feat: add iterator support

go.mod

@@ -1,6 +1,6 @@
 module go.bug.st/f
 
-go 1.22.3
+go 1.25
 
 require github.com/stretchr/testify v1.9.0
 

iter.go

@@ -0,0 +1,41 @@
+package f
+
+import "iter"
+
+// FilterIter takes an iterator and a matcher and returns only those elements that satisfy the matcher.
+func FilterIter[T any](values iter.Seq[T], matcher Matcher[T]) iter.Seq[T] {
+	return func(yield func(x T) bool) {
+		for x := range values {
+			if matcher(x) {
+				if !yield(x) {
+					return
+				}
+			}
+		}
+	}
+}
+
+// Map applies the Mapper function to each element of the iterator.
+func MapIter[T, U any](values iter.Seq[T], mapper Mapper[T, U]) iter.Seq[U] {
+	return func(yield func(x U) bool) {
+		for x := range values {
+			if !yield(mapper(x)) {
+				return
+			}
+		}
+	}
+}
+
+// Reducer is a function that reduces an iterator's elements to a single value.
+func ReduceIter[T any](values iter.Seq[T], reducer Reducer[T], initialValue ...T) T {
+	var result T
+	if len(initialValue) > 1 {
+		panic("initialValue must be a single value")
+	} else if len(initialValue) == 1 {
+		result = initialValue[0]
+	}
+	for v := range values {
+		result = reducer(result, v)
+	}
+	return result
+}

iter_test.go

@@ -0,0 +1,117 @@
+package f
+
+import (
+	"slices"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestFilterIter(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    []int
+		matcher  Matcher[int]
+		expected []int
+	}{
+		{
+			name:     "even numbers",
+			input:    []int{1, 2, 3, 4, 5},
+			matcher:  func(x int) bool { return x%2 == 0 },
+			expected: []int{2, 4},
+		},
+		{
+			name:     "odd numbers",
+			input:    []int{1, 2, 3, 4, 5},
+			matcher:  func(x int) bool { return x%2 == 1 },
+			expected: []int{1, 3, 5},
+		},
+		{
+			name:     "none",
+			input:    []int{2, 4, 6},
+			matcher:  func(x int) bool { return x < 0 },
+			expected: nil,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			values := slices.Values(tt.input)
+			result := slices.Collect(FilterIter(values, tt.matcher))
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}
+
+func TestMapIter(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    []int
+		mapper   Mapper[int, int]
+		expected []int
+	}{
+		{
+			name:     "double",
+			input:    []int{1, 2, 3},
+			mapper:   func(x int) int { return x * 2 },
+			expected: []int{2, 4, 6},
+		},
+		{
+			name:     "negate",
+			input:    []int{1, -2, 3},
+			mapper:   func(x int) int { return -x },
+			expected: []int{-1, 2, -3},
+		},
+		{
+			name:     "identity",
+			input:    []int{1, 2, 3},
+			mapper:   func(x int) int { return x },
+			expected: []int{1, 2, 3},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			values := slices.Values(tt.input)
+			result := slices.Collect(MapIter(values, tt.mapper))
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}
+
+func TestReduceIter(t *testing.T) {
+	tests := []struct {
+		name         string
+		input        []int
+		reducer      Reducer[int]
+		initialValue int
+		expected     int
+	}{
+		{
+			name:         "sum",
+			input:        []int{1, 2, 3, 4},
+			reducer:      func(a, b int) int { return a + b },
+			initialValue: 0,
+			expected:     10,
+		},
+		{
+			name:         "product",
+			input:        []int{1, 2, 3, 4},
+			reducer:      func(a, b int) int { return a * b },
+			initialValue: 1,
+			expected:     24,
+		},
+		{
+			name:         "subtract",
+			input:        []int{10, 2, 3},
+			reducer:      func(a, b int) int { return a - b },
+			initialValue: 20,
+			expected:     5,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			values := slices.Values(tt.input)
+			result := ReduceIter(values, tt.reducer, tt.initialValue)
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}

slices.go

@@ -9,6 +9,7 @@
 package f
 
 import (
+	"iter"
 	"runtime"
 	"sync"
 	"sync/atomic"
@@ -124,3 +125,16 @@ func Count[T any](in []T, matcher Matcher[T]) int {
 	}
 	return count
 }
+
+// RefIter takes a slice of type []T and returns an iterator that yields
+// pointers to each element of the slice.
+func RefIter[T any](slice []T) iter.Seq[*T] {
+	return func(yield func(*T) bool) {
+		for i := range slice {
+			if !yield(&slice[i]) {
+				return
+			}
+		}
+	}
+}
+

slices_test.go

@@ -9,9 +9,11 @@
 package f_test
 
 import (
+	"slices"
 	"strings"
 	"testing"
 
+	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	f "go.bug.st/f"
 )
@@ -100,3 +102,35 @@ func TestCount(t *testing.T) {
 	require.Equal(t, 0, f.Count(a, f.Equals("ddd")))
 	require.Equal(t, 3, f.Count(a, f.NotEquals("ddd")))
 }
+
+func TestRefIter(t *testing.T) {
+	type foo struct {
+		value int
+	}
+	values := []foo{
+		{value: 1},
+		{value: 2},
+		{value: 3},
+	}
+
+	t.Run("not working for range", func(t *testing.T) {
+		for _, v := range values {
+			v.value *= 10
+		}
+		assert.Equal(t, []foo{{1}, {2}, {3}}, values)
+	})
+
+	t.Run("not working slices.Values", func(t *testing.T) {
+		for v := range slices.Values(values) {
+			v.value *= 10
+		}
+		assert.Equal(t, []foo{{1}, {2}, {3}}, values)
+	})
+
+	t.Run("working RefIter", func(t *testing.T) {
+		for v := range f.RefIter(values) {
+			v.value *= 10
+		}
+		assert.Equal(t, []foo{{10}, {20}, {30}}, values)
+	})
+}