cmmTopCodeGen no longer takes DynFlags as an argument

[ghc-hetmet.git] / compiler / utils / Maybes.lhs

diff --git a/compiler/utils/Maybes.lhs b/compiler/utils/Maybes.lhs
index c977494..39e6185 100644 (file)
--- a/compiler/utils/Maybes.lhs
+++ b/compiler/utils/Maybes.lhs
@@ -10,14 +10,15 @@ module Maybes (
         MaybeErr(..), -- Instance of Monad
         failME, isSuccess,
 
+        fmapM_maybe,
         orElse,
         mapCatMaybes,
         allMaybes,
-        firstJust,
+        firstJust, firstJusts,
         expectJust,
         maybeToBool,
 
-        thenMaybe, seqMaybe, returnMaybe, failMaybe, fmapMMaybe
+        MaybeT(..)
     ) where
 
 import Data.Maybe
@@ -35,14 +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
@@ -50,16 +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}
@@ -77,31 +71,38 @@ mapCatMaybes f (x:xs) = case f x of
                         Nothing -> mapCatMaybes f xs
 \end{code}
 
-The Maybe monad
-~~~~~~~~~~~~~~~
 \begin{code}
-seqMaybe :: Maybe a -> Maybe a -> Maybe a
-seqMaybe (Just x) _  = Just x
-seqMaybe Nothing  my = my
-
-thenMaybe :: Maybe a -> (a -> Maybe b) -> Maybe b
-thenMaybe ma mb = case ma of
-                  Just x  -> mb x
-                  Nothing -> Nothing
-
-returnMaybe :: a -> Maybe a
-returnMaybe = Just
-
-failMaybe :: Maybe a
-failMaybe = Nothing
 
 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
 
-fmapMMaybe :: Monad m => (a -> m b) -> Maybe a -> m (Maybe b)
-fmapMMaybe _ Nothing  = return Nothing
-fmapMMaybe f (Just x) = f x >>= \x' -> return (Just 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}