(*
 * Copyright (c) 1997-1999 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
 *
 *)

(* monad with built-in memoizing capabilities *)
open StateMonad

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

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

