expectJust,
maybeToBool,
+ MaybeT(..)
) where
import Data.Maybe
Nothing `orElse` y = y
\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
+
+instance Monad m => Monad (MaybeT m) where
+ return = MaybeT . return . Just
+ x >>= f = MaybeT $ runMaybeT x >>= maybe (return Nothing) (runMaybeT . f)
+ fail _ = MaybeT $ return Nothing
+
+\end{code}
+
%************************************************************************
%* *