type Func<T extends any[], R> = (...a: T) => R
/**
* Composes single-argument functions from right to left. The rightmost
* function can take multiple arguments as it provides the signature for the
* resulting composite function.
*
* @param funcs The functions to compose.
* @returns A function obtained by composing the argument functions from right
* to left. For example, `compose(f, g, h)` is identical to doing
* `(...args) => f(g(h(...args)))`.
*/
export default function compose(): <R>(a: R) => R
export default function compose<F extends Function>(f: F): F
/* two functions */
export default function compose<A, T extends any[], R>(
f1: (a: A) => R,
f2: Func<T, A>
): Func<T, R>
/* three functions */
export default function compose<A, B, T extends any[], R>(
f1: (b: B) => R,
f2: (a: A) => B,
f3: Func<T, A>
): Func<T, R>
/* four functions */
export default function compose<A, B, C, T extends any[], R>(
f1: (c: C) => R,
f2: (b: B) => C,
f3: (a: A) => B,
f4: Func<T, A>
): Func<T, R>
/* rest */
export default function compose<R>(
f1: (a: any) => R,
...funcs: Function[]
): (...args: any[]) => R
export default function compose<R>(...funcs: Function[]): (...args: any[]) => R
export default function compose(...funcs: Function[]) {
if (funcs.length === 0) {
// infer the argument type so it is usable in inference down the line
return <T>(arg: T) => arg
}
if (funcs.length === 1) {
return funcs[0]
}
return funcs.reduce(
(a, b) =>
(...args: any) =>
a(b(...args))
)
}