+\end{code}
+
+\begin{code}
+fmapM_maybe :: Monad m => (a -> m b) -> Maybe a -> m (Maybe b)
+fmapM_maybe _ Nothing = return Nothing
+fmapM_maybe f (Just x) = do
+ x' <- f x
+ return $ Just x'
+\end{code}
+
+%************************************************************************
+%* *
+\subsection[MaybeT type]{The @MaybeT@ monad transformer}
+%* *
+%************************************************************************
+
+\begin{code}
+
+newtype MaybeT m a = MaybeT {runMaybeT :: m (Maybe a)}
+
+instance Functor m => Functor (MaybeT m) where
+ fmap f x = MaybeT $ fmap (fmap f) $ runMaybeT x