feat(collections/unstable): add cycle iterator utility

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##             main    #6386      +/-   ##
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+ Coverage   96.07%   96.09%   +0.02%     
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  Files         558      559       +1     
  Lines       42190    42205      +15     
  Branches     6374     6378       +4     
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[kt3k]

We have policy to accept new features as unstable features (see https://github.com/denoland/std/blob/main/.github/CONTRIBUTING.md#new-features-in-stable-packages-version--100). Can you prefix the files with unstable_ and export it from the path ./unstable-cycle?

I'm not completely sure whether this is aligned with other APIs in std/collections. What do people think?

[BlackAsLight]

It seems a bit weird to me for the function to ask for an Iterable when most iterables aren't 'cycle-able'. I think it would make more sense to ask for types that are guaranteed to be cycle-able, like Arrays and Maps.

The documentation mentions that if the iterable is empty then the code loops indefinitely. That is a memory leak. The code should check if its empty and either exit or throw an error.

export function* cycle<T>(iterable: Iterable<T>): Generator<T> {
  let isEmpty = false;
  while (true) {
    for (const x of iterable) {
      isEmpty = false;
      yield x;
    }
    if (isEmpty) break; // Or throw an error as iterable is not cycle-able.
    isEmpty = true;
  }
}

[canac]

If you use yield* this could be much more concise.

Suggested change

	let iterator = iterable[Symbol.iterator]();
	while (true) {
	const result = iterator.next();
	if (result.done) {
	iterator = iterable[Symbol.iterator]();
	} else {
	yield result.value;
	}
	}
	while (true) {
	yield* iterable;
	}

[timreichen]

I wonder what some use cases are for these, I've never encountered one myself.
Seems like lodash doesn't provide any cycle() method, lodash-contrib does. So this function seems kinda niche.