Typescript 任意数量的泛型参数

考虑任意数量的函数，每个函数接受一个参数并返回一个值（不要担心它们的作用，而不是重点）：

这些函数可以这样链接（fp中可能有更好的术语），因为每个函数都将前一个函数的返回类型作为参数：

type Fn<I, O> = (input: I) => O

function chain(...fns: Fn<any, any>[]): Fn<any, any> {
  return (input: any) => {
    let result = input
    for (const fn of fns) {
      result = fn(result)
    }
    return result
  }
}

const chained = chain(toNumber, toBoolean, toString)

const result = chained('5') // returns "true"

---

但是有更好的方法吗？

我不确定我会称之为更好的方法，但可以将此规则编码到类型系统中。然而，如果没有良好的文档，大多数观察者可能很难理解它的作用

---

下面的方法使用了一个泛型参数，

扩展了readonlyray，这太神奇了，我对类型系统有太多不知道的地方。。。
type Fn<I, O> = (input: I) => O

function chain(...fns: Fn<any, any>[]): Fn<any, any> {
  return (input: any) => {
    let result = input
    for (const fn of fns) {
      result = fn(result)
    }
    return result
  }
}

const chained = chain(toNumber, toBoolean, toString)

const result = chained('5') // returns "true"

declare function chain<S, T1>(fn: Fn<S, T1>): Fn<S, T1>
declare function chain<S, T1, T2>(fn1: Fn<S, T1>, fn2: Fn<T1, T2>): Fn<S, T2>
declare function chain<S, T1, T2, T3>(fn1: Fn<S, T1>, fn2: Fn<T1, T2>, fn3: Fn<T2, T3>): Fn<S, T3>

type Fn<I, O> = (input: I) => O;

type Input<F> = F extends Fn<infer U, any> ? U : never;

// or you can use ReturnType<T>, which is provided by typescript
type Output<F> = F extends Fn<any, infer U> ? U : never;

// first elem of tuple type
type Head<T> = T extends [infer U, ...unknown[]] ? U : never;

// all but first elem of tuple type
type Tail<T> = T extends [unknown, ...(infer U)] ? U : never;

// last element of array, only works on typescript 4.2+
type Last<T> = T extends [...unknown[], infer U] ? U : never;

// `true` if T can be composed with U, `false` if not, and `never` if the types are wrong
type CanCompose<T, U> = T extends Fn<any, infer Output> 
    ? (
        U extends Fn<infer Input, any>
            ? (Input extends Output ? true : false) 
            : never
    ) 
    : never;

// Evaluates to a tuple of booleans. If any element in the tuple is false, you cannot compose the whole array of T.
type IsComposable<T> = T extends Fn<any, any>[] 
    ? CanCompose<Head<T>, Head<Tail<T>>> extends true 
        ? (
            Tail<T> extends never[]
                ? [true] 
                : [true, ...(IsComposable<Tail<T>>)]
        ) 
        : [false]
    : [false];

type Composable<T> = IsComposable<T> extends true[] ? T : never[];
type ComposedFunction<T> = IsComposable<T> extends true[] 
    ? Fn<Input<Head<T>>, Output<Last<T>>>
    : Fn<never, never>;

function chain<T extends ReadonlyArray<Fn<any, any>>>(...fns: Composable<T>): ComposedFunction<T> {
    return ((x: Input<Head<T>>) => {
        let result = x;
        for (const fn of fns) {
            result = fn(result);
        }
        return result;
    }) as ComposedFunction<T>
}