Typing compose function in TypeScript
Tags: typescript, functional, javascript, tutorial
Published: 2021-06-22 18:07:42

Let's write some crazy, unreadable and unmaintainable typings for `compose` function. Maybe you will learn smth new.

Let's define some base types and utils.
```typescript
type Fn = (a: any) => any

type Head<T extends any[]> =
    T extends [infer H, ...infer _]
    ? H
    : never;

type Last<T extends any[]> =
    T extends [infer _]
    ? never : T extends [...infer _, infer Tl]
    ? Tl
    : never;

type Foo = typeof foo
type Bar = typeof bar
type Baz = typeof baz
```
Our main goal is to make `compose` without any arguments length restriction.

For example, take a look on lodash compose typings:
```typescript

    interface LodashFlowRight {
        <A extends any[], R1, R2, R3, R4, R5, R6, R7>(f7: (a: R6) => R7, f6: (a: R5) => R6, f5: (a: R4) => R5, f4: (a: R3) => R4, f3: (a: R2) => R3, f2: (a: R1) => R2, f1: (...args: A) => R1): (...args: A) => R7;
        <A extends any[], R1, R2, R3, R4, R5, R6>(f6: (a: R5) => R6, f5: (a: R4) => R5, f4: (a: R3) => R4, f3: (a: R2) => R3, f2: (a: R1) => R2, f1: (...args: A) => R1): (...args: A) => R6;
        <A extends any[], R1, R2, R3, R4, R5>(f5: (a: R4) => R5, f4: (a: R3) => R4, f3: (a: R2) => R3, f2: (a: R1) => R2, f1: (...args: A) => R1): (...args: A) => R5;
        <A extends any[], R1, R2, R3, R4>(f4: (a: R3) => R4, f3: (a: R2) => R3, f2: (a: R1) => R2, f1: (...args: A) => R1): (...args: A) => R4;
        <A extends any[], R1, R2, R3>(f3: (a: R2) => R3, f2: (a: R1) => R2, f1: (...args: A) => R1): (...args: A) => R3;
        <A extends any[], R1, R2>(f2: (a: R1) => R2, f1: (...args: A) => R1): (...args: A) => R2;
        (...func: Array<lodash.Many<(...args: any[]) => any>>): (...args: any[]) => any;
    }
```
There is a limit for arguments.

Let's try to write function without any limits, at least explicit limits. Please keep in mind TS has his own recursion limits, so we have to live with that

I will start with validation logic:
```typescript
type Allowed<
    T extends Fn[],
    Cache extends Fn[] = []
    > =
    T extends []
    ? Cache
    : T extends [infer Lst]
    ? Lst extends Fn
    ? Allowed<[], [...Cache, Lst]> : never
    : T extends [infer Fst, ...infer Lst]
    ? Fst extends Fn
    ? Lst extends Fn[]
    ? Head<Lst> extends Fn
    ? Head<Parameters<Fst>> extends ReturnType<Head<Lst>>
    ? Allowed<Lst, [...Cache, Fst]>
    : never
    : never
    : never
    : never
    : never;
```

Above type iterates through every function in the array and checks if argument of current function is assignable to return type of next function `Head<Parameters<Fst>> extends ReturnType<Head<Lst>>`

Next, we can define simple helpers:

```typescript

type LastParameterOf<T extends Fn[]> =
    Last<T> extends Fn
    ? Head<Parameters<Last<T>>>
    : never

type Return<T extends Fn[]> =
    Head<T> extends Fn
    ? ReturnType<Head<T>>
    : never

```

Finally, our compose function:

```typescript

function compose<T extends Fn, Fns extends T[], Allow extends {
    0: [never],
    1: [LastParameterOf<Fns>]
}[Allowed<Fns> extends never ? 0 : 1]>
    (...args: [...Fns]): (...data: Allow) => Return<Fns>

function compose<
    T extends Fn,
    Fns extends T[], Allow extends unknown[]
>(...args: [...Fns]) {
    return (...data: Allow) =>
        args.reduceRight((acc, elem) => elem(acc), data)
}

```

As you might have noticed, I have defined only one overload, this is considered a bad practice. We should always define at least two. Sorry for that.

And full example for copy/paste:

```typescript

type Foo = typeof foo
type Bar = typeof bar
type Baz = typeof baz


type Fn = (a: any) => any

type Head<T extends any[]> =
    T extends [infer H, ...infer _]
    ? H
    : never;

type Last<T extends any[]> =
    T extends [infer _]
    ? never : T extends [...infer _, infer Tl]
    ? Tl
    : never;


type Allowed<
    T extends Fn[],
    Cache extends Fn[] = []
    > =
    T extends []
    ? Cache
    : T extends [infer Lst]
    ? Lst extends Fn
    ? Allowed<[], [...Cache, Lst]> : never
    : T extends [infer Fst, ...infer Lst]
    ? Fst extends Fn
    ? Lst extends Fn[]
    ? Head<Lst> extends Fn
    ? Head<Parameters<Fst>> extends ReturnType<Head<Lst>>
    ? Allowed<Lst, [...Cache, Fst]>
    : never
    : never
    : never
    : never
    : never;

type LastParameterOf<T extends Fn[]> =
    Last<T> extends Fn
    ? Head<Parameters<Last<T>>>
    : never

type Return<T extends Fn[]> =
    Head<T> extends Fn
    ? ReturnType<Head<T>>
    : never


function compose<T extends Fn, Fns extends T[], Allow extends {
    0: [never],
    1: [LastParameterOf<Fns>]
}[Allowed<Fns> extends never ? 0 : 1]>
    (...args: [...Fns]): (...data: Allow) => Return<Fns>

function compose<
    T extends Fn,
    Fns extends T[], Allow extends unknown[]
>(...args: [...Fns]) {
    return (...data: Allow) =>
        args.reduceRight((acc, elem) => elem(acc), data)
}

const foo = (arg: 1 | 2) => [1, 2, 3]
const bar = (arg: string) => arg.length > 10 ? 1 : 2
const baz = (arg: number[]) => 'hello'

const check = compose(foo, bar, baz)([1, 2, 3]) // [number]
const check2 = compose(bar, foo)(1) // expected error

```