On Tue, 7 Apr 2009, Stephan Friedrichs wrote:
My solution is this transformer:
newtype ConsumerT c m a
= ConsumerT { runConsumerT :: [c] -> m (a, [c]) }
instance (Monad m) => Monad (ConsumerT c m) where
return x = ConsumerT $ \cs -> return (x, cs)
m >>= f = ConsumerT $ \cs -> do
~(x, cs') <- runConsumerT m cs
runConsumerT (f x) cs'
fail msg = ConsumerT $ const (fail msg)
But this is precisely the StateT, wrapped in a newtype and with restricted
operations on it. You could as well define
newtype ConsumerT c m a =
ConsumerT { runConsumerT :: StateT [c] m a }
instance (Monad m) => Monad (ConsumerT c m) where
return x = ConsumerT $ return x
m >>= f = ConsumerT $ runConsumerT . f =<< runConsumerT m
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