inrick

(*
  Example how OCaml's structurally typed modules can be combined and used. Note
  the overlap of [return] in [APPLICATIVE] and [MONAD] and how the signatures
  combine.
*)

let id x = x

module type FUNCTOR = sig
  type 'a t

inrick

(** Modules sharing type constraint in functor. *)

module type MONOID = sig
  type t
  val op : t -> t -> t
  val id : t
end

module type GROUP = sig
  include MONOID

inrick

(* for OCaml <4.03.0 define:
  type ('a, 'b) result = Ok of 'a | Error of 'b *)

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

module Id = struct