Do syntax for js

Do.js

// nested chaining is a common occurence for monads:

// type Maybe = Just(a) | Nothing

Just(5)
  .chain(num => Just(num * 2)
    .chain(num2 => Just(num2 + 3)
       .map(num3 => ({num, num2, num3}))
    )
  )

// this yields:
// Just({
//   num: 5,
//   num2: 10,
//   num3: 13
// })


// To avoid nesting, we can implement a function Do, which takes
// instances of Monads, and an arbitrary list of functions with
// increasing parameter count, and chains them automatically:

Do(
  Just(5),
  (num) => Just(num * 2),
  (num, num2) => Just(num2 + 3),
  (num, num2, num3) => ({num, num2, num3})
)

// which yields:
// Just({
//   num: 5,
//   num2: 10,
//   num3: 13
// })