0c27b3fe77ac1d5094ba3521e8142d9e7973133fMark Andrews{- |

ca44fe49bec16436cd95ace0af2e244f2096b284Brian WellingtonModule : $Header$

0c27b3fe77ac1d5094ba3521e8142d9e7973133fMark AndrewsDescription : State type from Control.Monad.State but no class MonadState

0c27b3fe77ac1d5094ba3521e8142d9e7973133fMark AndrewsCopyright : C. Maeder and Uni Bremen 2002-2005

0c27b3fe77ac1d5094ba3521e8142d9e7973133fMark AndrewsLicense : similar to LGPL, see HetCATS/LICENSE.txt or LIZENZ.txt

ca44fe49bec16436cd95ace0af2e244f2096b284Brian Wellington

70e5a7403f0e0a3bd292b8287c5fed5772c15270Automatic UpdaterMaintainer : Christian.Maeder@dfki.de

ca44fe49bec16436cd95ace0af2e244f2096b284Brian WellingtonStability : experimental

ca44fe49bec16436cd95ace0af2e244f2096b284Brian WellingtonPortability : portable

ca44fe49bec16436cd95ace0af2e244f2096b284Brian Wellington

ca44fe49bec16436cd95ace0af2e244f2096b284Brian WellingtonState type from Control.Monad.State but no class MonadState

ca44fe49bec16436cd95ace0af2e244f2096b284Brian Wellington

ca44fe49bec16436cd95ace0af2e244f2096b284Brian WellingtonThis module may be replaced by the (non-nhc98 module) Control.Monad.State

ca44fe49bec16436cd95ace0af2e244f2096b284Brian Wellington

ca44fe49bec16436cd95ace0af2e244f2096b284Brian Wellington-}

ca44fe49bec16436cd95ace0af2e244f2096b284Brian Wellington

ca44fe49bec16436cd95ace0af2e244f2096b284Brian Wellington

ca44fe49bec16436cd95ace0af2e244f2096b284Brian Wellingtonmodule Common.Lib.State where

ca44fe49bec16436cd95ace0af2e244f2096b284Brian Wellington

ca44fe49bec16436cd95ace0af2e244f2096b284Brian Wellington-- ---------------------------------------------------------------------------

ca44fe49bec16436cd95ace0af2e244f2096b284Brian Wellington-- Our fixed state monad

ca44fe49bec16436cd95ace0af2e244f2096b284Brian Wellingtonnewtype State s a = State { runState :: s -> (a, s) }

ca44fe49bec16436cd95ace0af2e244f2096b284Brian Wellington

ca44fe49bec16436cd95ace0af2e244f2096b284Brian Wellingtoninstance Functor (State s) where

ca44fe49bec16436cd95ace0af2e244f2096b284Brian Wellington fmap f m = State $ \s -> let

ca44fe49bec16436cd95ace0af2e244f2096b284Brian Wellington (a, s') = runState m s

ca44fe49bec16436cd95ace0af2e244f2096b284Brian Wellington in (f a, s')

ca44fe49bec16436cd95ace0af2e244f2096b284Brian Wellington

instance Monad (State s) where

return a = State $ \s -> (a, s)

m >>= k = State $ \s -> let

(a, s') = runState m s

in runState (k a) s'

-- put and get are non-overloaded here!

get :: State s s

get = State $ \s -> (s, s)

put :: s -> State s ()

put s = State $ \_ -> ((), s)

modify :: (s -> s) -> State s ()

modify f = get >>= put . f

gets :: (s -> a) -> State s a

gets f = fmap f get

evalState :: State s a -> s -> a

evalState m = fst . runState m

execState :: State s a -> s -> s

execState m = snd . runState m

mapState :: ((a, s) -> (b, s)) -> State s a -> State s b

mapState f m = State $ f . runState m

withState :: (s -> s) -> State s a -> State s a

withState f m = State $ runState m . f