Either in TypeScript

Either.ts

module Functional {
    export class Either<L, R> {
        constructor(public apply: <TResult>(funcL: (p: L) => TResult, funcR: (p: R) => TResult) => TResult) { }

        static left<L, R>(obj: L): Either<L, R> {
            return new Either<L, R>(<TResult>(funcL: (p: L) => TResult, funcR: (p: R) => TResult) => funcL(obj));
        }

        static right<L, R>(obj: R): Either<L, R> {
            return new Either<L, R>(<TResult>(funcL: (p: L) => TResult, funcR: (p: R) => TResult) => funcR(obj));
        }
    }
}

usage.ts

var either = Functional.Either.left<number[], string>([2, 3, 4]);

// gets sum for a array of number or length for a string
// 数列なら合計を、文字列なら長さを返す
var result = either.apply(
    nums => nums.reduce((a,b) => a+b),
    str => str.length
    );

// prints enumeration for a array of number or a string
// 数列なら列挙表示、文字列ならそのまま表示
either.apply(
    nums => alert(nums.join(",")),
    str => alert(str)
    );