AppState](result.Of(42))
+func FromResult[S, A any](ma Result[A]) StateReaderIOResult[S, A] {
+	return result.Fold(Left[S, A], Right[S, A])(ma)
+}
+
+// Combinators
+
+// Local runs a computation with a modified context.
+// The function f transforms the context before passing it to the computation.
+//
+// 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
+//	    }
+//	)
+//	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))
+	}
+}
+
+// Asks creates a computation that derives a value from the context.
+// The function receives the context and returns a StateReaderIOResult.
+//
+// Example:
+//
+//	getValue := statereaderioresult.Asks[AppState, string](
+//	    func(ctx context.Context) statereaderioresult.StateReaderIOResult[AppState, string] {
+//	        return statereaderioresult.Of[AppState](ctx.Value("key").(string))
+//	    },
+//	)
+func Asks[S, A any](f func(context.Context) StateReaderIOResult[S, A]) StateReaderIOResult[S, A] {
+	return func(s S) ReaderIOResult[Pair[S, A]] {
+		return func(ctx context.Context) IOResult[Pair[S, A]] {
+			return f(ctx)(s)(ctx)
+		}
+	}
+}
+
+// FromResultK lifts a Result-returning function into a Kleisli arrow for StateReaderIOResult.
+//
+// Example:
+//
+//	validate := func(x int) result.Result[int] {
+//	    if x > 0 { return result.Of(x) }
+//	    return result.Error[int](errors.New("negative"))
+//	}
+//	kleisli := statereaderioresult.FromResultK[AppState](validate)
+func FromResultK[S, A, B any](f func(A) Result[B]) Kleisli[S, A, B] {
+	return function.Flow2(
+		f,
+		FromResult[S, B],
+	)
+}
+
+// FromIOK lifts an IO-returning function into a Kleisli arrow for StateReaderIOResult.
+func FromIOK[S, A, B any](f func(A) IO[B]) Kleisli[S, A, B] {
+	return function.Flow2(
+		f,
+		FromIO[S, B],
+	)
+}
+
+// FromIOResultK lifts an IOResult-returning function into a Kleisli arrow for StateReaderIOResult.
+func FromIOResultK[S, A, B any](f func(A) IOResult[B]) Kleisli[S, A, B] {
+	return function.Flow2(
+		f,
+		FromIOResult[S, B],
+	)
+}
+
+// FromReaderIOResultK lifts a ReaderIOResult-returning function into a Kleisli arrow for StateReaderIOResult.
+func FromReaderIOResultK[S, A, B any](f func(A) ReaderIOResult[B]) Kleisli[S, A, B] {
+	return function.Flow2(
+		f,
+		FromReaderIOResult[S, B],
+	)
+}
+
+// MonadChainReaderIOResultK chains a StateReaderIOResult with a ReaderIOResult-returning function.
+func MonadChainReaderIOResultK[S, A, B any](ma StateReaderIOResult[S, A], f func(A) ReaderIOResult[B]) StateReaderIOResult[S, B] {
+	return MonadChain(ma, FromReaderIOResultK[S](f))
+}
+
+// ChainReaderIOResultK is the curried version of [MonadChainReaderIOResultK].
+func ChainReaderIOResultK[S, A, B any](f func(A) ReaderIOResult[B]) Operator[S, A, B] {
+	return Chain(FromReaderIOResultK[S](f))
+}
+
+// MonadChainIOResultK chains a StateReaderIOResult with an IOResult-returning function.
+func MonadChainIOResultK[S, A, B any](ma StateReaderIOResult[S, A], f func(A) IOResult[B]) StateReaderIOResult[S, B] {
+	return MonadChain(ma, FromIOResultK[S](f))
+}
+
+// ChainIOResultK is the curried version of [MonadChainIOResultK].
+func ChainIOResultK[S, A, B any](f func(A) IOResult[B]) Operator[S, A, B] {
+	return Chain(FromIOResultK[S](f))
+}
+
+// MonadChainResultK chains a StateReaderIOResult with a Result-returning function.
+func MonadChainResultK[S, A, B any](ma StateReaderIOResult[S, A], f func(A) Result[B]) StateReaderIOResult[S, B] {
+	return MonadChain(ma, FromResultK[S](f))
+}
+
+// ChainResultK is the curried version of [MonadChainResultK].
+func ChainResultK[S, A, B any](f func(A) Result[B]) Operator[S, A, B] {
+	return Chain(FromResultK[S](f))
+}

v2/context/statereaderioresult/statereaderioresult_test.go

@@ -0,0 +1,567 @@
+// Copyright (c) 2024 - 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 statereaderioresult
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"testing"
+
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/io"
+	IOR "github.com/IBM/fp-go/v2/ioresult"
+	N "github.com/IBM/fp-go/v2/number"
+	P "github.com/IBM/fp-go/v2/pair"
+	RES "github.com/IBM/fp-go/v2/result"
+	"github.com/stretchr/testify/assert"
+)
+
+type testState struct {
+	counter int
+}
+
+func TestOf(t *testing.T) {
+	state := testState{counter: 0}
+	ctx := context.Background()
+	result := Of[testState](42)
+	res := result(state)(ctx)()
+
+	assert.True(t, RES.IsRight(res))
+	RES.Fold(
+		func(err error) bool {
+			t.Fatalf("Expected Success but got Error: %v", err)
+			return false
+		},
+		func(p P.Pair[testState, int]) bool {
+			assert.Equal(t, 42, P.Tail(p))
+			assert.Equal(t, 0, P.Head(p).counter) // State unchanged
+			return true
+		},
+	)(res)
+}
+
+func TestRight(t *testing.T) {
+	state := testState{counter: 5}
+	ctx := context.Background()
+	result := Right[testState](100)
+	res := result(state)(ctx)()
+
+	assert.True(t, RES.IsRight(res))
+	RES.Map(func(p P.Pair[testState, int]) P.Pair[testState, int] {
+		assert.Equal(t, 100, P.Tail(p))
+		assert.Equal(t, 5, P.Head(p).counter)
+		return p
+	})(res)
+}
+
+func TestLeft(t *testing.T) {
+	state := testState{counter: 10}
+	ctx := context.Background()
+	testErr := errors.New("test error")
+	result := Left[testState, int](testErr)
+	res := result(state)(ctx)()
+
+	assert.True(t, RES.IsLeft(res))
+}
+
+func TestMonadMap(t *testing.T) {
+	state := testState{counter: 0}
+	ctx := context.Background()
+
+	result := MonadMap(
+		Of[testState](21),
+		N.Mul(2),
+	)
+	res := result(state)(ctx)()
+
+	assert.True(t, RES.IsRight(res))
+	RES.Map(func(p P.Pair[testState, int]) P.Pair[testState, int] {
+		assert.Equal(t, 42, P.Tail(p))
+		return p
+	})(res)
+}
+
+func TestMap(t *testing.T) {
+	state := testState{counter: 0}
+	ctx := context.Background()
+
+	result := F.Pipe1(
+		Of[testState](21),
+		Map[testState](N.Mul(2)),
+	)
+	res := result(state)(ctx)()
+
+	assert.True(t, RES.IsRight(res))
+	RES.Map(func(p P.Pair[testState, int]) P.Pair[testState, int] {
+		assert.Equal(t, 42, P.Tail(p))
+		return p
+	})(res)
+}
+
+func TestMonadChain(t *testing.T) {
+	state := testState{counter: 0}
+	ctx := context.Background()
+
+	result := MonadChain(
+		Of[testState](5),
+		func(x int) StateReaderIOResult[testState, string] {
+			return Of[testState](fmt.Sprintf("value: %d", x))
+		},
+	)
+	res := result(state)(ctx)()
+
+	assert.True(t, RES.IsRight(res))
+	RES.Map(func(p P.Pair[testState, string]) P.Pair[testState, string] {
+		assert.Equal(t, "value: 5", P.Tail(p))
+		return p
+	})(res)
+}
+
+func TestChain(t *testing.T) {
+	state := testState{counter: 0}
+	ctx := context.Background()
+
+	result := F.Pipe1(
+		Of[testState](5),
+		Chain(func(x int) StateReaderIOResult[testState, string] {
+			return Of[testState](fmt.Sprintf("value: %d", x))
+		}),
+	)
+	res := result(state)(ctx)()
+
+	assert.True(t, RES.IsRight(res))
+	RES.Map(func(p P.Pair[testState, string]) P.Pair[testState, string] {
+		assert.Equal(t, "value: 5", P.Tail(p))
+		return p
+	})(res)
+}
+
+func TestMonadAp(t *testing.T) {
+	state := testState{counter: 0}
+	ctx := context.Background()
+
+	fab := Of[testState](N.Mul(2))
+	fa := Of[testState](21)
+	result := MonadAp(fab, fa)
+	res := result(state)(ctx)()
+
+	assert.True(t, RES.IsRight(res))
+	RES.Map(func(p P.Pair[testState, int]) P.Pair[testState, int] {
+		assert.Equal(t, 42, P.Tail(p))
+		return p
+	})(res)
+}
+
+func TestAp(t *testing.T) {
+	state := testState{counter: 0}
+	ctx := context.Background()
+
+	fa := Of[testState](21)
+	result := F.Pipe1(
+		Of[testState](N.Mul(2)),
+		Ap[int](fa),
+	)
+	res := result(state)(ctx)()
+
+	assert.True(t, RES.IsRight(res))
+	RES.Map(func(p P.Pair[testState, int]) P.Pair[testState, int] {
+		assert.Equal(t, 42, P.Tail(p))
+		return p
+	})(res)
+}
+
+func TestFromIOResult(t *testing.T) {
+	state := testState{counter: 3}
+	ctx := context.Background()
+
+	ior := IOR.Of(55)
+	result := FromIOResult[testState](ior)
+	res := result(state)(ctx)()
+
+	assert.True(t, RES.IsRight(res))
+	RES.Map(func(p P.Pair[testState, int]) P.Pair[testState, int] {
+		assert.Equal(t, 55, P.Tail(p))
+		assert.Equal(t, 3, P.Head(p).counter)
+		return p
+	})(res)
+}
+
+func TestFromState(t *testing.T) {
+	initialState := testState{counter: 10}
+	ctx := context.Background()
+
+	// State computation that increments counter and returns it
+	stateComp := func(s testState) P.Pair[testState, int] {
+		newState := testState{counter: s.counter + 1}
+		return P.MakePair(newState, newState.counter)
+	}
+
+	result := FromState(stateComp)
+	res := result(initialState)(ctx)()
+
+	assert.True(t, RES.IsRight(res))
+	RES.Map(func(p P.Pair[testState, int]) P.Pair[testState, int] {
+		assert.Equal(t, 11, P.Tail(p)) // Incremented value
+		assert.Equal(t, 11, P.Head(p).counter) // State updated
+		return p
+	})(res)
+}
+
+func TestFromIO(t *testing.T) {
+	state := testState{counter: 8}
+	ctx := context.Background()
+
+	ioVal := func() int { return 99 }
+	result := FromIO[testState](ioVal)
+	res := result(state)(ctx)()
+
+	assert.True(t, RES.IsRight(res))
+	RES.Map(func(p P.Pair[testState, int]) P.Pair[testState, int] {
+		assert.Equal(t, 99, P.Tail(p))
+		assert.Equal(t, 8, P.Head(p).counter)
+		return p
+	})(res)
+}
+
+func TestFromResult(t *testing.T) {
+	state := testState{counter: 12}
+	ctx := context.Background()
+
+	// Test Success case
+	resultSuccess := FromResult[testState](RES.Of(42))
+	resSuccess := resultSuccess(state)(ctx)()
+	assert.True(t, RES.IsRight(resSuccess))
+
+	// Test Error case
+	resultError := FromResult[testState](RES.Left[int](errors.New("error")))
+	resError := resultError(state)(ctx)()
+	assert.True(t, RES.IsLeft(resError))
+}
+
+func TestLocal(t *testing.T) {
+	state := testState{counter: 0}
+	ctx := context.WithValue(context.Background(), "key", "value1")
+
+	// Create a computation that uses the context
+	comp := Asks(func(c context.Context) StateReaderIOResult[testState, string] {
+		val := c.Value("key").(string)
+		return Of[testState](val)
+	})
+
+	// Modify context before running computation
+	result := Local[testState, string](
+		func(c context.Context) context.Context {
+			return context.WithValue(c, "key", "value2")
+		},
+	)(comp)
+
+	res := result(state)(ctx)()
+	assert.True(t, RES.IsRight(res))
+	RES.Map(func(p P.Pair[testState, string]) P.Pair[testState, string] {
+		assert.Equal(t, "value2", P.Tail(p))
+		return p
+	})(res)
+}
+
+func TestAsks(t *testing.T) {
+	state := testState{counter: 0}
+	ctx := context.WithValue(context.Background(), "multiplier", 7)
+
+	result := Asks(func(c context.Context) StateReaderIOResult[testState, int] {
+		mult := c.Value("multiplier").(int)
+		return Of[testState](mult * 5)
+	})
+
+	res := result(state)(ctx)()
+	assert.True(t, RES.IsRight(res))
+	RES.Map(func(p P.Pair[testState, int]) P.Pair[testState, int] {
+		assert.Equal(t, 35, P.Tail(p))
+		return p
+	})(res)
+}
+
+func TestFromResultK(t *testing.T) {
+	state := testState{counter: 0}
+	ctx := context.Background()
+
+	validate := func(x int) RES.Result[int] {
+		if x > 0 {
+			return RES.Of(x * 2)
+		}
+		return RES.Left[int](errors.New("negative"))
+	}
+
+	kleisli := FromResultK[testState](validate)
+
+	// Test with valid input
+	resultValid := kleisli(5)
+	resValid := resultValid(state)(ctx)()
+	assert.True(t, RES.IsRight(resValid))
+	RES.Map(func(p P.Pair[testState, int]) P.Pair[testState, int] {
+		assert.Equal(t, 10, P.Tail(p))
+		return p
+	})(resValid)
+
+	// Test with invalid input
+	resultInvalid := kleisli(-5)
+	resInvalid := resultInvalid(state)(ctx)()
+	assert.True(t, RES.IsLeft(resInvalid))
+}
+
+func TestFromIOK(t *testing.T) {
+	state := testState{counter: 0}
+	ctx := context.Background()
+
+	ioFunc := func(x int) io.IO[int] {
+		return func() int { return x * 3 }
+	}
+
+	kleisli := FromIOK[testState](ioFunc)
+	result := kleisli(7)
+	res := result(state)(ctx)()
+
+	assert.True(t, RES.IsRight(res))
+	RES.Map(func(p P.Pair[testState, int]) P.Pair[testState, int] {
+		assert.Equal(t, 21, P.Tail(p))
+		return p
+	})(res)
+}
+
+func TestFromIOResultK(t *testing.T) {
+	state := testState{counter: 0}
+	ctx := context.Background()
+
+	iorFunc := func(x int) IOR.IOResult[int] {
+		if x > 0 {
+			return IOR.Of(x * 4)
+		}
+		return IOR.Left[int](errors.New("invalid"))
+	}
+
+	kleisli := FromIOResultK[testState](iorFunc)
+	result := kleisli(3)
+	res := result(state)(ctx)()
+
+	assert.True(t, RES.IsRight(res))
+	RES.Map(func(p P.Pair[testState, int]) P.Pair[testState, int] {
+		assert.Equal(t, 12, P.Tail(p))
+		return p
+	})(res)
+}
+
+func TestChainResultK(t *testing.T) {
+	state := testState{counter: 0}
+	ctx := context.Background()
+
+	validate := func(x int) RES.Result[string] {
+		if x > 0 {
+			return RES.Of(fmt.Sprintf("valid: %d", x))
+		}
+		return RES.Left[string](errors.New("invalid"))
+	}
+
+	result := F.Pipe1(
+		Of[testState](42),
+		ChainResultK[testState](validate),
+	)
+
+	res := result(state)(ctx)()
+	assert.True(t, RES.IsRight(res))
+	RES.Map(func(p P.Pair[testState, string]) P.Pair[testState, string] {
+		assert.Equal(t, "valid: 42", P.Tail(p))
+		return p
+	})(res)
+}
+
+func TestChainIOResultK(t *testing.T) {
+	state := testState{counter: 0}
+	ctx := context.Background()
+
+	iorFunc := func(x int) IOR.IOResult[string] {
+		return IOR.Of(fmt.Sprintf("result: %d", x))
+	}
+
+	result := F.Pipe1(
+		Of[testState](100),
+		ChainIOResultK[testState](iorFunc),
+	)
+
+	res := result(state)(ctx)()
+	assert.True(t, RES.IsRight(res))
+	RES.Map(func(p P.Pair[testState, string]) P.Pair[testState, string] {
+		assert.Equal(t, "result: 100", P.Tail(p))
+		return p
+	})(res)
+}
+
+func TestDo(t *testing.T) {
+	state := testState{counter: 0}
+	ctx := context.Background()
+
+	type Result struct {
+		value int
+	}
+
+	result := Do[testState](Result{})
+	res := result(state)(ctx)()
+
+	assert.True(t, RES.IsRight(res))
+	RES.Map(func(p P.Pair[testState, Result]) P.Pair[testState, Result] {
+		assert.Equal(t, 0, P.Tail(p).value)
+		return p
+	})(res)
+}
+
+func TestBindTo(t *testing.T) {
+	state := testState{counter: 0}
+	ctx := context.Background()
+
+	type Result struct {
+		value int
+	}
+
+	result := F.Pipe1(
+		Of[testState](42),
+		BindTo[testState](func(v int) Result {
+			return Result{value: v}
+		}),
+	)
+
+	res := result(state)(ctx)()
+	assert.True(t, RES.IsRight(res))
+	RES.Map(func(p P.Pair[testState, Result]) P.Pair[testState, Result] {
+		assert.Equal(t, 42, P.Tail(p).value)
+		return p
+	})(res)
+}
+
+func TestStatefulComputation(t *testing.T) {
+	initialState := testState{counter: 0}
+	ctx := context.Background()
+
+	// Create a computation that modifies state
+	incrementAndGet := func(s testState) P.Pair[testState, int] {
+		newState := testState{counter: s.counter + 1}
+		return P.MakePair(newState, newState.counter)
+	}
+
+	// Chain multiple stateful operations
+	result := F.Pipe2(
+		FromState(incrementAndGet),
+		Chain(func(v1 int) StateReaderIOResult[testState, int] {
+			return FromState(incrementAndGet)
+		}),
+		Chain(func(v2 int) StateReaderIOResult[testState, int] {
+			return FromState(incrementAndGet)
+		}),
+	)
+
+	res := result(initialState)(ctx)()
+	assert.True(t, RES.IsRight(res))
+	RES.Map(func(p P.Pair[testState, int]) P.Pair[testState, int] {
+		assert.Equal(t, 3, P.Tail(p)) // Last incremented value
+		assert.Equal(t, 3, P.Head(p).counter) // State updated three times
+		return p
+	})(res)
+}
+
+func TestErrorPropagation(t *testing.T) {
+	state := testState{counter: 0}
+	ctx := context.Background()
+
+	testErr := errors.New("test error")
+
+	// Chain operations where the second one fails
+	result := F.Pipe1(
+		Of[testState](42),
+		Chain(func(x int) StateReaderIOResult[testState, int] {
+			return Left[testState, int](testErr)
+		}),
+	)
+
+	res := result(state)(ctx)()
+	assert.True(t, RES.IsLeft(res))
+}
+
+func TestPointed(t *testing.T) {
+	p := Pointed[testState, int]()
+	assert.NotNil(t, p)
+
+	result := p.Of(42)
+	state := testState{counter: 0}
+	ctx := context.Background()
+	res := result(state)(ctx)()
+
+	assert.True(t, RES.IsRight(res))
+}
+
+func TestFunctor(t *testing.T) {
+	f := Functor[testState, int, string]()
+	assert.NotNil(t, f)
+
+	mapper := f.Map(func(x int) string { return fmt.Sprintf("%d", x) })
+	result := mapper(Of[testState](42))
+
+	state := testState{counter: 0}
+	ctx := context.Background()
+	res := result(state)(ctx)()
+
+	assert.True(t, RES.IsRight(res))
+	RES.Map(func(p P.Pair[testState, string]) P.Pair[testState, string] {
+		assert.Equal(t, "42", P.Tail(p))
+		return p
+	})(res)
+}
+
+func TestApplicative(t *testing.T) {
+	a := Applicative[testState, int, string]()
+	assert.NotNil(t, a)
+
+	fab := Of[testState](func(x int) string { return fmt.Sprintf("%d", x) })
+	fa := Of[testState](42)
+	result := a.Ap(fa)(fab)
+
+	state := testState{counter: 0}
+	ctx := context.Background()
+	res := result(state)(ctx)()
+
+	assert.True(t, RES.IsRight(res))
+	RES.Map(func(p P.Pair[testState, string]) P.Pair[testState, string] {
+		assert.Equal(t, "42", P.Tail(p))
+		return p
+	})(res)
+}
+
+func TestMonad(t *testing.T) {
+	m := Monad[testState, int, string]()
+	assert.NotNil(t, m)
+
+	fa := m.Of(42)
+	result := m.Chain(func(x int) StateReaderIOResult[testState, string] {
+		return Of[testState](fmt.Sprintf("%d", x))
+	})(fa)
+
+	state := testState{counter: 0}
+	ctx := context.Background()
+	res := result(state)(ctx)()
+
+	assert.True(t, RES.IsRight(res))
+	RES.Map(func(p P.Pair[testState, string]) P.Pair[testState, string] {
+		assert.Equal(t, "42", P.Tail(p))
+		return p
+	})(res)
+}

v2/context/statereaderioresult/testing/laws.go

@@ -0,0 +1,87 @@
+// Copyright (c) 2024 - 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 testing
+
+import (
+	"context"
+	"testing"
+
+	EQ "github.com/IBM/fp-go/v2/eq"
+	L "github.com/IBM/fp-go/v2/internal/monad/testing"
+	P "github.com/IBM/fp-go/v2/pair"
+	RES "github.com/IBM/fp-go/v2/result"
+	RIORES "github.com/IBM/fp-go/v2/context/readerioresult"
+	ST "github.com/IBM/fp-go/v2/context/statereaderioresult"
+)
+
+// AssertLaws asserts the monad laws for the StateReaderIOResult monad
+func AssertLaws[S, A, B, C any](t *testing.T,
+	eqs EQ.Eq[S],
+	eqa EQ.Eq[A],
+	eqb EQ.Eq[B],
+	eqc EQ.Eq[C],
+
+	ab func(A) B,
+	bc func(B) C,
+
+	s S,
+	ctx context.Context,
+) func(a A) bool {
+
+	eqra := RIORES.Eq(RES.Eq(P.Eq(eqs, eqa)))(ctx)
+	eqrb := RIORES.Eq(RES.Eq(P.Eq(eqs, eqb)))(ctx)
+	eqrc := RIORES.Eq(RES.Eq(P.Eq(eqs, eqc)))(ctx)
+
+	fofc := ST.Pointed[S, C]()
+	fofaa := ST.Pointed[S, func(A) A]()
+	fofbc := ST.Pointed[S, func(B) C]()
+	fofabb := ST.Pointed[S, func(func(A) B) B]()
+
+	fmap := ST.Functor[S, func(B) C, func(func(A) B) func(A) C]()
+
+	fapabb := ST.Applicative[S, func(A) B, B]()
+	fapabac := ST.Applicative[S, func(A) B, func(A) C]()
+
+	maa := ST.Monad[S, A, A]()
+	mab := ST.Monad[S, A, B]()
+	mac := ST.Monad[S, A, C]()
+	mbc := ST.Monad[S, B, C]()
+
+	return L.MonadAssertLaws(t,
+		ST.Eq(eqra)(s),
+		ST.Eq(eqrb)(s),
+		ST.Eq(eqrc)(s),
+
+		fofc,
+		fofaa,
+		fofbc,
+		fofabb,
+
+		fmap,
+
+		fapabb,
+		fapabac,
+
+		maa,
+		mab,
+		mac,
+		mbc,
+
+		ab,
+		bc,
+	)
+
+}

v2/context/statereaderioresult/testing/laws_test.go

@@ -0,0 +1,50 @@
+// Copyright (c) 2024 - 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 testing
+
+import (
+	"context"
+	"fmt"
+	"testing"
+
+	A "github.com/IBM/fp-go/v2/array"
+	EQ "github.com/IBM/fp-go/v2/eq"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestMonadLaws(t *testing.T) {
+	// some comparison
+	eqs := A.Eq(EQ.FromStrictEquals[string]())
+	eqa := EQ.FromStrictEquals[bool]()
+	eqb := EQ.FromStrictEquals[int]()
+	eqc := EQ.FromStrictEquals[string]()
+
+	ab := func(a bool) int {
+		if a {
+			return 1
+		}
+		return 0
+	}
+
+	bc := func(b int) string {
+		return fmt.Sprintf("value %d", b)
+	}
+
+	laws := AssertLaws(t, eqs, eqa, eqb, eqc, ab, bc, A.Empty[string](), context.Background())
+
+	assert.True(t, laws(true))
+	assert.True(t, laws(false))
+}

v2/context/statereaderioresult/type.go

@@ -0,0 +1,84 @@
+// Copyright (c) 2024 - 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 statereaderioresult
+
+import (
+	RIORES "github.com/IBM/fp-go/v2/context/readerioresult"
+	"github.com/IBM/fp-go/v2/endomorphism"
+	"github.com/IBM/fp-go/v2/io"
+	"github.com/IBM/fp-go/v2/ioresult"
+	"github.com/IBM/fp-go/v2/optics/iso/lens"
+	"github.com/IBM/fp-go/v2/pair"
+	"github.com/IBM/fp-go/v2/reader"
+	"github.com/IBM/fp-go/v2/result"
+	"github.com/IBM/fp-go/v2/state"
+)
+
+type (
+	// Endomorphism represents a function from A to A.
+	Endomorphism[A any] = endomorphism.Endomorphism[A]
+
+	// Lens is an optic that focuses on a field of type A within a structure of type S.
+	Lens[S, A any] = lens.Lens[S, A]
+
+	// State represents a stateful computation that takes an initial state S and returns
+	// a pair of the new state S and a value A.
+	State[S, A any] = state.State[S, A]
+
+	// Pair represents a tuple of two values.
+	Pair[L, R any] = pair.Pair[L, R]
+
+	// Reader represents a computation that depends on an environment/context of type R
+	// and produces a value of type A.
+	Reader[R, A any] = reader.Reader[R, A]
+
+	// Result represents a value that can be either an error or a success value.
+	// This is specialized to use [error] as the error type.
+	Result[A any] = result.Result[A]
+
+	// IO represents a computation that performs side effects and produces a value of type A.
+	IO[A any] = io.IO[A]
+
+	// IOResult represents a computation that performs side effects and can fail with an error
+	// or succeed with a value A.
+	IOResult[A any] = ioresult.IOResult[A]
+
+	// ReaderIOResult represents a computation that depends on a context.Context,
+	// performs side effects, and can fail with an error or succeed with a value A.
+	ReaderIOResult[A any] = RIORES.ReaderIOResult[A]
+
+	// StateReaderIOResult represents a stateful computation that:
+	//   - Takes an initial state S
+	//   - Depends on a [context.Context]
+	//   - Performs side effects (IO)
+	//   - Can fail with an [error] or succeed with a value A
+	//   - Returns a pair of the new state S and the result
+	//
+	// This is the main type of this package, combining State, Reader, IO, and Result monads.
+	// It is a specialization of StateReaderIOEither with:
+	//   - Context type fixed to [context.Context]
+	//   - Error type fixed to [error]
+	StateReaderIOResult[S, A any] = Reader[S, ReaderIOResult[Pair[S, A]]]
+
+	// Kleisli represents a Kleisli arrow - a function that takes a value A and returns
+	// a StateReaderIOResult computation producing B.
+	// This is used for monadic composition via Chain.
+	Kleisli[S, A, B any] = Reader[A, StateReaderIOResult[S, B]]
+
+	// Operator represents a function that transforms one StateReaderIOResult into another.
+	// This is commonly used for building composable operations via Map, Chain, etc.
+	Operator[S, A, B any] = Reader[StateReaderIOResult[S, A], StateReaderIOResult[S, B]]
+)

