A simple attempt at implementing iterators using golang/go#61405.

go.mod

module test

go 1.21

iter.go

package main

import (
	"fmt"
	"strconv"
)

type Iter[T any] func(func(T) bool) bool

func I[T any](i Iter[T]) Iter[T] { return i }

func (i Iter[T]) Limit(n int) Iter[T] {
	return func(yield func(T) bool) bool {
		return i(func(v T) bool {
			if n <= 0 {
				return false
			}
			n--

			return yield(v)
		})
	}
}

func Map[F, T any](i Iter[F], f func(F) T) Iter[T] {
	return func(yield func(T) bool) bool {
		return i(func(v F) bool {
			return yield(f(v))
		})
	}
}

func Reduce[T, R any](i Iter[T], initial R, reducer func(R, T) R) R {
	for v := range i {
		initial = reducer(initial, v)
	}
	return initial
}

type Addable interface {
	~int | ~int8 | ~int16 | ~int32 | ~int64 | ~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr | ~string | ~complex64 | ~complex128
}

func Sum[T Addable](i Iter[T]) (v T) {
	return Reduce(i, v, func(acc, cur T) T { return acc + cur })
}

func Generate(yield func(int) bool) bool {
	for i := 0; ; i++ {
		if !yield(i) {
			return false
		}
	}
}

func main() {
	iter := I(Generate)
	iter2 := iter.Limit(5)
	iter3 := Map(iter2, func(v int) string { return strconv.FormatInt(int64(v), 10) })
	fmt.Printf("%#v\n", Sum(iter3))
}