fantasy-do

pseudo.js

const Task = require('data.task')

// Using the Task Monad as an example, but any Monad would work.
const getAnswer = () => Task.of(42)
const even = n => Task.of((n % 2) === 0)

const result = fy(function * () {
  // Yield monads to get back their values.
  const answer = yield getAnswer()
  const halfAnswer = answer / 2
  // The returned value is passed out.
  return even(halfAnswer)
})

result.fork(
  e => console.log('ERROR:', e),
  x => console.log(x)
)

function fy (generatorFunction) {
  const generator = generatorFunction()
  const step = (nextValue) => {
    const { value, done } = generator.next(nextValue)
    if (done) {
      return value
    } else {
      return value.chain(step)
    }
  }
  return step()
}