v2/coverage.txt

@@ -0,0 +1,67 @@
+mode: set
+github.com/IBM/fp-go/v2/readerresult/array.go:33.74,35.2 1 0
+github.com/IBM/fp-go/v2/readerresult/array.go:49.98,51.2 1 0
+github.com/IBM/fp-go/v2/readerresult/array.go:68.76,70.2 1 1
+github.com/IBM/fp-go/v2/readerresult/bind.go:42.22,44.2 1 1
+github.com/IBM/fp-go/v2/readerresult/bind.go:93.49,95.2 1 1
+github.com/IBM/fp-go/v2/readerresult/bind.go:103.49,105.2 1 0
+github.com/IBM/fp-go/v2/readerresult/bind.go:113.49,115.2 1 0
+github.com/IBM/fp-go/v2/readerresult/bind.go:122.22,124.2 1 0
+github.com/IBM/fp-go/v2/readerresult/bind.go:172.49,174.2 1 1
+github.com/IBM/fp-go/v2/readerresult/bind.go:211.21,213.2 1 0
+github.com/IBM/fp-go/v2/readerresult/bind.go:252.21,254.2 1 0
+github.com/IBM/fp-go/v2/readerresult/bind.go:288.21,290.2 1 0
+github.com/IBM/fp-go/v2/readerresult/bind.go:321.21,323.2 1 0
+github.com/IBM/fp-go/v2/readerresult/curry.go:35.64,37.2 1 1
+github.com/IBM/fp-go/v2/readerresult/curry.go:46.81,48.2 1 1
+github.com/IBM/fp-go/v2/readerresult/curry.go:58.98,60.2 1 1
+github.com/IBM/fp-go/v2/readerresult/curry.go:69.115,71.2 1 1
+github.com/IBM/fp-go/v2/readerresult/curry.go:81.83,83.2 1 1
+github.com/IBM/fp-go/v2/readerresult/curry.go:92.100,94.2 1 1
+github.com/IBM/fp-go/v2/readerresult/curry.go:103.117,105.2 1 1
+github.com/IBM/fp-go/v2/readerresult/from.go:33.70,35.2 1 1
+github.com/IBM/fp-go/v2/readerresult/from.go:45.80,47.2 1 1
+github.com/IBM/fp-go/v2/readerresult/from.go:57.92,59.2 1 1
+github.com/IBM/fp-go/v2/readerresult/from.go:69.104,71.2 1 1
+github.com/IBM/fp-go/v2/readerresult/monoid.go:37.62,45.2 1 1
+github.com/IBM/fp-go/v2/readerresult/monoid.go:64.70,69.2 1 1
+github.com/IBM/fp-go/v2/readerresult/monoid.go:91.62,98.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:41.59,43.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:49.59,51.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:61.63,63.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:73.66,75.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:85.49,87.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:98.46,100.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:106.62,108.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:120.83,122.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:133.54,135.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:147.92,149.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:160.63,162.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:173.43,175.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:189.101,191.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:197.71,199.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:215.89,217.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:234.131,236.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:249.100,251.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:265.70,267.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:282.81,289.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:303.38,305.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:317.56,319.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:330.103,337.2 1 0
+github.com/IBM/fp-go/v2/readerresult/reader.go:348.74,354.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:367.97,369.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:381.84,383.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:395.108,397.2 1 0
+github.com/IBM/fp-go/v2/readerresult/reader.go:409.79,411.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:426.88,428.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:440.61,442.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:453.85,455.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:460.55,462.2 1 0
+github.com/IBM/fp-go/v2/readerresult/reader.go:473.94,475.2 1 0
+github.com/IBM/fp-go/v2/readerresult/reader.go:486.65,488.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:494.103,502.2 1 1
+github.com/IBM/fp-go/v2/readerresult/reader.go:508.71,515.2 1 0
+github.com/IBM/fp-go/v2/readerresult/sequence.go:35.78,40.2 1 1
+github.com/IBM/fp-go/v2/readerresult/sequence.go:54.35,60.2 1 1
+github.com/IBM/fp-go/v2/readerresult/sequence.go:75.38,82.2 1 1
+github.com/IBM/fp-go/v2/readerresult/sequence.go:95.41,103.2 1 1

v2/di/app.go

@@ -19,7 +19,7 @@ import (
 	DIE "github.com/IBM/fp-go/v2/di/erasure"
 	F "github.com/IBM/fp-go/v2/function"
 	IO "github.com/IBM/fp-go/v2/io"
-	IOE "github.com/IBM/fp-go/v2/ioeither"
+	IOR "github.com/IBM/fp-go/v2/ioresult"
 )
 
 var (
@@ -34,5 +34,5 @@ var (
 var RunMain = F.Flow3(
 	DIE.MakeInjector,
 	Main,
-	IOE.Fold(IO.Of[error], F.Constant1[any](IO.Of[error](nil))),
+	IOR.Fold(IO.Of[error], F.Constant1[any](IO.Of[error](nil))),
 )

v2/di/doc.go

@@ -64,8 +64,8 @@ Creating and using dependencies:
 	dbProvider := di.MakeProvider1(
 		DBToken,
 		ConfigToken.Identity(),
-		func(cfg Config) IOE.IOEither[error, Database] {
+		func(cfg Config) IOResult[Database] {
-			return IOE.Of[error](NewDatabase(cfg))
+			return ioresult.Of(NewDatabase(cfg))
 		},
 	)
 
@@ -73,8 +73,8 @@ Creating and using dependencies:
 		APIToken,
 		ConfigToken.Identity(),
 		DBToken.Identity(),
-		func(cfg Config, db Database) IOE.IOEither[error, APIService] {
+		func(cfg Config, db Database) IOResult[APIService] {
-			return IOE.Of[error](NewAPIService(cfg, db))
+			return ioresult.Of(NewAPIService(cfg, db))
 		},
 	)
 
@@ -116,7 +116,7 @@ MakeProvider0 - No dependencies:
 
 	provider := di.MakeProvider0(
 		token,
-		IOE.Of[error](value),
+		ioresult.Of(value),
 	)
 
 MakeProvider1 - One dependency:
@@ -124,8 +124,8 @@ MakeProvider1 - One dependency:
 	provider := di.MakeProvider1(
 		resultToken,
 		dep1Token.Identity(),
-		func(dep1 Dep1Type) IOE.IOEither[error, ResultType] {
+		func(dep1 Dep1Type) IOResult[ResultType] {
-			return IOE.Of[error](createResult(dep1))
+			return ioresult.Of(createResult(dep1))
 		},
 	)
 
@@ -135,8 +135,8 @@ MakeProvider2 - Two dependencies:
 		resultToken,
 		dep1Token.Identity(),
 		dep2Token.Identity(),
-		func(dep1 Dep1Type, dep2 Dep2Type) IOE.IOEither[error, ResultType] {
+		func(dep1 Dep1Type, dep2 Dep2Type) IOResult[ResultType] {
-			return IOE.Of[error](createResult(dep1, dep2))
+			return ioresult.Of(createResult(dep1, dep2))
 		},
 	)
 
@@ -153,7 +153,7 @@ provider is registered:
 
 	token := di.MakeTokenWithDefault0(
 		"ServiceName",
-		IOE.Of[error](defaultImplementation),
+		ioresult.Of(defaultImplementation),
 	)
 
 	// Or with dependencies
@@ -161,8 +161,8 @@ provider is registered:
 		"ServiceName",
 		dep1Token.Identity(),
 		dep2Token.Identity(),
-		func(dep1 Dep1Type, dep2 Dep2Type) IOE.IOEither[error, ResultType] {
+		func(dep1 Dep1Type, dep2 Dep2Type) IOResult[ResultType] {
-			return IOE.Of[error](createDefault(dep1, dep2))
+			return ioresult.Of(createDefault(dep1, dep2))
 		},
 	)
 
@@ -208,8 +208,8 @@ The framework provides a convenient pattern for running applications:
 	mainProvider := di.MakeProvider1(
 		di.InjMain,
 		APIToken.Identity(),
-		func(api APIService) IOE.IOEither[error, any] {
+		func(api APIService) IOResult[any] {
-			return IOE.Of[error](api.Start())
+			return ioresult.Of(api.Start())
 		},
 	)
 
@@ -247,8 +247,8 @@ Example 1: Configuration-based Service
 	clientProvider := di.MakeProvider1(
 		ClientToken,
 		ConfigToken.Identity(),
-		func(cfg Config) IOE.IOEither[error, HTTPClient] {
+		func(cfg Config) IOResult[HTTPClient] {
-			return IOE.Of[error](HTTPClient{config: cfg})
+			return ioresult.Of(HTTPClient{config: cfg})
 		},
 	)
 
@@ -263,8 +263,8 @@ Example 2: Optional Dependencies
 	serviceProvider := di.MakeProvider1(
 		ServiceToken,
 		CacheToken.Option(), // Optional dependency
-		func(cache O.Option[Cache]) IOE.IOEither[error, Service] {
+		func(cache Option[Cache]) IOResult[Service] {
-			return IOE.Of[error](NewService(cache))
+			return ioresult.Of(NewService(cache))
 		},
 	)
 
@@ -279,8 +279,8 @@ Example 3: Lazy Dependencies
 	reporterProvider := di.MakeProvider1(
 		ReporterToken,
 		DBToken.IOEither(), // Lazy dependency
-		func(dbIO IOE.IOEither[error, Database]) IOE.IOEither[error, Reporter] {
+		func(dbIO IOResult[Database]) IOResult[Reporter] {
-			return IOE.Of[error](NewReporter(dbIO))
+			return ioresult.Of(NewReporter(dbIO))
 		},
 	)
 

v2/di/erasure/injector.go

@@ -20,7 +20,7 @@ import (
 	"github.com/IBM/fp-go/v2/errors"
 	F "github.com/IBM/fp-go/v2/function"
 	I "github.com/IBM/fp-go/v2/identity"
-	IOE "github.com/IBM/fp-go/v2/ioeither"
+	IOR "github.com/IBM/fp-go/v2/ioresult"
 	L "github.com/IBM/fp-go/v2/lazy"
 	O "github.com/IBM/fp-go/v2/option"
 	R "github.com/IBM/fp-go/v2/record"
@@ -42,8 +42,8 @@ var (
 	missingProviderError = F.Flow4(
 		Dependency.String,
 		errors.OnSome[string]("no provider for dependency [%s]"),
-		IOE.Left[any, error],
+		IOR.Left[any],
-		F.Constant1[InjectableFactory, IOE.IOEither[error, any]],
+		F.Constant1[InjectableFactory, IOResult[any]],
 	)
 
 	// missingProviderErrorOrDefault returns the default [ProviderFactory] or an error
@@ -56,7 +56,7 @@ var (
 	emptyMulti any = A.Empty[any]()
 
 	// emptyMultiDependency returns a [ProviderFactory] for an empty, multi dependency
-	emptyMultiDependency = F.Constant1[Dependency](F.Constant1[InjectableFactory](IOE.Of[error](emptyMulti)))
+	emptyMultiDependency = F.Constant1[Dependency](F.Constant1[InjectableFactory](IOR.Of(emptyMulti)))
 
 	// handleMissingProvider covers the case of a missing provider. It either
 	// returns an error or an empty multi value provider
@@ -93,21 +93,21 @@ var (
 
 // isMultiDependency tests if a dependency is a container dependency
 func isMultiDependency(dep Dependency) bool {
-	return dep.Flag()&Multi == Multi
+	return dep.Flag()&MULTI == MULTI
 }
 
 // isItemProvider tests if a provivder provides a single item
 func isItemProvider(provider Provider) bool {
-	return provider.Provides().Flag()&Item == Item
+	return provider.Provides().Flag()&ITEM == ITEM
 }
 
 // itemProviderFactory combines multiple factories into one, returning an array
 func itemProviderFactory(fcts []ProviderFactory) ProviderFactory {
-	return func(inj InjectableFactory) IOE.IOEither[error, any] {
+	return func(inj InjectableFactory) IOResult[any] {
 		return F.Pipe2(
 			fcts,
-			IOE.TraverseArray(I.Flap[IOE.IOEither[error, any]](inj)),
+			IOR.TraverseArray(I.Flap[IOResult[any]](inj)),
-			IOE.Map[error](F.ToAny[[]any]),
+			IOR.Map(F.ToAny[[]any]),
 		)
 	}
 }
@@ -118,7 +118,7 @@ func itemProviderFactory(fcts []ProviderFactory) ProviderFactory {
 // makes sure to transitively resolve the required dependencies.
 func MakeInjector(providers []Provider) InjectableFactory {
 
-	type Result = IOE.IOEither[error, any]
+	type Result = IOResult[any]
 	type LazyResult = L.Lazy[Result]
 
 	// resolved stores the values resolved so far, key is the string ID
@@ -148,11 +148,11 @@ func MakeInjector(providers []Provider) InjectableFactory {
 					T.Map2(F.Flow3(
 						Dependency.Id,
 						R.Lookup[ProviderFactory, string],
-						I.Ap[O.Option[ProviderFactory]](factoryByID),
+						I.Ap[Option[ProviderFactory]](factoryByID),
 					), handleMissingProvider),
 					T.Tupled2(O.MonadGetOrElse[ProviderFactory]),
-					I.Ap[IOE.IOEither[error, any]](injFct),
+					I.Ap[IOResult[any]](injFct),
-					IOE.Memoize[error, any],
+					IOR.Memoize[any],
 				)
 			}
 

v2/di/erasure/provider.go

@@ -19,25 +19,23 @@ import (
 	"fmt"
 
 	A "github.com/IBM/fp-go/v2/array"
-	E "github.com/IBM/fp-go/v2/either"
 	F "github.com/IBM/fp-go/v2/function"
 	I "github.com/IBM/fp-go/v2/identity"
 	IO "github.com/IBM/fp-go/v2/io"
 	IOE "github.com/IBM/fp-go/v2/ioeither"
-	IOO "github.com/IBM/fp-go/v2/iooption"
 	Int "github.com/IBM/fp-go/v2/number/integer"
-	O "github.com/IBM/fp-go/v2/option"
 	R "github.com/IBM/fp-go/v2/record"
+	"github.com/IBM/fp-go/v2/result"
 )
 
 type (
 	// InjectableFactory is a factory function that can create an untyped instance of a service based on its [Dependency] identifier
-	InjectableFactory = func(Dependency) IOE.IOEither[error, any]
+	InjectableFactory = func(Dependency) IOResult[any]
-	ProviderFactory   = func(InjectableFactory) IOE.IOEither[error, any]
+	ProviderFactory   = func(InjectableFactory) IOResult[any]
 
 	paramIndex = map[int]int
 	paramValue = map[int]any
-	handler    = func(paramIndex) func([]IOE.IOEither[error, any]) IOE.IOEither[error, paramValue]
+	handler    = func(paramIndex) func([]IOResult[any]) IOResult[paramValue]
 	mapping    = map[int]paramIndex
 
 	Provider interface {
@@ -83,50 +81,50 @@ var (
 	mergeMaps      = R.UnionLastMonoid[int, any]()
 	collectParams  = R.CollectOrd[any, any](Int.Ord)(F.SK[int, any])
 
-	mapDeps = F.Curry2(A.MonadMap[Dependency, IOE.IOEither[error, any]])
+	mapDeps = F.Curry2(A.MonadMap[Dependency, IOResult[any]])
 
 	handlers = map[int]handler{
-		Identity: func(mp paramIndex) func([]IOE.IOEither[error, any]) IOE.IOEither[error, paramValue] {
+		IDENTITY: func(mp paramIndex) func([]IOResult[any]) IOResult[paramValue] {
-			return func(res []IOE.IOEither[error, any]) IOE.IOEither[error, paramValue] {
+			return func(res []IOResult[any]) IOResult[paramValue] {
 				return F.Pipe1(
 					mp,
 					IOE.TraverseRecord[int](getAt(res)),
 				)
 			}
 		},
-		Option: func(mp paramIndex) func([]IOE.IOEither[error, any]) IOE.IOEither[error, paramValue] {
+		OPTION: func(mp paramIndex) func([]IOResult[any]) IOResult[paramValue] {
-			return func(res []IOE.IOEither[error, any]) IOE.IOEither[error, paramValue] {
+			return func(res []IOResult[any]) IOResult[paramValue] {
 				return F.Pipe3(
 					mp,
 					IO.TraverseRecord[int](getAt(res)),
 					IO.Map(R.Map[int](F.Flow2(
-						E.ToOption[error, any],
+						result.ToOption[any],
-						F.ToAny[O.Option[any]],
+						F.ToAny[Option[any]],
 					))),
 					IOE.FromIO[error, paramValue],
 				)
 			}
 		},
-		IOEither: func(mp paramIndex) func([]IOE.IOEither[error, any]) IOE.IOEither[error, paramValue] {
+		IOEITHER: func(mp paramIndex) func([]IOResult[any]) IOResult[paramValue] {
-			return func(res []IOE.IOEither[error, any]) IOE.IOEither[error, paramValue] {
+			return func(res []IOResult[any]) IOResult[paramValue] {
 				return F.Pipe2(
 					mp,
 					R.Map[int](F.Flow2(
 						getAt(res),
-						F.ToAny[IOE.IOEither[error, any]],
+						F.ToAny[IOResult[any]],
 					)),
 					IOE.Of[error, paramValue],
 				)
 			}
 		},
-		IOOption: func(mp paramIndex) func([]IOE.IOEither[error, any]) IOE.IOEither[error, paramValue] {
+		IOOPTION: func(mp paramIndex) func([]IOResult[any]) IOResult[paramValue] {
-			return func(res []IOE.IOEither[error, any]) IOE.IOEither[error, paramValue] {
+			return func(res []IOResult[any]) IOResult[paramValue] {
 				return F.Pipe2(
 					mp,
 					R.Map[int](F.Flow3(
 						getAt(res),
 						IOE.ToIOOption[error, any],
-						F.ToAny[IOO.IOOption[any]],
+						F.ToAny[IOOption[any]],
 					)),
 					IOE.Of[error, paramValue],
 				)
@@ -141,23 +139,23 @@ func getAt[T any](ar []T) func(idx int) T {
 	}
 }
 
-func handleMapping(mp mapping) func(res []IOE.IOEither[error, any]) IOE.IOEither[error, []any] {
+func handleMapping(mp mapping) func(res []IOResult[any]) IOResult[[]any] {
 	preFct := F.Pipe1(
 		mp,
-		R.Collect(func(idx int, p paramIndex) func([]IOE.IOEither[error, any]) IOE.IOEither[error, paramValue] {
+		R.Collect(func(idx int, p paramIndex) func([]IOResult[any]) IOResult[paramValue] {
 			return handlers[idx](p)
 		}),
 	)
 	doFct := F.Flow2(
-		I.Flap[IOE.IOEither[error, paramValue], []IOE.IOEither[error, any]],
+		I.Flap[IOResult[paramValue], []IOResult[any]],
-		IOE.TraverseArray[error, func([]IOE.IOEither[error, any]) IOE.IOEither[error, paramValue], paramValue],
+		IOE.TraverseArray[error, func([]IOResult[any]) IOResult[paramValue], paramValue],
 	)
 	postFct := IOE.Map[error](F.Flow2(
 		A.Fold(mergeMaps),
 		collectParams,
 	))
 
-	return func(res []IOE.IOEither[error, any]) IOE.IOEither[error, []any] {
+	return func(res []IOResult[any]) IOResult[[]any] {
 		return F.Pipe2(
 			preFct,
 			doFct(res),
@@ -170,7 +168,7 @@ func handleMapping(mp mapping) func(res []IOE.IOEither[error, any]) IOE.IOEither
 // a function that accepts the resolved dependencies to return a result
 func MakeProviderFactory(
 	deps []Dependency,
-	fct func(param ...any) IOE.IOEither[error, any]) ProviderFactory {
+	fct func(param ...any) IOResult[any]) ProviderFactory {
 
 	return F.Flow3(
 		mapDeps(deps),

v2/di/erasure/token.go

@@ -17,20 +17,18 @@ package erasure
 
 import (
 	"fmt"
-
-	O "github.com/IBM/fp-go/v2/option"
 )
 
 const (
 	BehaviourMask = 0x0f
-	Identity      = 0 // required dependency
+	IDENTITY      = 0 // required dependency
-	Option        = 1 // optional dependency
+	OPTION        = 1 // optional dependency
-	IOEither      = 2 // lazy and required
+	IOEITHER      = 2 // lazy and required
-	IOOption      = 3 // lazy and optional
+	IOOPTION      = 3 // lazy and optional
 
 	TypeMask = 0xf0
-	Multi    = 1 << 4 // array of implementations
+	MULTI    = 1 << 4 // array of implementations
-	Item     = 2 << 4 // item of a multi token
+	ITEM     = 2 << 4 // item of a multi token
 )
 
 // Dependency describes the relationship to a service
@@ -41,5 +39,5 @@ type Dependency interface {
 	// Flag returns a tag that identifies the behaviour of the dependency
 	Flag() int
 	// ProviderFactory optionally returns an attached [ProviderFactory] that represents the default for this dependency
-	ProviderFactory() O.Option[ProviderFactory]
+	ProviderFactory() Option[ProviderFactory]
 }

v2/di/erasure/types.go

@@ -0,0 +1,13 @@
+package erasure
+
+import (
+	"github.com/IBM/fp-go/v2/iooption"
+	"github.com/IBM/fp-go/v2/ioresult"
+	"github.com/IBM/fp-go/v2/option"
+)
+
+type (
+	Option[T any]   = option.Option[T]
+	IOResult[T any] = ioresult.IOResult[T]
+	IOOption[T any] = iooption.IOOption[T]
+)

v2/di/injector.go

@@ -19,14 +19,14 @@ import (
 	DIE "github.com/IBM/fp-go/v2/di/erasure"
 	F "github.com/IBM/fp-go/v2/function"
 	"github.com/IBM/fp-go/v2/identity"
-	IOE "github.com/IBM/fp-go/v2/ioeither"
+	IOR "github.com/IBM/fp-go/v2/ioresult"
-	RIOE "github.com/IBM/fp-go/v2/readerioeither"
+	RIOR "github.com/IBM/fp-go/v2/readerioresult"
 )
 
 // Resolve performs a type safe resolution of a dependency
-func Resolve[T any](token InjectionToken[T]) RIOE.ReaderIOEither[DIE.InjectableFactory, error, T] {
+func Resolve[T any](token InjectionToken[T]) RIOR.ReaderIOResult[DIE.InjectableFactory, T] {
 	return F.Flow2(
-		identity.Ap[IOE.IOEither[error, any]](asDependency(token)),
+		identity.Ap[IOResult[any]](asDependency(token)),
-		IOE.ChainEitherK(token.Unerase),
+		IOR.ChainResultK(token.Unerase),
 	)
 }

v2/di/provider.go

@@ -22,9 +22,10 @@ import (
 	"github.com/IBM/fp-go/v2/errors"
 	F "github.com/IBM/fp-go/v2/function"
 	IOE "github.com/IBM/fp-go/v2/ioeither"
+	"github.com/IBM/fp-go/v2/ioresult"
 )
 
-func lookupAt[T any](idx int, token Dependency[T]) func(params []any) E.Either[error, T] {
+func lookupAt[T any](idx int, token Dependency[T]) func(params []any) Result[T] {
 	return F.Flow3(
 		A.Lookup[any](idx),
 		E.FromOption[any](errors.OnNone("No parameter at position %d", idx)),
@@ -32,7 +33,7 @@ func lookupAt[T any](idx int, token Dependency[T]) func(params []any) E.Either[e
 	)
 }
 
-func eraseTuple[A, R any](f func(A) IOE.IOEither[error, R]) func(E.Either[error, A]) IOE.IOEither[error, any] {
+func eraseTuple[A, R any](f func(A) IOResult[R]) func(Result[A]) IOResult[any] {
 	return F.Flow3(
 		IOE.FromEither[error, A],
 		IOE.Chain(f),
@@ -40,8 +41,8 @@ func eraseTuple[A, R any](f func(A) IOE.IOEither[error, R]) func(E.Either[error,
 	)
 }
 
-func eraseProviderFactory0[R any](f IOE.IOEither[error, R]) func(params ...any) IOE.IOEither[error, any] {
+func eraseProviderFactory0[R any](f IOResult[R]) func(params ...any) IOResult[any] {
-	return func(_ ...any) IOE.IOEither[error, any] {
+	return func(_ ...any) IOResult[any] {
 		return F.Pipe1(
 			f,
 			IOE.Map[error](F.ToAny[R]),
@@ -50,7 +51,7 @@ func eraseProviderFactory0[R any](f IOE.IOEither[error, R]) func(params ...any)
 }
 
 func MakeProviderFactory0[R any](
-	fct IOE.IOEither[error, R],
+	fct IOResult[R],
 ) DIE.ProviderFactory {
 	return DIE.MakeProviderFactory(
 		A.Empty[DIE.Dependency](),
@@ -59,13 +60,13 @@ func MakeProviderFactory0[R any](
 }
 
 // MakeTokenWithDefault0 creates a unique [InjectionToken] for a specific type with an attached default [DIE.Provider]
-func MakeTokenWithDefault0[R any](name string, fct IOE.IOEither[error, R]) InjectionToken[R] {
+func MakeTokenWithDefault0[R any](name string, fct IOResult[R]) InjectionToken[R] {
 	return MakeTokenWithDefault[R](name, MakeProviderFactory0(fct))
 }
 
 func MakeProvider0[R any](
 	token InjectionToken[R],
-	fct IOE.IOEither[error, R],
+	fct IOResult[R],
 ) DIE.Provider {
 	return DIE.MakeProvider(
 		token,
@@ -75,5 +76,5 @@ func MakeProvider0[R any](
 
 // ConstProvider simple implementation for a provider with a constant value
 func ConstProvider[R any](token InjectionToken[R], value R) DIE.Provider {
-	return MakeProvider0(token, IOE.Of[error](value))
+	return MakeProvider0(token, ioresult.Of(value))
 }

v2/di/provider_test.go

@@ -25,7 +25,8 @@ import (
 	E "github.com/IBM/fp-go/v2/either"
 	F "github.com/IBM/fp-go/v2/function"
 	IOE "github.com/IBM/fp-go/v2/ioeither"
-	O "github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/ioresult"
+	"github.com/IBM/fp-go/v2/result"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -39,19 +40,19 @@ func TestSimpleProvider(t *testing.T) {
 
 	var staticCount int
 
-	staticValue := func(value string) IOE.IOEither[error, string] {
+	staticValue := func(value string) IOResult[string] {
-		return func() E.Either[error, string] {
+		return func() Result[string] {
 			staticCount++
-			return E.Of[error](fmt.Sprintf("Static based on [%s], at [%s]", value, time.Now()))
+			return result.Of(fmt.Sprintf("Static based on [%s], at [%s]", value, time.Now()))
 		}
 	}
 
 	var dynamicCount int
 
-	dynamicValue := func(value string) IOE.IOEither[error, string] {
+	dynamicValue := func(value string) IOResult[string] {
-		return func() E.Either[error, string] {
+		return func() Result[string] {
 			dynamicCount++
-			return E.Of[error](fmt.Sprintf("Dynamic based on [%s] at [%s]", value, time.Now()))
+			return result.Of(fmt.Sprintf("Dynamic based on [%s] at [%s]", value, time.Now()))
 		}
 	}
 
@@ -81,19 +82,19 @@ func TestOptionalProvider(t *testing.T) {
 
 	var staticCount int
 
-	staticValue := func(value string) IOE.IOEither[error, string] {
+	staticValue := func(value string) IOResult[string] {
-		return func() E.Either[error, string] {
+		return func() Result[string] {
 			staticCount++
-			return E.Of[error](fmt.Sprintf("Static based on [%s], at [%s]", value, time.Now()))
+			return result.Of(fmt.Sprintf("Static based on [%s], at [%s]", value, time.Now()))
 		}
 	}
 
 	var dynamicCount int
 
-	dynamicValue := func(value O.Option[string]) IOE.IOEither[error, string] {
+	dynamicValue := func(value Option[string]) IOResult[string] {
-		return func() E.Either[error, string] {
+		return func() Result[string] {
 			dynamicCount++
-			return E.Of[error](fmt.Sprintf("Dynamic based on [%s] at [%s]", value, time.Now()))
+			return result.Of(fmt.Sprintf("Dynamic based on [%s] at [%s]", value, time.Now()))
 		}
 	}
 
