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typescriptgenericscurrying
NSjonas
NSjonas

Reputation: 12032

Typescript Typings for generic curry functions

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

Upvotes: 4

Views: 534

Answers (1)

Николай Гольцев
Николай Гольцев

Reputation: 1337

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.

Upvotes: 3

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