functs either

type Either<E, A> = Left<E> | Right<A>;
interface Left<E> {
    _tag: 'Left',
    value: E
}
interface Right<A> {
    _tag: 'Right',
    value: A
}

const left = <E, A=never>(e: E): Either<E, A> => ({
    _tag: 'Left',
    value: e
})

const right = <A, E=never>(a: A): Either<E, A> => ({
    _tag: 'Right',
    value: a
})

function div2ifeven(num: number): Either<string, number> {
    if (num === 0) {
        return left('/0')
    }
    if (num & 1) {
        return left('odd nr')
    }
    return right(2 / num)
}

console.log(div2ifeven(2))
console.log(div2ifeven(0))
console.log(div2ifeven(1))