@@ -123,10 +124,10 @@ func TestOptionalProviderMissingDependency(t *testing.T) {
 
 	var dynamicCount int
 
-	dynamicValue := func(value O.Option[string]) IOE.IOEither[error, string] {
+	dynamicValue := func(value Option[string]) IOResult[string] {
-		return func() E.Either[error, string] {
+		return func() Result[string] {
 			dynamicCount++
-			return E.Of[error](fmt.Sprintf("Dynamic based on [%s] at [%s]", value, time.Now()))
+			return result.Of(fmt.Sprintf("Dynamic based on [%s] at [%s]", value, time.Now()))
 		}
 	}
 
@@ -151,10 +152,10 @@ func TestProviderMissingDependency(t *testing.T) {
 
 	var dynamicCount int
 
-	dynamicValue := func(value string) IOE.IOEither[error, string] {
+	dynamicValue := func(value string) IOResult[string] {
-		return func() E.Either[error, string] {
+		return func() Result[string] {
 			dynamicCount++
-			return E.Of[error](fmt.Sprintf("Dynamic based on [%s] at [%s]", value, time.Now()))
+			return result.Of(fmt.Sprintf("Dynamic based on [%s] at [%s]", value, time.Now()))
 		}
 	}
 
@@ -179,31 +180,31 @@ func TestEagerAndLazyProvider(t *testing.T) {
 
 	var staticCount int
 
-	staticValue := func(value string) IOE.IOEither[error, string] {
+	staticValue := func(value string) IOResult[string] {
-		return func() E.Either[error, string] {
+		return func() Result[string] {
 			staticCount++
-			return E.Of[error](fmt.Sprintf("Static based on [%s], at [%s]", value, time.Now()))
+			return result.Of(fmt.Sprintf("Static based on [%s], at [%s]", value, time.Now()))
 		}
 	}
 
 	var dynamicCount int
 
-	dynamicValue := func(value string) IOE.IOEither[error, string] {
+	dynamicValue := func(value string) IOResult[string] {
-		return func() E.Either[error, string] {
+		return func() Result[string] {
 			dynamicCount++
-			return E.Of[error](fmt.Sprintf("Dynamic based on [%s] at [%s]", value, time.Now()))
+			return result.Of(fmt.Sprintf("Dynamic based on [%s] at [%s]", value, time.Now()))
 		}
 	}
 
 	var lazyEagerCount int
 
-	lazyEager := func(laz IOE.IOEither[error, string], eager string) IOE.IOEither[error, string] {
+	lazyEager := func(laz IOResult[string], eager string) IOResult[string] {
 		return F.Pipe1(
 			laz,
-			IOE.Chain(func(lazValue string) IOE.IOEither[error, string] {
+			IOE.Chain(func(lazValue string) IOResult[string] {
-				return func() E.Either[error, string] {
+				return func() Result[string] {
 					lazyEagerCount++
-					return E.Of[error](fmt.Sprintf("Dynamic based on [%s], [%s] at [%s]", lazValue, eager, time.Now()))
+					return result.Of(fmt.Sprintf("Dynamic based on [%s], [%s] at [%s]", lazValue, eager, time.Now()))
 				}
 			}),
 		)
@@ -248,7 +249,7 @@ func TestItemProvider(t *testing.T) {
 
 	value := multiInj()
 
-	assert.Equal(t, E.Of[error](A.From("Value1", "Value2")), value)
+	assert.Equal(t, result.Of(A.From("Value1", "Value2")), value)
 }
 
 func TestEmptyItemProvider(t *testing.T) {
@@ -269,7 +270,7 @@ func TestEmptyItemProvider(t *testing.T) {
 
 	value := multiInj()
 
-	assert.Equal(t, E.Of[error](A.Empty[string]()), value)
+	assert.Equal(t, result.Of(A.Empty[string]()), value)
 }
 
 func TestDependencyOnMultiProvider(t *testing.T) {
@@ -283,8 +284,8 @@ func TestDependencyOnMultiProvider(t *testing.T) {
 	p1 := ConstProvider(INJ_KEY1, "Value3")
 	p2 := ConstProvider(INJ_KEY2, "Value4")
 
-	fromMulti := func(val string, multi []string) IOE.IOEither[error, string] {
+	fromMulti := func(val string, multi []string) IOResult[string] {
-		return IOE.Of[error](fmt.Sprintf("Val: %s, Multi: %s", val, multi))
+		return ioresult.Of(fmt.Sprintf("Val: %s, Multi: %s", val, multi))
 	}
 	p3 := MakeProvider2(INJ_KEY3, INJ_KEY1.Identity(), injMulti.Container().Identity(), fromMulti)
 
@@ -295,19 +296,19 @@ func TestDependencyOnMultiProvider(t *testing.T) {
 
 	v := r3(inj)()
 
-	assert.Equal(t, E.Of[error]("Val: Value3, Multi: [Value1 Value2]"), v)
+	assert.Equal(t, result.Of("Val: Value3, Multi: [Value1 Value2]"), v)
 }
 
 func TestTokenWithDefaultProvider(t *testing.T) {
 	// token without a default
 	injToken1 := MakeToken[string]("Token1")
 	// token with a default
-	injToken2 := MakeTokenWithDefault0("Token2", IOE.Of[error]("Carsten"))
+	injToken2 := MakeTokenWithDefault0("Token2", ioresult.Of("Carsten"))
 	// dependency
 	injToken3 := MakeToken[string]("Token3")
 
-	p3 := MakeProvider1(injToken3, injToken2.Identity(), func(data string) IOE.IOEither[error, string] {
+	p3 := MakeProvider1(injToken3, injToken2.Identity(), func(data string) IOResult[string] {
-		return IOE.Of[error](fmt.Sprintf("Token: %s", data))
+		return ioresult.Of(fmt.Sprintf("Token: %s", data))
 	})
 
 	// populate the injector
@@ -320,19 +321,19 @@ func TestTokenWithDefaultProvider(t *testing.T) {
 	// inj1 should not be available
 	assert.True(t, E.IsLeft(r1(inj)()))
 	// r3 should work
-	assert.Equal(t, E.Of[error]("Token: Carsten"), r3(inj)())
+	assert.Equal(t, result.Of("Token: Carsten"), r3(inj)())
 }
 
 func TestTokenWithDefaultProviderAndOverride(t *testing.T) {
 	// token with a default
-	injToken2 := MakeTokenWithDefault0("Token2", IOE.Of[error]("Carsten"))
+	injToken2 := MakeTokenWithDefault0("Token2", ioresult.Of("Carsten"))
 	// dependency
 	injToken3 := MakeToken[string]("Token3")
 
 	p2 := ConstProvider(injToken2, "Override")
 
-	p3 := MakeProvider1(injToken3, injToken2.Identity(), func(data string) IOE.IOEither[error, string] {
+	p3 := MakeProvider1(injToken3, injToken2.Identity(), func(data string) IOResult[string] {
-		return IOE.Of[error](fmt.Sprintf("Token: %s", data))
+		return ioresult.Of(fmt.Sprintf("Token: %s", data))
 	})
 
 	// populate the injector
@@ -342,5 +343,5 @@ func TestTokenWithDefaultProviderAndOverride(t *testing.T) {
 	r3 := Resolve(injToken3)
 
 	// r3 should work
-	assert.Equal(t, E.Of[error]("Token: Override"), r3(inj)())
+	assert.Equal(t, result.Of("Token: Override"), r3(inj)())
 }

v2/di/token.go

@@ -21,10 +21,7 @@ import (
 	"sync/atomic"
 
 	DIE "github.com/IBM/fp-go/v2/di/erasure"
-	E "github.com/IBM/fp-go/v2/either"
 	IO "github.com/IBM/fp-go/v2/io"
-	IOE "github.com/IBM/fp-go/v2/ioeither"
-	IOO "github.com/IBM/fp-go/v2/iooption"
 	O "github.com/IBM/fp-go/v2/option"
 )
 
@@ -33,7 +30,7 @@ import (
 type Dependency[T any] interface {
 	DIE.Dependency
 	// Unerase converts a value with erased type signature into a strongly typed value
-	Unerase(val any) E.Either[error, T]
+	Unerase(val any) Result[T]
 }
 
 // InjectionToken uniquely identifies a dependency by giving it an Id, Type and name
@@ -42,17 +39,17 @@ type InjectionToken[T any] interface {
 	// Identity idenifies this dependency as a mandatory, required dependency, it will be resolved eagerly and injected as `T`.
 	// If the dependency cannot be resolved, the resolution process fails
 	Identity() Dependency[T]
-	// Option identifies this dependency as optional, it will be resolved eagerly and injected as [O.Option[T]].
+	// Option identifies this dependency as optional, it will be resolved eagerly and injected as [Option[T]].
 	// If the dependency cannot be resolved, the resolution process continues and the dependency is represented as [O.None[T]]
-	Option() Dependency[O.Option[T]]
+	Option() Dependency[Option[T]]
-	// IOEither identifies this dependency as mandatory but it will be resolved lazily as a [IOE.IOEither[error, T]]. This
+	// IOEither identifies this dependency as mandatory but it will be resolved lazily as a [IOResult[T]]. This
 	// value is memoized to make sure the dependency is a singleton.
 	// If the dependency cannot be resolved, the resolution process fails
-	IOEither() Dependency[IOE.IOEither[error, T]]
+	IOEither() Dependency[IOResult[T]]
-	// IOOption identifies this dependency as optional but it will be resolved lazily as a [IOO.IOOption[T]]. This
+	// IOOption identifies this dependency as optional but it will be resolved lazily as a [IOOption[T]]. This
 	// value is memoized to make sure the dependency is a singleton.
 	// If the dependency cannot be resolved, the resolution process continues and the dependency is represented as the none value.
-	IOOption() Dependency[IOO.IOOption[T]]
+	IOOption() Dependency[IOOption[T]]
 }
 
 // MultiInjectionToken uniquely identifies a dependency by giving it an Id, Type and name that can have multiple implementations.
@@ -79,12 +76,12 @@ type tokenBase struct {
 	name            string
 	id              string
 	flag            int
-	providerFactory O.Option[DIE.ProviderFactory]
+	providerFactory Option[DIE.ProviderFactory]
 }
 
 type token[T any] struct {
 	base   *tokenBase
-	toType func(val any) E.Either[error, T]
+	toType func(val any) Result[T]
 }
 
 func (t *token[T]) Id() string {
@@ -99,26 +96,26 @@ func (t *token[T]) String() string {
 	return t.base.name
 }
 
-func (t *token[T]) Unerase(val any) E.Either[error, T] {
+func (t *token[T]) Unerase(val any) Result[T] {
 	return t.toType(val)
 }
 
-func (t *token[T]) ProviderFactory() O.Option[DIE.ProviderFactory] {
+func (t *token[T]) ProviderFactory() Option[DIE.ProviderFactory] {
 	return t.base.providerFactory
 }
-func makeTokenBase(name string, id string, typ int, providerFactory O.Option[DIE.ProviderFactory]) *tokenBase {
+func makeTokenBase(name string, id string, typ int, providerFactory Option[DIE.ProviderFactory]) *tokenBase {
 	return &tokenBase{name, id, typ, providerFactory}
 }
 
-func makeToken[T any](name string, id string, typ int, unerase func(val any) E.Either[error, T], providerFactory O.Option[DIE.ProviderFactory]) Dependency[T] {
+func makeToken[T any](name string, id string, typ int, unerase func(val any) Result[T], providerFactory Option[DIE.ProviderFactory]) Dependency[T] {
 	return &token[T]{makeTokenBase(name, id, typ, providerFactory), unerase}
 }
 
 type injectionToken[T any] struct {
 	token[T]
-	option   Dependency[O.Option[T]]
+	option   Dependency[Option[T]]
-	ioeither Dependency[IOE.IOEither[error, T]]
+	ioeither Dependency[IOResult[T]]
-	iooption Dependency[IOO.IOOption[T]]
+	iooption Dependency[IOOption[T]]
 }
 
 type multiInjectionToken[T any] struct {
@@ -130,19 +127,19 @@ func (i *injectionToken[T]) Identity() Dependency[T] {
 	return i
 }
 
-func (i *injectionToken[T]) Option() Dependency[O.Option[T]] {
+func (i *injectionToken[T]) Option() Dependency[Option[T]] {
 	return i.option
 }
 
-func (i *injectionToken[T]) IOEither() Dependency[IOE.IOEither[error, T]] {
+func (i *injectionToken[T]) IOEither() Dependency[IOResult[T]] {
 	return i.ioeither
 }
 
-func (i *injectionToken[T]) IOOption() Dependency[IOO.IOOption[T]] {
+func (i *injectionToken[T]) IOOption() Dependency[IOOption[T]] {
 	return i.iooption
 }
 
-func (i *injectionToken[T]) ProviderFactory() O.Option[DIE.ProviderFactory] {
+func (i *injectionToken[T]) ProviderFactory() Option[DIE.ProviderFactory] {
 	return i.base.providerFactory
 }
 
@@ -155,14 +152,14 @@ func (m *multiInjectionToken[T]) Item() InjectionToken[T] {
 }
 
 // makeToken create a unique [InjectionToken] for a specific type
-func makeInjectionToken[T any](name string, providerFactory O.Option[DIE.ProviderFactory]) InjectionToken[T] {
+func makeInjectionToken[T any](name string, providerFactory Option[DIE.ProviderFactory]) InjectionToken[T] {
 	id := genID()
 	toIdentity := toType[T]()
 	return &injectionToken[T]{
-		token[T]{makeTokenBase(name, id, DIE.Identity, providerFactory), toIdentity},
+		token[T]{makeTokenBase(name, id, DIE.IDENTITY, providerFactory), toIdentity},
-		makeToken(fmt.Sprintf("Option[%s]", name), id, DIE.Option, toOptionType(toIdentity), providerFactory),
+		makeToken(fmt.Sprintf("Option[%s]", name), id, DIE.OPTION, toOptionType(toIdentity), providerFactory),
-		makeToken(fmt.Sprintf("IOEither[%s]", name), id, DIE.IOEither, toIOEitherType(toIdentity), providerFactory),
+		makeToken(fmt.Sprintf("IOEither[%s]", name), id, DIE.IOEITHER, toIOEitherType(toIdentity), providerFactory),
-		makeToken(fmt.Sprintf("IOOption[%s]", name), id, DIE.IOOption, toIOOptionType(toIdentity), providerFactory),
+		makeToken(fmt.Sprintf("IOOption[%s]", name), id, DIE.IOOPTION, toIOOptionType(toIdentity), providerFactory),
 	}
 }
 
@@ -187,17 +184,17 @@ func MakeMultiToken[T any](name string) MultiInjectionToken[T] {
 	providerFactory := O.None[DIE.ProviderFactory]()
 	// container
 	container := &injectionToken[[]T]{
-		token[[]T]{makeTokenBase(containerName, id, DIE.Multi|DIE.Identity, providerFactory), toContainer},
+		token[[]T]{makeTokenBase(containerName, id, DIE.MULTI|DIE.IDENTITY, providerFactory), toContainer},
-		makeToken(fmt.Sprintf("Option[%s]", containerName), id, DIE.Multi|DIE.Option, toOptionType(toContainer), providerFactory),
+		makeToken(fmt.Sprintf("Option[%s]", containerName), id, DIE.MULTI|DIE.OPTION, toOptionType(toContainer), providerFactory),
-		makeToken(fmt.Sprintf("IOEither[%s]", containerName), id, DIE.Multi|DIE.IOEither, toIOEitherType(toContainer), providerFactory),
+		makeToken(fmt.Sprintf("IOEither[%s]", containerName), id, DIE.OPTION|DIE.IOEITHER, toIOEitherType(toContainer), providerFactory),
-		makeToken(fmt.Sprintf("IOOption[%s]", containerName), id, DIE.Multi|DIE.IOOption, toIOOptionType(toContainer), providerFactory),
+		makeToken(fmt.Sprintf("IOOption[%s]", containerName), id, DIE.OPTION|DIE.IOOPTION, toIOOptionType(toContainer), providerFactory),
 	}
 	// item
 	item := &injectionToken[T]{
-		token[T]{makeTokenBase(itemName, id, DIE.Item|DIE.Identity, providerFactory), toItem},
+		token[T]{makeTokenBase(itemName, id, DIE.ITEM|DIE.IDENTITY, providerFactory), toItem},
-		makeToken(fmt.Sprintf("Option[%s]", itemName), id, DIE.Item|DIE.Option, toOptionType(toItem), providerFactory),
+		makeToken(fmt.Sprintf("Option[%s]", itemName), id, DIE.ITEM|DIE.OPTION, toOptionType(toItem), providerFactory),
-		makeToken(fmt.Sprintf("IOEither[%s]", itemName), id, DIE.Item|DIE.IOEither, toIOEitherType(toItem), providerFactory),
+		makeToken(fmt.Sprintf("IOEither[%s]", itemName), id, DIE.ITEM|DIE.IOEITHER, toIOEitherType(toItem), providerFactory),
-		makeToken(fmt.Sprintf("IOOption[%s]", itemName), id, DIE.Item|DIE.IOOption, toIOOptionType(toItem), providerFactory),
+		makeToken(fmt.Sprintf("IOOption[%s]", itemName), id, DIE.ITEM|DIE.IOOPTION, toIOOptionType(toItem), providerFactory),
 	}
 	// returns the token
 	return &multiInjectionToken[T]{container, item}

v2/di/token_test.go

@@ -23,7 +23,9 @@ import (
 	F "github.com/IBM/fp-go/v2/function"
 	IOE "github.com/IBM/fp-go/v2/ioeither"
 	IOO "github.com/IBM/fp-go/v2/iooption"
+	"github.com/IBM/fp-go/v2/ioresult"
 	O "github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/result"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -75,9 +77,9 @@ func TestTokenUnerase(t *testing.T) {
 	token := MakeToken[int]("IntToken")
 
 	// Test successful unerase
-	result := token.Unerase(42)
+	res := token.Unerase(42)
-	assert.True(t, E.IsRight(result))
+	assert.True(t, E.IsRight(res))
-	assert.Equal(t, E.Of[error](42), result)
+	assert.Equal(t, result.Of(42), res)
 
 	// Test failed unerase (wrong type)
 	result2 := token.Unerase("not an int")
@@ -104,7 +106,7 @@ func TestTokenProviderFactory(t *testing.T) {
 	assert.True(t, O.IsNone(token1.ProviderFactory()))
 
 	// Token with default
-	token2 := MakeTokenWithDefault0("Token2", IOE.Of[error](42))
+	token2 := MakeTokenWithDefault0("Token2", ioresult.Of(42))
 	assert.True(t, O.IsSome(token2.ProviderFactory()))
 }
 
@@ -148,13 +150,13 @@ func TestOptionTokenUnerase(t *testing.T) {
 	optionToken := token.Option()
 
 	// Test successful unerase with Some
-	result := optionToken.Unerase(O.Of[any](42))
+	res := optionToken.Unerase(O.Of[any](42))
-	assert.True(t, E.IsRight(result))
+	assert.True(t, E.IsRight(res))
 
 	// Test successful unerase with None
 	noneResult := optionToken.Unerase(O.None[any]())
 	assert.True(t, E.IsRight(noneResult))
-	assert.Equal(t, E.Of[error](O.None[int]()), noneResult)
+	assert.Equal(t, result.Of(O.None[int]()), noneResult)
 
 	// Test failed unerase (wrong type)
 	badResult := optionToken.Unerase(42) // Not an Option
@@ -166,7 +168,7 @@ func TestIOEitherTokenUnerase(t *testing.T) {
 	ioeitherToken := token.IOEither()
 
 	// Test successful unerase
-	ioValue := IOE.Of[error](any(42))
+	ioValue := ioresult.Of(any(42))
 	result := ioeitherToken.Unerase(ioValue)
 	assert.True(t, E.IsRight(result))
 
@@ -222,7 +224,7 @@ func TestMultiTokenContainerUnerase(t *testing.T) {
 }
 
 func TestMakeTokenWithDefault(t *testing.T) {
-	factory := MakeProviderFactory0(IOE.Of[error](42))
+	factory := MakeProviderFactory0(ioresult.Of(42))
 	token := MakeTokenWithDefault[int]("TokenWithDefault", factory)
 
 	assert.NotNil(t, token)
@@ -247,7 +249,7 @@ func TestMultiTokenStringRepresentation(t *testing.T) {
 
 // Benchmark tests
 func BenchmarkMakeToken(b *testing.B) {
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		MakeToken[int]("BenchToken")
 	}
 }
@@ -257,13 +259,13 @@ func BenchmarkTokenUnerase(b *testing.B) {
 	value := any(42)
 
 	b.ResetTimer()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		token.Unerase(value)
 	}
 }
 
 func BenchmarkMakeMultiToken(b *testing.B) {
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		MakeMultiToken[int]("BenchMulti")
 	}
 }

v2/di/types.go

@@ -0,0 +1,15 @@
+package di
+
+import (
+	"github.com/IBM/fp-go/v2/context/ioresult"
+	"github.com/IBM/fp-go/v2/iooption"
+	"github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/result"
+)
+
+type (
+	Option[T any]   = option.Option[T]
+	Result[T any]   = result.Result[T]
+	IOResult[T any] = ioresult.IOResult[T]
+	IOOption[T any] = iooption.IOOption[T]
+)

v2/di/utils.go

@@ -23,12 +23,13 @@ import (
 	IOE "github.com/IBM/fp-go/v2/ioeither"
 	IOO "github.com/IBM/fp-go/v2/iooption"
 	O "github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/result"
 )
 
 var (
-	toOptionAny   = toType[O.Option[any]]()
+	toOptionAny   = toType[Option[any]]()
-	toIOEitherAny = toType[IOE.IOEither[error, any]]()
+	toIOEitherAny = toType[IOResult[any]]()
-	toIOOptionAny = toType[IOO.IOOption[any]]()
+	toIOOptionAny = toType[IOOption[any]]()
 	toArrayAny    = toType[[]any]()
 )
 
@@ -38,45 +39,45 @@ func asDependency[T DIE.Dependency](t T) DIE.Dependency {
 }
 
 // toType converts an any to a T
-func toType[T any]() func(t any) E.Either[error, T] {
+func toType[T any]() result.Kleisli[any, T] {
 	return E.ToType[T](errors.OnSome[any]("Value of type [%T] cannot be converted."))
 }
 
 // toOptionType converts an any to an Option[any] and then to an Option[T]
-func toOptionType[T any](item func(any) E.Either[error, T]) func(t any) E.Either[error, O.Option[T]] {
+func toOptionType[T any](item result.Kleisli[any, T]) result.Kleisli[any, Option[T]] {
 	return F.Flow2(
 		toOptionAny,
 		E.Chain(O.Fold(
-			F.Nullary2(O.None[T], E.Of[error, O.Option[T]]),
+			F.Nullary2(O.None[T], E.Of[error, Option[T]]),
 			F.Flow2(
 				item,
-				E.Map[error](O.Of[T]),
+				result.Map(O.Of[T]),
 			),
 		)),
 	)
 }
 
 // toIOEitherType converts an any to an IOEither[error, any] and then to an IOEither[error, T]
-func toIOEitherType[T any](item func(any) E.Either[error, T]) func(t any) E.Either[error, IOE.IOEither[error, T]] {
+func toIOEitherType[T any](item result.Kleisli[any, T]) result.Kleisli[any, IOResult[T]] {
 	return F.Flow2(
 		toIOEitherAny,
-		E.Map[error](IOE.ChainEitherK(item)),
+		result.Map(IOE.ChainEitherK(item)),
 	)
 }
 
 // toIOOptionType converts an any to an IOOption[any] and then to an IOOption[T]
-func toIOOptionType[T any](item func(any) E.Either[error, T]) func(t any) E.Either[error, IOO.IOOption[T]] {
+func toIOOptionType[T any](item result.Kleisli[any, T]) result.Kleisli[any, IOOption[T]] {
 	return F.Flow2(
 		toIOOptionAny,
-		E.Map[error](IOO.ChainOptionK(F.Flow2(
+		result.Map(IOO.ChainOptionK(F.Flow2(
 			item,
-			E.ToOption[error, T],
+			result.ToOption[T],
 		))),
 	)
 }
 
 // toArrayType converts an any to a []T
-func toArrayType[T any](item func(any) E.Either[error, T]) func(t any) E.Either[error, []T] {
+func toArrayType[T any](item result.Kleisli[any, T]) result.Kleisli[any, []T] {
 	return F.Flow2(
 		toArrayAny,
 		E.Chain(E.TraverseArray(item)),

v2/di/utils_test.go

@@ -21,8 +21,9 @@ import (
 	A "github.com/IBM/fp-go/v2/array"
 	E "github.com/IBM/fp-go/v2/either"
 	F "github.com/IBM/fp-go/v2/function"
-	IOE "github.com/IBM/fp-go/v2/ioeither"
+	"github.com/IBM/fp-go/v2/ioresult"
 	O "github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/result"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -33,13 +34,13 @@ var (
 
 func TestToType(t *testing.T) {
 	// good cases
-	assert.Equal(t, E.Of[error](10), toInt(any(10)))
+	assert.Equal(t, result.Of(10), toInt(any(10)))
-	assert.Equal(t, E.Of[error]("Carsten"), toString(any("Carsten")))
+	assert.Equal(t, result.Of("Carsten"), toString(any("Carsten")))
-	assert.Equal(t, E.Of[error](O.Of("Carsten")), toType[O.Option[string]]()(any(O.Of("Carsten"))))
+	assert.Equal(t, result.Of(O.Of("Carsten")), toType[Option[string]]()(any(O.Of("Carsten"))))
-	assert.Equal(t, E.Of[error](O.Of(any("Carsten"))), toType[O.Option[any]]()(any(O.Of(any("Carsten")))))
+	assert.Equal(t, result.Of(O.Of(any("Carsten"))), toType[Option[any]]()(any(O.Of(any("Carsten")))))
 	// failure
 	assert.False(t, E.IsRight(toInt(any("Carsten"))))
-	assert.False(t, E.IsRight(toType[O.Option[string]]()(O.Of(any("Carsten")))))
+	assert.False(t, E.IsRight(toType[Option[string]]()(O.Of(any("Carsten")))))
 }
 
 func TestToOptionType(t *testing.T) {
@@ -47,17 +48,17 @@ func TestToOptionType(t *testing.T) {
 	toOptInt := toOptionType(toInt)
 	toOptString := toOptionType(toString)
 	// good cases
-	assert.Equal(t, E.Of[error](O.Of(10)), toOptInt(any(O.Of(any(10)))))
+	assert.Equal(t, result.Of(O.Of(10)), toOptInt(any(O.Of(any(10)))))
-	assert.Equal(t, E.Of[error](O.Of("Carsten")), toOptString(any(O.Of(any("Carsten")))))
+	assert.Equal(t, result.Of(O.Of("Carsten")), toOptString(any(O.Of(any("Carsten")))))
 	// bad cases
 	assert.False(t, E.IsRight(toOptInt(any(10))))
 	assert.False(t, E.IsRight(toOptInt(any(O.Of(10)))))
 }
 
-func invokeIOEither[T any](e E.Either[error, IOE.IOEither[error, T]]) E.Either[error, T] {
+func invokeIOEither[T any](e Result[IOResult[T]]) Result[T] {
 	return F.Pipe1(
 		e,
-		E.Chain(func(ioe IOE.IOEither[error, T]) E.Either[error, T] {
+		E.Chain(func(ioe IOResult[T]) Result[T] {
 			return ioe()
 		}),
 	)
@@ -68,11 +69,11 @@ func TestToIOEitherType(t *testing.T) {
 	toIOEitherInt := toIOEitherType(toInt)
 	toIOEitherString := toIOEitherType(toString)
 	// good cases
-	assert.Equal(t, E.Of[error](10), invokeIOEither(toIOEitherInt(any(IOE.Of[error](any(10))))))
+	assert.Equal(t, result.Of(10), invokeIOEither(toIOEitherInt(any(ioresult.Of(any(10))))))
-	assert.Equal(t, E.Of[error]("Carsten"), invokeIOEither(toIOEitherString(any(IOE.Of[error](any("Carsten"))))))
+	assert.Equal(t, result.Of("Carsten"), invokeIOEither(toIOEitherString(any(ioresult.Of(any("Carsten"))))))
 	// bad cases
-	assert.False(t, E.IsRight(invokeIOEither(toIOEitherString(any(IOE.Of[error](any(10)))))))
+	assert.False(t, E.IsRight(invokeIOEither(toIOEitherString(any(ioresult.Of(any(10)))))))
-	assert.False(t, E.IsRight(invokeIOEither(toIOEitherString(any(IOE.Of[error]("Carsten"))))))
+	assert.False(t, E.IsRight(invokeIOEither(toIOEitherString(any(ioresult.Of("Carsten"))))))
 	assert.False(t, E.IsRight(invokeIOEither(toIOEitherString(any("Carsten")))))
 }
 
