X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Futils%2FMaybes.lhs;h=d4f8346f1495cb003b6b89cc49abbe715e96c13c;hb=f5d4c3239e57b0396672ffc302961f84398d730e;hp=c977494d2b5f10d38d7e73504504470f0d1ad2c0;hpb=38ac36a32032a538db346411b119c47e4019c08f;p=ghc-hetmet.git

diff --git a/compiler/utils/Maybes.lhs b/compiler/utils/Maybes.lhs
index c977494..d4f8346 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
@@ -77,31 +77,29 @@ 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}