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(*
* 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
|