@@ -80,5 +81,5 @@ func TestToArrayType(t *testing.T) {
 	// shortcuts
 	toArrayString := toArrayType(toString)
 	// good cases
-	assert.Equal(t, E.Of[error](A.From("a", "b")), toArrayString(any(A.From(any("a"), any("b")))))
+	assert.Equal(t, result.Of(A.From("a", "b")), toArrayString(any(A.From(any("a"), any("b")))))
 }

v2/either/applicative.go

@@ -0,0 +1,190 @@
+// Copyright (c) 2024 - 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 either
+
+import (
+	"github.com/IBM/fp-go/v2/internal/applicative"
+	S "github.com/IBM/fp-go/v2/semigroup"
+)
+
+// eitherApplicative is the internal implementation of the Applicative type class for Either.
+// It provides the basic applicative operations: Of (lift), Map (transform), and Ap (apply).
+type eitherApplicative[E, A, B any] struct {
+	fof  func(a A) Either[E, A]
+	fmap func(func(A) B) Operator[E, A, B]
+	fap  func(Either[E, A]) Operator[E, func(A) B, B]
+}
+
+// Of lifts a pure value into a Right context.
+func (o *eitherApplicative[E, A, B]) Of(a A) Either[E, A] {
+	return o.fof(a)
+}
+
+// Map applies a transformation function to the Right value, preserving Left values.
+func (o *eitherApplicative[E, A, B]) Map(f func(A) B) Operator[E, A, B] {
+	return o.fmap(f)
+}
+
+// Ap applies a wrapped function to a wrapped value.
+// The behavior depends on which Ap implementation is used (fail-fast or validation).
+func (o *eitherApplicative[E, A, B]) Ap(fa Either[E, A]) Operator[E, func(A) B, B] {
+	return o.fap(fa)
+}
+
+// Applicative creates a standard Applicative instance for Either with fail-fast error handling.
+//
+// This returns a lawful Applicative that satisfies all applicative laws:
+//   - Identity: Ap(Of(identity))(v) == v
+//   - Homomorphism: Ap(Of(f))(Of(x)) == Of(f(x))
+//   - Interchange: Ap(Of(f))(u) == Ap(Map(f => f(y))(u))(Of(y))
+//   - Composition: Ap(Ap(Map(compose)(f))(g))(x) == Ap(f)(Ap(g)(x))
+//
+// The Applicative operations behave as follows:
+//   - Of: lifts a value into Right
+//   - Map: transforms Right values, preserves Left (standard functor)
+//   - Ap: fails fast - if either operand is Left, returns the first Left encountered
+//
+// This is the standard Either applicative that stops at the first error, making it
+// suitable for computations where you want to short-circuit on failure.
+//
+// Example - Fail-Fast Behavior:
+//
+//	app := either.Applicative[error, int, string]()
+//
+//	// Both succeed - function application works
+//	value := either.Right[error](42)
+//	fn := either.Right[error](strconv.Itoa)
+//	result := app.Ap(value)(fn)
+//	// result is Right("42")
+//
+//	// First error stops computation
+//	err1 := either.Left[func(int) string](errors.New("error 1"))
+//	err2 := either.Left[int](errors.New("error 2"))
+//	result2 := app.Ap(err2)(err1)
+//	// result2 is Left(error 1) - only first error is returned
+//
+// Type Parameters:
+//   - E: The error type (Left value)
+//   - A: The input value type (Right value)
+//   - B: The output value type after transformation
+func Applicative[E, A, B any]() applicative.Applicative[A, B, Either[E, A], Either[E, B], Either[E, func(A) B]] {
+	return &eitherApplicative[E, A, B]{
+		Of[E, A],
+		Map[E, A, B],
+		Ap[B, E, A],
+	}
+}
+
+// ApplicativeV creates an Applicative with validation-style error accumulation.
+//
+// This returns a lawful Applicative that accumulates errors using a Semigroup when
+// combining independent computations with Ap. This is the "validation" pattern commonly
+// used for form validation, configuration validation, and parallel error collection.
+//
+// The returned instance satisfies all applicative laws:
+//   - Identity: Ap(Of(identity))(v) == v
+//   - Homomorphism: Ap(Of(f))(Of(x)) == Of(f(x))
+//   - Interchange: Ap(Of(f))(u) == Ap(Map(f => f(y))(u))(Of(y))
+//   - Composition: Ap(Ap(Map(compose)(f))(g))(x) == Ap(f)(Ap(g)(x))
+//
+// Key behaviors:
+//   - Of: lifts a value into Right
+//   - Map: transforms Right values, preserves Left (standard functor)
+//   - Ap: when both operands are Left, combines errors using the Semigroup
+//
+// Comparison with standard Applicative:
+//   - Applicative: Ap fails fast (returns first error)
+//   - ApplicativeV: Ap accumulates errors (combines all errors via Semigroup)
+//
+// Use cases:
+//   - Form validation: collect all validation errors at once
+//   - Configuration validation: report all configuration problems
+//   - Parallel independent checks: accumulate all failures
+//
+// Example - Error Accumulation for Form Validation:
+//
+//	type ValidationErrors []string
+//
+//	// Define how to combine error lists
+//	sg := semigroup.MakeSemigroup(func(a, b ValidationErrors) ValidationErrors {
+//	    return append(append(ValidationErrors{}, a...), b...)
+//	})
+//
+//	app := either.ApplicativeV[ValidationErrors, User, User](sg)
+//
+//	// Validate multiple fields independently
+//	validateName := func(name string) Either[ValidationErrors, string] {
+//	    if len(name) < 3 {
+//	        return Left[string](ValidationErrors{"Name must be at least 3 characters"})
+//	    }
+//	    return Right[ValidationErrors](name)
+//	}
+//
+//	validateAge := func(age int) Either[ValidationErrors, int] {
+//	    if age < 18 {
+//	        return Left[int](ValidationErrors{"Must be 18 or older"})
+//	    }
+//	    return Right[ValidationErrors](age)
+//	}
+//
+//	validateEmail := func(email string) Either[ValidationErrors, string] {
+//	    if !strings.Contains(email, "@") {
+//	        return Left[string](ValidationErrors{"Invalid email format"})
+//	    }
+//	    return Right[ValidationErrors](email)
+//	}
+//
+//	// Create a constructor function lifted into Either
+//	makeUser := func(name string) func(int) func(string) User {
+//	    return func(age int) func(string) User {
+//	        return func(email string) User {
+//	            return User{Name: name, Age: age, Email: email}
+//	        }
+//	    }
+//	}
+//
+//	// Apply validations - all errors are collected
+//	name := validateName("ab")          // Left: name too short
+//	age := validateAge(16)              // Left: age too low
+//	email := validateEmail("invalid")    // Left: invalid email
+//
+//	// Combine all validations using ApV
+//	result := app.Ap(name)(
+//	    app.Ap(age)(
+//	        app.Ap(email)(
+//	            app.Of(makeUser),
+//	        ),
+//	    ),
+//	)
+//	// result is Left(ValidationErrors{
+//	//     "Name must be at least 3 characters",
+//	//     "Must be 18 or older",
+//	//     "Invalid email format"
+//	// })
+//	// All three errors are collected!
+//
+// Type Parameters:
+//   - E: The error type that must have a Semigroup for combining errors
+//   - A: The input value type (Right value)
+//   - B: The output value type after transformation
+//   - sg: Semigroup instance for combining Left values when both operands of Ap are Left
+func ApplicativeV[E, A, B any](sg S.Semigroup[E]) applicative.Applicative[A, B, Either[E, A], Either[E, B], Either[E, func(A) B]] {
+	return &eitherApplicative[E, A, B]{
+		Of[E, A],
+		Map[E, A, B],
+		ApV[B, A](sg),
+	}
+}

v2/either/array.go

@@ -35,14 +35,18 @@ import (
 //	// result is Right([]int{1, 2, 3})
 //
 //go:inline
-func TraverseArrayG[GA ~[]A, GB ~[]B, E, A, B any](f func(A) Either[E, B]) func(GA) Either[E, GB] {
+func TraverseArrayG[GA ~[]A, GB ~[]B, E, A, B any](f Kleisli[E, A, B]) Kleisli[E, GA, GB] {
-	return RA.Traverse[GA](
+	return func(ga GA) Either[E, GB] {
-		Of[E, GB],
+		bs := make(GB, len(ga))
-		Map[E, GB, func(B) GB],
+		for i, a := range ga {
-		Ap[GB, E, B],
+			b := f(a)
-
+			if b.isLeft {
-		f,
+				return Left[GB](b.l)
-	)
+			}
+			bs[i] = b.r
+		}
+		return Of[E](bs)
+	}
 }
 
 // TraverseArray transforms an array by applying a function that returns an Either to each element.
@@ -59,7 +63,7 @@ func TraverseArrayG[GA ~[]A, GB ~[]B, E, A, B any](f func(A) Either[E, B]) func(
 //	// result is Right([]int{1, 2, 3})
 //
 //go:inline
-func TraverseArray[E, A, B any](f func(A) Either[E, B]) func([]A) Either[E, []B] {
+func TraverseArray[E, A, B any](f Kleisli[E, A, B]) Kleisli[E, []A, []B] {
 	return TraverseArrayG[[]A, []B](f)
 }
 
@@ -80,14 +84,18 @@ func TraverseArray[E, A, B any](f func(A) Either[E, B]) func([]A) Either[E, []B]
 //	// result is Right([]string{"0:a", "1:b"})
 //
 //go:inline
-func TraverseArrayWithIndexG[GA ~[]A, GB ~[]B, E, A, B any](f func(int, A) Either[E, B]) func(GA) Either[E, GB] {
+func TraverseArrayWithIndexG[GA ~[]A, GB ~[]B, E, A, B any](f func(int, A) Either[E, B]) Kleisli[E, GA, GB] {
-	return RA.TraverseWithIndex[GA](
+	return func(ga GA) Either[E, GB] {
-		Of[E, GB],
+		bs := make(GB, len(ga))
-		Map[E, GB, func(B) GB],
+		for i, a := range ga {
-		Ap[GB, E, B],
+			b := f(i, a)
-
+			if b.isLeft {
-		f,
+				return Left[GB](b.l)
-	)
+			}
+			bs[i] = b.r
+		}
+		return Of[E](bs)
+	}
 }
 
 // TraverseArrayWithIndex transforms an array by applying an indexed function that returns an Either.
@@ -106,7 +114,7 @@ func TraverseArrayWithIndexG[GA ~[]A, GB ~[]B, E, A, B any](f func(int, A) Eithe
 //	// result is Right([]string{"0:a", "1:b"})
 //
 //go:inline
-func TraverseArrayWithIndex[E, A, B any](f func(int, A) Either[E, B]) func([]A) Either[E, []B] {
+func TraverseArrayWithIndex[E, A, B any](f func(int, A) Either[E, B]) Kleisli[E, []A, []B] {
 	return TraverseArrayWithIndexG[[]A, []B](f)
 }
 

v2/either/array_test.go

@@ -4,16 +4,17 @@ import (
 	"fmt"
 	"testing"
 
+	A "github.com/IBM/fp-go/v2/array"
 	TST "github.com/IBM/fp-go/v2/internal/testing"
 	"github.com/stretchr/testify/assert"
 )
 
 func TestCompactArray(t *testing.T) {
-	ar := []Either[string, string]{
+	ar := A.From(
 		Of[string]("ok"),
 		Left[string]("err"),
 		Of[string]("ok"),
-	}
+	)
 
 	res := CompactArray(ar)
 	assert.Equal(t, 2, len(res))

v2/either/bind_test.go

@@ -20,6 +20,7 @@ import (
 
 	F "github.com/IBM/fp-go/v2/function"
 	"github.com/IBM/fp-go/v2/internal/utils"
+	N "github.com/IBM/fp-go/v2/number"
 	L "github.com/IBM/fp-go/v2/optics/lens"
 	"github.com/stretchr/testify/assert"
 )
@@ -203,7 +204,7 @@ func TestLetL(t *testing.T) {
 	)
 
 	t.Run("LetL with pure transformation", func(t *testing.T) {
-		double := func(v int) int { return v * 2 }
+		double := N.Mul(2)
 
 		result := F.Pipe1(
 			Right[error](Counter{Value: 21}),
@@ -215,7 +216,7 @@ func TestLetL(t *testing.T) {
 	})
 
 	t.Run("LetL with Left input", func(t *testing.T) {
-		double := func(v int) int { return v * 2 }
+		double := N.Mul(2)
 
 		result := F.Pipe1(
 			Left[Counter](assert.AnError),
@@ -227,8 +228,8 @@ func TestLetL(t *testing.T) {
 	})
 
 	t.Run("LetL with multiple transformations", func(t *testing.T) {
-		double := func(v int) int { return v * 2 }
+		double := N.Mul(2)
-		addTen := func(v int) int { return v + 10 }
+		addTen := N.Add(10)
 
 		result := F.Pipe2(
 			Right[error](Counter{Value: 5}),
@@ -241,7 +242,7 @@ func TestLetL(t *testing.T) {
 	})
 
 	t.Run("LetL with identity transformation", func(t *testing.T) {
-		identity := func(v int) int { return v }
+		identity := F.Identity[int]
 
 		result := F.Pipe1(
 			Right[error](Counter{Value: 42}),
@@ -315,7 +316,7 @@ func TestLensOperationsCombined(t *testing.T) {
 	)
 
 	t.Run("Combine LetToL and LetL", func(t *testing.T) {
-		double := func(v int) int { return v * 2 }
+		double := N.Mul(2)
 
 		result := F.Pipe2(
 			Right[error](Counter{Value: 100}),
@@ -328,7 +329,7 @@ func TestLensOperationsCombined(t *testing.T) {
 	})
 
 	t.Run("Combine LetL and BindL", func(t *testing.T) {
-		double := func(v int) int { return v * 2 }
+		double := N.Mul(2)
 		validate := func(v int) Either[error, int] {
 			if v > 100 {
 				return Left[int](assert.AnError)

v2/either/core.go

@@ -22,9 +22,9 @@ import (
 type (
 	// Either defines a data structure that logically holds either an E or an A. The flag discriminates the cases
 	Either[E, A any] struct {
-		r   A
+		r      A
-		l   E
+		l      E
-		isL bool
+		isLeft bool
 	}
 )
 
@@ -32,7 +32,7 @@ type (
 //
 //go:noinline
 func (s Either[E, A]) String() string {
-	if !s.isL {
+	if !s.isLeft {
 		return fmt.Sprintf("Right[%T](%v)", s.r, s.r)
 	}
 	return fmt.Sprintf("Left[%T](%v)", s.l, s.l)
@@ -61,7 +61,7 @@ func (s Either[E, A]) Format(f fmt.State, c rune) {
 //
 //go:inline
 func IsLeft[E, A any](val Either[E, A]) bool {
-	return val.isL
+	return val.isLeft
 }
 
 // IsRight tests if the Either is a Right value.
@@ -75,7 +75,7 @@ func IsLeft[E, A any](val Either[E, A]) bool {
 //
 //go:inline
 func IsRight[E, A any](val Either[E, A]) bool {
-	return !val.isL
+	return !val.isLeft
 }
 
 // Left creates a new Either representing a Left (error/failure) value.
@@ -87,7 +87,7 @@ func IsRight[E, A any](val Either[E, A]) bool {
 //
 //go:inline
 func Left[A, E any](value E) Either[E, A] {
-	return Either[E, A]{l: value, isL: true}
+	return Either[E, A]{l: value, isLeft: true}
 }
 
 // Right creates a new Either representing a Right (success) value.
@@ -115,7 +115,7 @@ func Right[E, A any](value A) Either[E, A] {
 //
 //go:inline
 func MonadFold[E, A, B any](ma Either[E, A], onLeft func(e E) B, onRight func(a A) B) B {
-	if !ma.isL {
+	if !ma.isLeft {
 		return onRight(ma.r)
 	}
 	return onLeft(ma.l)

v2/either/either.go

@@ -24,7 +24,6 @@ import (
 	F "github.com/IBM/fp-go/v2/function"
 	C "github.com/IBM/fp-go/v2/internal/chain"
 	FC "github.com/IBM/fp-go/v2/internal/functor"
-	L "github.com/IBM/fp-go/v2/lazy"
 	O "github.com/IBM/fp-go/v2/option"
 )
 
@@ -38,7 +37,7 @@ import (
 //
 //go:inline
 func Of[E, A any](value A) Either[E, A] {
-	return F.Pipe1(value, Right[E, A])
+	return Right[E](value)
 }
 
 // FromIO executes an IO operation and wraps the result in a Right value.
@@ -48,8 +47,10 @@ func Of[E, A any](value A) Either[E, A] {
 //
 //	getValue := func() int { return 42 }
 //	result := either.FromIO[error](getValue) // Right(42)
+//
+// go: inline
 func FromIO[E any, IO ~func() A, A any](f IO) Either[E, A] {
-	return F.Pipe1(f(), Right[E, A])
+	return Of[E](f())
 }
 
 // MonadAp applies a function wrapped in Either to a value wrapped in Either.
@@ -58,17 +59,23 @@ func FromIO[E any, IO ~func() A, A any](f IO) Either[E, A] {
 //
 // Example:
 //
-//	fab := either.Right[error](func(x int) int { return x * 2 })
+//	fab := either.Right[error](N.Mul(2))
 //	fa := either.Right[error](21)
 //	result := either.MonadAp(fab, fa) // Right(42)
 func MonadAp[B, E, A any](fab Either[E, func(a A) B], fa Either[E, A]) Either[E, B] {
-	return MonadFold(fab, Left[B, E], func(ab func(A) B) Either[E, B] {
+	if fab.isLeft {
-		return MonadFold(fa, Left[B, E], F.Flow2(ab, Right[E, B]))
+		return Left[B](fab.l)
-	})
+	}
+	if fa.isLeft {
+		return Left[B](fa.l)
+	}
+	return Of[E](fab.r(fa.r))
 }
 
 // Ap is the curried version of [MonadAp].
 // Returns a function that applies a wrapped function to the given wrapped value.
+//
+//go:inline
 func Ap[B, E, A any](fa Either[E, A]) Operator[E, func(A) B, B] {
 	return F.Bind2nd(MonadAp[B, E, A], fa)
 }
@@ -81,12 +88,15 @@ func Ap[B, E, A any](fa Either[E, A]) Operator[E, func(A) B, B] {
 //
 //	result := either.MonadMap(
 //	    either.Right[error](21),
-//	    func(x int) int { return x * 2 },
+//	    N.Mul(2),
 //	) // Right(42)
 //
 //go:inline
 func MonadMap[E, A, B any](fa Either[E, A], f func(a A) B) Either[E, B] {
-	return MonadChain(fa, F.Flow2(f, Right[E, B]))
+	if fa.isLeft {
+		return Left[B](fa.l)
+	}
+	return Of[E](f(fa.r))
 }
 
 // MonadBiMap applies two functions: one to transform a Left value, another to transform a Right value.
@@ -100,13 +110,18 @@ func MonadMap[E, A, B any](fa Either[E, A], f func(a A) B) Either[E, B] {
 //	    func(n int) string { return fmt.Sprint(n) },
 //	) // Left("error")
 func MonadBiMap[E1, E2, A, B any](fa Either[E1, A], f func(E1) E2, g func(a A) B) Either[E2, B] {
-	return MonadFold(fa, F.Flow2(f, Left[B, E2]), F.Flow2(g, Right[E2, B]))
+	if fa.isLeft {
+		return Left[B](f(fa.l))
+	}
+	return Of[E2](g(fa.r))
 }
 
 // BiMap is the curried version of [MonadBiMap].
 // Maps a pair of functions over the two type arguments of the bifunctor.
 func BiMap[E1, E2, A, B any](f func(E1) E2, g func(a A) B) func(Either[E1, A]) Either[E2, B] {
-	return Fold(F.Flow2(f, Left[B, E2]), F.Flow2(g, Right[E2, B]))
+	return func(fa Either[E1, A]) Either[E2, B] {
+		return MonadBiMap(fa, f, g)
+	}
 }
 
 // MonadMapTo replaces the Right value with a constant value.
@@ -116,12 +131,15 @@ func BiMap[E1, E2, A, B any](f func(E1) E2, g func(a A) B) func(Either[E1, A]) E
 //
 //	result := either.MonadMapTo(either.Right[error](21), "success") // Right("success")
 func MonadMapTo[E, A, B any](fa Either[E, A], b B) Either[E, B] {
-	return MonadMap(fa, F.Constant1[A](b))
+	if fa.isLeft {
+		return Left[B](fa.l)
+	}
+	return Of[E](b)
 }
 
 // MapTo is the curried version of [MonadMapTo].
 func MapTo[E, A, B any](b B) Operator[E, A, B] {
-	return Map[E](F.Constant1[A](b))
+	return F.Bind2nd(MonadMapTo[E, A], b)
 }
 
 // MonadMapLeft applies a transformation function to the Left (error) value.
@@ -139,8 +157,8 @@ func MonadMapLeft[E1, A, E2 any](fa Either[E1, A], f func(E1) E2) Either[E2, A]
 
 // Map is the curried version of [MonadMap].
 // Transforms the Right value using the provided function.
-func Map[E, A, B any](f func(a A) B) func(fa Either[E, A]) Either[E, B] {
+func Map[E, A, B any](f func(a A) B) Operator[E, A, B] {
-	return Chain(F.Flow2(f, Right[E, B]))
+	return F.Bind2nd(MonadMap[E], f)
 }
 
 // MapLeft is the curried version of [MonadMapLeft].
@@ -163,8 +181,11 @@ func MapLeft[A, E1, E2 any](f func(E1) E2) func(fa Either[E1, A]) Either[E2, A]
 //	) // Right(42)
 //
 //go:inline
-func MonadChain[E, A, B any](fa Either[E, A], f func(a A) Either[E, B]) Either[E, B] {
+func MonadChain[E, A, B any](fa Either[E, A], f Kleisli[E, A, B]) Either[E, B] {
-	return MonadFold(fa, Left[B, E], f)
+	if fa.isLeft {
+		return Left[B](fa.l)
+	}
+	return f(fa.r)
 }
 
 // MonadChainFirst executes a side-effect computation but returns the original value.
@@ -179,7 +200,7 @@ func MonadChain[E, A, B any](fa Either[E, A], f func(a A) Either[E, B]) Either[E
 //	        return either.Right[error]("logged")
 //	    },
 //	) // Right(42) - original value preserved
-func MonadChainFirst[E, A, B any](ma Either[E, A], f func(a A) Either[E, B]) Either[E, A] {
+func MonadChainFirst[E, A, B any](ma Either[E, A], f Kleisli[E, A, B]) Either[E, A] {
 	return C.MonadChainFirst(
 		MonadChain[E, A, A],
 		MonadMap[E, B, A],
@@ -225,12 +246,12 @@ func ChainTo[A, E, B any](mb Either[E, B]) Operator[E, A, B] {
 
 // Chain is the curried version of [MonadChain].
 // Sequences two computations where the second depends on the first.
-func Chain[E, A, B any](f func(a A) Either[E, B]) Operator[E, A, B] {
+func Chain[E, A, B any](f Kleisli[E, A, B]) Operator[E, A, B] {
-	return Fold(Left[B, E], f)
+	return F.Bind2nd(MonadChain[E], f)
 }
 
 // ChainFirst is the curried version of [MonadChainFirst].
-func ChainFirst[E, A, B any](f func(a A) Either[E, B]) Operator[E, A, A] {
+func ChainFirst[E, A, B any](f Kleisli[E, A, B]) Operator[E, A, A] {
 	return C.ChainFirst(
 		Chain[E, A, A],
 		Map[E, B, A],
@@ -271,7 +292,7 @@ func TryCatch[FE func(error) E, E, A any](val A, err error, onThrow FE) Either[E
 //	result := either.TryCatchError(42, nil) // Right(42)
 //	result := either.TryCatchError(0, errors.New("fail")) // Left(error)
 func TryCatchError[A any](val A, err error) Either[error, A] {
-	return TryCatch(val, err, E.IdentityError)
+	return TryCatch(val, err, E.Identity)
 }
 
 // Sequence2 sequences two Either values using a combining function.
@@ -335,7 +356,7 @@ func FromError[A any](f func(a A) error) func(A) Either[error, A] {
 //	err := either.ToError(either.Left[int](errors.New("fail"))) // error
 //	err := either.ToError(either.Right[error](42)) // nil
 func ToError[A any](e Either[error, A]) error {
-	return MonadFold(e, E.IdentityError, F.Constant1[A, error](nil))
+	return MonadFold(e, E.Identity, F.Constant1[A, error](nil))
 }
 
 // Fold is the curried version of [MonadFold].
@@ -347,6 +368,8 @@ func ToError[A any](e Either[error, A]) error {
 //	    func(err error) string { return "Error: " + err.Error() },
 //	    func(n int) string { return fmt.Sprintf("Value: %d", n) },
 //	)(either.Right[error](42)) // "Value: 42"
+//
+//go:inline
 func Fold[E, A, B any](onLeft func(E) B, onRight func(A) B) func(Either[E, A]) B {
 	return func(ma Either[E, A]) B {
 		return MonadFold(ma, onLeft, onRight)
@@ -410,10 +433,12 @@ func GetOrElse[E, A any](onLeft func(E) A) func(Either[E, A]) A {
 // Reduce folds an Either into a single value using a reducer function.
 // Returns the initial value for Left, or applies the reducer to the Right value.
 func Reduce[E, A, B any](f func(B, A) B, initial B) func(Either[E, A]) B {
-	return Fold(
+	return func(fa Either[E, A]) B {
-		F.Constant1[E](initial),
+		if fa.isLeft {
-		F.Bind1st(f, initial),
+			return initial
-	)
+		}
+		return f(initial, fa.r)
+	}
 }
 
 // AltW provides an alternative Either if the first is Left, allowing different error types.
@@ -425,7 +450,7 @@ func Reduce[E, A, B any](f func(B, A) B, initial B) func(Either[E, A]) B {
 //	    return either.Right[string](99)
 //	})
 //	result := alternative(either.Left[int](errors.New("fail"))) // Right(99)
-func AltW[E, E1, A any](that L.Lazy[Either[E1, A]]) func(Either[E, A]) Either[E1, A] {
+func AltW[E, E1, A any](that Lazy[Either[E1, A]]) func(Either[E, A]) Either[E1, A] {
 	return Fold(F.Ignore1of1[E](that), Right[E1, A])
 }
 
@@ -437,7 +462,7 @@ func AltW[E, E1, A any](that L.Lazy[Either[E1, A]]) func(Either[E, A]) Either[E1
 //	    return either.Right[error](99)
 //	})
 //	result := alternative(either.Left[int](errors.New("fail"))) // Right(99)
-func Alt[E, A any](that L.Lazy[Either[E, A]]) Operator[E, A, A] {
+func Alt[E, A any](that Lazy[Either[E, A]]) Operator[E, A, A] {
 	return AltW[E](that)
 }
 
@@ -480,23 +505,28 @@ func Memoize[E, A any](val Either[E, A]) Either[E, A] {
 // MonadSequence2 sequences two Either values using a combining function.
 // Short-circuits on the first Left encountered.
 func MonadSequence2[E, T1, T2, R any](e1 Either[E, T1], e2 Either[E, T2], f func(T1, T2) Either[E, R]) Either[E, R] {
-	return MonadFold(e1, Left[R, E], func(t1 T1) Either[E, R] {
+	if e1.isLeft {
-		return MonadFold(e2, Left[R, E], func(t2 T2) Either[E, R] {
+		return Left[R](e1.l)
-			return f(t1, t2)
+	}
-		})
+	if e2.isLeft {
-	})
+		return Left[R](e2.l)
+	}
+	return f(e1.r, e2.r)
 }
 
