X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2Futils%2FMaybes.lhs;h=39e6185a19eedbc355a31573e951e5ac0f05994b;hp=1d43365dce467b2e9291027b75dd34f393c7ed85;hb=8133a9f47b99f4e65ed30551de32ad72c6b61b27;hpb=da6f25437e59e97791a1fea7ce2f9ce4dccc57f6

diff --git a/compiler/utils/Maybes.lhs b/compiler/utils/Maybes.lhs
index 1d43365..39e6185 100644
--- a/compiler/utils/Maybes.lhs
+++ b/compiler/utils/Maybes.lhs
@@ -10,13 +10,15 @@ module Maybes (
         MaybeErr(..), -- Instance of Monad
         failME, isSuccess,
 
+        fmapM_maybe,
         orElse,
         mapCatMaybes,
         allMaybes,
-        firstJust,
+        firstJust, firstJusts,
         expectJust,
         maybeToBool,
 
+        MaybeT(..)
     ) where
 
 import Data.Maybe
@@ -34,14 +36,9 @@ infixr 4 `orElse`
 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
@@ -49,16 +46,14 @@ allMaybes (Just x  : ms) = case allMaybes ms of
                            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}
@@ -77,11 +72,40 @@ mapCatMaybes f (x:xs) = case f x of
 \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}
+
 
 %************************************************************************
 %*                                                                      *