TypeScript Maybe<T> monad

maybe_monad.ts

type Maybe<T> = {
    map: <T2>(func: (t: T) => T2) => Maybe<T2>,
    then: <T2>(func: (t: T) => Maybe<T2>) => Maybe<T2>,
    value: (fallback: T) => T
};
const maybe = function <T>(a: T | undefined | null): Maybe<T> {
    const value: [T] | [] = (a !== null && a !== void 0) ? [a] : [];
    return {
        map: function <T2>(func: (t: T) => T2): Maybe<T2> {
            return value.length
                ? maybe(func(value[0]))
                : maybe<T2>(null)
        },
        then: function <T2>(func: (t: T) => Maybe<T2>): Maybe<T2> {
            return value.length
                ? func(value[0])
                : maybe<T2>(null)
        },
        value: function (fallback: T): T {
            return value.length ? value[0] : fallback;
        }
    };
};


const v = 5;
const m = maybe<number>(v);
console.log(m.map(x => x * 2).map(x => `--> ${x} <<--`).value('no value'))