this is transducers in golang using golang generics (didn't implement cat or filter sorry)

transducer.go

package main

import "fmt"

type InitFn[A any] func() A
type IdentityFn[A any] func(x A) A
type ReduceFn[A any, B any] func(acc A, cur B) A

type Reducer[A any, B any] struct {
	Init     InitFn[A]
	Identity IdentityFn[B]
	Reduce   ReduceFn[A, B]
}

func NewReducer[A any, B any](init InitFn[A], rfn ReduceFn[A, B]) Reducer[A, B] {
	return Reducer[A, B]{
		Init:     init,
		Identity: func(x B) B { return x },
		Reduce:   rfn,
	}
}

// Transducer is a reversed reducer order because that's how it unwinds itself
// when it is "transduced".
type Transducer[X any, A any, Y any, B any] func(Reducer[Y, B]) Reducer[X, A]

type Iterator[T any] func() (x T, done bool)

func NewIterator[T any](x []T) func() (x T, done bool) {
	var i int
	var zero T
	return func() (T, bool) {
		if i < len(x) {
			i += 1
			return x[i-1], false
		}
		return zero, true
	}
}

func Map[A any, B any, T any](fn func(a A) B) Transducer[T, A, T, B] {
	return func(r Reducer[T, B]) Reducer[T, A] {
		return NewReducer(
			func() (z T) { return z },
			func(acc T, cur A) T { return r.Reduce(acc, fn(cur)) },
		)
	}
}

func Comp[X any, A any, Y any, B any, Z any, C any](tx1 Transducer[X, A, Y, B], tx2 Transducer[Y, B, Z, C]) Transducer[X, A, Z, C] {
	return func(r Reducer[Z, C]) Reducer[X, A] {
		return tx1(tx2(r))
	}
}

func Push[A any]() Reducer[[]A, A] {
	return NewReducer(
		func() (z []A) { return z },
		func(acc []A, cur A) []A { return append(acc, cur) },
	)
}

func Transduce[X any, A any, Y any, B any](xform Transducer[X, A, Y, B], reducer Reducer[Y, B], xs Iterator[A]) X {
	r := xform(reducer)
	acc := r.Init()
	for x, done := xs(); !done; x, done = xs() {
		acc = r.Reduce(acc, x)
	}
	return acc
}

func main() {
	xx := make([]int, 8)
	for i := range xx {
		xx[i] = i
	}

	type T float32

	xform1 := Map[int, float64, []T](func(x int) float64 { return float64(x) * 2.5 })
	xform2 := Map[float64, T, []T](func(x float64) T { return T(x) / -2 })
	xform := Comp(
		xform1,
		xform2,
	)
	yy := Transduce(
		xform,
		Push[T](),
		NewIterator(xx),
	)
	fmt.Printf("xx = %v\n", xx)
	fmt.Printf("yy = %v\n", yy)
	fmt.Printf("typeof(yy[0]) = %T\n", yy[0])
}