base: MRP-refactoring of AMP instances

This refactors `(>>)`/`(*>)`/`return`/`pure` methods into normal form.

The redundant explicit `return` method definitions are dropped
altogether.

The explicit `(>>) = (*>)` definitions can't be removed yet, as
the default implementation of `(>>)` is still in terms of `(*>)`
(even though that should have been changed according to the AMP but
wasn't -- see note in GHC.Base for details why this had to be postponed)

A nofib comparision shows this refactoring to result in minor runtime
improvements (unless those are within normal measurement fluctuations):

        Program           Size    Allocs   Runtime   Elapsed  TotalMem
  -------------------------------------------------------------------------
            Min          -0.0%     -0.0%     -1.6%     -3.9%     -1.1%
            Max          -0.0%     +0.0%     +0.5%     +0.5%      0.0%
  Geometric Mean         -0.0%     -0.0%     -0.4%     -0.5%     -0.0%

Full `nofib` report at https://phabricator.haskell.org/P68

Reviewers: quchen, alanz, austin, #core_libraries_committee, bgamari

Reviewed By: bgamari

Differential Revision: https://phabricator.haskell.org/D1316

2 files changed, 3 insertions, 7 deletions

diff --git a/libraries/base/Text/ParserCombinators/ReadP.hs b/libraries/base/Text/ParserCombinators/ReadP.hs
index 034411d6bf..bae2abc90e 100644
--- a/libraries/base/Text/ParserCombinators/ReadP.hs
+++ b/libraries/base/Text/ParserCombinators/ReadP.hs
@@ -103,7 +103,7 @@ data P a
 -- Monad, MonadPlus
 
 instance Applicative P where
-  pure  = return
+  pure x = Result x Fail
   (<*>) = ap
 
 instance MonadPlus P where
@@ -111,8 +111,6 @@ instance MonadPlus P where
   mplus = (<|>)
 
 instance Monad P where
-  return x = Result x Fail
-
   (Get f)      >>= k = Get (\c -> f c >>= k)
   (Look f)     >>= k = Look (\s -> f s >>= k)
   Fail         >>= _ = Fail
@@ -161,11 +159,10 @@ instance Functor ReadP where
   fmap h (R f) = R (\k -> f (k . h))
 
 instance Applicative ReadP where
-    pure = return
+    pure x = R (\k -> k x)
     (<*>) = ap
 
 instance Monad ReadP where
-  return x  = R (\k -> k x)
   fail _    = R (\_ -> Fail)
   R m >>= f = R (\k -> m (\a -> let R m' = f a in m' k))
 
diff --git a/libraries/base/Text/ParserCombinators/ReadPrec.hs b/libraries/base/Text/ParserCombinators/ReadPrec.hs
index 027648d9e8..02268364ca 100644
--- a/libraries/base/Text/ParserCombinators/ReadPrec.hs
+++ b/libraries/base/Text/ParserCombinators/ReadPrec.hs
@@ -75,11 +75,10 @@ instance Functor ReadPrec where
   fmap h (P f) = P (\n -> fmap h (f n))
 
 instance Applicative ReadPrec where
-    pure = return
+    pure x  = P (\_ -> pure x)
     (<*>) = ap
 
 instance Monad ReadPrec where
-  return x  = P (\_ -> return x)
   fail s    = P (\_ -> fail s)
   P f >>= k = P (\n -> do a <- f n; let P f' = k a in f' n)