A Hack Styled Operator

map-ala-hack.ts

type Func<A, B> = (input: A) => B;

const _$ = undefined;

//  If f and the input are provided, evalulate and return B[]
function map<A, B>(input: A[], f: Func<A, B>): B[];
//  If input is not provided, return a function ala partial application
function map<A, B>(input: undefined, f: Func<A, B>): Func<A[], B[]>;

function map<A, B>(input: A[] | undefined, f: Func<A, B>) {
    if (input == undefined) {
        return (input: A[]) => mapImpl(f, input);
    }
    return mapImpl(f, input);
}

function mapImpl<A, B>(f: Func<A, B>, input: A[]) {
    const result: B[] = [];
    for (const item of input) {
        result.push(f(item));
    }
    return result;
}

// Use the _$ placeholder to return a function instead of an array
const doubleArray = map(_$, (x: number) => 2 * x);
// Passing an array returns an array 
const aDoubledArray = map([1, 2, 3, 4], (x: number) => 2 * x);