Either.ts

interface Either<V> {
    map: <T>(f: (V) => T) => Either<T>
    fold: <L,R>(l: (V) => L, r: (V) => R) => L | R
}

function left<V>(x: V): Either<V> {
    return {
        map: _ => left(x),
        fold: (l, r) => l(x)
    }
}

function right<V>(x: V): Either<V> {
    return {
        map: <T>(f: (V) => T)  => right<T>(f(x)),
        fold: (l, r) => r(x)
    }
}

let rightFour: Either<Number> = right(4)
let rightFive = rightFour.map(x => x + 1)
let five = rightFive.fold(l => l, r => r)

let result = document.createElement('h1');
result.textContent = five.toString();


document.body.appendChild(result);