Type Iterator#next() argument (yield expression)

[pauldraper]

Search Terms

Iterator.next() type, yield expression type

Suggestion

Type Iterator.next()/yield. I suggest Iterator<A, B>, where A is the input type, and B is the output type.

Mentioned in #10148 and #14883.

Required by #32283.

Alternative proposal in #32523, those this requires more drastic changes to TypeScript's type model.

Use Cases

Pretty self-explanatory; I want typed expressions. It's why I use TypeScript.

And currently, all yield ... expressions have no typing. (They are typed as any).

Examples

function* totalLength() {
  let result = 0;
  while (true) {
    result += (yield result).length;
}

const t = totalLength();
t.next('abc'); // 0
t.next('def'); // 3
t.return(''); // 6

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

[pauldraper]

And actually, Iterator has additional problems, like return?(value?: any): IteratorResult<T> which is just simply wrong.

To be more concrete, the types should be

interface Iterator<T> {
    next(): IteratorResult<T>;
}

interface Generator<A, B> extends Iterator<B> {
    next(value?: A): IteratorResult<B>;
    return<T>(value?: T): IteratorResult<T>;
    throw(e?: any): IteratorResult<B>;
}

The optional argument is problematic for having non-undefined values inside the generator, but I don't have a better suggestion.

[AlCalzone]

And currently, all yield ... expressions have no typing. (They are typed as any).

I think you should take a look at the TS 3.6 release notes
https://devblogs.microsoft.com/typescript/announcing-typescript-3-6-rc/

[RyanCavanaugh]

See #30790 and its linked issues

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.

[pauldraper]

Excellent, thanks @AlCalzone for linking to the 3.6 RC notes.

@typescript-bot I think you might not be working correctly (you didn't actually close).