1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
|
module StateMonad where
data StateT s m a = StateT { runStateT :: s -> m (a,s) }
instance (Monad m) => Monad (StateT s m) where
return a = StateT $ \ s -> return (a, s)
m >>= k = StateT $ \ s -> do
~(a, s') <- runStateT m s
runStateT (k a) s'
instance Monad m => Functor (StateT s m) where
fmap = liftM
class MonadTrans t where
lift :: (Monad m) => m a -> t m a
instance MonadTrans (StateT s) where
lift m = StateT $ \s -> do
a <- m
return (a, s)
get :: (Monad m) => StateT s m s
get = StateT $ \s -> return (s, s)
gets :: (Monad m) => (s -> a) -> StateT s m a
gets f = StateT $ \s -> return (f s, s)
put :: (Monad m) => s -> StateT s m ()
put x = StateT $ \_ -> return ((), x)
modify :: (Monad m) => (s -> s) -> StateT s m ()
modify f = get >>= \s -> put (f s)
concatMapM :: (Monad m) => (a -> m [b]) -> [a] -> m [b]
concatMapM f xs = liftM concat (mapM f xs)
|