In addition to what's already been pointed out, note that this:
do t <- readTVar p case t of Empty -> return Nothing Trie l m r -> do <stuff>
Is a case of the (non-existent) MaybeT transformer:
do Trie l m r <- readTVar p <stuff with slight modifications>
The modifications being something like 'return . Just' => 'return', and 'return Nothing' => mzero.
newtype MaybeT m a = MaybeT { runMaybeT :: m (Maybe a) }
instance Functor f => Functor (MaybeT f) where fmap f = MaybeT . fmap (fmap f) . runMaybeT
instance Monad m => Monad (MaybeT m) where return = MaybeT . return . Just m >>= f = MaybeT $ runMaybeT m >>= (runMaybeT . f) fail _ = MaybeT $ return Nothing
instance Monad m => MonadPlus (MaybeT m) where mzero = MaybeT $ return Nothing m1 `mplus` m2 = MaybeT $ liftM2 mplus (runMaybeT m1) (runMaybeT m2)
(I haven't tested the code, but that's approximately what it looks like; let me know if I did something wrong and you need it fixed; I suspect you won't, as I'm not sure it simplifies the remained of <stuff> any :)) -- Dan