Improve type inference for generic curried functions

[masaeedu]

// #-------------- This works

type Repeat = (n: number) => (s: string) => string;
export const repeat: Repeat = n => s => s.repeat(n); // Sweet! s and n are both inferred!

// -------------------------#


// #----- Problems begin here

type Repeat2 = (n: number) => <T extends { repeat(n: number): T }>(repeatable: T) => T;

// Only n is inferred as number. Won't work with noImplicitAny enabled
export const repeat2Broken: Repeat2 = n => r => r.repeat(n); 
// Workaround is to redeclare everything from the type signature
export const repeat2Ugly: Repeat2 =
 n => <T extends { repeat(n: number): T }>(r: T) => r.repeat(n);

// -------------------------#


// #----------- It gets worse

// Type inference breaks for all args following the generic one, so even if you use
// the workaround, the next args won't regain inference
type Repeat3 = 
 (n: number) 
 => <T extends { repeat(n: number): T }>(repeatable: T) 
 => (reverse: boolean) 
 => T;

// Even if you explicitly provide the boilerplate for the generic arg, the boolean arg 
// won't be inferred. So the following won't work with noImplicitAny enabled
export const repeat3UglyAndBroken: Repeat3 =
 n => <T extends { repeat(n: number): T }>(r: T) => b => { ... }

// You have to add typing for the last arg
export const repeat3Uglier: Repeat3 =
 n => <T extends { repeat(n: number): T }>(r: T) => (b: boolean) => { ... }

// -------------------------#

I'd really appreciate better support for good inference in this kind of scenario, whether it is solved by supporting partial generic application or addition of participation of return-types in inference or whatever.

JS libraries that use a functional programming paradigm often end up loosely or any-ly typed, because you find yourself fighting the type system and adding repetitive type noise all over your code to derive any safety. A couple of examples where better support for FP would be useful:

• Libraries like React or Apollo make use of curried higher-order functions for solving cross-cutting concerns. In-fact, my original use case comes from React higher-order components
• Popular FP libraries like Ramda become tedious to use in a noImplicitAny project because any generic functions passed or received from the library tend to quickly devolve into {} unless you repeatedly hand-hold the type system

Here is my original use-case, if interested:

export type GetState<TState> = () => TState;
export type SetState<TState> = (newState: TState) => void;
export type StatelessComponent<TProp> = (prop: TProp) => React.ReactElement<TProp>
export type StatefulComponent<TProp, TState> = 
 (stateAccess: { getState: GetState<TState>, setState: SetState<TState> })
 => StatelessComponent<TProp>;

type ScrollableHOC = 
 (options: ScrollableOptions) 
 => <T extends ScrollableProps>(Child: ReactComponent<T>) 
 => StatefulComponent<T, ScrollableState>;

// The introduction of the Child arg breaks type inference for getState, setState, props
const scrollable: ScrollableHOC = opts => Child => ({ getState, setState }) => props => {
 // ...
}

[HerringtonDarkholme]

It does not need a high order function to produce the behavior. Type inference fails when generic bound occurs.

type GenBound = <T extends string>(r: T) => T
var firstOrder: GenBound = r => r.length

https://www.typescriptlang.org/play/index.html#src=type%20GenBound%20%3D%20%3CT%20extends%20string%3E(r%3A%20T)%20%3D%3E%20T%0D%0Avar%20firstOrder%3A%20GenBound%20%3D%20r%20%3D%3E%20r.length

[masaeedu]

Herrington Darkholme (@HerringtonDarkholme) This is a contrived example to demonstrate type inference degradation when generics are involved. The request is to make type inference work for Haskell-style curried functions, I don't actually care about the Repeatable example here or how else it could be implemented.

The actual use case is React higher order components, as illustrated in the second snippet. As the name suggests, they necessarily involve higher-order functions.

[HerringtonDarkholme]

Asad Saeeduddin (@masaeedu) My point here is that the root cause of this problem is not curried function but generic.

Note the spec has already said:

A contextual signature S is extracted from a function type T as follows:
If T is a function type with exactly one call signature, and if that call signature is non-generic, S is that signature.

https://github.com/Microsoft/TypeScript/blob/02547fe664a1b5d1f07ea459f054c34e356d3746/doc/spec.md#410-function-expressions

So this works as intended. Yet, TypeScript has changed a lot since 1.8. It is not impossible to change the spec.

