Define type to array behavior

Question

I need to crate a type that can hold an array where each element is a function that gets the previous output as an input. For example, a valid array is:

const pipe: SpecialArray = [
    () => string,
    (prev:string) => number,
    (prev:number) => boolean
];

An invalid array would be:

const pipe: SpecialArray = [
    () => string,
    (prev:number) => number, // <-- Here number should be string
    (prev:number) => boolean
];

Is there a way to define such structure in TypeScript?

score 0 · Answer 1 · answered May 29 '22 at 05:52

0

I believe no, since Array is homogenous, you'll have to define in the form [(a: string) => number, (b: number) => boolean], and you can't really enumerate all (unless a code generator is used).

answered May 29 '22 at 05:52

equt

192
1
8

My use case is to pass such an array to generate a value. I can't define all possible inputs. – Naor May 29 '22 at 05:53
Is the array-like use case a must? If not, how about a type like `type Pipe`, and then combine it? – equt May 29 '22 at 05:58
Sound good, can you elaborate? What is Pipe<>? – Naor May 29 '22 at 06:07
This? `type Pipe = [(input:A) => B, (input:B)=>C] ` – Naor May 29 '22 at 06:08
No, I'm referring to something like the [`pipe` function in the fp-ts library](https://gcanti.github.io/fp-ts/modules/function.ts.html#pipe), and I'm assuming that you're doing function composition like `.` in Haskell, which is you don't really care the the middle values, but only the start & end one. – equt May 29 '22 at 06:16

score 0 · Answer 2 · answered May 29 '22 at 08:23

It is impossible in TypeScript to create standalone type for such kind of array. In order to do that you should create compose function, infer each function from arguments and validate it. Please see my article and questions/answers on SO: first , second, third.

Also you can check this compose function in fnts lib

Define type to array behavior

2 Answers2