what.js

interface Functor (F) {
  map: (a => b) => F(a) => F(b)
}

interface Applicative(M) > Monad (M) {
  andThen: M(a) => (a => M(b)) => M(b)
}

type Maybe (a) {
  data Nothing
  data Just(a)

  implements Functor {
    map: match {
      (fn, Nothing()) => Nothing
      (fn, Just(x)) => Just(fn(x))
    }
  }
  
  implements Monad {
    andThen: match {
      (Just(x), k) => k(x)
      (Nothing(), x) => Nothing()
    }
  }

}

const j = Just(1).map(add1)