(* https://cs3110.github.io/textbook/chapters/ds/monads.html *)

module type Monad = sig
  type 'a t
  val return: 'a -> 'a t
  val (>>=): 'a t -> ('a -> 'b t) -> 'b t
end

module Maybe : Monad = struct
  type 'a t = 'a option
  let return a = Some a
  let (>>=) a f =
    match a with
    | None -> None
    | Some a' -> f a'
end