1 -----------------------------------------------------------------------------
3 -- Module : Control.Monad.Reader
4 -- Copyright : (c) Andy Gill 2001,
5 -- (c) Oregon Graduate Institute of Science and Technology, 2001
6 -- License : BSD-style (see the file libraries/core/LICENSE)
8 -- Maintainer : libraries@haskell.org
9 -- Stability : experimental
10 -- Portability : non-portable ( requires mulit-parameter type classes,
11 -- requires functional dependencies )
13 -- Declaration of the Monoid class,and instances for list and functions
15 -- Inspired by the paper
16 -- \em{Functional Programming with Overloading and
17 -- Higher-Order Polymorphism},
18 -- \A[HREF="http://www.cse.ogi.edu/~mpj"]{Mark P Jones},
19 -- Advanced School of Functional Programming, 1995.}
20 -----------------------------------------------------------------------------
22 module Control.Monad.Reader (
34 module Control.Monad.Fix,
35 module Control.Monad.Trans,
41 import Control.Monad.Fix
42 import Control.Monad.Trans
44 -- ----------------------------------------------------------------------------
46 -- asks for the internal (non-mutable) state.
48 class (Monad m) => MonadReader r m | m -> r where
50 local :: (r -> r) -> m a -> m a
52 -- This allows you to provide a projection function.
54 asks :: (MonadReader r m) => (r -> a) -> m a
59 -- ----------------------------------------------------------------------------
60 -- The partially applied function type is a simple reader monad
62 instance Functor ((->) r) where
65 instance Monad ((->) r) where
67 m >>= k = \r -> k (m r) r
69 instance MonadFix ((->) r) where
70 mfix f = \r -> let a = f a r in a
72 instance MonadReader r ((->) r) where
76 -- ---------------------------------------------------------------------------
77 -- Our parameterizable reader monad
79 newtype Reader r a = Reader { runReader :: r -> a }
81 instance Functor (Reader r) where
82 fmap f m = Reader $ \r -> f (runReader m r)
84 instance Monad (Reader r) where
85 return a = Reader $ \_ -> a
86 m >>= k = Reader $ \r -> runReader (k (runReader m r)) r
88 instance MonadFix (Reader r) where
89 mfix f = Reader $ \r -> let a = runReader (f a) r in a
91 instance MonadReader r (Reader r) where
93 local f m = Reader $ runReader m . f
95 mapReader :: (a -> b) -> Reader r a -> Reader r b
96 mapReader f m = Reader $ f . runReader m
98 -- This is a more general version of local.
100 withReader :: (r' -> r) -> Reader r a -> Reader r' a
101 withReader f m = Reader $ runReader m . f
103 -- ---------------------------------------------------------------------------
104 -- Our parameterizable reader monad, with an inner monad
106 newtype ReaderT r m a = ReaderT { runReaderT :: r -> m a }
108 instance (Monad m) => Functor (ReaderT r m) where
109 fmap f m = ReaderT $ \r -> do
113 instance (Monad m) => Monad (ReaderT r m) where
114 return a = ReaderT $ \_ -> return a
115 m >>= k = ReaderT $ \r -> do
118 fail msg = ReaderT $ \_ -> fail msg
120 instance (MonadPlus m) => MonadPlus (ReaderT r m) where
121 mzero = ReaderT $ \_ -> mzero
122 m `mplus` n = ReaderT $ \r -> runReaderT m r `mplus` runReaderT n r
124 instance (MonadFix m) => MonadFix (ReaderT r m) where
125 mfix f = ReaderT $ \r -> mfix $ \a -> runReaderT (f a) r
127 instance (Monad m) => MonadReader r (ReaderT r m) where
129 local f m = ReaderT $ \r -> runReaderT m (f r)
131 instance MonadTrans (ReaderT r) where
132 lift m = ReaderT $ \_ -> m
134 instance (MonadIO m) => MonadIO (ReaderT r m) where
135 liftIO = lift . liftIO
137 mapReaderT :: (m a -> n b) -> ReaderT w m a -> ReaderT w n b
138 mapReaderT f m = ReaderT $ f . runReaderT m
140 withReaderT :: (r' -> r) -> ReaderT r m a -> ReaderT r' m a
141 withReaderT f m = ReaderT $ runReaderT m . f