maybe.ex

defmodule Just do
  defstruct value: nil
end

defmodule Nothing do
  defstruct value: nil
end

defmodule Maybe do
  defstruct id: :nothing, value: nil
  import Just
  import Nothing

  def init(value \\ nil)
  def init(value) when value == nil, do: %Nothing{}
  def init(value), do: %Just{value: value}
end

defprotocol Monad do
  def bind(ma, func)
end

defimpl Monad, for: Just do
  def bind(%Just{value: value}, func) do
    %Just{value: func.(value)}
  end
end

defimpl Monad, for: Nothing do
  def bind(_val, _func), do: %Nothing{}
end