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

2 files changed, 6 insertions, 5 deletions

diff --git a/compiler/stgSyn/CoreToStg.hs b/compiler/stgSyn/CoreToStg.hs
index dc70851205..e5954ab440 100644
--- a/compiler/stgSyn/CoreToStg.hs
+++ b/compiler/stgSyn/CoreToStg.hs
@@ -990,11 +990,11 @@ instance Functor LneM where
     fmap = liftM
 
 instance Applicative LneM where
-    pure = return
+    pure = returnLne
     (<*>) = ap
 
 instance Monad LneM where
-    return = returnLne
+    return = pure
     (>>=)  = thenLne
 
 instance MonadFix LneM where
diff --git a/compiler/stgSyn/StgLint.hs b/compiler/stgSyn/StgLint.hs
index b415b4f2d9..ef5dd9237a 100644
--- a/compiler/stgSyn/StgLint.hs
+++ b/compiler/stgSyn/StgLint.hs
@@ -314,13 +314,14 @@ instance Functor LintM where
       fmap = liftM
 
 instance Applicative LintM where
-      pure = return
+      pure a = LintM $ \_loc _scope errs -> (a, errs)
       (<*>) = ap
+      (*>)  = thenL_
 
 instance Monad LintM where
-    return a = LintM $ \_loc _scope errs -> (a, errs)
+    return = pure
     (>>=) = thenL
-    (>>)  = thenL_
+    (>>)  = (*>)
 
 thenL :: LintM a -> (a -> LintM b) -> LintM b
 thenL m k = LintM $ \loc scope errs