FR: a way to have the return type of `yield` inferred from the yielded type

[dktapps]

Feature request

I thought it would be nice to be able to implement an await-like pattern using generic Promises and generators.

Sadly, I don't seem to be able to make PHPStan understand that the result of yield is dependent on the type of promise yielded.

Is this something that would be good for PHPStan itself, or is there a way that this could be implemented by an extension?

https://phpstan.org/r/235a7674-8514-4c68-969c-c97740151188

[ondrejmirtes]

One mistake there, fixed sample: https://phpstan.org/r/cab54e25-383c-4f18-996a-dbbe7f07371d

I need an expected output here since I don't understand what's currently wrong with types reported by PHPStan and what should be there instead.

[dktapps]

The Promise<TValue> wasn't a mistake.

This isn't exactly a bug since PHPStan doesn't really consider yield as a function. I want to use it as an async function of sorts, but the way generator types are specified and inferred is too limiting.

Its synchronous equivalent would have a signature something like this:

/**
 * @template TValue
 * @param Promise<TValue> $promise
 * @return TValue
 */
function syncYield(Promise $promise);

[dktapps]

Essentially I want to implement something like TypeScript async/await, but I can't find a way to do it that PHPStan can understand properly.

[ondrejmirtes]

I need an expected output of the playground here since I don't understand what's currently wrong with types reported by PHPStan and what should be there instead.

[dktapps]

I've simplified the example to just include the problematic parts: https://phpstan.org/r/791605fb-85b9-4595-a8ac-53de50efdae8

Like I said, this isn't technically a bug. I'm looking for a way for (in this specific case) to have the return type of yield be inferred from the yielded promise.

[ondrejmirtes]

Can you please modify this example https://phpstan.org/r/791605fb-85b9-4595-a8ac-53de50efdae8 with the rest of the code that actually calls send/next/onResolve, and that actually works in runtime? So 'getIntPromise()' should return new Promise(1) and 'getStringPromise()' should return new Promise('foo')...

I'm trying to understand it but I still don't have the full picture.

[ondrejmirtes]

And also, how is this specific example solved in languages that have async/await...

[dktapps]

TS use pretty much the same concept to implement async/await using ES6 generators: https://devblogs.microsoft.com/typescript/what-about-async-await/
https://basarat.gitbook.io/typescript/future-javascript/async-await#async-await-support-in-typescript

I'll try to put together an example.

[ondrejmirtes]

The challenge here is that in case of TS, the static analysis part is straightforward - you just extract the T from Promise of the awaited function.

But because here we're trying to do the same thing with userland code, it's much harder to reason about that code using types. I'd say that the equivalent would be to try to type the __generator and __awaiter functions.

I understand you're trying to extract int when yielding Promise<int>, but I feel link we'd need some new syntax/PHPDoc tags to make it work.

Something like:

@generator-send-template TValue
@generator-send Promise<TValue>
@generator-send-value TValue

...above the function.

[muglug]

Psalm supports @psalm-yield TValue: https://psalm.dev/r/799b92d11a

[muglug]

And here's your code with @psalm-yield: https://psalm.dev/r/95a731bb98

[muglug]

Amphp uses this (I added the feature for that library) and it means Psalm's own invocations of Amp are properly typed :)

[ondrejmirtes]

It's a feasible solution, but my problem with it is that it's controlled by the yielded object, but whether it actually works in runtime is controlled by the caller of the generator.

Additionally, there would have to be some checks whether there's some conflict between TSend of the Generator above the function, and @phpstan-yield above the class of the yielded object. I'm not even sure how to choose which one should win :)

And there would also have to be a check to see if the thing in @phpstan-yield exists - if it's a valid type or template type.