How can I make a Maybe-Transformer MaybeT into an instance of MonadWriter?
- by martingw
I am trying to build a MaybeT-Transformer Monad, based on the example in the Real World Haskell, Chapter Monad Transformers:
data MaybeT m a = MaybeT { runMT :: m (Maybe a) }
instance (Monad m) => Monad (MaybeT m) where
m >>= f = MaybeT $ do a <- runMT m
case a of
Just x -> runMT (f x)
Nothing -> return Nothing
return a = MaybeT $ return (Just a)
instance MonadTrans MaybeT where
lift m = MaybeT $ do
a <- m
return (Just a)
This works fine, but now I want to make MaybeT an instance of MonadWriter:
instance (MonadWriter w m) => MonadWriter w (MaybeT m) where
tell = lift . tell
listen m = MaybeT $ do unwrapped <- listen (runMT m)
return (Just unwrapped)
The tell is ok, but I can't get the listen function right. The best I could come up with after 1 1/2 days of constructor origami is the one you see above: unwrapped is supposed to be a tuple of (Maybe a, w), and that I want to wrap up in a Maybe-Type and put the whole thing in an empty MonadWriter.
But the compiler complains with:
Occurs check: cannot construct the infinite type: a = Maybe a
When generalising the type(s) for `listen'
In the instance declaration for `MonadWriter w (MaybeT m)'
What am I missing?
