{-# LANGUAGE CPP #-} {-# LANGUAGE UndecidableInstances #-} -- Search for UndecidableInstances to see why this is needed {- | Module : Control.Monad.Reader.Class Copyright : (c) Andy Gill 2001, (c) Oregon Graduate Institute of Science and Technology 2001, (c) Jeff Newbern 2003-2007, (c) Andriy Palamarchuk 2007 License : BSD-style (see the file LICENSE) Maintainer : libraries@haskell.org Stability : experimental Portability : non-portable (multi-param classes, functional dependencies) [Computation type:] Computations which read values from a shared environment. [Binding strategy:] Monad values are functions from the environment to a value. The bound function is applied to the bound value, and both have access to the shared environment. [Useful for:] Maintaining variable bindings, or other shared environment. [Zero and plus:] None. [Example type:] @'Reader' [(String,Value)] a@ The 'Reader' monad (also called the Environment monad). Represents a computation, which can read values from a shared environment, pass values from function to function, and execute sub-computations in a modified environment. Using 'Reader' monad for such computations is often clearer and easier than using the 'Control.Monad.State.State' monad. Inspired by the paper /Functional Programming with Overloading and Higher-Order Polymorphism/, Mark P Jones (<http://web.cecs.pdx.edu/~mpj/>) Advanced School of Functional Programming, 1995. -} module Control.Monad.Reader.Class ( MonadReader(..), asks, ) where import Control.Monad.Trans.Cont as Cont import Control.Monad.Trans.Except import Control.Monad.Trans.Error import Control.Monad.Trans.Identity import Control.Monad.Trans.List import Control.Monad.Trans.Maybe import Control.Monad.Trans.Reader (ReaderT) import qualified Control.Monad.Trans.Reader as ReaderT (ask, local, reader) import qualified Control.Monad.Trans.RWS.Lazy as LazyRWS (RWST, ask, local, reader) import qualified Control.Monad.Trans.RWS.Strict as StrictRWS (RWST, ask, local, reader) import Control.Monad.Trans.State.Lazy as Lazy import Control.Monad.Trans.State.Strict as Strict import Control.Monad.Trans.Writer.Lazy as Lazy import Control.Monad.Trans.Writer.Strict as Strict import Control.Monad.Trans.Class (lift) import Control.Monad import Data.Monoid -- ---------------------------------------------------------------------------- -- class MonadReader -- asks for the internal (non-mutable) state. -- | See examples in "Control.Monad.Reader". -- Note, the partially applied function type @(->) r@ is a simple reader monad. -- See the @instance@ declaration below. class Monad m => MonadReader r m | m -> r where #if defined(__GLASGOW_HASKELL__) && __GLASGOW_HASKELL__ >= 707 {-# MINIMAL (ask | reader), local #-} #endif -- | Retrieves the monad environment. ask :: m r ask = reader id -- | Executes a computation in a modified environment. local :: (r -> r) -- ^ The function to modify the environment. -> m a -- ^ @Reader@ to run in the modified environment. -> m a -- | Retrieves a function of the current environment. reader :: (r -> a) -- ^ The selector function to apply to the environment. -> m a reader f = do r <- ask return (f r) -- | Retrieves a function of the current environment. asks :: MonadReader r m => (r -> a) -- ^ The selector function to apply to the environment. -> m a asks = reader -- ---------------------------------------------------------------------------- -- The partially applied function type is a simple reader monad instance MonadReader r ((->) r) where ask = id local f m = m . f reader = id instance Monad m => MonadReader r (ReaderT r m) where ask = ReaderT.ask local = ReaderT.local reader = ReaderT.reader instance (Monad m, Monoid w) => MonadReader r (LazyRWS.RWST r w s m) where ask = LazyRWS.ask local = LazyRWS.local reader = LazyRWS.reader instance (Monad m, Monoid w) => MonadReader r (StrictRWS.RWST r w s m) where ask = StrictRWS.ask local = StrictRWS.local reader = StrictRWS.reader -- --------------------------------------------------------------------------- -- Instances for other mtl transformers -- -- All of these instances need UndecidableInstances, -- because they do not satisfy the coverage condition. instance MonadReader r' m => MonadReader r' (ContT r m) where ask = lift ask local = Cont.liftLocal ask local reader = lift . reader instance (Error e, MonadReader r m) => MonadReader r (ErrorT e m) where ask = lift ask local = mapErrorT . local reader = lift . reader instance MonadReader r m => MonadReader r (ExceptT e m) where ask = lift ask local = mapExceptT . local reader = lift . reader instance MonadReader r m => MonadReader r (IdentityT m) where ask = lift ask local = mapIdentityT . local reader = lift . reader instance MonadReader r m => MonadReader r (ListT m) where ask = lift ask local = mapListT . local reader = lift . reader instance MonadReader r m => MonadReader r (MaybeT m) where ask = lift ask local = mapMaybeT . local reader = lift . reader instance MonadReader r m => MonadReader r (Lazy.StateT s m) where ask = lift ask local = Lazy.mapStateT . local reader = lift . reader instance MonadReader r m => MonadReader r (Strict.StateT s m) where ask = lift ask local = Strict.mapStateT . local reader = lift . reader instance (Monoid w, MonadReader r m) => MonadReader r (Lazy.WriterT w m) where ask = lift ask local = Lazy.mapWriterT . local reader = lift . reader instance (Monoid w, MonadReader r m) => MonadReader r (Strict.WriterT w m) where ask = lift ask local = Strict.mapWriterT . local reader = lift . reader