Yield Overrides

[arcanis]

This PR implements #43632 and provides a way to annotate values in such a way that, when yielded, they return a specific other type. This allows libraries built around generators to provide proper typings:

declare function select<T>(fn: () => T): YieldType<T>;

function* generator() {
  const squareValue = yield select(state => state.number ** 2);
  assertEqual<number>()(squareValue);

  const isGreaterThan10 = yield select(state => state.number > 10);
  assertEqual<boolean>()(isGreaterThan10);

  const hexValue = yield select(state => state.number.toString(16));
  assertEqual<string>()(hexValue;
}

Fixes #43632

cc @andrewbranch for the playground link

[andrewbranch]

@typescript-bot pack this

[typescript-bot]

Heya @andrewbranch, I've started to run the tarball bundle task on this PR at 9fc3cc5. You can monitor the build here.

[arcanis]

Sorry about that, tests should be fixed; can you try to re-run the pack?

[andrewbranch]

@typescript-bot pack this

[typescript-bot]

Heya @andrewbranch, I've started to run the tarball bundle task on this PR at c9f8bb2. You can monitor the build here.

[typescript-bot]

Hey @andrewbranch, I've packed this into an installable tgz. You can install it for testing by referencing it in your package.json like so:

{
    "devDependencies": {
        "typescript": "https://typescript.visualstudio.com/cf7ac146-d525-443c-b23c-0d58337efebc/_apis/build/builds/118156/artifacts?artifactName=tgz&fileId=49D5863517A089D4E367A13C0EC34BE731BA8B8BB714FF38756C82B1AA70226002&fileName=/typescript-4.6.0-insiders.20220124.tgz"
    }
}

and then running npm install.

---

There is also a playground for this build and an npm module you can use via "typescript": "npm:@typescript-deploys/[email protected]".;

[yume-chan]

Maybe use type YieldType<T> = intrinsic;? (see #46836 for ThisType above)

[neurosnap]

I'd love to help in anyway possible to get this across the finish line. Is there anything I can do to help?

[arcanis]

It should already be ok apart from a couple of extra tests; next blocker is to find someone to champion it in the TS team.

[neurosnap]

How can we find someone to champion this PR?

[andrewbranch]

To be clear, nobody on the TypeScript team ever indicated that this approach to solving this problem would be mergeable. @arcanis wrote it as an experiment purely to help the discussion in #43632 along. Given the discussion there, I personally feel that this is not the right approach—this is essentially just a type assertion, but piggy-backing on non-type-assertion syntax in such a way that nobody will recognize that it’s just as unsafe. The next step is not finding someone to champion this PR; we are still on the very first step: exploring possible solutions that could be driven to consensus.