feat: add ReduceBy

slice/slice.go

@@ -492,6 +492,18 @@ func Reduce[T any](slice []T, iteratee func(index int, item1, item2 T) T, initia
 	return result
 }
 
+// ReduceBy produces a value from slice by accumulating the result of each element as passed through the reducer function.
+// Play: todo
+func ReduceBy[T any, U any](slice []T, initial U, reducer func(index int, item T, agg U) U) U {
+	accumulator := initial
+
+	for i, v := range slice {
+		accumulator = reducer(i, v, accumulator)
+	}
+
+	return accumulator
+}
+
 // Replace returns a copy of the slice with the first n non-overlapping instances of old replaced by new.
 // Play: https://go.dev/play/p/P5mZp7IhOFo
 func Replace[T comparable](slice []T, old T, new T, n int) []T {

slice/slice_example_test.go

@@ -460,6 +460,23 @@ func ExampleReduce() {
 	// 6
 }
 
+func ExampleReduceBy() {
+	result1 := ReduceBy([]int{1, 2, 3, 4}, 0, func(_ int, item int, agg int) int {
+		return agg + item
+	})
+
+	result2 := ReduceBy([]int{1, 2, 3, 4}, "", func(_ int, item int, agg string) string {
+		return agg + fmt.Sprintf("%v", item)
+	})
+
+	fmt.Println(result1)
+	fmt.Println(result2)
+
+	// Output:
+	// 10
+	// 1234
+}
+
 func ExampleReplace() {
 	strs := []string{"a", "b", "c", "a"}
 

slice/slice_test.go

@@ -405,6 +405,22 @@ func TestReduce(t *testing.T) {
 	}
 }
 
+func TestReduceBy(t *testing.T) {
+	assert := internal.NewAssert(t, "TestReduce2")
+
+	result1 := ReduceBy([]int{1, 2, 3, 4}, 0, func(_ int, item int, agg int) int {
+		return agg + item
+	})
+
+	result2 := ReduceBy([]int{1, 2, 3, 4}, "", func(_ int, item int, agg string) string {
+		return agg + fmt.Sprintf("%v", item)
+	})
+
+	assert.Equal(10, result1)
+	assert.Equal("1234", result2)
+
+}
+
 func TestIntSlice(t *testing.T) {
 	var nums []any
 	nums = append(nums, 1, 2, 3)