feat: support arrow function type shorthand

[aaron-harvey]

This PR adds support for arrow expression type shorthand for params and return type. I believe this is a valid way to declare arrow types (flow isn't complaining about it, and flow-ide picks it up).

It also includes a few minor performance enhancements, ie, moving some context things outside of the forEach loop.

// example of what is now supported
type FnType = (a: string, b: number) => number
const arrowFunction: FnType = (a, b) => { return 42; }

[danharper]

Thanks :)

Instead of checking each param in the type annotation, what do you think about just allowing any arrow whose types come from an annotation on the variable?

e.g. const f: Fn = (a) => a * 2 should pass blindly, instead of checking each param on the arrow? Fn can't be defined without param types anyway, so it's kinda enforced by Flow itself. Also resolves an potential issue where Fn is defined in another file.

[aaron-harvey]

@danharper I think this is a good idea - I hadn't considered the case where a type was defined elsewhere.
It also simplifies the code.

[danharper]

Could you add a valid test for each with a type from another file (e.g. just const f: Foo = (a, b) => 42;)? I haven't tried it, but I don't think that'd pass, because of getTypeAliases which can be removed.

I think it should just be able to check parent.id.typeAnnotation now?

[aaron-harvey]

Good catch. I simplified the check and added a test to cover types in another file.

[gajus]

Good catch. I simplified the check and added a test to cover types in another file.

Is this still applicable?

Resolved.

[gajus]

🎉 This PR is included in version 3.4.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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