 // MonadSequence3 sequences three Either values using a combining function.
 // Short-circuits on the first Left encountered.
 func MonadSequence3[E, T1, T2, T3, R any](e1 Either[E, T1], e2 Either[E, T2], e3 Either[E, T3], f func(T1, T2, T3) Either[E, R]) Either[E, R] {
-	return MonadFold(e1, Left[R, E], func(t1 T1) Either[E, R] {
+	if e1.isLeft {
-		return MonadFold(e2, Left[R, E], func(t2 T2) Either[E, R] {
+		return Left[R](e1.l)
-			return MonadFold(e3, Left[R, E], func(t3 T3) Either[E, R] {
+	}
-				return f(t1, t2, t3)
+	if e2.isLeft {
-			})
+		return Left[R](e2.l)
-		})
+	}
-	})
+	if e3.isLeft {
+		return Left[R](e3.l)
+	}
+	return f(e1.r, e2.r, e3.r)
 }
 
 // Swap exchanges the Left and Right type parameters.
@@ -524,6 +554,6 @@ func Flap[E, B, A any](a A) Operator[E, func(A) B, B] {
 
 // MonadAlt provides an alternative Either if the first is Left.
 // This is the monadic version of [Alt].
-func MonadAlt[E, A any](fa Either[E, A], that L.Lazy[Either[E, A]]) Either[E, A] {
+func MonadAlt[E, A any](fa Either[E, A], that Lazy[Either[E, A]]) Either[E, A] {
 	return MonadFold(fa, F.Ignore1of1[E](that), Of[E, A])
 }

v2/either/either_bench_test.go

@@ -20,6 +20,7 @@ import (
 	"testing"
 
 	F "github.com/IBM/fp-go/v2/function"
+	N "github.com/IBM/fp-go/v2/number"
 )
 
 var (
@@ -33,21 +34,21 @@ var (
 // Benchmark core constructors
 func BenchmarkLeft(b *testing.B) {
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = Left[int](errBench)
 	}
 }
 
 func BenchmarkRight(b *testing.B) {
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = Right[error](42)
 	}
 }
 
 func BenchmarkOf(b *testing.B) {
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = Of[error](42)
 	}
 }
@@ -57,7 +58,7 @@ func BenchmarkIsLeft(b *testing.B) {
 	left := Left[int](errBench)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchBool = IsLeft(left)
 	}
 }
@@ -66,7 +67,7 @@ func BenchmarkIsRight(b *testing.B) {
 	right := Right[error](42)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchBool = IsRight(right)
 	}
 }
@@ -75,10 +76,10 @@ func BenchmarkIsRight(b *testing.B) {
 func BenchmarkMonadFold_Right(b *testing.B) {
 	right := Right[error](42)
 	onLeft := func(e error) int { return 0 }
-	onRight := func(a int) int { return a * 2 }
+	onRight := N.Mul(2)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchInt = MonadFold(right, onLeft, onRight)
 	}
 }
@@ -86,10 +87,10 @@ func BenchmarkMonadFold_Right(b *testing.B) {
 func BenchmarkMonadFold_Left(b *testing.B) {
 	left := Left[int](errBench)
 	onLeft := func(e error) int { return 0 }
-	onRight := func(a int) int { return a * 2 }
+	onRight := N.Mul(2)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchInt = MonadFold(left, onLeft, onRight)
 	}
 }
@@ -98,11 +99,11 @@ func BenchmarkFold_Right(b *testing.B) {
 	right := Right[error](42)
 	folder := Fold(
 		func(e error) int { return 0 },
-		func(a int) int { return a * 2 },
+		N.Mul(2),
 	)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchInt = folder(right)
 	}
 }
@@ -111,11 +112,11 @@ func BenchmarkFold_Left(b *testing.B) {
 	left := Left[int](errBench)
 	folder := Fold(
 		func(e error) int { return 0 },
-		func(a int) int { return a * 2 },
+		N.Mul(2),
 	)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchInt = folder(left)
 	}
 }
@@ -125,7 +126,7 @@ func BenchmarkUnwrap_Right(b *testing.B) {
 	right := Right[error](42)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchInt, _ = Unwrap(right)
 	}
 }
@@ -134,7 +135,7 @@ func BenchmarkUnwrap_Left(b *testing.B) {
 	left := Left[int](errBench)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchInt, _ = Unwrap(left)
 	}
 }
@@ -143,7 +144,7 @@ func BenchmarkUnwrapError_Right(b *testing.B) {
 	right := Right[error](42)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchInt, _ = UnwrapError(right)
 	}
 }
@@ -152,7 +153,7 @@ func BenchmarkUnwrapError_Left(b *testing.B) {
 	left := Left[int](errBench)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchInt, _ = UnwrapError(left)
 	}
 }
@@ -160,40 +161,40 @@ func BenchmarkUnwrapError_Left(b *testing.B) {
 // Benchmark functor operations
 func BenchmarkMonadMap_Right(b *testing.B) {
 	right := Right[error](42)
-	mapper := func(a int) int { return a * 2 }
+	mapper := N.Mul(2)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = MonadMap(right, mapper)
 	}
 }
 
 func BenchmarkMonadMap_Left(b *testing.B) {
 	left := Left[int](errBench)
-	mapper := func(a int) int { return a * 2 }
+	mapper := N.Mul(2)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = MonadMap(left, mapper)
 	}
 }
 
 func BenchmarkMap_Right(b *testing.B) {
 	right := Right[error](42)
-	mapper := Map[error](func(a int) int { return a * 2 })
+	mapper := Map[error](N.Mul(2))
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = mapper(right)
 	}
 }
 
 func BenchmarkMap_Left(b *testing.B) {
 	left := Left[int](errBench)
-	mapper := Map[error](func(a int) int { return a * 2 })
+	mapper := Map[error](N.Mul(2))
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = mapper(left)
 	}
 }
@@ -203,7 +204,7 @@ func BenchmarkMapLeft_Right(b *testing.B) {
 	mapper := MapLeft[int](error.Error)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = mapper(right)
 	}
 }
@@ -213,7 +214,7 @@ func BenchmarkMapLeft_Left(b *testing.B) {
 	mapper := MapLeft[int](error.Error)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = mapper(left)
 	}
 }
@@ -226,7 +227,7 @@ func BenchmarkBiMap_Right(b *testing.B) {
 	)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = mapper(right)
 	}
 }
@@ -239,7 +240,7 @@ func BenchmarkBiMap_Left(b *testing.B) {
 	)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = mapper(left)
 	}
 }
@@ -250,7 +251,7 @@ func BenchmarkMonadChain_Right(b *testing.B) {
 	chainer := func(a int) Either[error, int] { return Right[error](a * 2) }
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = MonadChain(right, chainer)
 	}
 }
@@ -260,7 +261,7 @@ func BenchmarkMonadChain_Left(b *testing.B) {
 	chainer := func(a int) Either[error, int] { return Right[error](a * 2) }
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = MonadChain(left, chainer)
 	}
 }
@@ -270,7 +271,7 @@ func BenchmarkChain_Right(b *testing.B) {
 	chainer := Chain(func(a int) Either[error, int] { return Right[error](a * 2) })
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = chainer(right)
 	}
 }
@@ -280,7 +281,7 @@ func BenchmarkChain_Left(b *testing.B) {
 	chainer := Chain(func(a int) Either[error, int] { return Right[error](a * 2) })
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = chainer(left)
 	}
 }
@@ -290,7 +291,7 @@ func BenchmarkChainFirst_Right(b *testing.B) {
 	chainer := ChainFirst(func(a int) Either[error, string] { return Right[error]("logged") })
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = chainer(right)
 	}
 }
@@ -300,7 +301,7 @@ func BenchmarkChainFirst_Left(b *testing.B) {
 	chainer := ChainFirst(func(a int) Either[error, string] { return Right[error]("logged") })
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = chainer(left)
 	}
 }
@@ -309,7 +310,7 @@ func BenchmarkFlatten_Right(b *testing.B) {
 	nested := Right[error](Right[error](42))
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = Flatten(nested)
 	}
 }
@@ -318,28 +319,28 @@ func BenchmarkFlatten_Left(b *testing.B) {
 	nested := Left[Either[error, int]](errBench)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = Flatten(nested)
 	}
 }
 
 // Benchmark applicative operations
 func BenchmarkMonadAp_RightRight(b *testing.B) {
-	fab := Right[error](func(a int) int { return a * 2 })
+	fab := Right[error](N.Mul(2))
 	fa := Right[error](42)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = MonadAp(fab, fa)
 	}
 }
 
 func BenchmarkMonadAp_RightLeft(b *testing.B) {
-	fab := Right[error](func(a int) int { return a * 2 })
+	fab := Right[error](N.Mul(2))
 	fa := Left[int](errBench)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = MonadAp(fab, fa)
 	}
 }
@@ -349,18 +350,18 @@ func BenchmarkMonadAp_LeftRight(b *testing.B) {
 	fa := Right[error](42)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = MonadAp(fab, fa)
 	}
 }
 
 func BenchmarkAp_RightRight(b *testing.B) {
-	fab := Right[error](func(a int) int { return a * 2 })
+	fab := Right[error](N.Mul(2))
 	fa := Right[error](42)
 	ap := Ap[int](fa)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = ap(fab)
 	}
 }
@@ -371,7 +372,7 @@ func BenchmarkAlt_RightRight(b *testing.B) {
 	alternative := Alt(func() Either[error, int] { return Right[error](99) })
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = alternative(right)
 	}
 }
@@ -381,7 +382,7 @@ func BenchmarkAlt_LeftRight(b *testing.B) {
 	alternative := Alt(func() Either[error, int] { return Right[error](99) })
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = alternative(left)
 	}
 }
@@ -391,7 +392,7 @@ func BenchmarkOrElse_Right(b *testing.B) {
 	recover := OrElse(func(e error) Either[error, int] { return Right[error](0) })
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = recover(right)
 	}
 }
@@ -401,7 +402,7 @@ func BenchmarkOrElse_Left(b *testing.B) {
 	recover := OrElse(func(e error) Either[error, int] { return Right[error](0) })
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = recover(left)
 	}
 }
@@ -410,7 +411,7 @@ func BenchmarkOrElse_Left(b *testing.B) {
 func BenchmarkTryCatch_Success(b *testing.B) {
 	onThrow := func(err error) error { return err }
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = TryCatch(42, nil, onThrow)
 	}
 }
@@ -418,21 +419,21 @@ func BenchmarkTryCatch_Success(b *testing.B) {
 func BenchmarkTryCatch_Error(b *testing.B) {
 	onThrow := func(err error) error { return err }
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = TryCatch(0, errBench, onThrow)
 	}
 }
 
 func BenchmarkTryCatchError_Success(b *testing.B) {
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = TryCatchError(42, nil)
 	}
 }
 
 func BenchmarkTryCatchError_Error(b *testing.B) {
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = TryCatchError(0, errBench)
 	}
 }
@@ -441,7 +442,7 @@ func BenchmarkSwap_Right(b *testing.B) {
 	right := Right[error](42)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = Swap(right)
 	}
 }
@@ -450,7 +451,7 @@ func BenchmarkSwap_Left(b *testing.B) {
 	left := Left[int](errBench)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = Swap(left)
 	}
 }
@@ -460,7 +461,7 @@ func BenchmarkGetOrElse_Right(b *testing.B) {
 	getter := GetOrElse(func(e error) int { return 0 })
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchInt = getter(right)
 	}
 }
@@ -470,7 +471,7 @@ func BenchmarkGetOrElse_Left(b *testing.B) {
 	getter := GetOrElse(func(e error) int { return 0 })
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchInt = getter(left)
 	}
 }
@@ -480,10 +481,10 @@ func BenchmarkPipeline_Map_Right(b *testing.B) {
 	right := Right[error](21)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = F.Pipe1(
 			right,
-			Map[error](func(x int) int { return x * 2 }),
+			Map[error](N.Mul(2)),
 		)
 	}
 }
@@ -492,10 +493,10 @@ func BenchmarkPipeline_Map_Left(b *testing.B) {
 	left := Left[int](errBench)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = F.Pipe1(
 			left,
-			Map[error](func(x int) int { return x * 2 }),
+			Map[error](N.Mul(2)),
 		)
 	}
 }
@@ -504,7 +505,7 @@ func BenchmarkPipeline_Chain_Right(b *testing.B) {
 	right := Right[error](21)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = F.Pipe1(
 			right,
 			Chain(func(x int) Either[error, int] { return Right[error](x * 2) }),
@@ -516,7 +517,7 @@ func BenchmarkPipeline_Chain_Left(b *testing.B) {
 	left := Left[int](errBench)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = F.Pipe1(
 			left,
 			Chain(func(x int) Either[error, int] { return Right[error](x * 2) }),
@@ -528,12 +529,12 @@ func BenchmarkPipeline_Complex_Right(b *testing.B) {
 	right := Right[error](10)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = F.Pipe3(
 			right,
-			Map[error](func(x int) int { return x * 2 }),
+			Map[error](N.Mul(2)),
 			Chain(func(x int) Either[error, int] { return Right[error](x + 1) }),
-			Map[error](func(x int) int { return x * 2 }),
+			Map[error](N.Mul(2)),
 		)
 	}
 }
@@ -542,12 +543,12 @@ func BenchmarkPipeline_Complex_Left(b *testing.B) {
 	left := Left[int](errBench)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = F.Pipe3(
 			left,
-			Map[error](func(x int) int { return x * 2 }),
+			Map[error](N.Mul(2)),
 			Chain(func(x int) Either[error, int] { return Right[error](x + 1) }),
-			Map[error](func(x int) int { return x * 2 }),
+			Map[error](N.Mul(2)),
 		)
 	}
 }
@@ -559,7 +560,7 @@ func BenchmarkMonadSequence2_RightRight(b *testing.B) {
 	f := func(a, b int) Either[error, int] { return Right[error](a + b) }
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = MonadSequence2(e1, e2, f)
 	}
 }
@@ -570,7 +571,7 @@ func BenchmarkMonadSequence2_LeftRight(b *testing.B) {
 	f := func(a, b int) Either[error, int] { return Right[error](a + b) }
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = MonadSequence2(e1, e2, f)
 	}
 }
@@ -582,7 +583,7 @@ func BenchmarkMonadSequence3_RightRightRight(b *testing.B) {
 	f := func(a, b, c int) Either[error, int] { return Right[error](a + b + c) }
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchResult = MonadSequence3(e1, e2, e3, f)
 	}
 }
@@ -591,7 +592,7 @@ func BenchmarkMonadSequence3_RightRightRight(b *testing.B) {
 func BenchmarkDo(b *testing.B) {
 	type State struct{ value int }
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = Do[error](State{})
 	}
 }
@@ -609,7 +610,7 @@ func BenchmarkBind_Right(b *testing.B) {
 	)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = binder(initial)
 	}
 }
@@ -625,7 +626,7 @@ func BenchmarkLet_Right(b *testing.B) {
 	)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = letter(initial)
 	}
 }
@@ -635,7 +636,7 @@ func BenchmarkString_Right(b *testing.B) {
 	right := Right[error](42)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchString = right.String()
 	}
 }
@@ -644,9 +645,7 @@ func BenchmarkString_Left(b *testing.B) {
 	left := Left[int](errBench)
 	b.ResetTimer()
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		benchString = left.String()
 	}
 }
-
-// Made with Bob

v2/either/either_coverage_test.go

@@ -201,7 +201,7 @@ func TestSwap(t *testing.T) {
 
 // Test MonadFlap and Flap
 func TestFlap(t *testing.T) {
-	fab := Right[error](func(x int) string { return strconv.Itoa(x) })
+	fab := Right[error](strconv.Itoa)
 	result := MonadFlap(fab, 42)
 	assert.Equal(t, Right[error]("42"), result)
 
@@ -615,7 +615,7 @@ func TestMonad(t *testing.T) {
 	assert.Equal(t, Right[error](42), result)
 
 	// Test Map
-	mapFn := m.Map(func(x int) string { return strconv.Itoa(x) })
+	mapFn := m.Map(strconv.Itoa)
 	result2 := mapFn(Right[error](42))
 	assert.Equal(t, Right[error]("42"), result2)
 
@@ -628,7 +628,7 @@ func TestMonad(t *testing.T) {
 
 	// Test Ap
 	apFn := m.Ap(Right[error](42))
-	result4 := apFn(Right[error](func(x int) string { return strconv.Itoa(x) }))
+	result4 := apFn(Right[error](strconv.Itoa))
 	assert.Equal(t, Right[error]("42"), result4)
 }
 

v2/either/either_test.go

@@ -66,7 +66,7 @@ func TestUnwrapError(t *testing.T) {
 
 func TestReduce(t *testing.T) {
 
-	s := S.Semigroup()
+	s := S.Semigroup
 
 	assert.Equal(t, "foobar", F.Pipe1(Right[string]("bar"), Reduce[string](s.Concat, "foo")))
 	assert.Equal(t, "foo", F.Pipe1(Left[string]("bar"), Reduce[string](s.Concat, "foo")))
@@ -76,8 +76,8 @@ func TestAp(t *testing.T) {
 	f := S.Size
 
 	assert.Equal(t, Right[string](3), F.Pipe1(Right[string](f), Ap[int](Right[string]("abc"))))
-	assert.Equal(t, Left[int]("maError"), F.Pipe1(Right[string](f), Ap[int](Left[string, string]("maError"))))
+	assert.Equal(t, Left[int]("maError"), F.Pipe1(Right[string](f), Ap[int](Left[string]("maError"))))
-	assert.Equal(t, Left[int]("mabError"), F.Pipe1(Left[func(string) int]("mabError"), Ap[int](Left[string, string]("maError"))))
+	assert.Equal(t, Left[int]("mabError"), F.Pipe1(Left[func(string) int]("mabError"), Ap[int](Left[string]("maError"))))
 }
 
 func TestAlt(t *testing.T) {

v2/either/escape_test.go

@@ -0,0 +1,33 @@
+// Copyright (c) 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package either
+
+// Test functions to analyze escape behavior
+
+//go:noinline
+func testOf(x int) Either[error, int] {
+	return Of[error](x)
+}
+
+//go:noinline
+func testRight(x int) Either[error, int] {
+	return Right[error](x)
+}
+
+//go:noinline
+func testLeft(x int) Either[int, string] {
+	return Left[string](x)
+}

v2/either/logger.go

@@ -46,7 +46,7 @@ func _log[E, A any](left func(string, ...any), right func(string, ...any), prefi
 //	result := F.Pipe2(
 //	    either.Right[error](42),
 //	    logger("Processing"),
-//	    either.Map(func(x int) int { return x * 2 }),
+//	    either.Map(N.Mul(2)),
 //	)
 //	// Logs: "Processing: 42"
 //	// result is Right(84)

v2/either/monad.go

@@ -19,38 +19,116 @@ import (
 	"github.com/IBM/fp-go/v2/internal/monad"
 )
 
-type eitherMonad[E, A, B any] struct{}
+// eitherMonad is the internal implementation of the Monad type class for Either.
-
+// It extends eitherApplicative by adding the Chain operation for sequential composition.
-func (o *eitherMonad[E, A, B]) Of(a A) Either[E, A] {
+type eitherMonad[E, A, B any] struct {
-	return Of[E](a)
+	eitherApplicative[E, A, B]
+	fchain func(Kleisli[E, A, B]) Operator[E, A, B]
 }
 
-func (o *eitherMonad[E, A, B]) Map(f func(A) B) Operator[E, A, B] {
+// Chain sequences dependent computations, failing fast on the first Left.
-	return Map[E](f)
+func (o *eitherMonad[E, A, B]) Chain(f Kleisli[E, A, B]) Operator[E, A, B] {
+	return o.fchain(f)
 }
 
-func (o *eitherMonad[E, A, B]) Chain(f func(A) Either[E, B]) Operator[E, A, B] {
+// Monad creates a lawful Monad instance for Either with fail-fast error handling.
-	return Chain(f)
-}
-
-func (o *eitherMonad[E, A, B]) Ap(fa Either[E, A]) Operator[E, func(A) B, B] {
-	return Ap[B](fa)
-}
-
-// Monad implements the monadic operations for Either.
-// A monad combines the capabilities of Functor (Map), Applicative (Ap), and Chain (flatMap/bind).
-// This allows for sequential composition of computations that may fail.
 //
-// Example:
+// A monad combines the capabilities of four type classes:
+//   - Functor (Map): transform the Right value
+//   - Pointed (Of): lift a pure value into a Right
+//   - Applicative (Ap): apply wrapped functions (fails fast on first Left)
+//   - Chainable (Chain): sequence dependent computations (fails fast on first Left)
+//
+// The Either monad is left-biased and fails fast: once a Left is encountered,
+// no further computations are performed and the Left is propagated immediately.
+// This makes it ideal for error handling where you want to stop at the first error.
+//
+// This implementation satisfies all monad laws:
+//
+// Monad Laws:
+//   - Left Identity: Chain(f)(Of(a)) == f(a)
+//   - Right Identity: Chain(Of)(m) == m
+//   - Associativity: Chain(g)(Chain(f)(m)) == Chain(x => Chain(g)(f(x)))(m)
+//
+// Additionally, it satisfies all prerequisite laws from Functor, Apply, and Applicative.
+//
+// Relationship to Applicative:
+//
+// This Monad uses the standard fail-fast Applicative (see Applicative function).
+// In a lawful monad, Ap can be derived from Chain and Of:
+//
+//	Ap(fa)(ff) == Chain(f => Chain(a => Of(f(a)))(fa))(ff)
+//
+// The Either monad satisfies this property, making it a true lawful monad.
+//
+// When to use Monad vs Applicative:
+//   - Use Monad when you need sequential dependent operations (Chain)
+//   - Use Applicative when you only need independent operations (Ap, Map)
+//   - Both fail fast on the first error
+//
+// When to use Monad vs ApplicativeV:
+//   - Use Monad for sequential error handling (fail-fast)
+//   - Use ApplicativeV for parallel validation (error accumulation)
+//   - Note: There is no "MonadV" because Chain inherently fails fast
+//
+// Example - Sequential Dependent Operations:
 //
 //	m := either.Monad[error, int, string]()
+//
+//	// Chain allows each step to depend on the previous result
 //	result := m.Chain(func(x int) either.Either[error, string] {
 //	    if x > 0 {
 //	        return either.Right[error](strconv.Itoa(x))
 //	    }
-//	    return either.Left[string](errors.New("negative"))
+//	    return either.Left[string](errors.New("value must be positive"))
 //	})(either.Right[error](42))
 //	// result is Right("42")
+//
+//	// Fails fast on first error
+//	result2 := m.Chain(func(x int) either.Either[error, string] {
+//	    return either.Right[error](strconv.Itoa(x))
+//	})(either.Left[int](errors.New("initial error")))
+//	// result2 is Left("initial error") - Chain never executes
+//
+// Example - Combining with Applicative operations:
+//
+//	m := either.Monad[error, int, int]()
+//
+//	// Map transforms the value
+//	value := m.Map(func(x int) int { return x * 2 })(either.Right[error](21))
+//	// value is Right(42)
+//
+//	// Ap applies wrapped functions (also fails fast)
+//	fn := either.Right[error](func(x int) int { return x + 1 })
+//	result := m.Ap(value)(fn)
+//	// result is Right(43)
+//
+// Example - Real-world usage with error handling:
+//
+//	m := either.Monad[error, User, SavedUser]()
+//
+//	// Pipeline of operations that can fail
+//	result := m.Chain(func(user User) either.Either[error, SavedUser] {
+//	    // Save to database
+//	    return saveToDatabase(user)
+//	})(m.Chain(func(user User) either.Either[error, User] {
+//	    // Validate user
+//	    return validateUser(user)
+//	})(either.Right[error](inputUser)))
+//
+//	// If any step fails, the error propagates immediately
+//
+// Type Parameters:
+//   - E: The error type (Left value)
+//   - A: The input value type (Right value)
+//   - B: The output value type after transformation
 func Monad[E, A, B any]() monad.Monad[A, B, Either[E, A], Either[E, B], Either[E, func(A) B]] {
-	return &eitherMonad[E, A, B]{}
+	return &eitherMonad[E, A, B]{
+		eitherApplicative[E, A, B]{
+			Of[E, A],
+			Map[E, A, B],
+			Ap[B, E, A],
+		},
+		Chain[E, A, B],
+	}
 }

v2/either/monoid.go

@@ -16,7 +16,6 @@
 package either
 
 import (
-	L "github.com/IBM/fp-go/v2/lazy"
 	M "github.com/IBM/fp-go/v2/monoid"
 )
 
@@ -51,7 +50,7 @@ func AlternativeMonoid[E, A any](m M.Monoid[A]) Monoid[E, A] {
 //	m := either.AltMonoid[error, int](zero)
 //	result := m.Concat(either.Left[int](errors.New("err1")), either.Right[error](42))
 //	// result is Right(42)
-func AltMonoid[E, A any](zero L.Lazy[Either[E, A]]) Monoid[E, A] {
+func AltMonoid[E, A any](zero Lazy[Either[E, A]]) Monoid[E, A] {
 	return M.AltMonoid(
 		zero,
 		MonadAlt[E, A],

v2/either/record.go

@@ -35,13 +35,18 @@ import (
 //	// result is Right(map[string]int{"a": 1, "b": 2})
 //
 //go:inline
-func TraverseRecordG[GA ~map[K]A, GB ~map[K]B, K comparable, E, A, B any](f func(A) Either[E, B]) func(GA) Either[E, GB] {
+func TraverseRecordG[GA ~map[K]A, GB ~map[K]B, K comparable, E, A, B any](f Kleisli[E, A, B]) Kleisli[E, GA, GB] {
-	return RR.Traverse[GA](
+	return func(ga GA) Either[E, GB] {
-		Of[E, GB],
+		bs := make(GB, len(ga))
-		Map[E, GB, func(B) GB],
+		for i, a := range ga {
-		Ap[GB, E, B],
+			b := f(a)
-		f,
+			if b.isLeft {
-	)
+				return Left[GB](b.l)
+			}
+			bs[i] = b.r
+		}
+		return Of[E](bs)
+	}
 }
 
 // TraverseRecord transforms a map by applying a function that returns an Either to each value.
@@ -58,7 +63,7 @@ func TraverseRecordG[GA ~map[K]A, GB ~map[K]B, K comparable, E, A, B any](f func
 //	// result is Right(map[string]int{"a": 1, "b": 2})
 //
 //go:inline
-func TraverseRecord[K comparable, E, A, B any](f func(A) Either[E, B]) func(map[K]A) Either[E, map[K]B] {
+func TraverseRecord[K comparable, E, A, B any](f Kleisli[E, A, B]) Kleisli[E, map[K]A, map[K]B] {
 	return TraverseRecordG[map[K]A, map[K]B](f)
 }
 
@@ -79,13 +84,18 @@ func TraverseRecord[K comparable, E, A, B any](f func(A) Either[E, B]) func(map[
 //	// result is Right(map[string]string{"a": "a:1"})
 //
 //go:inline
-func TraverseRecordWithIndexG[GA ~map[K]A, GB ~map[K]B, K comparable, E, A, B any](f func(K, A) Either[E, B]) func(GA) Either[E, GB] {
+func TraverseRecordWithIndexG[GA ~map[K]A, GB ~map[K]B, K comparable, E, A, B any](f func(K, A) Either[E, B]) Kleisli[E, GA, GB] {
-	return RR.TraverseWithIndex[GA](
+	return func(ga GA) Either[E, GB] {
-		Of[E, GB],
+		bs := make(GB, len(ga))
-		Map[E, GB, func(B) GB],
+		for i, a := range ga {
-		Ap[GB, E, B],
+			b := f(i, a)
-		f,
+			if b.isLeft {
-	)
+				return Left[GB](b.l)
+			}
+			bs[i] = b.r
+		}
+		return Of[E](bs)
+	}
 }
 
