Add mergeMapConcurrently type definition

[gitusp]

mergeMapConcurrently seems to be missing its type definition.
I have checked that copying-and-pasting these definitions into my project's node_modules/@most/core/type-definitions/combinator/mergeConcurrently.d.ts seemed suggest types as documented, but not tested seriously.

[TylorS]

Hey @gitusp thank you for taking the time to open this PR! I have a few small comments that I think will make these definitions ready for everyone.

1. The 2nd and 3rd overload should be reversed. If 3 parameters are not given, TypeScript will always match the 2nd overload given that one will always be able to match the first argument. Here's a quick example of what I mean. In general you'll want the most initial params declared towards the top and the least initial params declared towards the bottom.

2. After the current 2nd and 3rd overloads are switched, the then 3rd & 4th overloads likely should be merged in order to properly take advantage of them. Here's one more example of how the situation (after addressing the first point) that the types get us into when trying to use the parameters one-by-one. I believe the 3rd overload should become something like

export function mergeMapConcurrently<A, B>(f: (a: A) => Stream<B>): {
  (concurrency: number, s: Stream<A>) => Stream<B>
  (concurrency: number): (s: Stream<A>) => Stream<B>
}

This will allow TypeScript to follow the overloads like a tree structure and allow all valid use of the function.

[briancavalier]

Hey @gitusp, thanks! I just opened #291 to fix an issue with the mergeMapConcurrently flow defs. It uses the same partial application technique @TylorS recommends, so even though it's Flow rather than TS, may be a good reference.

[gitusp]

Thanks so much @TylorS and @briancavalier! I played around @TylorS's examples and confirmed what you mentioned. That was really good to know.

I ported @briancavalier's new flow defs into TypeScript and confirmed that the new definitions works on all the forms of partial application.

Before

After

[briancavalier]

👍 Looks good. Thanks!