1 -----------------------------------------------------------------------------
3 -- Module : Control.Monad.RWS
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 (multi-param classes, functional dependencies)
12 -- Declaration of the MonadRWS class.
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.RWS (
32 module Control.Monad.Reader,
33 module Control.Monad.Writer,
34 module Control.Monad.State,
40 import Control.Monad.Fix
41 import Control.Monad.Trans
42 import Control.Monad.Reader
43 import Control.Monad.Writer
44 import Control.Monad.State
47 newtype RWS r w s a = RWS { runRWS :: r -> s -> (a, s, w) }
49 instance Functor (RWS r w s) where
50 fmap f m = RWS $ \r s -> let
51 (a, s', w) = runRWS m r s
54 instance (Monoid w) => Monad (RWS r w s) where
55 return a = RWS $ \_ s -> (a, s, mempty)
56 m >>= k = RWS $ \r s -> let
57 (a, s', w) = runRWS m r s
58 (b, s'', w') = runRWS (k a) r s'
59 in (b, s'', w `mappend` w')
61 instance (Monoid w) => MonadFix (RWS r w s) where
62 mfix f = RWS $ \r s -> let (a, s', w) = runRWS (f a) r s in (a, s', w)
64 instance (Monoid w) => MonadReader r (RWS r w s) where
65 ask = RWS $ \r s -> (r, s, mempty)
66 local f m = RWS $ \r s -> runRWS m (f r) s
68 instance (Monoid w) => MonadWriter w (RWS r w s) where
69 tell w = RWS $ \_ s -> ((), s, w)
70 listen m = RWS $ \r s -> let
71 (a, s', w) = runRWS m r s
73 pass m = RWS $ \r s -> let
74 ((a, f), s', w) = runRWS m r s
77 instance (Monoid w) => MonadState s (RWS r w s) where
78 get = RWS $ \_ s -> (s, s, mempty)
79 put s = RWS $ \_ _ -> ((), s, mempty)
82 evalRWS :: RWS r w s a -> r -> s -> (a, w)
84 (a, _, w) = runRWS m r s
87 execRWS :: RWS r w s a -> r -> s -> (s, w)
89 (_, s', w) = runRWS m r s
92 mapRWS :: ((a, s, w) -> (b, s, w')) -> RWS r w s a -> RWS r w' s b
93 mapRWS f m = RWS $ \r s -> f (runRWS m r s)
95 withRWS :: (r' -> s -> (r, s)) -> RWS r w s a -> RWS r' w s a
96 withRWS f m = RWS $ \r s -> uncurry (runRWS m) (f r s)
99 newtype RWST r w s m a = RWST { runRWST :: r -> s -> m (a, s, w) }
101 instance (Monad m) => Functor (RWST r w s m) where
102 fmap f m = RWST $ \r s -> do
103 (a, s', w) <- runRWST m r s
106 instance (Monoid w, Monad m) => Monad (RWST r w s m) where
107 return a = RWST $ \_ s -> return (a, s, mempty)
108 m >>= k = RWST $ \r s -> do
109 (a, s', w) <- runRWST m r s
110 (b, s'',w') <- runRWST (k a) r s'
111 return (b, s'', w `mappend` w')
112 fail msg = RWST $ \_ _ -> fail msg
114 instance (Monoid w, MonadPlus m) => MonadPlus (RWST r w s m) where
115 mzero = RWST $ \_ _ -> mzero
116 m `mplus` n = RWST $ \r s -> runRWST m r s `mplus` runRWST n r s
118 instance (Monoid w, MonadFix m) => MonadFix (RWST r w s m) where
119 mfix f = RWST $ \r s -> mfix $ \ ~(a, _, _) -> runRWST (f a) r s
121 instance (Monoid w, Monad m) => MonadReader r (RWST r w s m) where
122 ask = RWST $ \r s -> return (r, s, mempty)
123 local f m = RWST $ \r s -> runRWST m (f r) s
125 instance (Monoid w, Monad m) => MonadWriter w (RWST r w s m) where
126 tell w = RWST $ \_ s -> return ((),s,w)
127 listen m = RWST $ \r s -> do
128 (a, s', w) <- runRWST m r s
129 return ((a, w), s', w)
130 pass m = RWST $ \r s -> do
131 ((a, f), s', w) <- runRWST m r s
134 instance (Monoid w, Monad m) => MonadState s (RWST r w s m) where
135 get = RWST $ \_ s -> return (s, s, mempty)
136 put s = RWST $ \_ _ -> return ((), s, mempty)
138 instance (Monoid w) => MonadTrans (RWST r w s) where
139 lift m = RWST $ \_ s -> do
141 return (a, s, mempty)
143 instance (Monoid w, MonadIO m) => MonadIO (RWST r w s m) where
144 liftIO = lift . liftIO
147 evalRWST :: (Monad m) => RWST r w s m a -> r -> s -> m (a, w)
149 (a, _, w) <- runRWST m r s
152 execRWST :: (Monad m) => RWST r w s m a -> r -> s -> m (s, w)
154 (_, s', w) <- runRWST m r s
157 mapRWST :: (m (a, s, w) -> n (b, s, w')) -> RWST r w s m a -> RWST r w' s n b
158 mapRWST f m = RWST $ \r s -> f (runRWST m r s)
160 withRWST :: (r' -> s -> (r, s)) -> RWST r w s m a -> RWST r' w s m a
161 withRWST f m = RWST $ \r s -> uncurry (runRWST m) (f r s)