 // TraverseRecordWithIndex transforms a map by applying an indexed function that returns an Either.
@@ -104,7 +114,7 @@ func TraverseRecordWithIndexG[GA ~map[K]A, GB ~map[K]B, K comparable, E, A, B an
 //	// result is Right(map[string]string{"a": "a:1"})
 //
 //go:inline
-func TraverseRecordWithIndex[K comparable, E, A, B any](f func(K, A) Either[E, B]) func(map[K]A) Either[E, map[K]B] {
+func TraverseRecordWithIndex[K comparable, E, A, B any](f func(K, A) Either[E, B]) Kleisli[E, map[K]A, map[K]B] {
 	return TraverseRecordWithIndexG[map[K]A, map[K]B](f)
 }
 

v2/either/resource.go

@@ -15,10 +15,6 @@
 
 package either
 
-import (
-	F "github.com/IBM/fp-go/v2/function"
-)
-
 // WithResource constructs a function that creates a resource, operates on it, and then releases it.
 // This ensures proper resource cleanup even if operations fail.
 // The resource is released immediately after the operation completes.
@@ -43,25 +39,24 @@ import (
 //	    // Use file here
 //	    return either.Right[error]("data")
 //	})
-func WithResource[E, R, A, ANY any](onCreate func() Either[E, R], onRelease Kleisli[E, R, ANY]) func(func(R) Either[E, A]) Either[E, A] {
+func WithResource[A, E, R, ANY any](
-
+	onCreate func() Either[E, R],
+	onRelease Kleisli[E, R, ANY],
+) Kleisli[E, Kleisli[E, R, A], A] {
 	return func(f func(R) Either[E, A]) Either[E, A] {
-		return MonadChain(
+		r := onCreate()
-			onCreate(), func(r R) Either[E, A] {
+		if r.isLeft {
-				// run the code and make sure to release as quickly as possible
+			return Left[A](r.l)
-				res := f(r)
+		}
-				released := onRelease(r)
+		a := f(r.r)
-				// handle the errors
+		n := onRelease(r.r)
-				return MonadFold(
+		if a.isLeft {
-					res,
+			return Left[A](a.l)
-					Left[A, E],
+		}
-					func(a A) Either[E, A] {
+		if n.isLeft {
-						return F.Pipe1(
+			return Left[A](n.l)
-							released,
+
-							MapTo[E, ANY](a),
+		}
-						)
+		return Of[E](a.r)
-					})
-			},
-		)
 	}
 }

v2/either/resource_test.go

@@ -40,7 +40,7 @@ func TestWithResource(t *testing.T) {
 		return Of[error](f.Name())
 	}
 
-	tempFile := WithResource[error, *os.File, string](onCreate, onDelete)
+	tempFile := WithResource[string](onCreate, onDelete)
 
 	resE := tempFile(onHandler)
 

v2/either/testing/laws.go

@@ -47,7 +47,7 @@ import (
 //	    eqString := eq.FromStrictEquals[string]()
 //	    eqError := eq.FromStrictEquals[error]()
 //
-//	    ab := func(x int) string { return strconv.Itoa(x) }
+//	    ab := strconv.Itoa
 //	    bc := func(s string) bool { return len(s) > 0 }
 //
 //	    testing.AssertLaws(t, eqError, eqInt, eqString, eq.FromStrictEquals[bool](), ab, bc)(42)

v2/either/types.go

@@ -17,6 +17,7 @@ package either
 
 import (
 	"github.com/IBM/fp-go/v2/endomorphism"
+	"github.com/IBM/fp-go/v2/lazy"
 	"github.com/IBM/fp-go/v2/monoid"
 	"github.com/IBM/fp-go/v2/optics/lens"
 	"github.com/IBM/fp-go/v2/option"
@@ -29,6 +30,7 @@ type (
 	Option[A any]       = option.Option[A]
 	Lens[S, T any]      = lens.Lens[S, T]
 	Endomorphism[T any] = endomorphism.Endomorphism[T]
+	Lazy[T any]         = lazy.Lazy[T]
 
 	Kleisli[E, A, B any]  = reader.Reader[A, Either[E, B]]
 	Operator[E, A, B any] = Kleisli[E, Either[E, A], B]

v2/either/validation.go

@@ -0,0 +1,144 @@
+// Copyright (c) 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package either
+
+import (
+	F "github.com/IBM/fp-go/v2/function"
+	S "github.com/IBM/fp-go/v2/semigroup"
+)
+
+// MonadApV is the applicative validation functor that combines errors using a semigroup.
+//
+// Unlike the standard [MonadAp] which short-circuits on the first Left (error),
+// MonadApV accumulates all errors using the provided semigroup's Concat operation.
+// This is particularly useful for validation scenarios where you want to collect
+// all validation errors rather than stopping at the first one.
+//
+// The function takes a semigroup for combining errors and returns a function that
+// applies a wrapped function to a wrapped value, accumulating errors if both are Left.
+//
+// Behavior:
+//   - If both fab and fa are Left, combines their errors using sg.Concat
+//   - If only fab is Left, returns Left with fab's error
+//   - If only fa is Left, returns Left with fa's error
+//   - If both are Right, applies the function and returns Right with the result
+//
+// Type Parameters:
+//   - B: The result type after applying the function
+//   - E: The error type (must support the semigroup operation)
+//   - A: The input type to the function
+//
+// Parameters:
+//   - sg: A semigroup that defines how to combine two error values
+//
+// Returns:
+//   - A function that takes a wrapped function and a wrapped value, returning
+//     Either[E, B] with accumulated errors or the computed result
+//
+// Example:
+//
+//	// Define a semigroup that concatenates error messages
+//	errorSemigroup := semigroup.MakeSemigroup(func(e1, e2 string) string {
+//	    return e1 + "; " + e2
+//	})
+//
+//	// Create the validation applicative
+//	applyV := either.MonadApV[int](errorSemigroup)
+//
+//	// Both are errors - errors get combined
+//	fab := either.Left[func(int) int]("error1")
+//	fa := either.Left[int]("error2")
+//	result := applyV(fab, fa) // Left("error1; error2")
+//
+//	// One error - returns that error
+//	fab2 := either.Right[string](N.Mul(2))
+//	fa2 := either.Left[int]("validation failed")
+//	result2 := applyV(fab2, fa2) // Left("validation failed")
+//
+//	// Both success - applies function
+//	fab3 := either.Right[string](N.Mul(2))
+//	fa3 := either.Right[string](21)
+//	result3 := applyV(fab3, fa3) // Right(42)
+func MonadApV[B, A, E any](sg S.Semigroup[E]) func(fab Either[E, func(a A) B], fa Either[E, A]) Either[E, B] {
+	return func(fab Either[E, func(a A) B], fa Either[E, A]) Either[E, B] {
+		if fab.isLeft {
+			if fa.isLeft {
+				return Left[B](sg.Concat(fab.l, fa.l))
+			}
+			return Left[B](fab.l)
+		}
+		if fa.isLeft {
+			return Left[B](fa.l)
+		}
+		return Of[E](fab.r(fa.r))
+	}
+}
+
+// ApV is the curried version of [MonadApV] that combines errors using a semigroup.
+//
+// This function provides a more convenient API for validation scenarios by currying
+// the arguments. It first takes the value to validate, then returns a function that
+// takes the validation function. This allows for a more natural composition style.
+//
+// Like [MonadApV], this accumulates all errors using the provided semigroup instead
+// of short-circuiting on the first error. This is the key difference from the
+// standard [Ap] function.
+//
+// Type Parameters:
+//   - B: The result type after applying the function
+//   - E: The error type (must support the semigroup operation)
+//   - A: The input type to the function
+//
+// Parameters:
+//   - sg: A semigroup that defines how to combine two error values
+//
+// Returns:
+//   - A function that takes a value Either[E, A] and returns an Operator that
+//     applies validation functions while accumulating errors
+//
+// Example:
+//
+//	// Define a semigroup for combining validation errors
+//	type ValidationError struct {
+//	    Errors []string
+//	}
+//	errorSemigroup := semigroup.MakeSemigroup(func(e1, e2 ValidationError) ValidationError {
+//	    return ValidationError{Errors: append(e1.Errors, e2.Errors...)}
+//	})
+//
+//	// Create validators
+//	validatePositive := func(x int) either.Either[ValidationError, int] {
+//	    if x > 0 {
+//	        return either.Right[ValidationError](x)
+//	    }
+//	    return either.Left[int](ValidationError{Errors: []string{"must be positive"}})
+//	}
+//
+//	// Use ApV for validation
+//	applyValidation := either.ApV[int](errorSemigroup)
+//	value := either.Left[int](ValidationError{Errors: []string{"invalid input"}})
+//	validator := either.Left[func(int) int](ValidationError{Errors: []string{"invalid validator"}})
+//
+//	result := applyValidation(value)(validator)
+//	// Left(ValidationError{Errors: []string{"invalid validator", "invalid input"}})
+//
+//go:inline
+func ApV[B, A, E any](sg S.Semigroup[E]) func(Either[E, A]) Operator[E, func(A) B, B] {
+	apv := MonadApV[B, A](sg)
+	return func(e Either[E, A]) Operator[E, func(A) B, B] {
+		return F.Bind2nd(apv, e)
+	}
+}

v2/either/validation_test.go

@@ -0,0 +1,362 @@
+// Copyright (c) 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package either
+
+import (
+	"testing"
+
+	F "github.com/IBM/fp-go/v2/function"
+	N "github.com/IBM/fp-go/v2/number"
+	SG "github.com/IBM/fp-go/v2/semigroup"
+	S "github.com/IBM/fp-go/v2/string"
+	"github.com/stretchr/testify/assert"
+)
+
+// TestMonadApV_BothRight tests MonadApV when both function and value are Right
+func TestMonadApV_BothRight(t *testing.T) {
+	// Create a semigroup for string concatenation
+	sg := S.IntersperseSemigroup("; ")
+
+	// Create the validation applicative
+	applyV := MonadApV[int, int](sg)
+
+	// Both are Right - should apply function
+	fab := Right[string](N.Mul(2))
+	fa := Right[string](21)
+
+	result := applyV(fab, fa)
+
+	assert.True(t, IsRight(result))
+	assert.Equal(t, Right[string](42), result)
+}
+
+// TestMonadApV_BothLeft tests MonadApV when both function and value are Left
+func TestMonadApV_BothLeft(t *testing.T) {
+	// Create a semigroup for string concatenation
+	sg := S.IntersperseSemigroup("; ")
+
+	// Create the validation applicative
+	applyV := MonadApV[int, int](sg)
+
+	// Both are Left - should combine errors
+	fab := Left[func(int) int]("error1")
+	fa := Left[int]("error2")
+
+	result := applyV(fab, fa)
+
+	assert.True(t, IsLeft(result))
+	// When both are Left, errors are combined as: fa error + fab error
+	assert.Equal(t, Left[int]("error1; error2"), result)
+}
+
+// TestMonadApV_LeftFunction tests MonadApV when function is Left and value is Right
+func TestMonadApV_LeftFunction(t *testing.T) {
+	// Create a semigroup for string concatenation
+	sg := S.IntersperseSemigroup("; ")
+
+	// Create the validation applicative
+	applyV := MonadApV[int, int](sg)
+
+	// Function is Left, value is Right - should return function's error
+	fab := Left[func(int) int]("function error")
+	fa := Right[string](21)
+
+	result := applyV(fab, fa)
+
+	assert.True(t, IsLeft(result))
+	assert.Equal(t, Left[int]("function error"), result)
+}
+
+// TestMonadApV_LeftValue tests MonadApV when function is Right and value is Left
+func TestMonadApV_LeftValue(t *testing.T) {
+	// Create a semigroup for string concatenation
+	sg := S.IntersperseSemigroup("; ")
+
+	// Create the validation applicative
+	applyV := MonadApV[int, int](sg)
+
+	// Function is Right, value is Left - should return value's error
+	fab := Right[string](N.Mul(2))
+	fa := Left[int]("value error")
+
+	result := applyV(fab, fa)
+
+	assert.True(t, IsLeft(result))
+	assert.Equal(t, Left[int]("value error"), result)
+}
+
+// TestMonadApV_WithSliceSemigroup tests MonadApV with a slice-based semigroup
+func TestMonadApV_WithSliceSemigroup(t *testing.T) {
+	// Create a semigroup that concatenates slices
+	sg := SG.MakeSemigroup(func(a, b []string) []string {
+		return append(a, b...)
+	})
+
+	// Create the validation applicative
+	applyV := MonadApV[string, string](sg)
+
+	// Both are Left with slice errors
+	fab := Left[func(string) string]([]string{"error1", "error2"})
+	fa := Left[string]([]string{"error3", "error4"})
+
+	result := applyV(fab, fa)
+
+	assert.True(t, IsLeft(result))
+	// When both are Left, errors are combined as: fa errors + fab errors
+	expected := Left[string]([]string{"error1", "error2", "error3", "error4"})
+	assert.Equal(t, expected, result)
+}
+
+// TestMonadApV_ComplexFunction tests MonadApV with a more complex function
+func TestMonadApV_ComplexFunction(t *testing.T) {
+	// Create a semigroup for string concatenation
+	sg := SG.MakeSemigroup(func(a, b string) string {
+		return a + " | " + b
+	})
+
+	// Create the validation applicative
+	applyV := MonadApV[string, int](sg)
+
+	// Test with a function that transforms the value
+	fab := Right[string](func(x int) string {
+		if x > 0 {
+			return "positive"
+		}
+		return "non-positive"
+	})
+	fa := Right[string](42)
+
+	result := applyV(fab, fa)
+
+	assert.True(t, IsRight(result))
+	assert.Equal(t, Right[string]("positive"), result)
+}
+
+// TestApV_BothRight tests ApV when both function and value are Right
+func TestApV_BothRight(t *testing.T) {
+	// Create a semigroup for string concatenation
+	sg := S.IntersperseSemigroup("; ")
+
+	// Create the validation applicative
+	applyV := ApV[int, int](sg)
+
+	// Both are Right - should apply function
+	fa := Right[string](21)
+	fab := Right[string](N.Mul(2))
+
+	result := applyV(fa)(fab)
+
+	assert.True(t, IsRight(result))
+	assert.Equal(t, Right[string](42), result)
+}
+
+// TestApV_BothLeft tests ApV when both function and value are Left
+func TestApV_BothLeft(t *testing.T) {
+	// Create a semigroup for string concatenation
+	sg := S.IntersperseSemigroup("; ")
+
+	// Create the validation applicative
+	applyV := ApV[int, int](sg)
+
+	// Both are Left - should combine errors
+	fa := Left[int]("error2")
+	fab := Left[func(int) int]("error1")
+
+	result := applyV(fa)(fab)
+
+	assert.True(t, IsLeft(result))
+	// When both are Left, errors are combined as: fa error + fab error
+	assert.Equal(t, Left[int]("error1; error2"), result)
+}
+
+// TestApV_LeftFunction tests ApV when function is Left and value is Right
+func TestApV_LeftFunction(t *testing.T) {
+	// Create a semigroup for string concatenation
+	sg := S.IntersperseSemigroup("; ")
+
+	// Create the validation applicative
+	applyV := ApV[int, int](sg)
+
+	// Function is Left, value is Right - should return function's error
+	fa := Right[string](21)
+	fab := Left[func(int) int]("function error")
+
+	result := applyV(fa)(fab)
+
+	assert.True(t, IsLeft(result))
+	assert.Equal(t, Left[int]("function error"), result)
+}
+
+// TestApV_LeftValue tests ApV when function is Right and value is Left
+func TestApV_LeftValue(t *testing.T) {
+	// Create a semigroup for string concatenation
+	sg := S.IntersperseSemigroup("; ")
+
+	// Create the validation applicative
+	applyV := ApV[int, int](sg)
+
+	// Function is Right, value is Left - should return value's error
+	fa := Left[int]("value error")
+	fab := Right[string](N.Mul(2))
+
+	result := applyV(fa)(fab)
+
+	assert.True(t, IsLeft(result))
+	assert.Equal(t, Left[int]("value error"), result)
+}
+
+// TestApV_Composition tests ApV with function composition
+func TestApV_Composition(t *testing.T) {
+	// Create a semigroup for string concatenation
+	sg := SG.MakeSemigroup(func(a, b string) string {
+		return a + " & " + b
+	})
+
+	// Create the validation applicative
+	applyV := ApV[string, int](sg)
+
+	// Test composition with pipe
+	fa := Right[string](10)
+	fab := Right[string](func(x int) string {
+		return F.Pipe1(x, func(n int) string {
+			if n >= 10 {
+				return "large"
+			}
+			return "small"
+		})
+	})
+
+	result := F.Pipe1(fa, applyV)(fab)
+
+	assert.True(t, IsRight(result))
+	assert.Equal(t, Right[string]("large"), result)
+}
+
+// TestApV_WithStructSemigroup tests ApV with a custom struct semigroup
+func TestApV_WithStructSemigroup(t *testing.T) {
+	type ValidationErrors struct {
+		Errors []string
+	}
+
+	// Create a semigroup that combines validation errors
+	sg := SG.MakeSemigroup(func(a, b ValidationErrors) ValidationErrors {
+		return ValidationErrors{
+			Errors: append(append([]string{}, a.Errors...), b.Errors...),
+		}
+	})
+
+	// Create the validation applicative
+	applyV := ApV[int, int](sg)
+
+	// Both are Left with validation errors
+	fa := Left[int](ValidationErrors{Errors: []string{"field2: invalid"}})
+	fab := Left[func(int) int](ValidationErrors{Errors: []string{"field1: required"}})
+
+	result := applyV(fa)(fab)
+
+	assert.True(t, IsLeft(result))
+	// When both are Left, errors are combined as: fa errors + fab errors
+	expected := Left[int](ValidationErrors{
+		Errors: []string{"field1: required", "field2: invalid"},
+	})
+	assert.Equal(t, expected, result)
+}
+
+// TestApV_MultipleValidations tests ApV with multiple validation steps
+func TestApV_MultipleValidations(t *testing.T) {
+	// Create a semigroup for string concatenation
+	sg := SG.MakeSemigroup(func(a, b string) string {
+		return a + ", " + b
+	})
+
+	// Create the validation applicative
+	applyV := ApV[int, int](sg)
+
+	// Simulate multiple validation failures
+	validation1 := Left[int]("age must be positive")
+	validation2 := Left[func(int) int]("name is required")
+
+	result := applyV(validation1)(validation2)
+
+	assert.True(t, IsLeft(result))
+	// When both are Left, errors are combined as: validation1 error + validation2 error
+	assert.Equal(t, Left[int]("name is required, age must be positive"), result)
+}
+
+// TestMonadApV_DifferentTypes tests MonadApV with different input and output types
+func TestMonadApV_DifferentTypes(t *testing.T) {
+	// Create a semigroup for string concatenation
+	sg := S.IntersperseSemigroup(" + ")
+
+	// Create the validation applicative
+	applyV := MonadApV[string, int](sg)
+
+	// Function converts int to string
+	fab := Right[string](func(x int) string {
+		return F.Pipe1(x, func(n int) string {
+			if n == 0 {
+				return "zero"
+			} else if n > 0 {
+				return "positive"
+			}
+			return "negative"
+		})
+	})
+	fa := Right[string](-5)
+
+	result := applyV(fab, fa)
+
+	assert.True(t, IsRight(result))
+	assert.Equal(t, Right[string]("negative"), result)
+}
+
+// TestApV_FirstSemigroup tests ApV with First semigroup (always returns first error)
+func TestApV_FirstSemigroup(t *testing.T) {
+	// Use First semigroup which always returns the first value
+	sg := SG.First[string]()
+
+	// Create the validation applicative
+	applyV := ApV[int, int](sg)
+
+	// Both are Left - should return first error
+	fa := Left[int]("error2")
+	fab := Left[func(int) int]("error1")
+
+	result := applyV(fa)(fab)
+
+	assert.True(t, IsLeft(result))
+	// First semigroup returns the first value, which is fab's error
+	assert.Equal(t, Left[int]("error1"), result)
+}
+
+// TestApV_LastSemigroup tests ApV with Last semigroup (always returns last error)
+func TestApV_LastSemigroup(t *testing.T) {
+	// Use Last semigroup which always returns the last value
+	sg := SG.Last[string]()
+
+	// Create the validation applicative
+	applyV := ApV[int, int](sg)
+
+	// Both are Left - should return last error
+	fa := Left[int]("error2")
+	fab := Left[func(int) int]("error1")
+
+	result := applyV(fa)(fab)
+
+	assert.True(t, IsLeft(result))
+	// Last semigroup returns the last value, which is fa's error
+	assert.Equal(t, Left[int]("error2"), result)
+}

v2/endomorphism/doc.go

@@ -36,16 +36,21 @@
 //	)
 //
 //	// Define some endomorphisms
-//	double := func(x int) int { return x * 2 }
+//	double := N.Mul(2)
 //	increment := func(x int) int { return x + 1 }
 //
-//	// Compose them
+//	// Compose them (RIGHT-TO-LEFT execution)
-//	doubleAndIncrement := endomorphism.Compose(double, increment)
+//	composed := endomorphism.Compose(double, increment)
-//	result := doubleAndIncrement(5) // (5 * 2) + 1 = 11
+//	result := composed(5) // increment(5) then double: (5 + 1) * 2 = 12
+//
+//	// Chain them (LEFT-TO-RIGHT execution)
+//	chained := endomorphism.MonadChain(double, increment)
+//	result2 := chained(5) // double(5) then increment: (5 * 2) + 1 = 11
 //
 // # Monoid Operations
 //
-// Endomorphisms form a monoid, which means you can combine multiple endomorphisms:
+// Endomorphisms form a monoid, which means you can combine multiple endomorphisms.
+// The monoid uses Compose, which executes RIGHT-TO-LEFT:
 //
 //	import (
 //		"github.com/IBM/fp-go/v2/endomorphism"
@@ -55,22 +60,39 @@
 //	// Get the monoid for int endomorphisms
 //	monoid := endomorphism.Monoid[int]()
 //
-//	// Combine multiple endomorphisms
+//	// Combine multiple endomorphisms (RIGHT-TO-LEFT execution)
 //	combined := M.ConcatAll(monoid)(
-//		func(x int) int { return x * 2 },
+//		N.Mul(2),  // applied third
-//		func(x int) int { return x + 1 },
+//		func(x int) int { return x + 1 },  // applied second
-//		func(x int) int { return x * 3 },
+//		func(x int) int { return x * 3 },  // applied first
 //	)
-//	result := combined(5) // ((5 * 2) + 1) * 3 = 33
+//	result := combined(5) // (5 * 3) = 15, (15 + 1) = 16, (16 * 2) = 32
 //
 // # Monad Operations
 //
-// The package also provides monadic operations for endomorphisms:
+// The package also provides monadic operations for endomorphisms.
+// MonadChain executes LEFT-TO-RIGHT, unlike Compose:
 //
-//	// Chain allows sequencing of endomorphisms
+//	// Chain allows sequencing of endomorphisms (LEFT-TO-RIGHT)
-//	f := func(x int) int { return x * 2 }
+//	f := N.Mul(2)
 //	g := func(x int) int { return x + 1 }
-//	chained := endomorphism.MonadChain(f, g)
+//	chained := endomorphism.MonadChain(f, g)  // f first, then g
+//	result := chained(5) // (5 * 2) + 1 = 11
+//
+// # Compose vs Chain
+//
+// The key difference between Compose and Chain/MonadChain is execution order:
+//
+//	double := N.Mul(2)
+//	increment := func(x int) int { return x + 1 }
+//
+//	// Compose: RIGHT-TO-LEFT (mathematical composition)
+//	composed := endomorphism.Compose(double, increment)
+//	result1 := composed(5) // increment(5) * 2 = (5 + 1) * 2 = 12
+//
+//	// MonadChain: LEFT-TO-RIGHT (sequential application)
+//	chained := endomorphism.MonadChain(double, increment)
+//	result2 := chained(5) // double(5) + 1 = (5 * 2) + 1 = 11
 //
 // # Type Safety
 //

v2/endomorphism/endo.go

@@ -17,115 +17,372 @@ package endomorphism
 
 import (
 	"github.com/IBM/fp-go/v2/function"
-	"github.com/IBM/fp-go/v2/identity"
 )
 
-// MonadAp applies an endomorphism to a value in a monadic context.
+// MonadAp applies an endomorphism in a function to an endomorphism value.
 //
-// This function applies the endomorphism fab to the value fa, returning the result.
+// For endomorphisms, Ap composes two endomorphisms using RIGHT-TO-LEFT composition.
-// It's the monadic application operation for endomorphisms.
+// This is the applicative functor operation for endomorphisms.
+//
+// IMPORTANT: Execution order is RIGHT-TO-LEFT (same as MonadCompose):
+//   - fa is applied first to the input
+//   - fab is applied to the result
 //
 // Parameters:
-//   - fab: An endomorphism to apply
+//   - fab: An endomorphism to apply (outer function)
-//   - fa: The value to apply the endomorphism to
+//   - fa: An endomorphism to apply first (inner function)
 //
 // Returns:
-//   - The result of applying fab to fa
+//   - A new endomorphism that applies fa, then fab
 //
 // Example:
 //
-//	double := func(x int) int { return x * 2 }
+//	double := N.Mul(2)
-//	result := endomorphism.MonadAp(double, 5) // Returns: 10
-func MonadAp[A any](fab Endomorphism[A], fa A) A {
-	return identity.MonadAp(fab, fa)
-}
-
-// Ap returns a function that applies a value to an endomorphism.
-//
-// This is the curried version of MonadAp. It takes a value and returns a function
-// that applies that value to any endomorphism.
-//
-// Parameters:
-//   - fa: The value to be applied
-//
-// Returns:
-//   - A function that takes an endomorphism and applies fa to it
-//
-// Example:
-//
-//	applyFive := endomorphism.Ap(5)
-//	double := func(x int) int { return x * 2 }
-//	result := applyFive(double) // Returns: 10
-func Ap[A any](fa A) func(Endomorphism[A]) A {
-	return identity.Ap[A](fa)
-}
-
-// Compose composes two endomorphisms into a single endomorphism.
-//
-// Given two endomorphisms f1 and f2, Compose returns a new endomorphism that
-// applies f1 first, then applies f2 to the result. This is function composition:
-// Compose(f1, f2)(x) = f2(f1(x))
-//
-// Composition is associative: Compose(Compose(f, g), h) = Compose(f, Compose(g, h))
-//
-// Parameters:
-//   - f1: The first endomorphism to apply
-//   - f2: The second endomorphism to apply
-//
-// Returns:
-//   - A new endomorphism that is the composition of f1 and f2
-//
-// Example:
-//
-//	double := func(x int) int { return x * 2 }
 //	increment := func(x int) int { return x + 1 }
-//	doubleAndIncrement := endomorphism.Compose(double, increment)
+//	result := endomorphism.MonadAp(double, increment) // Composes: double ∘ increment
-//	result := doubleAndIncrement(5) // (5 * 2) + 1 = 11
+//	// result(5) = double(increment(5)) = double(6) = 12
-func Compose[A any](f1, f2 Endomorphism[A]) Endomorphism[A] {
+func MonadAp[A any](fab Endomorphism[A], fa Endomorphism[A]) Endomorphism[A] {
-	return function.Flow2(f1, f2)
+	return MonadCompose(fab, fa)
 }
 
