MaybeErr(..), -- Instance of Monad
failME, isSuccess,
+ fmapM_maybe,
orElse,
mapCatMaybes,
allMaybes,
- firstJust,
+ firstJust, firstJusts,
expectJust,
maybeToBool,
+ MaybeT(..)
) where
import Data.Maybe
maybeToBool :: Maybe a -> Bool
maybeToBool Nothing = False
maybeToBool (Just _) = True
-\end{code}
-
-@catMaybes@ takes a list of @Maybe@s and returns a list of
-the contents of all the @Just@s in it. @allMaybes@ collects
-a list of @Justs@ into a single @Just@, returning @Nothing@ if there
-are any @Nothings@.
-\begin{code}
+-- | Collects a list of @Justs@ into a single @Just@, returning @Nothing@ if
+-- there are any @Nothings@.
allMaybes :: [Maybe a] -> Maybe [a]
allMaybes [] = Just []
allMaybes (Nothing : _) = Nothing
Nothing -> Nothing
Just xs -> Just (x:xs)
-\end{code}
-
-@firstJust@ takes a list of @Maybes@ and returns the
-first @Just@ if there is one, or @Nothing@ otherwise.
+firstJust :: Maybe a -> Maybe a -> Maybe a
+firstJust (Just a) _ = Just a
+firstJust Nothing b = b
-\begin{code}
-firstJust :: [Maybe a] -> Maybe a
-firstJust [] = Nothing
-firstJust (Just x : _) = Just x
-firstJust (Nothing : ms) = firstJust ms
+-- | Takes a list of @Maybes@ and returns the first @Just@ if there is one, or
+-- @Nothing@ otherwise.
+firstJusts :: [Maybe a] -> Maybe a
+firstJusts = foldr firstJust Nothing
\end{code}
\begin{code}
\end{code}
\begin{code}
+
orElse :: Maybe a -> a -> a
(Just x) `orElse` _ = x
Nothing `orElse` y = y
\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
+
+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}
+
%************************************************************************
%* *