Make Monad/Applicative instances MRP-friendly

This patch refactors pure/(*>) and return/(>>) in MRP-friendly way, i.e.
such that the explicit definitions for `return` and `(>>)` match the
MRP-style default-implementation, i.e.

  return = pure

and

  (>>) = (*>)

This way, e.g. all `return = pure` definitions can easily be grepped and
removed in GHC 8.1;

Test Plan: Harbormaster

Reviewers: goldfire, alanz, bgamari, quchen, austin

Reviewed By: quchen, austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1312

1 files changed, 2 insertions, 1 deletions

diff --git a/compiler/utils/Exception.hs b/compiler/utils/Exception.hs
index 850393e359..8168992e00 100644
--- a/compiler/utils/Exception.hs
+++ b/compiler/utils/Exception.hs
@@ -6,6 +6,7 @@ module Exception
     )
     where
 
+import Control.Applicative as A
 import Control.Exception
 import Control.Monad.IO.Class
 
@@ -28,7 +29,7 @@ tryIO = try
 -- implementations of 'gbracket' and 'gfinally' use 'gmask'
 -- thus rarely require overriding.
 --
-class MonadIO m => ExceptionMonad m where
+class (A.Applicative m, MonadIO m) => ExceptionMonad m where
 
   -- | Generalised version of 'Control.Exception.catch', allowing an arbitrary
   -- exception handling monad instead of just 'IO'.