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|
-----------------------------------------------------------------------------
-- |
-- Module : Control.Monad.Reader
-- Copyright : (c) Andy Gill 2001,
-- (c) Oregon Graduate Institute of Science and Technology, 2001
-- License : BSD-style (see the file libraries/base/LICENSE)
--
-- Maintainer : libraries@haskell.org
-- Stability : experimental
-- Portability : non-portable (multi-param classes, functional dependencies)
--
-- Declaration of the Monoid class,and instances for list and functions
--
-- Inspired by the paper
-- /Functional Programming with Overloading and
-- Higher-Order Polymorphism/,
-- Mark P Jones (<http://www.cse.ogi.edu/~mpj/>)
-- Advanced School of Functional Programming, 1995.
-----------------------------------------------------------------------------
module Control.Monad.Reader (
MonadReader(..),
asks,
Reader(..),
mapReader,
withReader,
ReaderT(..),
mapReaderT,
withReaderT,
module Control.Monad,
module Control.Monad.Fix,
module Control.Monad.Trans,
) where
import Prelude
import Control.Monad
import Control.Monad.Fix
import Control.Monad.Trans
-- ----------------------------------------------------------------------------
-- class MonadReader
-- asks for the internal (non-mutable) state.
class (Monad m) => MonadReader r m | m -> r where
ask :: m r
local :: (r -> r) -> m a -> m a
-- This allows you to provide a projection function.
asks :: (MonadReader r m) => (r -> a) -> m a
asks f = do
r <- ask
return (f r)
-- ----------------------------------------------------------------------------
-- The partially applied function type is a simple reader monad
instance Functor ((->) r) where
fmap = (.)
instance Monad ((->) r) where
return = const
m >>= k = \r -> k (m r) r
instance MonadFix ((->) r) where
mfix f = \r -> let a = f a r in a
instance MonadReader r ((->) r) where
ask = id
local f m = m . f
-- ---------------------------------------------------------------------------
-- Our parameterizable reader monad
newtype Reader r a = Reader { runReader :: r -> a }
instance Functor (Reader r) where
fmap f m = Reader $ \r -> f (runReader m r)
instance Monad (Reader r) where
return a = Reader $ \_ -> a
m >>= k = Reader $ \r -> runReader (k (runReader m r)) r
instance MonadFix (Reader r) where
mfix f = Reader $ \r -> let a = runReader (f a) r in a
instance MonadReader r (Reader r) where
ask = Reader id
local f m = Reader $ runReader m . f
mapReader :: (a -> b) -> Reader r a -> Reader r b
mapReader f m = Reader $ f . runReader m
-- This is a more general version of local.
withReader :: (r' -> r) -> Reader r a -> Reader r' a
withReader f m = Reader $ runReader m . f
-- ---------------------------------------------------------------------------
-- Our parameterizable reader monad, with an inner monad
newtype ReaderT r m a = ReaderT { runReaderT :: r -> m a }
instance (Monad m) => Functor (ReaderT r m) where
fmap f m = ReaderT $ \r -> do
a <- runReaderT m r
return (f a)
instance (Monad m) => Monad (ReaderT r m) where
return a = ReaderT $ \_ -> return a
m >>= k = ReaderT $ \r -> do
a <- runReaderT m r
runReaderT (k a) r
fail msg = ReaderT $ \_ -> fail msg
instance (MonadPlus m) => MonadPlus (ReaderT r m) where
mzero = ReaderT $ \_ -> mzero
m `mplus` n = ReaderT $ \r -> runReaderT m r `mplus` runReaderT n r
instance (MonadFix m) => MonadFix (ReaderT r m) where
mfix f = ReaderT $ \r -> mfix $ \a -> runReaderT (f a) r
instance (Monad m) => MonadReader r (ReaderT r m) where
ask = ReaderT return
local f m = ReaderT $ \r -> runReaderT m (f r)
instance MonadTrans (ReaderT r) where
lift m = ReaderT $ \_ -> m
instance (MonadIO m) => MonadIO (ReaderT r m) where
liftIO = lift . liftIO
mapReaderT :: (m a -> n b) -> ReaderT w m a -> ReaderT w n b
mapReaderT f m = ReaderT $ f . runReaderT m
withReaderT :: (r' -> r) -> ReaderT r m a -> ReaderT r' m a
withReaderT f m = ReaderT $ runReaderT m . f
|
