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module T1735_Help.State where
import Control.Monad (ap, liftM)
newtype 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 MonadFail m => MonadFail (StateT s m) where
fail s = StateT $ \_ -> fail s
instance Monad m => Functor (StateT s m) where
fmap = liftM
instance Monad m => Applicative (StateT s m) where
pure = return
(<*>) = ap
get :: Monad m => StateT s m s
get = StateT $ \s -> return (s, s)
put :: Monad m => s -> StateT s m ()
put s = StateT $ \_ -> return ((), s)
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