class Identity {

  constructor(x) {

    this.run = () => x

    // this :: Identity x

    // (x -> y) -> Identity y

    this.map = f => new Identity(f(this.run()))

    // this :: Identity x

    // (x -> Identity y) -> Identity y

    this.bind = f => f(this.run())

    // this :: Identity x

    // (x -> (y || Identity y)) -> Identity y

    this.then = f => {

      const y = f(this.run())

      return y instanceof Identity ? y : new Identity(y)

    }

  }

}

​

Identity.pure = x => new Identity(x)

​

​

const result = Identity.pure(10)

  .then(x => x + 1) // map

  .then(x => new Identity(x * 2)) // bind

.run()

​

console.log(result)