relearn.res

// we write Maybe module (stolen from haskell)
module Maybe = {
  type t<'a> =
    | Just('a)
    | Nothing

  let fmap = (x, fn) =>
    switch x {
    | Just(x) => Just(x->fn)
    | Nothing => Nothing
    }

  let apply = (x, fn) =>
    switch x {
    | Nothing => Nothing
    | Just(x) =>
      switch fn {
      | Nothing => Nothing
      | Just(fn) => Just(x->fn)
      }
    }

  let bind = (x, fn) =>
    switch x {
    | Just(x) => x->fn
    | Nothing => Nothing
    }
}

let functor = Maybe.fmap(Just(5), x => x + 5) // Just(10)
let applicative = Maybe.apply(Just(2), Just(x => x + 3)) // Just(5)
let bind = Maybe.bind(Just(6), x => Just(x + 3)) // Just(9)

let apply = (x, fn) =>
    switch (x, fn) {
      | (Nothing, _) 
      | (_, Nothing) => Nothing
      | (Just(x), Just(fn)) => Just(x->fn)
    }