Chris@19: (*
Chris@19:  * Copyright (c) 1997-1999 Massachusetts Institute of Technology
Chris@19:  * Copyright (c) 2003, 2007-14 Matteo Frigo
Chris@19:  * Copyright (c) 2003, 2007-14 Massachusetts Institute of Technology
Chris@19:  *
Chris@19:  * This program is free software; you can redistribute it and/or modify
Chris@19:  * it under the terms of the GNU General Public License as published by
Chris@19:  * the Free Software Foundation; either version 2 of the License, or
Chris@19:  * (at your option) any later version.
Chris@19:  *
Chris@19:  * This program is distributed in the hope that it will be useful,
Chris@19:  * but WITHOUT ANY WARRANTY; without even the implied warranty of
Chris@19:  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
Chris@19:  * GNU General Public License for more details.
Chris@19:  *
Chris@19:  * You should have received a copy of the GNU General Public License
Chris@19:  * along with this program; if not, write to the Free Software
Chris@19:  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
Chris@19:  *
Chris@19:  *)
Chris@19: 
Chris@19: (*************************************************************
Chris@19:  *   Monads
Chris@19:  *************************************************************)
Chris@19: 
Chris@19: (*
Chris@19:  * Phil Wadler has many well written papers about monads.  See
Chris@19:  * http://cm.bell-labs.com/cm/cs/who/wadler/ 
Chris@19:  *)
Chris@19: (* vanilla state monad *)
Chris@19: module StateMonad = struct
Chris@19:   let returnM x = fun s -> (x, s)
Chris@19: 
Chris@19:   let (>>=) = fun m k -> 
Chris@19:     fun s ->
Chris@19:       let (a', s') = m s
Chris@19:       in let (a'', s'') = k a' s'
Chris@19:       in (a'', s'')
Chris@19: 
Chris@19:   let (>>) = fun m k ->
Chris@19:     m >>= fun _ -> k
Chris@19: 
Chris@19:   let rec mapM f = function
Chris@19:       [] -> returnM []
Chris@19:     | a :: b ->
Chris@19: 	f a >>= fun a' ->
Chris@19: 	  mapM f b >>= fun b' ->
Chris@19: 	    returnM (a' :: b')
Chris@19: 
Chris@19:   let runM m x initial_state =
Chris@19:     let (a, _) = m x initial_state
Chris@19:     in a
Chris@19: 
Chris@19:   let fetchState =
Chris@19:     fun s -> s, s
Chris@19: 
Chris@19:   let storeState newState =
Chris@19:     fun _ -> (), newState
Chris@19: end
Chris@19: 
Chris@19: (* monad with built-in memoizing capabilities *)
Chris@19: module MemoMonad =
Chris@19:   struct
Chris@19:     open StateMonad
Chris@19: 
Chris@19:     let memoizing lookupM insertM f k =
Chris@19:       lookupM k >>= fun vMaybe ->
Chris@19: 	match vMaybe with
Chris@19: 	  Some value -> returnM value
Chris@19: 	| None ->
Chris@19: 	    f k >>= fun value ->
Chris@19: 	      insertM k value >> returnM value
Chris@19: 
Chris@19:     let runM initial_state m x  = StateMonad.runM m x initial_state
Chris@19: end