OCaml Functors - small examples

func.ml

==> func0.ml <==

module Debug = struct
  let debug msg = Printf.eprintf "debug: %s\n" msg
  let error msg = Printf.eprintf "error: %s\n" msg
end


let main () =
  let argv = Array.to_list Sys.argv in
  let args = List.tl argv in
  ( List.iter print_endline args
  ; Debug.debug "all done"
  ; exit 0
  )

let () = if !Sys.interactive then () else main ()




==> func1.ml <==

module Debug = struct
  let debug prg msg = Printf.eprintf "%s debug: %s\n" prg msg
  let error prg msg = Printf.eprintf "%s error: %s\n" prg msg
end


let main () =
  let argv = Array.to_list Sys.argv in
  let this = List.hd argv in
  let args = List.tl argv in
  ( List.iter print_endline args
  ; Debug.debug this "all done"
  ; exit 0
  )

let () = if !Sys.interactive then () else main ()




==> func2.ml <==

module type PRG = sig
  val name: string
end

module Debug (Prg : PRG) = struct
  let debug msg = Printf.eprintf "%s debug: %s\n" Prg.name msg
  let error msg = Printf.eprintf "%s error: %s\n" Prg.name msg
end

module X = struct
  let name = "hello"
end

module D  = Debug(X)
module D' = Debug(struct let name = "hello" end)

let main () =
  let argv = Array.to_list Sys.argv in
  let args = List.tl argv in
  ( List.iter print_endline args
  ; D.debug "all done"
  ; exit 0
  )

let () = if !Sys.interactive then () else main ()




==> func3.ml <==

module type ELEMENT = sig
  type t
  val compare: t -> t -> int
end

module MySet (Element: ELEMENT ) = struct
  type set = Element.t list

  let add x set = x::set
  let mem x set = List.exists (fun y -> Element.compare x y = 0) set
end

(* /usr/lib/ocaml/string.mli:
  
type t = string
(** An alias for the type of strings. *)

val compare: t -> t -> int
(** The comparison function for strings, with the same specification as
    {!Pervasives.compare}.  Along with the type [t], this function [compare]
    allows the module [String] to be passed as argument to the functors
    {!Set.Make} and {!Map.Make}. *)
*)

module StringSet = MySet(String)