Javascript Monads

option.js

class Option {
  constructor(some, value) {
    this.some = some
    this.value = value
  }

  map(next) {
    return this.some ? next(this.value) : this
  }

  toString() {
    return this.some ? `Some(${this.value})` : 'None'
  }
}

function from(value) {
  return new Option(true, value)
}

const NONE = new Option(false)

console.log(
  from(15)
    .map(i => i % 2 === 0 ? from(i / 2) : NONE)
    .map(i => i % 3 === 0 ? from(i / 3) : NONE)
    .toString(),
  from(16)
    .map(i => i % 2 === 0 ? from(i / 2) : NONE)
    .map(i => i % 3 === 0 ? from(i / 3) : NONE)
    .toString(),
  from(17)
    .map(i => i % 2 === 0 ? from(i / 2) : NONE)
    .map(i => i % 3 === 0 ? from(i / 3) : NONE)
    .toString(),
  from(18)
    .map(i => i % 2 === 0 ? from(i / 2) : NONE)
    .map(i => i % 3 === 0 ? from(i / 3) : NONE)
    .toString()
)

reader.js

class Reader {
  constructor(get) {
    this.get = get
  }

  map(next) {
    return from(config => next(this.get(config)).get(config))
  }
}

function from(get) {
  return new Reader(get)
}

function cased(get) {
  return from(get).map(msg => from(({ upper }) => upper ? msg.toUpperCase() : msg))
}

console.log(
  cased(({ name }) => `Hello ${name}.`).map(greeting =>
    cased(({ pet }) => `Don't forget to feed ${pet}.`).map(reminder =>
      cased(({ name }) => `Have a nice day, ${name}!`).map(farewell =>
        from(() => `${greeting} ${reminder} ${farewell}`)
      )
    )
  )
    .get({
      upper: false,
      name: "Ken",
      pet: "the dog"
    })
)