> {-# LANGUAGE > MultiParamTypeClasses, > FlexibleInstances, > RankNTypes > #-} > > module CFLP.Control.Monad.Update ( > > MonadUpdate(..), Update(..), UpdateT > > ) where > > import Control.Monad.State > import Control.Monad.TransA monad that supports non-deterministic state updates is an instance of the class `MonadUpdate` that defines an operation to incorporate a monadic update-action into monadic computations.

> class MonadUpdate s m > where > update :: (forall m' . MonadPlus m' => s -> m' s) -> m ()An instance of `MonadPlus` that threads a state can update that state non-deterministically.

> instance MonadPlus m => MonadUpdate s (StateT s m) > where > update upd = get >>= upd >>= putWe also define an interface for monads that perform associated updates in a state that is threaded through a (possibly, but not necessarily different) monad. We use the state monad transformer `StateT` to thread the constraint store through the monad that returns the results.

> class Update s m m' > where > updateState :: m a -> StateT s m' aAgain, a state threading monad gives rise to a natural instance, where results are returned in the base monad.

> instance MonadPlus m => Update s (StateT s m) m > where > updateState = idState monads are a natural choice for a monad that updates state, but they have a drawback: monadic values are functions that are reexecuted for each shared occurrence of a monadic sub computation. Transforming Monads ------------------- We define a monad transformer `UpdateT` that adds the capability of non-deterministic state updates to arbitrary instances of `MonadPlus`. Monadic actions in the resulting monads are data terms if monadic actions are data terms in the base monad. As a consequence, they are evaluated only once if they are shared. The updating monadic action must be polymorphic in the used monad `m'`.

> newtype UpdateT s m a = UpdateT { unUpdateT :: m (WithUpdate s m a) } > data WithUpdate s m a > = Return a > | Update (forall m' . MonadPlus m' => s -> m' s) (UpdateT s m a)A transformed instance of `MonadPlus` is an instance of `MonadUpdate`.

> instance MonadPlus m => MonadUpdate s (UpdateT s m) > where > update upd = UpdateT (return (Update upd (return ())))It is also an instance of `Update` where results are returned in the base monad. In order to perform stored updates, we thread a state through the monadic computation.

> instance MonadPlus m => Update s (UpdateT s m) m > where > updateState = run > where > run :: MonadPlus m => UpdateT s m a -> StateT s m a > run x = lift (unUpdateT x) >>= doUpdate > > doUpdate (Return a) = return a > doUpdate (Update upd y) = do update upd; run yWe define another instance of `Update` where results are not returned in the base monad but in the transformed base monad. This instance is useful to support computations that may or may not consider the threaded store. All upcate actions are kept in the monadic values and threaded additionally.

> instance MonadPlus m => Update s (UpdateT s m) (UpdateT s m) > where > updateState = run > where > run :: MonadPlus m => UpdateT s m a -> StateT s (UpdateT s m) a > run x = lift (lift (unUpdateT x)) >>= doUpdate > > doUpdate (Return a) = return a > doUpdate (Update upd y) = do update upd; lift (update upd); run yWe specify that a transformed monad is indeed a monad, that it is an instance of `MonadPlus` if the base monad is, and that, `UpdateT` (with an arbitrary store `s`) is a monad transformer.

> instance Monad m => Monad (UpdateT s m) > where > return = UpdateT . return . Return > > x >>= f = UpdateT (unUpdateT x >>= g) > where g (Return a) = unUpdateT (f a) > g (Update upd y) = return (Update upd (y >>= f)) > > instance MonadPlus m => MonadPlus (UpdateT s m) > where > mzero = UpdateT mzero > x `mplus` y = UpdateT (unUpdateT x `mplus` unUpdateT y) > > instance MonadTrans (UpdateT s) > where > lift = UpdateT . liftM ReturnTo simplify debugging, we define `Show` instances for transformed list monads.

> instance Show a => Show (UpdateT s [] a) > where > show (UpdateT x) = show x > > instance Show a => Show (WithUpdate cs [] a) > where > show (Return x) = "(Return "++show x++")" > show (Update _ (UpdateT x)) = "(Update _ "++show x++")"