cannam@127: (*
cannam@127:  * Copyright (c) 1997-1999 Massachusetts Institute of Technology
cannam@127:  * Copyright (c) 2003, 2007-14 Matteo Frigo
cannam@127:  * Copyright (c) 2003, 2007-14 Massachusetts Institute of Technology
cannam@127:  *
cannam@127:  * This program is free software; you can redistribute it and/or modify
cannam@127:  * it under the terms of the GNU General Public License as published by
cannam@127:  * the Free Software Foundation; either version 2 of the License, or
cannam@127:  * (at your option) any later version.
cannam@127:  *
cannam@127:  * This program is distributed in the hope that it will be useful,
cannam@127:  * but WITHOUT ANY WARRANTY; without even the implied warranty of
cannam@127:  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
cannam@127:  * GNU General Public License for more details.
cannam@127:  *
cannam@127:  * You should have received a copy of the GNU General Public License
cannam@127:  * along with this program; if not, write to the Free Software
cannam@127:  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
cannam@127:  *
cannam@127:  *)
cannam@127: 
cannam@127: (*************************************************************
cannam@127:  *   Monads
cannam@127:  *************************************************************)
cannam@127: 
cannam@127: (*
cannam@127:  * Phil Wadler has many well written papers about monads.  See
cannam@127:  * http://cm.bell-labs.com/cm/cs/who/wadler/ 
cannam@127:  *)
cannam@127: (* vanilla state monad *)
cannam@127: module StateMonad = struct
cannam@127:   let returnM x = fun s -> (x, s)
cannam@127: 
cannam@127:   let (>>=) = fun m k -> 
cannam@127:     fun s ->
cannam@127:       let (a', s') = m s
cannam@127:       in let (a'', s'') = k a' s'
cannam@127:       in (a'', s'')
cannam@127: 
cannam@127:   let (>>) = fun m k ->
cannam@127:     m >>= fun _ -> k
cannam@127: 
cannam@127:   let rec mapM f = function
cannam@127:       [] -> returnM []
cannam@127:     | a :: b ->
cannam@127: 	f a >>= fun a' ->
cannam@127: 	  mapM f b >>= fun b' ->
cannam@127: 	    returnM (a' :: b')
cannam@127: 
cannam@127:   let runM m x initial_state =
cannam@127:     let (a, _) = m x initial_state
cannam@127:     in a
cannam@127: 
cannam@127:   let fetchState =
cannam@127:     fun s -> s, s
cannam@127: 
cannam@127:   let storeState newState =
cannam@127:     fun _ -> (), newState
cannam@127: end
cannam@127: 
cannam@127: (* monad with built-in memoizing capabilities *)
cannam@127: module MemoMonad =
cannam@127:   struct
cannam@127:     open StateMonad
cannam@127: 
cannam@127:     let memoizing lookupM insertM f k =
cannam@127:       lookupM k >>= fun vMaybe ->
cannam@127: 	match vMaybe with
cannam@127: 	  Some value -> returnM value
cannam@127: 	| None ->
cannam@127: 	    f k >>= fun value ->
cannam@127: 	      insertM k value >> returnM value
cannam@127: 
cannam@127:     let runM initial_state m x  = StateMonad.runM m x initial_state
cannam@127: end