flow-eff.js

export default class Eff<EA: Object, A> {
  run: () => A;
  constructor(run: () => A) {
    this.run = run
  }
  map<B>(f: (_: A) => B): Eff<EA, B> {
    return new Eff(() => f(this.run()))
  }
  chain<EB: Object, B>(f: (_: A) => Eff<EB, B>): Eff<EA & EB, B> {
    return Eff.join(this.map(f))
  }
  static join<EA: Object, EB: Object, B>(x: Eff<EB, Eff<EA, B>>): Eff<EA & EB, B> {
    return new Eff(() => x.run().run())
  }
  ...
}