either-monad.js

const Right = value => ({
  map: f => Right(f(value)), // Either[E,A].map(fn: A => B): Either[E,B]
  bind(f) { return this.map(f).join() }, // Either[E,A].bind(fn: A => Either[E,B]): Either[E,B]
  join: () => value,
  isRight: () => true,
  isLeft: () => false,
  cata: (l, r) => () => r(value)
})

const Left = value => ({
  map: f => Left(value),
  bind: f => Left(value),
  join: () => Left(value),
  isRight: () => false,
  isLeft: () => true,
  cata: (l, r) => () => l(value)
})

// Either[Left,Right]
const Either = val => Right(val)
Either.of = val => Either(val)