Typescript: Supporting generic type inference over the other higher-order functions

[falsandtru]

Currently, TypeScript infers generic types when referring its type immediately. Its behavior is undesirable for using higher-order functions. The type inference of generics should delay until it will be called.

function flip<a, b, c>(f: (a: a, b: b) => c): (b: b, a: a) => c {
  return (b: b, a: a) => f(a, b);
}
function zip<T, U>(x: T, y: U): [T, U] {
  return [x, y];
}
var expected: <T, U>(y: U, x: T) => [T, U] = flip(zip);
var actual: (y: {}, x: {}) => [{}, {}] = flip(zip);

[fdecampredon]

this issue is really problematic for functional paradigm users :

const map = <T, U>(transform: (t: T) => U) =>
    (arr: T[]) => arr.map(transform)

const identity = <T>(t: T) => t;

const identityStr = (t: string) => t;


const arr: string[] = map(identityStr)(['a']);
const arr1: string[] = map(identity)(['a']); // Type '{}[]' is not assignable to type 'string[]'.

[DanielRosenwasser]

(welcome back @fdecampredon!)

[fdecampredon]

(thanks ! :) )

[baio]

Yep, there is to many <any> in my Ramda code.

[Igorbek]

10247, #9949 for the reference. And see @gcnew's comment

[masaeedu]

Relevant paper: "Practical type inference for higher-rank types". Specifically the section on subsumption of parametrically polymorphic signatures.

Ultimately this comes down to better unification, which is what @gcnew has done some work on if you follow along in #9949. For the flip example above, the rank 1 function type <T, U>(x: T, y: U) => [T, U] can be substituted for the rank 0 function type (a: a) => (b: b) => c to obtain f: (x: a, y: b) => [a, b], because the former is more polymorphic than the latter. From there we need TypeScript's usual approach to unification to kick in and infer c = [a, b]. The tricky part is coming up with a general algorithm to do all of this.

[pelotom]

Another example:

const compose = <A, B, C>(g: (b: B) => C, f: (a: A) => B) => (a: A) => g(f(a))
const identity = <T>(x: T) => x
const lessThanTen = (x: number) => x < 10
const composed = compose(lessThanTen, identity)
//                       ^^^^^^^^^^^
// Argument of type '(x: number) => boolean' is not assignable to parameter of type '(b: {}) => boolean'.
//   Types of parameters 'x' and 'b' are incompatible.
//     Type '{}' is not assignable to type 'number'.

Can be fixed with explicit type parameters

const composed = compose<number, number, boolean>(lessThanTen, identity)

but would be really nice if this could just unify!

[Birowsky]

I would love to know if there is some movement here. Here's another usecase: https://stackoverflow.com/q/47934804/592641

[cameron-martin]

I've ran into this issue when using lodash's memoize function when the function being memoised is generic.

[steve8708]

This would also be really valuable for mixins on react components with proptypes

export class MyComponent extends MyMixin(React.Component<MyComponentProps>) {}

I _love_ the new mixins feature since 2.2 and I was really excited about refactoring my code to create a variety of mixins for my components until I realized as of right now it won't quite work as I'd hoped without a lot of manual type declaring 😭

[kpdonn]

Heads up I put up my attempt at implementing this at #24626. No idea if it'll go anywhere but it works for a lot of the cases I've tried so feel free to try it out if you're interested.

[nateabele]

@kpdonn 😭 It's so beautiful.

[sibelius]

Any progress on this?

[ahejlsberg]

Higher order function type inference now implemented in #30215.