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