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/base/LICENSE)
8 -- Maintainer : libraries@haskell.org
9 -- Stability : experimental
10 -- Portability : non-portable ( mulit-param classes, functional dependencies )
12 -- Declaration of the Monoid class,and instances for list and functions
14 -- Inspired by the paper
15 -- /Functional Programming with Overloading and
16 -- Higher-Order Polymorphism/,
17 -- Mark P Jones (<http://www.cse.ogi.edu/~mpj>)
18 -- Advanced School of Functional Programming, 1995.
19 -----------------------------------------------------------------------------
21 module Control.Monad.Reader (
33 module Control.Monad.Fix,
34 module Control.Monad.Trans,
40 import Control.Monad.Fix
41 import Control.Monad.Trans
43 -- ----------------------------------------------------------------------------
45 -- asks for the internal (non-mutable) state.
47 class (Monad m) => MonadReader r m | m -> r where
49 local :: (r -> r) -> m a -> m a
51 -- This allows you to provide a projection function.
53 asks :: (MonadReader r m) => (r -> a) -> m a
58 -- ----------------------------------------------------------------------------
59 -- The partially applied function type is a simple reader monad
61 instance Functor ((->) r) where
64 instance Monad ((->) r) where
66 m >>= k = \r -> k (m r) r
68 instance MonadFix ((->) r) where
69 mfix f = \r -> let a = f a r in a
71 instance MonadReader r ((->) r) where
75 -- ---------------------------------------------------------------------------
76 -- Our parameterizable reader monad
78 newtype Reader r a = Reader { runReader :: r -> a }
80 instance Functor (Reader r) where
81 fmap f m = Reader $ \r -> f (runReader m r)
83 instance Monad (Reader r) where
84 return a = Reader $ \_ -> a
85 m >>= k = Reader $ \r -> runReader (k (runReader m r)) r
87 instance MonadFix (Reader r) where
88 mfix f = Reader $ \r -> let a = runReader (f a) r in a
90 instance MonadReader r (Reader r) where
92 local f m = Reader $ runReader m . f
94 mapReader :: (a -> b) -> Reader r a -> Reader r b
95 mapReader f m = Reader $ f . runReader m
97 -- This is a more general version of local.
99 withReader :: (r' -> r) -> Reader r a -> Reader r' a
100 withReader f m = Reader $ runReader m . f
102 -- ---------------------------------------------------------------------------
103 -- Our parameterizable reader monad, with an inner monad
105 newtype ReaderT r m a = ReaderT { runReaderT :: r -> m a }
107 instance (Monad m) => Functor (ReaderT r m) where
108 fmap f m = ReaderT $ \r -> do
112 instance (Monad m) => Monad (ReaderT r m) where
113 return a = ReaderT $ \_ -> return a
114 m >>= k = ReaderT $ \r -> do
117 fail msg = ReaderT $ \_ -> fail msg
119 instance (MonadPlus m) => MonadPlus (ReaderT r m) where
120 mzero = ReaderT $ \_ -> mzero
121 m `mplus` n = ReaderT $ \r -> runReaderT m r `mplus` runReaderT n r
123 instance (MonadFix m) => MonadFix (ReaderT r m) where
124 mfix f = ReaderT $ \r -> mfix $ \a -> runReaderT (f a) r
126 instance (Monad m) => MonadReader r (ReaderT r m) where
128 local f m = ReaderT $ \r -> runReaderT m (f r)
130 instance MonadTrans (ReaderT r) where
131 lift m = ReaderT $ \_ -> m
133 instance (MonadIO m) => MonadIO (ReaderT r m) where
134 liftIO = lift . liftIO
136 mapReaderT :: (m a -> n b) -> ReaderT w m a -> ReaderT w n b
137 mapReaderT f m = ReaderT $ f . runReaderT m
139 withReaderT :: (r' -> r) -> ReaderT r m a -> ReaderT r' m a
140 withReaderT f m = ReaderT $ runReaderT m . f