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fftw-3.3.10/genfft/monads.ml

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+(*
+ * Copyright (c) 1997-1999 Massachusetts Institute of Technology
+ * Copyright (c) 2003, 2007-14 Matteo Frigo
+ * Copyright (c) 2003, 2007-14 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., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ *
+ *)
+
+(*************************************************************
+ *   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