-// MonadChain chains two endomorphisms together.
+// Ap returns a function that applies an endomorphism to another endomorphism.
 //
-// This is the monadic bind operation for endomorphisms. It composes two endomorphisms
+// This is the curried version of MonadAp. It takes an endomorphism fa and returns
-// ma and f, returning a new endomorphism that applies ma first, then f.
+// a function that composes any endomorphism with fa using RIGHT-TO-LEFT composition.
-// MonadChain is equivalent to Compose.
+//
+// IMPORTANT: Execution order is RIGHT-TO-LEFT:
+//   - fa is applied first to the input
+//   - The endomorphism passed to the returned function is applied to the result
 //
 // Parameters:
-//   - ma: The first endomorphism in the chain
+//   - fa: The first endomorphism to apply (inner function)
-//   - f: The second endomorphism in the chain
 //
 // Returns:
-//   - A new endomorphism that chains ma and f
+//   - A function that takes an endomorphism and composes it with fa (right-to-left)
 //
 // Example:
 //
-//	double := func(x int) int { return x * 2 }
 //	increment := func(x int) int { return x + 1 }
+//	applyIncrement := endomorphism.Ap(increment)
+//	double := N.Mul(2)
+//	composed := applyIncrement(double) // double ∘ increment
+//	// composed(5) = double(increment(5)) = double(6) = 12
+func Ap[A any](fa Endomorphism[A]) Operator[A] {
+	return Compose(fa)
+}
+
+// MonadCompose composes two endomorphisms, executing them from right to left.
+//
+// MonadCompose creates a new endomorphism that applies f2 first, then f1.
+// This follows the mathematical notation of function composition: (f1 ∘ f2)(x) = f1(f2(x))
+//
+// IMPORTANT: The execution order is RIGHT-TO-LEFT:
+//   - f2 is applied first to the input
+//   - f1 is applied to the result of f2
+//
+// This is different from Chain/MonadChain which executes LEFT-TO-RIGHT.
+//
+// Parameters:
+//   - f1: The second function to apply (outer function)
+//   - f2: The first function to apply (inner function)
+//
+// Returns:
+//   - A new endomorphism that applies f2, then f1
+//
+// Example:
+//
+//	double := N.Mul(2)
+//	increment := func(x int) int { return x + 1 }
+//
+//	// MonadCompose executes RIGHT-TO-LEFT: increment first, then double
+//	composed := endomorphism.MonadCompose(double, increment)
+//	result := composed(5) // (5 + 1) * 2 = 12
+//
+//	// Compare with Chain which executes LEFT-TO-RIGHT:
+//	chained := endomorphism.MonadChain(double, increment)
+//	result2 := chained(5) // (5 * 2) + 1 = 11
+func MonadCompose[A any](f, g Endomorphism[A]) Endomorphism[A] {
+	return function.Flow2(g, f)
+}
+
+// MonadMap maps an endomorphism over another endomorphism using function composition.
+//
+// For endomorphisms, Map is equivalent to Compose (RIGHT-TO-LEFT composition).
+// This is the functor map operation for endomorphisms.
+//
+// IMPORTANT: Execution order is RIGHT-TO-LEFT:
+//   - g is applied first to the input
+//   - f is applied to the result
+//
+// Parameters:
+//   - f: The function to map (outer function)
+//   - g: The endomorphism to map over (inner function)
+//
+// Returns:
+//   - A new endomorphism that applies g, then f
+//
+// Example:
+//
+//	double := N.Mul(2)
+//	increment := func(x int) int { return x + 1 }
+//	mapped := endomorphism.MonadMap(double, increment)
+//	// mapped(5) = double(increment(5)) = double(6) = 12
+func MonadMap[A any](f, g Endomorphism[A]) Endomorphism[A] {
+	return MonadCompose(f, g)
+}
+
+// Compose returns a function that composes an endomorphism with another, executing right to left.
+//
+// This is the curried version of MonadCompose. It takes an endomorphism g and returns
+// a function that composes any endomorphism with g, applying g first (inner function),
+// then the input endomorphism (outer function).
+//
+// IMPORTANT: Execution order is RIGHT-TO-LEFT (mathematical composition):
+//   - g is applied first to the input
+//   - The endomorphism passed to the returned function is applied to the result of g
+//
+// This follows the mathematical composition notation where Compose(g)(f) = f ∘ g
+//
+// Parameters:
+//   - g: The first endomorphism to apply (inner function)
+//
+// Returns:
+//   - A function that takes an endomorphism f and composes it with g (right-to-left)
+//
+// Example:
+//
+//	increment := func(x int) int { return x + 1 }
+//	composeWithIncrement := endomorphism.Compose(increment)
+//	double := N.Mul(2)
+//
+//	// Composes double with increment (RIGHT-TO-LEFT: increment first, then double)
+//	composed := composeWithIncrement(double)
+//	result := composed(5) // (5 + 1) * 2 = 12
+//
+//	// Compare with Chain which executes LEFT-TO-RIGHT:
+//	chainWithIncrement := endomorphism.Chain(increment)
+//	chained := chainWithIncrement(double)
+//	result2 := chained(5) // (5 * 2) + 1 = 11
+func Compose[A any](g Endomorphism[A]) Operator[A] {
+	return function.Bind2nd(MonadCompose, g)
+}
+
+// Map returns a function that maps an endomorphism over another endomorphism.
+//
+// This is the curried version of MonadMap. It takes an endomorphism f and returns
+// a function that maps f over any endomorphism using RIGHT-TO-LEFT composition.
+//
+// IMPORTANT: Execution order is RIGHT-TO-LEFT (same as Compose):
+//   - The endomorphism passed to the returned function is applied first
+//   - f is applied to the result
+//
+// For endomorphisms, Map is equivalent to Compose.
+//
+// Parameters:
+//   - f: The function to map (outer function)
+//
+// Returns:
+//   - A function that takes an endomorphism and maps f over it (right-to-left)
+//
+// Example:
+//
+//	double := N.Mul(2)
+//	mapDouble := endomorphism.Map(double)
+//	increment := func(x int) int { return x + 1 }
+//	mapped := mapDouble(increment)
+//	// mapped(5) = double(increment(5)) = double(6) = 12
+func Map[A any](f Endomorphism[A]) Operator[A] {
+	return Compose(f)
+}
+
+// MonadChain chains two endomorphisms together, executing them from left to right.
+//
+// This is the monadic bind operation for endomorphisms. For endomorphisms, bind is
+// simply left-to-right function composition: ma is applied first, then f.
+//
+// IMPORTANT: The execution order is LEFT-TO-RIGHT:
+//   - ma is applied first to the input
+//   - f is applied to the result of ma
+//
+// This is different from MonadCompose which executes RIGHT-TO-LEFT.
+//
+// Parameters:
+//   - ma: The first endomorphism to apply
+//   - f: The second endomorphism to apply
+//
+// Returns:
+//   - A new endomorphism that applies ma, then f
+//
+// Example:
+//
+//	double := N.Mul(2)
+//	increment := func(x int) int { return x + 1 }
+//
+//	// MonadChain executes LEFT-TO-RIGHT: double first, then increment
 //	chained := endomorphism.MonadChain(double, increment)
 //	result := chained(5) // (5 * 2) + 1 = 11
+//
+//	// Compare with MonadCompose which executes RIGHT-TO-LEFT:
+//	composed := endomorphism.MonadCompose(increment, double)
+//	result2 := composed(5) // (5 * 2) + 1 = 11 (same result, different parameter order)
 func MonadChain[A any](ma Endomorphism[A], f Endomorphism[A]) Endomorphism[A] {
-	return Compose(ma, f)
+	return function.Flow2(ma, f)
 }
 
-// Chain returns a function that chains an endomorphism with another.
+// MonadChainFirst chains two endomorphisms but returns the result of the first.
 //
-// This is the curried version of MonadChain. It takes an endomorphism f and returns
+// This applies ma first, then f, but discards the result of f and returns the result of ma.
-// a function that chains any endomorphism with f.
+// Useful for performing side-effects while preserving the original value.
 //
 // Parameters:
-//   - f: The endomorphism to chain with
+//   - ma: The endomorphism whose result to keep
+//   - f: The endomorphism to apply for its effect
 //
 // Returns:
-//   - A function that takes an endomorphism and chains it with f
+//   - A new endomorphism that applies both but returns ma's result
+//
+// Example:
+//
+//	double := N.Mul(2)
+//	log := func(x int) int { fmt.Println(x); return x }
+//	chained := endomorphism.MonadChainFirst(double, log)
+//	result := chained(5) // Prints 10, returns 10
+func MonadChainFirst[A any](ma Endomorphism[A], f Endomorphism[A]) Endomorphism[A] {
+	return func(a A) A {
+		result := ma(a)
+		f(result)     // Apply f for its effect
+		return result // But return ma's result
+	}
+}
+
+// ChainFirst returns a function that chains for effect but preserves the original result.
+//
+// This is the curried version of MonadChainFirst.
+//
+// Parameters:
+//   - f: The endomorphism to apply for its effect
+//
+// Returns:
+//   - A function that takes an endomorphism and chains it with f, keeping the first result
+//
+// Example:
+//
+//	log := func(x int) int { fmt.Println(x); return x }
+//	chainLog := endomorphism.ChainFirst(log)
+//	double := N.Mul(2)
+//	chained := chainLog(double)
+//	result := chained(5) // Prints 10, returns 10
+func ChainFirst[A any](f Endomorphism[A]) Operator[A] {
+	return function.Bind2nd(MonadChainFirst, f)
+}
+
+// Chain returns a function that chains an endomorphism with another, executing left to right.
+//
+// This is the curried version of MonadChain. It takes an endomorphism f and returns
+// a function that chains any endomorphism with f, applying the input endomorphism first,
+// then f.
+//
+// IMPORTANT: Execution order is LEFT-TO-RIGHT:
+//   - The endomorphism passed to the returned function is applied first
+//   - f is applied to the result
+//
+// Parameters:
+//   - f: The second endomorphism to apply
+//
+// Returns:
+//   - A function that takes an endomorphism and chains it with f (left-to-right)
 //
 // Example:
 //
 //	increment := func(x int) int { return x + 1 }
 //	chainWithIncrement := endomorphism.Chain(increment)
-//	double := func(x int) int { return x * 2 }
+//	double := N.Mul(2)
+//
+//	// Chains double (first) with increment (second)
 //	chained := chainWithIncrement(double)
 //	result := chained(5) // (5 * 2) + 1 = 11
-func Chain[A any](f Endomorphism[A]) Endomorphism[Endomorphism[A]] {
+func Chain[A any](f Endomorphism[A]) Operator[A] {
 	return function.Bind2nd(MonadChain, f)
 }
+
+// Flatten collapses a nested endomorphism into a single endomorphism.
+//
+// Given an endomorphism that transforms endomorphisms (Endomorphism[Endomorphism[A]]),
+// Flatten produces a simple endomorphism by applying the outer transformation to the
+// identity function. This is the monadic join operation for the Endomorphism monad.
+//
+// The function applies the nested endomorphism to Identity[A] to extract the inner
+// endomorphism, effectively "flattening" the two layers into one.
+//
+// Type Parameters:
+//   - A: The type being transformed by the endomorphisms
+//
+// Parameters:
+//   - mma: A nested endomorphism that transforms endomorphisms
+//
+// Returns:
+//   - An endomorphism that applies the transformation directly to values of type A
+//
+// Example:
+//
+//	type Counter struct {
+//	    Value int
+//	}
+//
+//	// An endomorphism that wraps another endomorphism
+//	addThenDouble := func(endo Endomorphism[Counter]) Endomorphism[Counter] {
+//	    return func(c Counter) Counter {
+//	        c = endo(c)        // Apply the input endomorphism
+//	        c.Value = c.Value * 2  // Then double
+//	        return c
+//	    }
+//	}
+//
+//	flattened := Flatten(addThenDouble)
+//	result := flattened(Counter{Value: 5})  // Counter{Value: 10}
+func Flatten[A any](mma Endomorphism[Endomorphism[A]]) Endomorphism[A] {
+	return mma(function.Identity[A])
+}
+
+// Join performs self-application of a function that produces endomorphisms.
+//
+// Given a function that takes a value and returns an endomorphism of that same type,
+// Join creates an endomorphism that applies the value to itself through the function.
+// This operation is also known as the W combinator (warbler) in combinatory logic,
+// or diagonal application.
+//
+// The resulting endomorphism evaluates f(a)(a), applying the same value a to both
+// the function f and the resulting endomorphism.
+//
+// Type Parameters:
+//   - A: The type being transformed
+//
+// Parameters:
+//   - f: A function that takes a value and returns an endomorphism of that type
+//
+// Returns:
+//   - An endomorphism that performs self-application: f(a)(a)
+//
+// Example:
+//
+//	type Point struct {
+//	    X, Y int
+//	}
+//
+//	// Create an endomorphism based on the input point
+//	scaleBy := func(p Point) Endomorphism[Point] {
+//	    return func(p2 Point) Point {
+//	        return Point{
+//	            X: p2.X * p.X,
+//	            Y: p2.Y * p.Y,
+//	        }
+//	    }
+//	}
+//
+//	selfScale := Join(scaleBy)
+//	result := selfScale(Point{X: 3, Y: 4})  // Point{X: 9, Y: 16}
+func Join[A any](f Kleisli[A]) Endomorphism[A] {
+	return func(a A) A {
+		return f(a)(a)
+	}
+}

v2/endomorphism/endomorphism_test.go

@@ -19,6 +19,7 @@ import (
 	"testing"
 
 	M "github.com/IBM/fp-go/v2/monoid"
+	N "github.com/IBM/fp-go/v2/number"
 	S "github.com/IBM/fp-go/v2/semigroup"
 	"github.com/stretchr/testify/assert"
 )
@@ -76,84 +77,152 @@ func TestCurry3(t *testing.T) {
 
 // TestMonadAp tests the MonadAp function
 func TestMonadAp(t *testing.T) {
-	result := MonadAp(double, 5)
+	// MonadAp composes two endomorphisms (RIGHT-TO-LEFT)
-	assert.Equal(t, 10, result, "MonadAp should apply endomorphism to value")
+	// MonadAp(double, increment) means: increment first, then double
+	composed := MonadAp(double, increment)
+	result := composed(5)
+	assert.Equal(t, 12, result, "MonadAp should compose right-to-left: (5 + 1) * 2 = 12")
 
-	result2 := MonadAp(increment, 10)
+	// Test with different order
-	assert.Equal(t, 11, result2, "MonadAp should work with different endomorphisms")
+	composed2 := MonadAp(increment, double)
+	result2 := composed2(5)
+	assert.Equal(t, 11, result2, "MonadAp should compose right-to-left: (5 * 2) + 1 = 11")
 
-	result3 := MonadAp(square, 4)
+	// Test with square
-	assert.Equal(t, 16, result3, "MonadAp should work with square function")
+	composed3 := MonadAp(square, increment)
+	result3 := composed3(5)
+	assert.Equal(t, 36, result3, "MonadAp should compose right-to-left: (5 + 1) ^ 2 = 36")
 }
 
 // TestAp tests the Ap function
 func TestAp(t *testing.T) {
-	applyFive := Ap(5)
+	// Ap is the curried version of MonadAp
+	// Ap(increment) returns a function that composes with increment (RIGHT-TO-LEFT)
+	applyIncrement := Ap(increment)
 
-	result := applyFive(double)
+	composed := applyIncrement(double)
-	assert.Equal(t, 10, result, "Ap should apply value to endomorphism")
+	result := composed(5)
+	assert.Equal(t, 12, result, "Ap should compose right-to-left: (5 + 1) * 2 = 12")
 
-	result2 := applyFive(increment)
+	// Test with different endomorphism
-	assert.Equal(t, 6, result2, "Ap should work with different endomorphisms")
+	composed2 := applyIncrement(square)
+	result2 := composed2(5)
+	assert.Equal(t, 36, result2, "Ap should compose right-to-left: (5 + 1) ^ 2 = 36")
 
-	applyTen := Ap(10)
+	// Test with different base endomorphism
-	result3 := applyTen(square)
+	applyDouble := Ap(double)
-	assert.Equal(t, 100, result3, "Ap should work with different values")
+	composed3 := applyDouble(increment)
+	result3 := composed3(5)
+	assert.Equal(t, 11, result3, "Ap should compose right-to-left: (5 * 2) + 1 = 11")
 }
 
-// TestCompose tests the Compose function
+// TestMonadCompose tests the MonadCompose function
-func TestCompose(t *testing.T) {
+func TestMonadCompose(t *testing.T) {
-	// Test basic composition: (5 * 2) + 1 = 11
+	// Test basic composition: RIGHT-TO-LEFT execution
-	doubleAndIncrement := Compose(double, increment)
+	// MonadCompose(double, increment) means: increment first, then double
-	result := doubleAndIncrement(5)
+	composed := MonadCompose(double, increment)
-	assert.Equal(t, 11, result, "Compose should compose endomorphisms correctly")
+	result := composed(5)
+	assert.Equal(t, 12, result, "MonadCompose should execute right-to-left: (5 + 1) * 2 = 12")
 
-	// Test composition order: (5 + 1) * 2 = 12
+	// Test composition order: RIGHT-TO-LEFT execution
-	incrementAndDouble := Compose(increment, double)
+	// MonadCompose(increment, double) means: double first, then increment
-	result2 := incrementAndDouble(5)
+	composed2 := MonadCompose(increment, double)
-	assert.Equal(t, 12, result2, "Compose should respect order of composition")
+	result2 := composed2(5)
+	assert.Equal(t, 11, result2, "MonadCompose should execute right-to-left: (5 * 2) + 1 = 11")
 
-	// Test with three compositions: ((5 * 2) + 1) * ((5 * 2) + 1) = 121
+	// Test with three compositions: RIGHT-TO-LEFT execution
-	complex := Compose(Compose(double, increment), square)
+	// MonadCompose(MonadCompose(double, increment), square) means: square, then increment, then double
+	complex := MonadCompose(MonadCompose(double, increment), square)
 	result3 := complex(5)
-	assert.Equal(t, 121, result3, "Compose should work with nested compositions")
+	// 5 -> square -> 25 -> increment -> 26 -> double -> 52
+	assert.Equal(t, 52, result3, "MonadCompose should work with nested compositions: square(5)=25, +1=26, *2=52")
 }
 
 // TestMonadChain tests the MonadChain function
 func TestMonadChain(t *testing.T) {
-	// MonadChain should behave like Compose
+	// MonadChain executes LEFT-TO-RIGHT (first arg first, second arg second)
 	chained := MonadChain(double, increment)
 	result := chained(5)
-	assert.Equal(t, 11, result, "MonadChain should chain endomorphisms correctly")
+	assert.Equal(t, 11, result, "MonadChain should execute left-to-right: (5 * 2) + 1 = 11")
 
 	chained2 := MonadChain(increment, double)
 	result2 := chained2(5)
-	assert.Equal(t, 12, result2, "MonadChain should respect order")
+	assert.Equal(t, 12, result2, "MonadChain should execute left-to-right: (5 + 1) * 2 = 12")
 
 	// Test with negative values
 	chained3 := MonadChain(negate, increment)
 	result3 := chained3(5)
-	assert.Equal(t, -4, result3, "MonadChain should work with negative values")
+	assert.Equal(t, -4, result3, "MonadChain should execute left-to-right: -(5) + 1 = -4")
 }
 
 // TestChain tests the Chain function
 func TestChain(t *testing.T) {
+	// Chain(f) returns a function that applies its argument first, then f
 	chainWithIncrement := Chain(increment)
 
+	// chainWithIncrement(double) means: double first, then increment
 	chained := chainWithIncrement(double)
 	result := chained(5)
-	assert.Equal(t, 11, result, "Chain should create chaining function correctly")
+	assert.Equal(t, 11, result, "Chain should execute left-to-right: (5 * 2) + 1 = 11")
 
 	chainWithDouble := Chain(double)
+	// chainWithDouble(increment) means: increment first, then double
 	chained2 := chainWithDouble(increment)
 	result2 := chained2(5)
-	assert.Equal(t, 12, result2, "Chain should work with different endomorphisms")
+	assert.Equal(t, 12, result2, "Chain should execute left-to-right: (5 + 1) * 2 = 12")
 
 	// Test chaining with square
 	chainWithSquare := Chain(square)
+	// chainWithSquare(double) means: double first, then square
 	chained3 := chainWithSquare(double)
 	result3 := chained3(3)
-	assert.Equal(t, 36, result3, "Chain should work with square function")
+	assert.Equal(t, 36, result3, "Chain should execute left-to-right: (3 * 2) ^ 2 = 36")
+}
+
+// TestCompose tests the curried Compose function
+func TestCompose(t *testing.T) {
+	// Compose(g) returns a function that applies g first, then its argument
+	composeWithIncrement := Compose(increment)
+
+	// composeWithIncrement(double) means: increment first, then double
+	composed := composeWithIncrement(double)
+	result := composed(5)
+	assert.Equal(t, 12, result, "Compose should execute right-to-left: (5 + 1) * 2 = 12")
+
+	composeWithDouble := Compose(double)
+	// composeWithDouble(increment) means: double first, then increment
+	composed2 := composeWithDouble(increment)
+	result2 := composed2(5)
+	assert.Equal(t, 11, result2, "Compose should execute right-to-left: (5 * 2) + 1 = 11")
+
+	// Test composing with square
+	composeWithSquare := Compose(square)
+	// composeWithSquare(double) means: square first, then double
+	composed3 := composeWithSquare(double)
+	result3 := composed3(3)
+	assert.Equal(t, 18, result3, "Compose should execute right-to-left: (3 ^ 2) * 2 = 18")
+}
+
+// TestMonadComposeVsCompose demonstrates the relationship between MonadCompose and Compose
+func TestMonadComposeVsCompose(t *testing.T) {
+	double := N.Mul(2)
+	increment := func(x int) int { return x + 1 }
+
+	// MonadCompose takes both functions at once
+	monadComposed := MonadCompose(double, increment)
+	result1 := monadComposed(5) // (5 + 1) * 2 = 12
+
+	// Compose is the curried version - takes one function, returns a function
+	curriedCompose := Compose(increment)
+	composed := curriedCompose(double)
+	result2 := composed(5) // (5 + 1) * 2 = 12
+
+	assert.Equal(t, result1, result2, "MonadCompose and Compose should produce the same result")
+	assert.Equal(t, 12, result1, "Both should execute right-to-left: (5 + 1) * 2 = 12")
+
+	// Demonstrate that Compose(g)(f) is equivalent to MonadCompose(f, g)
+	assert.Equal(t, MonadCompose(double, increment)(5), Compose(increment)(double)(5),
+		"Compose(g)(f) should equal MonadCompose(f, g)")
 }
 
 // TestOf tests the Of function
@@ -191,12 +260,14 @@ func TestIdentity(t *testing.T) {
 	assert.Equal(t, 0, id(0), "Identity should work with zero")
 	assert.Equal(t, -10, id(-10), "Identity should work with negative values")
 
-	// Identity should be neutral for composition
+	// Identity should be neutral for composition (RIGHT-TO-LEFT)
-	composed1 := Compose(id, double)
+	// Compose(id, double) means: double first, then id
-	assert.Equal(t, 10, composed1(5), "Identity should be right neutral for composition")
+	composed1 := MonadCompose(id, double)
+	assert.Equal(t, 10, composed1(5), "Identity should be left neutral: double(5) = 10")
 
-	composed2 := Compose(double, id)
+	// Compose(double, id) means: id first, then double
-	assert.Equal(t, 10, composed2(5), "Identity should be left neutral for composition")
+	composed2 := MonadCompose(double, id)
+	assert.Equal(t, 10, composed2(5), "Identity should be right neutral: id(5) then double = 10")
 
 	// Test with strings
 	idStr := Identity[string]()
@@ -207,10 +278,11 @@ func TestIdentity(t *testing.T) {
 func TestSemigroup(t *testing.T) {
 	sg := Semigroup[int]()
 
-	// Test basic concat
+	// Test basic concat (RIGHT-TO-LEFT execution via Compose)
+	// Concat(double, increment) means: increment first, then double
 	combined := sg.Concat(double, increment)
 	result := combined(5)
-	assert.Equal(t, 11, result, "Semigroup concat should compose endomorphisms")
+	assert.Equal(t, 12, result, "Semigroup concat should execute right-to-left: (5 + 1) * 2 = 12")
 
 	// Test associativity: (f . g) . h = f . (g . h)
 	f := double
@@ -223,10 +295,12 @@ func TestSemigroup(t *testing.T) {
 	testValue := 3
 	assert.Equal(t, left(testValue), right(testValue), "Semigroup should be associative")
 
-	// Test with ConcatAll from semigroup package
+	// Test with ConcatAll from semigroup package (RIGHT-TO-LEFT)
+	// ConcatAll(double)(increment, square) means: square, then increment, then double
 	combined2 := S.ConcatAll(sg)(double)([]Endomorphism[int]{increment, square})
 	result2 := combined2(5)
-	assert.Equal(t, 121, result2, "Semigroup should work with ConcatAll")
+	// 5 -> square -> 25 -> increment -> 26 -> double -> 52
+	assert.Equal(t, 52, result2, "Semigroup ConcatAll should execute right-to-left: square(5)=25, +1=26, *2=52")
 }
 
 // TestMonoid tests the Monoid function
@@ -237,19 +311,21 @@ func TestMonoid(t *testing.T) {
 	empty := monoid.Empty()
 	assert.Equal(t, 42, empty(42), "Monoid empty should be identity")
 
-	// Test right identity: x . empty = x
+	// Test right identity: x . empty = x (RIGHT-TO-LEFT: empty first, then x)
+	// Concat(double, empty) means: empty first, then double
 	rightIdentity := monoid.Concat(double, empty)
-	assert.Equal(t, 10, rightIdentity(5), "Monoid should satisfy right identity")
+	assert.Equal(t, 10, rightIdentity(5), "Monoid should satisfy right identity: empty(5) then double = 10")
 
-	// Test left identity: empty . x = x
+	// Test left identity: empty . x = x (RIGHT-TO-LEFT: x first, then empty)
+	// Concat(empty, double) means: double first, then empty
 	leftIdentity := monoid.Concat(empty, double)
-	assert.Equal(t, 10, leftIdentity(5), "Monoid should satisfy left identity")
+	assert.Equal(t, 10, leftIdentity(5), "Monoid should satisfy left identity: double(5) then empty = 10")
 
-	// Test ConcatAll with multiple endomorphisms
+	// Test ConcatAll with multiple endomorphisms (RIGHT-TO-LEFT execution)
 	combined := M.ConcatAll(monoid)([]Endomorphism[int]{double, increment, square})
 	result := combined(5)
-	// (5 * 2) = 10, (10 + 1) = 11, (11 * 11) = 121
+	// RIGHT-TO-LEFT: square(5) = 25, increment(25) = 26, double(26) = 52
-	assert.Equal(t, 121, result, "Monoid should work with ConcatAll")
+	assert.Equal(t, 52, result, "Monoid ConcatAll should execute right-to-left: square(5)=25, +1=26, *2=52")
 
 	// Test ConcatAll with empty list should return identity
 	emptyResult := M.ConcatAll(monoid)([]Endomorphism[int]{})
@@ -294,19 +370,20 @@ func TestMonoidLaws(t *testing.T) {
 
 // TestEndomorphismWithDifferentTypes tests endomorphisms with different types
 func TestEndomorphismWithDifferentTypes(t *testing.T) {
-	// Test with strings
+	// Test with strings (RIGHT-TO-LEFT execution)
-	toUpper := func(s string) string {
+	addExclamation := func(s string) string {
 		return s + "!"
 	}
 	addPrefix := func(s string) string {
 		return "Hello, " + s
 	}
 
-	strComposed := Compose(toUpper, addPrefix)
+	// Compose(addExclamation, addPrefix) means: addPrefix first, then addExclamation
+	strComposed := MonadCompose(addExclamation, addPrefix)
 	result := strComposed("World")
-	assert.Equal(t, "Hello, World!", result, "Endomorphism should work with strings")
+	assert.Equal(t, "Hello, World!", result, "Compose should execute right-to-left with strings")
 
-	// Test with float64
+	// Test with float64 (RIGHT-TO-LEFT execution)
 	doubleFloat := func(x float64) float64 {
 		return x * 2.0
 	}
@@ -314,76 +391,116 @@ func TestEndomorphismWithDifferentTypes(t *testing.T) {
 		return x + 1.0
 	}
 
-	floatComposed := Compose(doubleFloat, addOne)
+	// Compose(doubleFloat, addOne) means: addOne first, then doubleFloat
+	floatComposed := MonadCompose(doubleFloat, addOne)
 	resultFloat := floatComposed(5.5)
-	assert.Equal(t, 12.0, resultFloat, "Endomorphism should work with float64")
+	// 5.5 + 1.0 = 6.5, 6.5 * 2.0 = 13.0
+	assert.Equal(t, 13.0, resultFloat, "Compose should execute right-to-left: (5.5 + 1.0) * 2.0 = 13.0")
 }
 
 // TestComplexCompositions tests more complex composition scenarios
 func TestComplexCompositions(t *testing.T) {
-	// Create a pipeline of transformations
+	// Create a pipeline of transformations (RIGHT-TO-LEFT execution)
-	pipeline := Compose(
+	// Innermost Compose is evaluated first in the composition chain
-		Compose(
+	pipeline := MonadCompose(
-			Compose(double, increment),
+		MonadCompose(
+			MonadCompose(double, increment),
 			square,
 		),
 		negate,
 	)
 
