X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Futils%2FMaybes.lhs;h=8a6a5f441e2db9fc62b4345047c70d605984c803;hb=d436c70d43fb905c63220040168295e473f4b90a;hp=c977494d2b5f10d38d7e73504504470f0d1ad2c0;hpb=38ac36a32032a538db346411b119c47e4019c08f;p=ghc-hetmet.git

diff --git a/compiler/utils/Maybes.lhs b/compiler/utils/Maybes.lhs
index c977494..8a6a5f4 100644
--- a/compiler/utils/Maybes.lhs
+++ b/compiler/utils/Maybes.lhs
@@ -17,7 +17,7 @@ module Maybes (
         expectJust,
         maybeToBool,
 
-        thenMaybe, seqMaybe, returnMaybe, failMaybe, fmapMMaybe
+        MaybeT(..)
     ) where
 
 import Data.Maybe
@@ -35,14 +35,9 @@ 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
@@ -50,12 +45,8 @@ 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.
-
-\begin{code}
+-- | Takes a list of @Maybes@ and returns the first @Just@ if there is one, or
+-- @Nothing@ otherwise.
 firstJust :: [Maybe a] -> Maybe a
 firstJust [] = Nothing
 firstJust (Just x  : _)  = Just x
@@ -77,31 +68,29 @@ 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
+orElse :: Maybe a -> a -> a
+(Just x) `orElse` _ = x
+Nothing  `orElse` y = y
+\end{code}
 
-thenMaybe :: Maybe a -> (a -> Maybe b) -> Maybe b
-thenMaybe ma mb = case ma of
-                  Just x  -> mb x
-                  Nothing -> Nothing
+%************************************************************************
+%*									*
+\subsection[MaybeT type]{The @MaybeT@ monad transformer}
+%*									*
+%************************************************************************
 
-returnMaybe :: a -> Maybe a
-returnMaybe = Just
+\begin{code}
 
-failMaybe :: Maybe a
-failMaybe = Nothing
+newtype MaybeT m a = MaybeT {runMaybeT :: m (Maybe a)}
 
-orElse :: Maybe a -> a -> a
-(Just x) `orElse` _ = x
-Nothing  `orElse` y = y
+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}