Wrong typescript type for $.ap()

[vlinder]

most/type-definitions/most.d.ts

Line 75 in bfed148

ap<B, C>(fs: Stream<(a: A) => B>): Stream<C>;

Not sure how this can have survived for so long, but this type suggests that you should pass a stream of functions to f$.ap() when in fact you should pass a stream of values.

The typings below should be the correct ones.

    ap<
      B = A extends (x: infer B) => any ? B : never,
      C = A extends (x: any) => infer C ? C : never
    >(
      x$: Stream<B>
    ): Stream<C>;

This also works

    ap(
      x$: Stream<A extends (x: infer B) => any ? B : never>
    ): Stream<A extends (x: any) => infer C ? C : never>;

Should I make a PR?

[briancavalier]

Hey @vlinder 👋 . I think the existing types are as intended. Here's the implementation of ap.

Can you help me understand why you feel they should be different?

[vlinder]

Thanks for your response!

The docs state that you should pass a stream of values. Or rather that you call .ap() on the stream of functions.

most/docs/api.md

Line 701 in bfed148

#### `streamOfFunctions.ap(stream) -> Stream`

However the typings as I linked in the OP

most/type-definitions/most.d.ts

Line 75 in bfed148

ap<B, C>(fs: Stream<(a: A) => B>): Stream<C>;

states that the passed stream should be a stream of functions.

I wrote a short program to verify which was right, the docs or the types. Typescript playground here

import { periodic } from "most";
const add = (a: number, b: number) => a + b;
const value$ = periodic(1000).constant(1).scan(add, 0);
const f$ = periodic(5000)
  .constant(1)
  .scan(add, 0)
  .map((x) => (y: number) => x * y);
f$.ap(value$).forEach(console.log);

This code work well, but it does not type check since the .ap function is typed incorrectly. In our repository we have a package dedicated to fixing the type issues in the most npm package that override some of the functions we need to have correct or better typings for (arbitrary amount of arguments mostly and this #536). After adding my typings from OP it now both works and type checks.

I would gladly create a PR with those improved typings if there was a chance for them to be merged and released. But since my last PR that was very simple was never responded to I'm a bit hesitant to put in the effort if it is just going to be ignored. Let me know! :)