A brief snippet doing an iterable structure with recursion instead of loops

iterable.ts

type Iter<A> = () => A | null;

const array_to_iter = <A>(arr: A[]): Iter<A> => {
    let index = 0;

    return () => {
        const el = arr[index];
        index += 1;
        return el;
    }
};

const for_iter = <A>(iter: () => A | null, cb: (el: A) => unknown): void => {
    const el = iter();
    if(!el) return;
    
    cb(el);
    for_iter(iter, cb);
};

const map_iter = <A, B>(iter: () => A | null, cb: (el: A) => B): B[] => {
    let acc = [] satisfies B[];
    const el = iter();

    if(!el) return acc;

    return [cb(el), ...map_iter(iter, cb)];
}

const reduce_iter = <A, B>(iter: () => A | null, cb: (el: A, acc: B) => B, acc: B): B => {
    const el = iter();

    if(!el) return acc;

    return reduce_iter(iter, cb, cb(el, acc));
}

for_iter(array_to_iter(["a5", "b4", "c3", "d2", "e1"]), console.log);
console.log(map_iter(array_to_iter(["a5", "b4", "c3", "d2", "e1"]), (el) => `${el} - mapped`));
console.log(reduce_iter(array_to_iter([100, 10, 1]), (el, acc) => (el + acc), 0));