(*
 * Copyright (c) 1997-1999 Massachusetts Institute of Technology
 * Copyright (c) 2003, 2006 Matteo Frigo
 * Copyright (c) 2003, 2006 Massachusetts Institute of Technology
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 *
 *)
(* $Id: monads.ml,v 1.3 2006-01-05 03:04:27 stevenj Exp $ *)

(*************************************************************
 *   Monads
 *************************************************************)

(*
 * Phil Wadler has many well written papers about monads.  See
 * http://cm.bell-labs.com/cm/cs/who/wadler/ 
 *)
(* vanilla state monad *)
module StateMonad = struct
  let returnM x = fun s -> (x, s)

  let (>>=) = fun m k -> 
    fun s ->
      let (a', s') = m s
      in let (a'', s'') = k a' s'
      in (a'', s'')

  let (>>) = fun m k ->
    m >>= fun _ -> k

  let rec mapM f = function
      [] -> returnM []
    | a :: b ->
	f a >>= fun a' ->
	  mapM f b >>= fun b' ->
	    returnM (a' :: b')

  let runM m x initial_state =
    let (a, _) = m x initial_state
    in a

  let fetchState =
    fun s -> s, s

  let storeState newState =
    fun _ -> (), newState
end

(* monad with built-in memoizing capabilities *)
module MemoMonad =
  struct
    open StateMonad

    let memoizing lookupM insertM f k =
      lookupM k >>= fun vMaybe ->
	match vMaybe with
	  Some value -> returnM value
	| None ->
	    f k >>= fun value ->
	      insertM k value >> returnM value

    let runM initial_state m x  = StateMonad.runM m x initial_state
end