Can not call a generic function as an call "effect" (pattern in in redux-saga)

[kasperpeulen]

TypeScript Version: 4.0.2

Code

interface Foo {
 bla: string;
}

declare function map<T, R>(value: T[], transform: (t: T) => R): R[];

declare const foos: Foo[];

declare function call<Fn extends (...args: any[]) => any>(
 fn: Fn,
 ...args: Parameters<Fn>
): ReturnType<Fn>;

const a = call(map, foos, (foo: Foo) => foo.bla);

Expected behavior:
I expect this to give no compile errors.

Actual behavior:

Argument of type '(foo: Foo) => string' is not assignable to parameter of type '(t: unknown) => unknown'. Types of parameters 'foo' and 't' are incompatible. Type 'unknown' is not assignable to type 'Foo'

Playground Link:
Playground

[jack-williams]

This falls largely under the same design limitation as described here: #31811.

When using generics a good guideline to follow is to push generic parameters down such that they quantifier over as little structure as necessary. I think the best way to write this is:

declare function call<P extends unknown[], R>(
 fn: (...args: P) => R,
 ...args: P
): R;

[MartinJohns]

Jack Williams (@jack-williams) was a minute faster than me. :-D

#38964 and #37181 are also related.

[kasperpeulen]

Jack Williams (@jack-williams) Ah that is interesting thanks!

The next step would be if the callback (the third argument), could be inferred from the first two arguments.

interface Foo { bla: string; }
declare function map<T, R>(value: T[], transform: (t: T) => R): R[];
declare const foos: Foo[];

declare function call<P extends unknown[], R>(
 fn: (...args: P) => R,
 ...args: P
): R;

// Object is of type 'unknown'. vvv
const a = call(map, foos, (foo) => foo.bla);

This fails. With the error in the comment. However, if I write it like this:

declare function call<X, P extends unknown[], R>(
 fn: (x: X, ...args: P) => R,
 x: X,
 ...args: P
): R;

// Compiles, but a inferred as unknown[]
const a = call(map, foos, (foo) => foo.bla);

It does compile, if I hover over foo it is of type Foo, but a is inferred as unknown[]. Is this a "legit" bug?

[RyanCavanaugh]

Gonna have to link #30134 for that one

[typescript-bot]

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