[masaeedu]

Herrington Darkholme (@HerringtonDarkholme) I understand. This isn't a request for a specific change to type system's behavior. I'm simply trying to illustrate how a common use-case (defining curried functions with out-of-band type signatures) quickly becomes very tedious with the way the type system works right now.

[gcnew]

This earliest report of this issue that I know of is #9366. I've tried to implement a prototype solution using a Hindley-Milner style constraint solver, however I hit on several major problems:

• TypeScript has unconstrained union types (i.e. a union may be created between any two types). When unifying two unions, you don't really know which type var with which other one "should" unify. This means that either you have to try all solutions (backtracking, very expensive) or not supporting it at all. It turns out that it's getting used in existing user code and the currently implemented approach somewhat works. This means that the new implementation should be at least as powerful. A property I wasn't able to achieve.
• Error reporting. The current error reporting is very good. Languages using Hindley-Milner (especially Haskell) are known for their verbose and unhelpful messages. I couldn't figure out a way to keep the quality of the current errors, however, I do believe there are ways to come close.
• Inference. The current inference algorithm leaves a lot to be desired. For example, currently "contextually typed" functions are typed by inferring the rest of the expression, and then a second pass is made using the resulting type to check the contextually typed function. A better approach would be to infer a generic signature for the contextually typed function and allow unification to do its magic. Unfortunately, inferring a generic signature of a function is impeded by mutability, need for 'var arg' types and other constraints.

All in all, it's a difficult problem. Unless the team prioritises it and puts considerable effort into figuring it out, I don't think it could be resolved.

PS: If interested, you can see my work at https://github.com/gcnew/TypeScript/tree/polyFuncUnification. I tried many approaches, not all of which committed, thus I'm not sure how well the latest commit is working.

[masaeedu]

Marin Marinov (@gcnew) I really appreciate the work that must have gone into this investigation. You're probably better equipped to determine whether this is feasible than I am, but I was wondering whether something like F#'s automatic generalization would be possible for function signatures in TypeScript. The idea is simply to infer an implicit generic type parameter for any parameters. So that when I write:

 function f(foo, bar) {
 return foo;
 }

What actually gets recorded in the type system is:

 function f<T1, T2>(foo: T1, bar: T2) {
 return foo;
 }

instead of:

 function f(foo: any, bar: any) {
 return foo;
 }

If this were the case, getting inference to work in the scenario I described above would simply involve the type checker matching up the constraints on type parameters.

[gcnew]

It's actually very hard. You'd like these generics to be reduced to concrete types based on usage and that's far from trivial. You have to factor in overloads, mutability, variadic functions, open-ended objects, unions, subclassing, etc. If you give up on overloads, mutability and unconstrained unions (limit them to only predefined Tagged Unions (ADTs)) it becomes manageable.

[gcnew]

Ryan Cavanaugh (@RyanCavanaugh) commented on the issue yesterday, see the discussion on #15114.

[masaeedu]

Marin Marinov (@gcnew) The scenario I'm interested in is not really usage-site, i.e. I don't really care about inference of the type args when doing foo(a, b) (although you'd get much better inference there for free). The linked issue, for example, wants to infer type information based on usage of parameters in the body of the function, whereas the change I'm proposing doesn't involve any analysis of parameter usage. What I want is to fix inference when assigning a function to a variable of a particular function type.

You are right that overloading makes things hairier (as it does in nearly every feature). Could you provide an example of how unconstrained unions/mutability would interact with the proposed change? E.g. if the following:

function foo(a, b, c) {
}

simply desugared to:

function foo<T1, T2, T3>(a: T1, b: T2, c: T3) { }

How would unions and mutability affect this?

[masaeedu]

Marin Marinov (@gcnew) What I'm suggesting seems to be a dupe of #14078

[gcnew]

I think #3410 is the first issue stating the bug. There are many related issues, but the most important ones are #5616 and #8397. The root cause of the problem is stated here: #3410 (comment).

[npenin]

I think there is another usecase if it helps :

interface A
{
 add<T>(name: string, a: T, cb: TypedCallback<T>)
}

interface B
{
 add<T, U>(name: string, a: T, cb: Callback<T, U>)
}

type TypedCallback<T> = <U>(a1: T, a2: U) => void;
type Callback<T, U> = (a1: T, a2: U) => void;

var a: A, b: B;

a.add('a', { prop1: 'string' }, function (a1, a2)
{

})
b.add('b', { prop1: 'string' }, function (a1, a2)
{

})

in the case b, everything works as expected, but in case a, a1 is not inferred.