Re: [Haskell-cafe] Intro to monad transformers

instance Monad m => MonadPlus (MaybeT m) where mzero = MaybeT $ return Nothing mplus x y = MaybeT $ do maybe_value <- runMaybeT x case maybe_value of Nothing -> runMaybeT y Just _ -> return maybe_value

I've not run it so with some caution, I'd expect this to work - the last line is now returning the first answer with 'return' of the wrapped monad. I'm not sure if this is a lawful version of mplus though...