Reduce function for arrays/slices in Go

How to write a functional Reduce function in Go using generics, including a unit test. The foundation for many functional programming libraries!

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Reduce applies a binary function to each element in the input slice and an accumulator, returning the final value of the accumulator. The function f takes an accumulator of type B and an element of type A and returns a new accumulator of type B.

func Reduce[A any, B any](input []A, initial B, f func(B, A) B) B {
	accumulator := initial
	for _, v := range input {
		accumulator = f(accumulator, v)
	}
	return accumulator
}

and this is a unittest for the function:

func TestReduce(t *testing.T) {
	input := []int{1, 2, 3, 4, 5}
	initial := 0
	f := func(b int, a int) int {
		return b + a
	}
	expected := 15
	result := util.Reduce(input, initial, f)
	if expected != result {
		t.Errorf("Expected %v, got %v", expected, result)
	}
}