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)
consume :: (Monad m) => ConsumerT c m (Maybe c)
consume = ConsumerT $ \css -> case css of
[] -> return (Nothing, [])
(c:cs) -> return (Just c, cs)
consumeAll :: (Monad m) => ConsumerT c m [c]
consumeAll = ConsumerT $ \cs -> return (cs, [])
--
Früher hieß es ja: Ich denke, also bin ich.
Heute weiß man: Es geht auch so.
- Dieter Nuhr
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