An example of the coproduct type in TypeScript

gistfile1.ts

interface Sum<A,B>
{
    elim<C>(left: (value: A) => C, right: (value: B) => C): C;
}
function Left<A>(value: A): {elim<C>(left:(value:A)=>C,right:(value:any)=>C):C;}
{
    return {
        elim: <C>(left: (value: A) => C, right: (value: any) => C) => left(value)
    };
}
function Right<B>(value: B): {elim<C>(left:(value:any)=>C,right:(value:B)=>C):C;}
{
    return {
        elim: <C>(left: (value: any) => C, right: (value: B) => C) => right(value)
    };
}


// An example

function modulo2(x: number): Sum<number, number>
{
    if (x % 2 === 0) {
        return Left(x/2);
    } else {
        return Right((x-1)/2);
    }
}

var a = modulo2(42);
console.log(a.elim(
    x => "a is 2*" + x.toString(),
    y => "a is 2*" + y.toString() + "+1"
));


// Another example

String.prototype.repeat = String.prototype.repeat || function (n) { return (n=n>>>0)===0 ? "" : this + this.repeat(n-1); };

function helloNumber(a: number): string
{
    return "Hello world" + "!".repeat(a);
}
function helloString(a: string): string
{
    return "Hello " + a + "-san!";
}
function hello(a: Sum<number, string>): string
{
    return a.elim(helloNumber, helloString);
}

console.log(hello(Left(7)));
console.log(hello(Right("Fujikido Kenji")));