Maybe.js

class Maybe {
  constructor(isJust, value) {
    this.isJust = isJust
    if (isJust) {
      this.value = value
    }
  }

  static just(value) {
    return new Maybe(true, value)
  }

  static nothing = new Maybe(false)

  map(fun) {
    return this.isJust ? new Maybe(true, fun(this.value)) : this
  }

  static pure = Maybe.just

  ap(mx) {
    return this.isJust ? mx.map(this.value) : this
  }

  bind(fun) {
    return this.isJust ? fun(this.value) : this
  }

  static do(generatorFunction) {
    const generator = generatorFunction()
    const doImpl = (unwrappedValue) => {
      const { done, value } = generator.next(unwrappedValue)
      return done ? value : value.bind(doImpl)
    }
    return doImpl(undefined)
  }
}