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|
----------------------------------------------------------------------
-- |
-- Module : BacktrackM
-- Maintainer : PL
-- Stability : (stable)
-- Portability : (portable)
--
-- > CVS $Date: 2005/04/21 16:22:00 $
-- > CVS $Author: bringert $
-- > CVS $Revision: 1.4 $
--
-- Backtracking state monad, with r\/o environment
-----------------------------------------------------------------------------
{-# OPTIONS_GHC -fglasgow-exts #-}
module GF.Data.BacktrackM ( -- * the backtracking state monad
BacktrackM,
-- * controlling the monad
failure,
(|||),
-- * handling the state & environment
readState,
writeState,
-- * monad specific utilities
member,
-- * running the monad
foldBM, runBM,
foldSolutions, solutions,
foldFinalStates, finalStates
) where
import Data.List
import Control.Monad
----------------------------------------------------------------------
-- Combining endomorphisms and continuations
-- a la Ralf Hinze
-- BacktrackM = state monad transformer over the backtracking monad
newtype BacktrackM s a = BM (forall b . (a -> s -> b -> b) -> s -> b -> b)
-- * running the monad
runBM :: BacktrackM s a -> s -> [(s,a)]
runBM (BM m) s = m (\x s xs -> (s,x) : xs) s []
foldBM :: (a -> s -> b -> b) -> b -> BacktrackM s a -> s -> b
foldBM f b (BM m) s = m f s b
foldSolutions :: (a -> b -> b) -> b -> BacktrackM s a -> s -> b
foldSolutions f b (BM m) s = m (\x s b -> f x b) s b
solutions :: BacktrackM s a -> s -> [a]
solutions = foldSolutions (:) []
foldFinalStates :: (s -> b -> b) -> b -> BacktrackM s () -> s -> b
foldFinalStates f b (BM m) s = m (\x s b -> f s b) s b
finalStates :: BacktrackM s () -> s -> [s]
finalStates bm = map fst . runBM bm
-- * handling the state & environment
readState :: BacktrackM s s
readState = BM (\c s b -> c s s b)
writeState :: s -> BacktrackM s ()
writeState s = BM (\c _ b -> c () s b)
instance Monad (BacktrackM s) where
return a = BM (\c s b -> c a s b)
BM m >>= k = BM (\c s b -> m (\a s b -> unBM (k a) c s b) s b)
where unBM (BM m) = m
fail _ = failure
-- * controlling the monad
failure :: BacktrackM s a
failure = BM (\c s b -> b)
(|||) :: BacktrackM s a -> BacktrackM s a -> BacktrackM s a
(BM f) ||| (BM g) = BM (\c s b -> g c s $! f c s b)
instance MonadPlus (BacktrackM s) where
mzero = failure
mplus = (|||)
-- * specific functions on the backtracking monad
member :: [a] -> BacktrackM s a
member xs = BM (\c s b -> foldl' (\b x -> c x s b) b xs)
|
