[commit: ghc] master: Re-use `transformers`'s `MaybeT` rather than our own (0933331)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/09333313f32be975faf9158fcd3648489d78ad82/ghc

>---------------------------------------------------------------

commit 09333313f32be975faf9158fcd3648489d78ad82
Author: Herbert Valerio Riedel <hvr at gnu.org>
Date:   Mon Dec 7 17:32:23 2015 +0100

    Re-use `transformers`'s `MaybeT` rather than our own
    
    The now removed `MaybeT` type was originally added back in 2008
    via bc845b714132a897032502536fea8cd018ce325b
    
    Reviewed By: bgamari
    
    Differential Revision: https://phabricator.haskell.org/D1583


>---------------------------------------------------------------

09333313f32be975faf9158fcd3648489d78ad82
 compiler/utils/Maybes.hs | 60 ++++--------------------------------------------
 1 file changed, 4 insertions(+), 56 deletions(-)

diff --git a/compiler/utils/Maybes.hs b/compiler/utils/Maybes.hs
index 656f40a..ac51070 100644
--- a/compiler/utils/Maybes.hs
+++ b/compiler/utils/Maybes.hs
@@ -3,7 +3,6 @@
 (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
 -}
 
-{-# LANGUAGE CPP #-}
 module Maybes (
         module Data.Maybe,
 
@@ -18,11 +17,9 @@ module Maybes (
         MaybeT(..), liftMaybeT
     ) where
 
-import Control.Applicative
+import Control.Applicative as A
 import Control.Monad
-#if __GLASGOW_HASKELL__ > 710
-import Control.Monad.Fail
-#endif
+import Control.Monad.Trans.Maybe
 import Data.Maybe
 
 infixr 4 `orElse`
@@ -64,56 +61,7 @@ orElse = flip fromMaybe
 ************************************************************************
 -}
 
-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
-
-#if __GLASGOW_HASKELL__ < 710
--- Pre-AMP change
-instance (Monad m, Applicative m) => Applicative (MaybeT m) where
-#else
-instance (Monad m) => Applicative (MaybeT m) where
-#endif
-  pure = MaybeT . pure . Just
-  (<*>) = ap
-
-#if __GLASGOW_HASKELL__ < 710
--- Pre-AMP change
-instance (Monad m, Applicative m) => Monad (MaybeT m) where
-#else
-instance (Monad m) => Monad (MaybeT m) where
-#endif
-  return = pure
-  x >>= f = MaybeT $ runMaybeT x >>= maybe (pure Nothing) (runMaybeT . f)
-  fail _ = MaybeT $ pure Nothing
-
-
-#if __GLASGOW_HASKELL__ > 710
-instance Monad m => MonadFail (MaybeT m) where
-  fail _ = MaybeT $ return Nothing
-#endif
-
-#if __GLASGOW_HASKELL__ < 710
--- Pre-AMP change
-instance (Monad m, Applicative m) => Alternative (MaybeT m) where
-#else
-instance (Monad m) => Alternative (MaybeT m) where
-#endif
-  empty = mzero
-  (<|>) = mplus
-
-#if __GLASGOW_HASKELL__ < 710
--- Pre-AMP change
-instance (Monad m, Applicative m) => MonadPlus (MaybeT m) where
-#else
-instance Monad m => MonadPlus (MaybeT m) where
-#endif
-  mzero       = MaybeT $ pure Nothing
-  p `mplus` q = MaybeT $ do ma <- runMaybeT p
-                            case ma of
-                              Just a  -> pure (Just a)
-                              Nothing -> runMaybeT q
+-- We had our own MaybeT in the past. Now we reuse transformer's MaybeT
 
 liftMaybeT :: Monad m => m a -> MaybeT m a
 liftMaybeT act = MaybeT $ Just `liftM` act
@@ -136,7 +84,7 @@ instance Applicative (MaybeErr err) where
   (<*>) = ap
 
 instance Monad (MaybeErr err) where
-  return = pure
+  return = A.pure
   Succeeded v >>= k = k v
   Failed e    >>= _ = Failed e