Maybe

Maybe.ts

interface IFunctor<T> {
  map<U>(f: (x: T) => U): IFunctor<U>;
}

export class Maybe<T> implements IFunctor<T> {
  private readonly $value: T;

  static of<U>(x: U) {
    return new Maybe(x);
  }

  constructor(x: T) {
    this.$value = x;
  }

  map<U>(fn: (xx: T | undefined) => U): IFunctor<U | T> {
    return this.isNothing ? this! : Maybe.of(fn(this.$value));
  }

  get isNothing() {
    return this.$value === null || this.$value === undefined;
  }

  toString() {
    return this.isNothing ? "Nothing" : `Just(${this.$value})`;
  }
}