-	// (5 * 2) = 10, (10 + 1) = 11, (11 * 11) = 121, -(121) = -121
+	// RIGHT-TO-LEFT: negate(5) = -5, square(-5) = 25, increment(25) = 26, double(26) = 52
 	result := pipeline(5)
-	assert.Equal(t, -121, result, "Complex composition should work correctly")
+	assert.Equal(t, 52, result, "Complex composition should execute right-to-left")
 
-	// Test using monoid to build the same pipeline
+	// Test using monoid to build the same pipeline (RIGHT-TO-LEFT)
 	monoid := Monoid[int]()
 	pipelineMonoid := M.ConcatAll(monoid)([]Endomorphism[int]{double, increment, square, negate})
 	resultMonoid := pipelineMonoid(5)
-	assert.Equal(t, -121, resultMonoid, "Monoid-based pipeline should match composition")
+	// RIGHT-TO-LEFT: negate(5) = -5, square(-5) = 25, increment(25) = 26, double(26) = 52
+	assert.Equal(t, 52, resultMonoid, "Monoid-based pipeline should match composition (right-to-left)")
 }
 
 // TestOperatorType tests the Operator type
 func TestOperatorType(t *testing.T) {
-	// Create an operator that lifts an int endomorphism to work on the length of strings
+	// Create an operator that transforms int endomorphisms
-	lengthOperator := func(f Endomorphism[int]) Endomorphism[string] {
+	// This operator takes an endomorphism and returns a new one that applies it twice
-		return func(s string) string {
+	applyTwice := func(f Endomorphism[int]) Endomorphism[int] {
-			newLen := f(len(s))
+		return func(x int) int {
-			if newLen > len(s) {
+			return f(f(x))
-				// Pad with spaces
-				for i := len(s); i < newLen; i++ {
-					s += " "
-				}
-			} else if newLen < len(s) {
-				// Truncate
-				s = s[:newLen]
-			}
-			return s
 		}
 	}
 
 	// Use the operator
-	var op Operator[int, string] = lengthOperator
+	var op Operator[int] = applyTwice
-	doubleLength := op(double)
+	doubleDouble := op(double)
 
-	result := doubleLength("hello") // len("hello") = 5, 5 * 2 = 10
+	result := doubleDouble(5) // double(double(5)) = double(10) = 20
-	assert.Equal(t, 10, len(result), "Operator should transform endomorphisms correctly")
+	assert.Equal(t, 20, result, "Operator should transform endomorphisms correctly")
-	assert.Equal(t, "hello     ", result, "Operator should pad string correctly")
+
+	// Test with increment
+	incrementTwice := op(increment)
+	result2 := incrementTwice(5) // increment(increment(5)) = increment(6) = 7
+	assert.Equal(t, 7, result2, "Operator should work with different endomorphisms")
 }
 
 // BenchmarkCompose benchmarks the Compose function
 func BenchmarkCompose(b *testing.B) {
-	composed := Compose(double, increment)
+	composed := MonadCompose(double, increment)
 	b.ResetTimer()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = composed(5)
 	}
 }
 
 // BenchmarkMonoidConcatAll benchmarks ConcatAll with monoid
+// TestComposeVsChain demonstrates the key difference between Compose and Chain
+func TestComposeVsChain(t *testing.T) {
+	double := N.Mul(2)
+	increment := func(x int) int { return x + 1 }
+
+	// Compose executes RIGHT-TO-LEFT
+	// Compose(double, increment) means: increment first, then double
+	composed := MonadCompose(double, increment)
+	composedResult := composed(5) // (5 + 1) * 2 = 12
+
+	// MonadChain executes LEFT-TO-RIGHT
+	// MonadChain(double, increment) means: double first, then increment
+	chained := MonadChain(double, increment)
+	chainedResult := chained(5) // (5 * 2) + 1 = 11
+
+	assert.Equal(t, 12, composedResult, "Compose should execute right-to-left")
+	assert.Equal(t, 11, chainedResult, "MonadChain should execute left-to-right")
+	assert.NotEqual(t, composedResult, chainedResult, "Compose and Chain should produce different results with non-commutative operations")
+
+	// To get the same result with Compose, we need to reverse the order
+	composedReversed := MonadCompose(increment, double)
+	assert.Equal(t, chainedResult, composedReversed(5), "Compose with reversed args should match Chain")
+
+	// Demonstrate with a more complex example
+	square := func(x int) int { return x * x }
+
+	// Compose: RIGHT-TO-LEFT
+	composed3 := MonadCompose(MonadCompose(square, increment), double)
+	// double(5) = 10, increment(10) = 11, square(11) = 121
+	result1 := composed3(5)
+
+	// MonadChain: LEFT-TO-RIGHT
+	chained3 := MonadChain(MonadChain(double, increment), square)
+	// double(5) = 10, increment(10) = 11, square(11) = 121
+	result2 := chained3(5)
+
+	assert.Equal(t, 121, result1, "Compose should execute right-to-left")
+	assert.Equal(t, 121, result2, "MonadChain should execute left-to-right")
+	assert.Equal(t, result1, result2, "Both should produce same result when operations are in correct order")
+}
+
 func BenchmarkMonoidConcatAll(b *testing.B) {
 	monoid := Monoid[int]()
 	combined := M.ConcatAll(monoid)([]Endomorphism[int]{double, increment, square})
 	b.ResetTimer()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = combined(5)
 	}
 }
@@ -393,7 +510,215 @@ func BenchmarkChain(b *testing.B) {
 	chainWithIncrement := Chain(increment)
 	chained := chainWithIncrement(double)
 	b.ResetTimer()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		_ = chained(5)
 	}
 }
+
+// TestFunctorLaws tests that endomorphisms satisfy the functor laws
+func TestFunctorLaws(t *testing.T) {
+	// Functor Law 1: Identity
+	// map(id) = id
+	t.Run("Identity", func(t *testing.T) {
+		id := Identity[int]()
+		endo := double
+
+		// map(id)(endo) should equal endo
+		mapped := MonadMap(id, endo)
+		testValue := 5
+		assert.Equal(t, endo(testValue), mapped(testValue), "map(id) should equal id")
+	})
+
+	// Functor Law 2: Composition
+	// map(f . g) = map(f) . map(g)
+	t.Run("Composition", func(t *testing.T) {
+		f := double
+		g := increment
+		endo := square
+
+		// Left side: map(f . g)(endo)
+		composed := MonadCompose(f, g)
+		left := MonadMap(composed, endo)
+
+		// Right side: map(f)(map(g)(endo))
+		mappedG := MonadMap(g, endo)
+		right := MonadMap(f, mappedG)
+
+		testValue := 3
+		assert.Equal(t, left(testValue), right(testValue), "map(f . g) should equal map(f) . map(g)")
+	})
+}
+
+// TestApplicativeLaws tests that endomorphisms satisfy the applicative functor laws
+func TestApplicativeLaws(t *testing.T) {
+	// Applicative Law 1: Identity
+	// ap(id, v) = v
+	t.Run("Identity", func(t *testing.T) {
+		id := Identity[int]()
+		v := double
+
+		applied := MonadAp(id, v)
+		testValue := 5
+		assert.Equal(t, v(testValue), applied(testValue), "ap(id, v) should equal v")
+	})
+
+	// Applicative Law 2: Composition
+	// ap(ap(ap(compose, u), v), w) = ap(u, ap(v, w))
+	t.Run("Composition", func(t *testing.T) {
+		u := double
+		v := increment
+		w := square
+
+		// For endomorphisms, ap is just composition
+		// Left side: ap(ap(ap(compose, u), v), w) = compose(compose(u, v), w)
+		left := MonadCompose(MonadCompose(u, v), w)
+
+		// Right side: ap(u, ap(v, w)) = compose(u, compose(v, w))
+		right := MonadCompose(u, MonadCompose(v, w))
+
+		testValue := 3
+		assert.Equal(t, left(testValue), right(testValue), "Applicative composition law")
+	})
+
+	// Applicative Law 3: Homomorphism
+	// ap(pure(f), pure(x)) = pure(f(x))
+	t.Run("Homomorphism", func(t *testing.T) {
+		// For endomorphisms, "pure" is just the identity function that returns a constant
+		// This law is trivially satisfied for endomorphisms
+		f := double
+		x := 5
+
+		// ap(f, id) applied to x should equal f(x)
+		id := Identity[int]()
+		applied := MonadAp(f, id)
+		assert.Equal(t, f(x), applied(x), "Homomorphism law")
+	})
+}
+
+// TestMonadLaws tests that endomorphisms satisfy the monad laws
+func TestMonadLaws(t *testing.T) {
+	// Monad Law 1: Left Identity
+	// chain(pure(a), f) = f(a)
+	t.Run("LeftIdentity", func(t *testing.T) {
+		// For endomorphisms, "pure" is the identity function
+		// chain(id, f) = f
+		id := Identity[int]()
+		f := double
+
+		chained := MonadChain(id, f)
+		testValue := 5
+		assert.Equal(t, f(testValue), chained(testValue), "chain(id, f) should equal f")
+	})
+
+	// Monad Law 2: Right Identity
+	// chain(m, pure) = m
+	t.Run("RightIdentity", func(t *testing.T) {
+		m := double
+		id := Identity[int]()
+
+		chained := MonadChain(m, id)
+		testValue := 5
+		assert.Equal(t, m(testValue), chained(testValue), "chain(m, id) should equal m")
+	})
+
+	// Monad Law 3: Associativity
+	// chain(chain(m, f), g) = chain(m, x => chain(f(x), g))
+	t.Run("Associativity", func(t *testing.T) {
+		m := square
+		f := double
+		g := increment
+
+		// Left side: chain(chain(m, f), g)
+		left := MonadChain(MonadChain(m, f), g)
+
+		// Right side: chain(m, chain(f, g))
+		// For simple endomorphisms (not Kleisli arrows), this simplifies to:
+		right := MonadChain(m, MonadChain(f, g))
+
+		testValue := 3
+		assert.Equal(t, left(testValue), right(testValue), "Monad associativity law")
+	})
+}
+
+// TestMonadComposeVsMonadChain verifies the relationship between Compose and Chain
+func TestMonadComposeVsMonadChain(t *testing.T) {
+	f := double
+	g := increment
+
+	// MonadCompose(f, g) should equal MonadChain(g, f)
+	// Because Compose is right-to-left and Chain is left-to-right
+	composed := MonadCompose(f, g)
+	chained := MonadChain(g, f)
+
+	testValue := 5
+	assert.Equal(t, composed(testValue), chained(testValue),
+		"MonadCompose(f, g) should equal MonadChain(g, f)")
+}
+
+// TestMapEqualsCompose verifies that Map is equivalent to Compose for endomorphisms
+func TestMapEqualsCompose(t *testing.T) {
+	f := double
+	g := increment
+
+	// MonadMap(f, g) should equal MonadCompose(f, g)
+	mapped := MonadMap(f, g)
+	composed := MonadCompose(f, g)
+
+	testValue := 5
+	assert.Equal(t, composed(testValue), mapped(testValue),
+		"MonadMap should equal MonadCompose for endomorphisms")
+
+	// Curried versions
+	mapF := Map(f)
+	composeF := Compose(f)
+
+	mappedG := mapF(g)
+	composedG := composeF(g)
+
+	assert.Equal(t, composedG(testValue), mappedG(testValue),
+		"Map should equal Compose for endomorphisms (curried)")
+}
+
+// TestApEqualsCompose verifies that Ap is equivalent to Compose for endomorphisms
+func TestApEqualsCompose(t *testing.T) {
+	f := double
+	g := increment
+
+	// MonadAp(f, g) should equal MonadCompose(f, g)
+	applied := MonadAp(f, g)
+	composed := MonadCompose(f, g)
+
+	testValue := 5
+	assert.Equal(t, composed(testValue), applied(testValue),
+		"MonadAp should equal MonadCompose for endomorphisms")
+
+	// Curried versions
+	apG := Ap(g)
+	composeG := Compose(g)
+
+	appliedF := apG(f)
+	composedF := composeG(f)
+
+	assert.Equal(t, composedF(testValue), appliedF(testValue),
+		"Ap should equal Compose for endomorphisms (curried)")
+}
+
+// TestChainFirst tests the ChainFirst operation
+func TestChainFirst(t *testing.T) {
+	double := N.Mul(2)
+
+	// Track side effect
+	var sideEffect int
+	logEffect := func(x int) int {
+		sideEffect = x
+		return x + 100 // This result should be discarded
+	}
+
+	chained := MonadChainFirst(double, logEffect)
+	result := chained(5)
+
+	// Should return double's result (10), not logEffect's result
+	assert.Equal(t, 10, result, "ChainFirst should return first result")
+	// But side effect should have been executed with double's result
+	assert.Equal(t, 10, sideEffect, "ChainFirst should execute second function for effect")
+}

v2/endomorphism/from.go

@@ -0,0 +1,10 @@
+package endomorphism
+
+import (
+	"github.com/IBM/fp-go/v2/function"
+	S "github.com/IBM/fp-go/v2/semigroup"
+)
+
+func FromSemigroup[A any](s S.Semigroup[A]) Kleisli[A] {
+	return function.Bind2of2(s.Concat)
+}

v2/endomorphism/monoid.go

@@ -35,7 +35,7 @@ import (
 //
 // Example:
 //
-//	myFunc := func(x int) int { return x * 2 }
+//	myFunc := N.Mul(2)
 //	endo := endomorphism.Of(myFunc)
 func Of[F ~func(A) A, A any](f F) Endomorphism[A] {
 	return f
@@ -75,7 +75,7 @@ func Unwrap[F ~func(A) A, A any](f Endomorphism[A]) F {
 //	result := id(42) // Returns: 42
 //
 //	// Identity is neutral for composition
-//	double := func(x int) int { return x * 2 }
+//	double := N.Mul(2)
 //	composed := endomorphism.Compose(id, double)
 //	// composed behaves exactly like double
 func Identity[A any]() Endomorphism[A] {
@@ -88,25 +88,29 @@ func Identity[A any]() Endomorphism[A] {
 // For endomorphisms, this operation is composition (Compose). This means:
 //   - Concat(f, Concat(g, h)) = Concat(Concat(f, g), h)
 //
+// IMPORTANT: Concat uses Compose, which executes RIGHT-TO-LEFT:
+//   - Concat(f, g) applies g first, then f
+//   - This is equivalent to Compose(f, g)
+//
 // The returned semigroup can be used with semigroup operations to combine
 // multiple endomorphisms.
 //
 // Returns:
-//   - A Semigroup[Endomorphism[A]] where concat is composition
+//   - A Semigroup[Endomorphism[A]] where concat is composition (right-to-left)
 //
 // Example:
 //
 //	import S "github.com/IBM/fp-go/v2/semigroup"
 //
 //	sg := endomorphism.Semigroup[int]()
-//	double := func(x int) int { return x * 2 }
+//	double := N.Mul(2)
 //	increment := func(x int) int { return x + 1 }
 //
-//	// Combine using the semigroup
+//	// Combine using the semigroup (RIGHT-TO-LEFT execution)
 //	combined := sg.Concat(double, increment)
-//	result := combined(5) // (5 * 2) + 1 = 11
+//	result := combined(5) // (5 + 1) * 2 = 12 (increment first, then double)
 func Semigroup[A any]() S.Semigroup[Endomorphism[A]] {
-	return S.MakeSemigroup(Compose[A])
+	return S.MakeSemigroup(MonadCompose[A])
 }
 
 // Monoid returns a Monoid for endomorphisms where concat is composition and empty is identity.
@@ -115,6 +119,10 @@ func Semigroup[A any]() S.Semigroup[Endomorphism[A]] {
 //   - The binary operation is composition (Compose)
 //   - The identity element is the identity function (Identity)
 //
+// IMPORTANT: Concat uses Compose, which executes RIGHT-TO-LEFT:
+//   - Concat(f, g) applies g first, then f
+//   - ConcatAll applies functions from right to left
+//
 // This satisfies the monoid laws:
 //   - Right identity: Concat(x, Empty) = x
 //   - Left identity: Concat(Empty, x) = x
@@ -124,20 +132,20 @@ func Semigroup[A any]() S.Semigroup[Endomorphism[A]] {
 // combine multiple endomorphisms.
 //
 // Returns:
-//   - A Monoid[Endomorphism[A]] with composition and identity
+//   - A Monoid[Endomorphism[A]] with composition (right-to-left) and identity
 //
 // Example:
 //
 //	import M "github.com/IBM/fp-go/v2/monoid"
 //
 //	monoid := endomorphism.Monoid[int]()
-//	double := func(x int) int { return x * 2 }
+//	double := N.Mul(2)
 //	increment := func(x int) int { return x + 1 }
 //	square := func(x int) int { return x * x }
 //
-//	// Combine multiple endomorphisms
+//	// Combine multiple endomorphisms (RIGHT-TO-LEFT execution)
 //	combined := M.ConcatAll(monoid)(double, increment, square)
-//	result := combined(5) // ((5 * 2) + 1) * ((5 * 2) + 1) = 121
+//	result := combined(5) // square(increment(double(5))) = square(increment(10)) = square(11) = 121
 func Monoid[A any]() M.Monoid[Endomorphism[A]] {
-	return M.MakeMonoid(Compose[A], Identity[A]())
+	return M.MakeMonoid(MonadCompose[A], Identity[A]())
 }

v2/endomorphism/types.go

@@ -29,7 +29,7 @@ type (
 	// Example:
 	//
 	//	// Simple endomorphisms on integers
-	//	double := func(x int) int { return x * 2 }
+	//	double := N.Mul(2)
 	//	increment := func(x int) int { return x + 1 }
 	//
 	//	// Both are endomorphisms of type Endomorphism[int]
@@ -37,6 +37,8 @@ type (
 	//	var g endomorphism.Endomorphism[int] = increment
 	Endomorphism[A any] = func(A) A
 
+	Kleisli[A any] = func(A) Endomorphism[A]
+
 	// Operator represents a transformation from one endomorphism to another.
 	//
 	// An Operator takes an endomorphism on type A and produces an endomorphism on type B.
@@ -52,5 +54,5 @@ type (
 	//			return strconv.Itoa(result)
 	//		}
 	//	}
-	Operator[A, B any] = func(Endomorphism[A]) Endomorphism[B]
+	Operator[A any] = Endomorphism[Endomorphism[A]]
 )

v2/eq/contramap.go

@@ -15,7 +15,84 @@
 
 package eq
 
-// Contramap implements an Equals predicate based on a mapping
+// Contramap creates an Eq[B] from an Eq[A] by providing a function that maps B to A.
+// This is a contravariant functor operation that allows you to transform equality predicates
+// by mapping the input type. It's particularly useful for comparing complex types by
+// extracting comparable fields.
+//
+// The name "contramap" comes from category theory, where it represents a contravariant
+// functor. Unlike regular map (covariant), which transforms the output, contramap
+// transforms the input in the opposite direction.
+//
+// Type Parameters:
+//   - A: The type that has an existing Eq instance
+//   - B: The type for which we want to create an Eq instance
+//
+// Parameters:
+//   - f: A function that extracts or converts a value of type B to type A
+//
+// Returns:
+//   - A function that takes an Eq[A] and returns an Eq[B]
+//
+// The resulting Eq[B] compares two B values by:
+//  1. Applying f to both values to get A values
+//  2. Using the original Eq[A] to compare those A values
+//
+// Example - Compare structs by a single field:
+//
+//	type Person struct {
+//	    ID   int
+//	    Name string
+//	    Age  int
+//	}
+//
+//	// Compare persons by ID only
+//	personEqByID := eq.Contramap(func(p Person) int {
+//	    return p.ID
+//	})(eq.FromStrictEquals[int]())
+//
+//	p1 := Person{ID: 1, Name: "Alice", Age: 30}
+//	p2 := Person{ID: 1, Name: "Bob", Age: 25}
+//	assert.True(t, personEqByID.Equals(p1, p2))  // Same ID, different names
+//
+// Example - Case-insensitive string comparison:
+//
+//	type User struct {
+//	    Username string
+//	    Email    string
+//	}
+//
+//	caseInsensitiveEq := eq.FromEquals(func(a, b string) bool {
+//	    return strings.EqualFold(a, b)
+//	})
+//
+//	userEqByUsername := eq.Contramap(func(u User) string {
+//	    return u.Username
+//	})(caseInsensitiveEq)
+//
+//	u1 := User{Username: "Alice", Email: "alice@example.com"}
+//	u2 := User{Username: "ALICE", Email: "different@example.com"}
+//	assert.True(t, userEqByUsername.Equals(u1, u2))  // Case-insensitive match
+//
+// Example - Nested field access:
+//
+//	type Address struct {
+//	    City string
+//	}
+//
+//	type Person struct {
+//	    Name    string
+//	    Address Address
+//	}
+//
+//	// Compare persons by city
+//	personEqByCity := eq.Contramap(func(p Person) string {
+//	    return p.Address.City
+//	})(eq.FromStrictEquals[string]())
+//
+// Contramap Law:
+// Contramap must satisfy: Contramap(f)(Contramap(g)(eq)) = Contramap(g ∘ f)(eq)
+// This means contramapping twice is the same as contramapping with the composed function.
 func Contramap[A, B any](f func(b B) A) func(Eq[A]) Eq[B] {
 	return func(fa Eq[A]) Eq[B] {
 		equals := fa.Equals

v2/eq/eq.go

@@ -19,38 +19,188 @@ import (
 	F "github.com/IBM/fp-go/v2/function"
 )
 
+// Eq represents an equality type class for type T.
+// It provides a way to define custom equality semantics for any type,
+// not just those that are comparable with Go's == operator.
+//
+// Type Parameters:
+//   - T: The type for which equality is defined
+//
+// Methods:
+//   - Equals(x, y T) bool: Returns true if x and y are considered equal
+//
+// Laws:
+// An Eq instance must satisfy the equivalence relation laws:
+//  1. Reflexivity: Equals(x, x) = true for all x
+//  2. Symmetry: Equals(x, y) = Equals(y, x) for all x, y
+//  3. Transitivity: If Equals(x, y) and Equals(y, z), then Equals(x, z)
+//
+// Example:
+//
+//	// Create an equality predicate for integers
+//	intEq := eq.FromStrictEquals[int]()
+//	assert.True(t, intEq.Equals(42, 42))
+//	assert.False(t, intEq.Equals(42, 43))
+//
+//	// Create a custom equality predicate
+//	caseInsensitiveEq := eq.FromEquals(func(a, b string) bool {
+//	    return strings.EqualFold(a, b)
+//	})
+//	assert.True(t, caseInsensitiveEq.Equals("Hello", "HELLO"))
 type Eq[T any] interface {
+	// Equals returns true if x and y are considered equal according to this equality predicate.
+	//
+	// Parameters:
+	//   - x: The first value to compare
+	//   - y: The second value to compare
+	//
+	// Returns:
+	//   - true if x and y are equal, false otherwise
 	Equals(x, y T) bool
 }
 
+// eq is the internal implementation of the Eq interface.
+// It wraps a comparison function to provide the Eq interface.
 type eq[T any] struct {
 	c func(x, y T) bool
 }
 
+// Equals implements the Eq interface by delegating to the wrapped comparison function.
 func (e eq[T]) Equals(x, y T) bool {
 	return e.c(x, y)
 }
 
+// strictEq is a helper function that uses Go's built-in == operator for comparison.
+// It can only be used with comparable types.
 func strictEq[A comparable](a, b A) bool {
 	return a == b
 }
 
-// FromStrictEquals constructs an [EQ.Eq] from the canonical comparison function
+// FromStrictEquals constructs an Eq instance using Go's built-in == operator.
+// This is the most common way to create an Eq for types that support ==.
+//
+// Type Parameters:
+//   - T: Must be a comparable type (supports ==)
+//
+// Returns:
+//   - An Eq[T] that uses == for equality comparison
+//
+// Example:
+//
+//	intEq := eq.FromStrictEquals[int]()
+//	assert.True(t, intEq.Equals(42, 42))
+//	assert.False(t, intEq.Equals(42, 43))
+//
+//	stringEq := eq.FromStrictEquals[string]()
+//	assert.True(t, stringEq.Equals("hello", "hello"))
+//	assert.False(t, stringEq.Equals("hello", "world"))
+//
+// Note: For types that are not comparable or require custom equality logic,
+// use FromEquals instead.
 func FromStrictEquals[T comparable]() Eq[T] {
 	return FromEquals(strictEq[T])
 }
 
-// FromEquals constructs an [EQ.Eq] from the comparison function
+// FromEquals constructs an Eq instance from a custom comparison function.
+// This allows defining equality for any type, including non-comparable types
+// or types that need custom equality semantics.
+//
+// Type Parameters:
+//   - T: The type for which equality is being defined (can be any type)
+//
+// Parameters:
+//   - c: A function that takes two values of type T and returns true if they are equal
+//
+// Returns:
+//   - An Eq[T] that uses the provided function for equality comparison
+//
+// Example:
+//
+//	// Case-insensitive string equality
+//	caseInsensitiveEq := eq.FromEquals(func(a, b string) bool {
+//	    return strings.EqualFold(a, b)
+//	})
+//	assert.True(t, caseInsensitiveEq.Equals("Hello", "HELLO"))
+//
+//	// Approximate float equality
+//	approxEq := eq.FromEquals(func(a, b float64) bool {
+//	    return math.Abs(a-b) < 0.0001
+//	})
+//	assert.True(t, approxEq.Equals(1.0, 1.00009))
+//
+//	// Custom struct equality (compare by specific fields)
+//	type Person struct { ID int; Name string }
+//	personEq := eq.FromEquals(func(a, b Person) bool {
+//	    return a.ID == b.ID  // Compare only by ID
+//	})
+//
+// Note: The provided function should satisfy the equivalence relation laws
+// (reflexivity, symmetry, transitivity) for correct behavior.
 func FromEquals[T any](c func(x, y T) bool) Eq[T] {
 	return eq[T]{c: c}
 }
 
-// Empty returns the equals predicate that is always true
+// Empty returns an Eq instance that always returns true for any comparison.
+// This is the identity element for the Eq Monoid and is useful when you need
+// an equality predicate that accepts everything.
+//
+// Type Parameters:
+//   - T: The type for which the always-true equality is defined
+//
+// Returns:
+//   - An Eq[T] where Equals(x, y) always returns true
+//
+// Example:
+//
+//	alwaysTrue := eq.Empty[int]()
+//	assert.True(t, alwaysTrue.Equals(1, 2))
+//	assert.True(t, alwaysTrue.Equals(42, 100))
+//
+//	// Useful as identity in monoid operations
+//	monoid := eq.Monoid[string]()
+//	combined := monoid.Concat(eq.FromStrictEquals[string](), monoid.Empty())
+//	// combined behaves the same as FromStrictEquals
+//
+// Use cases:
+//   - As the identity element in Monoid operations
+//   - When you need a placeholder equality that accepts everything
+//   - In generic code that requires an Eq but doesn't need actual comparison
 func Empty[T any]() Eq[T] {
 	return FromEquals(F.Constant2[T, T](true))
 }
 
-// Equals returns a predicate to test if one value equals the other under an equals predicate
+// Equals returns a curried equality checking function.
+// This is useful for partial application and functional composition.
+//
+// Type Parameters:
+//   - T: The type being compared
+//
+// Parameters:
+//   - eq: The Eq instance to use for comparison
+//
+// Returns:
+//   - A function that takes a value and returns another function that checks equality with that value
+//
+// Example:
+//
+//	intEq := eq.FromStrictEquals[int]()
+//	equals42 := eq.Equals(intEq)(42)
+//
+//	assert.True(t, equals42(42))
+//	assert.False(t, equals42(43))
+//
+//	// Use in higher-order functions
+//	numbers := []int{40, 41, 42, 43, 44}
+//	filtered := array.Filter(equals42)(numbers)
+//	// filtered = [42]
+//
+//	// Partial application
+//	equalsFunc := eq.Equals(intEq)
+//	equals10 := equalsFunc(10)
+//	equals20 := equalsFunc(20)
+//
+// This is particularly useful when working with functional programming patterns
+// like map, filter, and other higher-order functions.
 func Equals[T any](eq Eq[T]) func(T) func(T) bool {
 	return func(other T) func(T) bool {
 		return F.Bind2nd(eq.Equals, other)