A Monad class in Flow

Monad.js

export default class Monad<V> {

  value: V;

  static of = <T>(value: T): Monad<T> => new Monad(value);

  constructor(value: V) {
    this.value = value;
  }

  get isNothing(): boolean {
    return !this.value;
  }

  map<T>(fn: (V) => T): Monad<T | null> {
    return this.isNothing
      ? Monad.of(null)
      : Monad.of(fn(this.value));
  }

  join(): V | Monad<null> {
    return this.isNothing
      ? Monad.of(null)
      : this.value;
  }

  chain<T>(fn: (V) => T): T | Monad<null> | null {
    return this.map(fn).join();
  }
}

MonadWithRamda.js

import R from 'ramda';

const nilOrEmpty = R.either(R.isNil, R.isEmpty);

export default class Monad<V> {

  value: V;

  static of = <T>(value: T): Monad<T> => new Monad(value);

  constructor(value: V) {
    this.value = value;
  }

  get isNothing(): boolean {
    return nilOrEmpty(this.value);
  }

  map<T>(fn: (V) => T): Monad<T | null> {
    return this.isNothing
      ? Monad.of(null)
      : Monad.of(fn(this.value));
  }

  join(): V | Monad<null> {
    return this.isNothing
      ? Monad.of(null)
      : this.value;
  }

  chain<T>(fn: (V) => T): T | Monad<null> | null {
    return this.map(fn).join();
  }
}