applicative_maybe.ex

defmodule ApplicativeMaybe do
  @type t(a) :: {:just, a} | :nothing

  @spec pure(val :: a) :: t(a) when a: var
  def pure(val), do: {:just, val}
  
  @spec fmap(f :: (a -> b), ma :: t(a)) :: t(b) when a: var, b: var
  def fmap(f, {:just, val}), do: {:just, f.(val)}
  def fmap(_, :nothing    ), do: :nothing
  
  @spec ap(mf :: t((a -> b)), ma :: t(a)) :: t(b) when a: var, b: var
  def ap({:just, f}, {:just, val}), do: {:just, f.(val)}
  def ap(_         , _           ), do: :nothing
end

alias ApplicativeMaybe, as: M

f = fn a -> fn b -> fn c -> fn d -> %{ab: a + b, cd: c <> d} end end end end

f |> M.fmap(M.pure(2)) |> M.ap(M.pure(3)) |> M.ap(M.pure("foo")) |> M.ap(M.pure("bar"))
# => {:just, %{ab: 5, cd: "foobar"}}

f |> M.fmap(M.pure(2)) |> M.ap(M.pure(3)) |> M.ap(:nothing) |> M.ap(M.pure("bar"))
# => :nothing