`pipe` loses generics

TypeScript Version: 3.4.1

Search Terms: generic rest parameters pipe compose

Code

I’m defining pipe using generic rest parameters as recommended here: https://github.com/Microsoft/TypeScript/issues/29904#issuecomment-471334674.

// Copied from https://github.com/Microsoft/TypeScript/issues/29904#issuecomment-471334674
declare function pipe<A extends any[], B>(ab: (...args: A) => B): (...args: A) => B;
declare function pipe<A extends any[], B, C>(
    ab: (...args: A) => B,
    bc: (b: B) => C,
): (...args: A) => C;
declare function pipe<A extends any[], B, C, D>(
    ab: (...args: A) => B,
    bc: (b: B) => C,
    cd: (c: C) => D,
): (...args: A) => D;

declare const myGenericFn: <T>(t: T) => string[];
declare const join: (strings: string[]) => string;

// Expected type: `<T>(t: T) => string`
// Actual type: `(t: any) => string`
const fn1 = pipe(
    myGenericFn,
    join,
);

// Workaround:
// Expected and actual type: `<T>(t: T) => string`
const fn2 = pipe(
    myGenericFn,
    strings => join(strings),
);

Playground Link: https://www.typescriptlang.org/play/index.html#src=declare function pipe<A extends any[]%2C B>(ab%3A (...args%3A A) %3D> B)%3A%20(…args%3A%20A)%20%3D%3E%20B%3B%0D%0Adeclare%20function%20pipe%3CA%20extends%20any%5B%5D%2C%20B%2C%20C%3E(%0D%0A%20%20%20%20ab%3A%20(…args%3A%20A)%20%3D%3E%20B%2C%0D%0A%20%20%20%20bc%3A%20(b%3A%20B)%20%3D%3E%20C%2C%0D%0A)%3A%20(…args%3A%20A)%20%3D%3E%20C%3B%0D%0Adeclare%20function%20pipe%3CA%20extends%20any%5B%5D%2C%20B%2C%20C%2C%20D%3E(%0D%0A%20%20%20%20ab%3A%20(…args%3A%20A)%20%3D%3E%20B%2C%0D%0A%20%20%20%20bc%3A%20(b%3A%20B)%20%3D%3E%20C%2C%0D%0A%20%20%20%20cd%3A%20(c%3A%20C)%20%3D%3E%20D%2C%0D%0A)%3A%20(…args%3A%20A)%20%3D%3E%20D%3B%0D%0A%0D%0Adeclare%20const%20myGenericFn%3A%20%3CT%3E(t%3A%20T)%20%3D%3E%20string%5B%5D%3B%0D%0Adeclare%20const%20join%3A%20(strings%3A%20string%5B%5D)%20%3D%3E%20string%3B%0D%0A%0D%0A%2F%2F%20Expected%20type%3A%20%60%3CT%3E(t%3A%20T)%20%3D%3E%20string%60%0D%0A%2F%2F%20Actual%20type%3A%20%60(t%3A%20any)%20%3D%3E%20string%60%0D%0Aconst%20fn1%20%3D%20pipe(%0D%0A%20%20%20%20myGenericFn%2C%0D%0A%20%20%20%20join%2C%0D%0A)%3B%0D%0A%0D%0A%2F%2F%20Workaround%3A%0D%0A%2F%2F%20Expected%20and%20actual%20type%3A%20%60%3CT%3E(t%3A%20T)%20%3D%3E%20string%60%0D%0Aconst%20fn2%20%3D%20pipe(%0D%0A%20%20%20%20myGenericFn%2C%0D%0A%20%20%20%20strings%20%3D%3E%20join(strings)%2C%0D%0A)%3B%0D%0A

Related Issues: https://github.com/Microsoft/TypeScript/issues/29904

/cc @ahejlsberg

[babakness]

Hey guys, for what its worth, I’ve come up with a recursive Pipe and Compose which preserves parameter names. I think this is a better experience than have variables names like a, b etc.

It tolerates generics better but this too has issues with generics.

Anyway, I just published this library

Type only:

https://github.com/babakness/pipe-and-compose-types

Implementation:

https://github.com/babakness/pipe-and-compose

and here is an article I wrote on it

https://dev.to/babak/introducing-the-recursive-pipe-and-compose-types-3g9o

[ahejlsberg]

Does the reasoning in this comment apply here too?

Yes, that reasoning applies to this example as well.