Typescript types for a pipe() function

Question

Consider the following TypeScript code:

type operator<T> = (input:T) => T

const pipe = <T>(...operators:operator<T>[]) => (input:T):T => operators.reduce((output, f) => f(output), input)

const add2:operator<number> = x => x+2

const times3:operator<number> = x => x*3

console.log(pipe(add2, times3)(1))    //output 9

The pipe function simply pipes the input of one operator into the result of the next operator.

Now consider this new definition of the operator type:

type operator<T, U> = (input:T) => U

How should the pipe function be rewritten in order for the IDE to let me know if I am using the types correctly?

Eg: consider these two operators:

const times3:operator<number, number> = x => x*3

const toStr:operator<number, string> = x => `${x}`

I would like this to work properly:

pipe(times3, toStr)(1)

And here I would like the IDE to warn me that the types are wrong:

pipe(toStr, times3)(1)

I can't figure this out, thank in advance.

Answer 1

Here is how RxJS does it :

pipe(): Observable<T>;
pipe<A>(op1: OperatorFunction<T, A>): Observable<A>;
pipe<A, B>(op1: OperatorFunction<T, A>, op2: OperatorFunction<A, B>): Observable<B>;
pipe<A, B, C>(op1: OperatorFunction<T, A>, op2: OperatorFunction<A, B>, op3: OperatorFunction<B, C>): Observable<C>;
pipe<A, B, C, D>(
    op1: OperatorFunction<T, A>,
    op2: OperatorFunction<A, B>,
    op3: OperatorFunction<B, C>,
    op4: OperatorFunction<C, D>
): Observable<D>;
pipe<A, B, C, D, E>(
    op1: OperatorFunction<T, A>,
    op2: OperatorFunction<A, B>,
    op3: OperatorFunction<B, C>,
    op4: OperatorFunction<C, D>,
    op5: OperatorFunction<D, E>
): Observable<E>;
pipe<A, B, C, D, E, F>(
    op1: OperatorFunction<T, A>,
    op2: OperatorFunction<A, B>,
    op3: OperatorFunction<B, C>,
    op4: OperatorFunction<C, D>,
    op5: OperatorFunction<D, E>,
    op6: OperatorFunction<E, F>
): Observable<F>;
pipe<A, B, C, D, E, F, G>(
    op1: OperatorFunction<T, A>,
    op2: OperatorFunction<A, B>,
    op3: OperatorFunction<B, C>,
    op4: OperatorFunction<C, D>,
    op5: OperatorFunction<D, E>,
    op6: OperatorFunction<E, F>,
    op7: OperatorFunction<F, G>
): Observable<G>;
pipe<A, B, C, D, E, F, G, H>(
    op1: OperatorFunction<T, A>,
    op2: OperatorFunction<A, B>,
    op3: OperatorFunction<B, C>,
    op4: OperatorFunction<C, D>,
    op5: OperatorFunction<D, E>,
    op6: OperatorFunction<E, F>,
    op7: OperatorFunction<F, G>,
    op8: OperatorFunction<G, H>
): Observable<H>;
pipe<A, B, C, D, E, F, G, H, I>(
    op1: OperatorFunction<T, A>,
    op2: OperatorFunction<A, B>,
    op3: OperatorFunction<B, C>,
    op4: OperatorFunction<C, D>,
    op5: OperatorFunction<D, E>,
    op6: OperatorFunction<E, F>,
    op7: OperatorFunction<F, G>,
    op8: OperatorFunction<G, H>,
    op9: OperatorFunction<H, I>
): Observable<I>;
pipe<A, B, C, D, E, F, G, H, I>(
    op1: OperatorFunction<T, A>,
    op2: OperatorFunction<A, B>,
    op3: OperatorFunction<B, C>,
    op4: OperatorFunction<C, D>,
    op5: OperatorFunction<D, E>,
    op6: OperatorFunction<E, F>,
    op7: OperatorFunction<F, G>,
    op8: OperatorFunction<G, H>,
    op9: OperatorFunction<H, I>,
    ...operations: OperatorFunction<any, any>[]
): Observable<unknown>;

It's not pretty, but it gets the job done.

Answer 2

I know it isn't the same exact function signature but... might I suggest using a builder pattern?

Typescript Playground example

 const pipe = <A, B>(fn: (a: A) => B) => { return { f: function<C>(g: (x: B) => C) { return pipe((arg: A) => g(fn(arg)))}, build: () => fn } } const compose = <A, B>(fn: (a: A) => B) => { return { f: function<C>(g: (x: C) => A) { return compose((arg: C) => fn(g(arg)))}, build: () => fn } } const add = (x: number) => (y: number) => x + y const format = (n: number) => `value: ${n.toString()}` const upper = (s: string) => s.toUpperCase() const process = pipe(add(2)).f(add(6)).f(format).f(upper).build() const process2 = compose(upper).f(format).f(add(6)).f(add(5)).build() console.log(process(6)) console.log(process2(6))

Answer 3

Goblinlord's answer is inspiring, if the runtime recursion is the concern, we may type erase the actual implementation so we can replace the recursion with iteration. Type erasing brings a risk that flaws could escape compile time type check but I think that is a price I am willing to pay.

 type Fn<T, U> = (i: T) => U type Pipe<T, U> = { f: <K>(fn: Fn<U, K>) => Pipe<T, K>, build: () => Fn<T, U> } function pipe<T, U>(fn: Fn<T, U>): Pipe<T, U> { const fns: Fn<any, any>[] = [fn] const p: Pipe<any, any> = { f: (fn) => { fns.push(fn); return p; }, build: () => { return (input) => fns.reduce((prev, curr) => curr(prev), input); } } return p; } const add = (x: number) => (y: number) => x + y const format = (n: number) => `value: ${n.toString()}` const upper = (s: string) => s.toUpperCase() const process = pipe(add(2)).f(add(6)).f(format).f(upper).build() console.log(process(1))

Answer 4

Insipired by Goblinlord , meriton andthis solution of Array Sort .

The code

type Reverse<Arr extends readonly any[]> = Arr extends [infer TFirst, ...infer TRest] ? [...Reverse<TRest>, TFirst] : Arr;

type Operator<A, B> = (value: A) => B
type OperatorA<T> = T extends Operator<infer A, any> ? A : never
type OperatorB<T> = T extends Operator<any, infer B> ? B : never

type PipeOperators<Operators extends unknown[], Input> =
  Operators extends [infer Item, ...infer Tail] ? (
    [
      Operator<Input, OperatorB<Item>>,
      ...PipeOperators<Tail, OperatorB<Item>>
    ]
  ) : Operators
type PipeOperatorsOutput<Operators extends unknown[]> = OperatorB<Reverse<Operators>[0]>

function pipe<Input, Operators extends unknown[]>(...operators: PipeOperators<Operators, Input>): (input: Input) => PipeOperatorsOutput<Operators> {
  return operators as never // Runtime implementation.
}



const add = (x: number) => (y: number) => x + y
const format = (n: number) => `value: ${n.toString()}`
const upper = (s: string) => s.toUpperCase()


const __TEST1__: string = pipe(add(2), format, upper)(1)
const __TEST2__: string = pipe(add(2), upper)(1) // Error: Type 'number' is not assignable to type 'string'.
const __TEST3__: string = pipe(add(2), format)("") // Error: Argument of type 'string' is not assignable to parameter of type 'number'.
const __TEST4__: string = pipe(add(2), format)(1)
const __TEST5__: number = pipe(add(2), add(2))(1)

There are some places where I used any and unknown , however it should be more exact type. But so far this is only way I could get the code working.

Please, don't beat much if it doesn't work properly.