Typescript Typings for generic curry functions

Question

I'm trying to get better with FP but struggling how to deal with typing generic "curry" functions.

For example, I've written a "Curry" version of reduce:

const reduce = <S, R>(fn: (result: R, obj: S, i?: number, arr?: S[]) => R, init: R) => 
(objects: S[]) => 
objects.reduce(fn, init);

const numToSum: number[] = [1,2,3,5,8,13];

const result = reduce((sum, n) => sum + n, 0)(numToSum);

The problem is that typescript obviously can't know the type of S until you actually call the curry function with numToSum.

It's more obvious when you see it without the curry call:

const sumNumbersFn = reduce((sum, n) => sum + n, 0);

In this case you can fix this by typing the arguments of the function itself with n: number or explicitly setting the generic parameters <number[], number>.

The former seems reasonable, but what I'm running into is that as things get more complex, I'm constantly having to provide generic signature.

I'm wondering if I am missing something, and it is actually possible to get typescript to "infer" the types from the later function calls?

Going back to the first example:

const result = reduce((sum, n) => sum + n, 0)(numToSum);

It seems like the compiler actually SHOULD have all the information needed to infer types.

Maybe my types are just off?

Update

Here's a more concrete/full example of the problem I'm running into

TS-Playground

I know that stackoverflow.com/questions/60203078/… is very similar (in title)... I think at it's core the question is different, but if you think this is a duplicate, my feelings won't be hurt — NSjonas
– NSjonas, Commented Nov 14, 2020 at 17:43
I'm also not sure that what I'm doing is even correct fp. I wouldn't be surprised if what I've written above is a incorrect application — NSjonas
– NSjonas, Commented Nov 14, 2020 at 17:44
I don't think TS can ever infer the generic types from the value you eventually pass to the function returned - you could assign the return value of reduce and call it with multiple different values, for example. — jonrsharpe
– jonrsharpe, Commented Nov 14, 2020 at 17:49
@jonrsharpe if you have a moment, check out the ts-playground link I just added... It probably does a better job of describing the problem I'm trying to overcome — NSjonas
– NSjonas, Commented Nov 14, 2020 at 17:57

Николай Гольцев · Accepted Answer · 2020-11-14 20:27:25Z

3

All you have to do is to declare a function which you return from select function as generic:

function select<T, S>(sFn: (obj: T) => S) {
  function computation<G>(fn: (obj: S) => G) {
    return (obj: T) => fn(sFn(obj));
  }

  return computation;
}

P.S: I don't know why such kind of syntax doesn't work in TS playground:

const select = <T, S>(sFn: (obj: T) => S) => <G>(fn: (obj: S) => G) => {
  return (obj: T) => fn(sFn(obj));
}

Maybe because of some TS config settings. Thus I wrote the solution as a function declaration instead of a function expression.

answered Nov 14, 2020 at 20:27

Николай Гольцев

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1 Comment

NSjonas Over a year ago

ah man... I trashed so hard and was sooo close. I actually tried the "function expression" version along the way (probably multiple times). Your a hero!

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