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

1 files changed, 8 insertions, 13 deletions

diff --git a/libraries/base/Data/Semigroup.hs b/libraries/base/Data/Semigroup.hs
index 661e513cba..f3f9f0b326 100644
--- a/libraries/base/Data/Semigroup.hs
+++ b/libraries/base/Data/Semigroup.hs
@@ -332,8 +332,7 @@ instance Applicative Min where
   Min f <*> Min x = Min (f x)
 
 instance Monad Min where
-  return = Min
-  _ >> a = a
+  (>>) = (*>)
   Min a >>= f = f a
 
 instance MonadFix Min where
@@ -389,8 +388,7 @@ instance Applicative Max where
   Max f <*> Max x = Max (f x)
 
 instance Monad Max where
-  return = Max
-  _ >> a = a
+  (>>) = (*>)
   Max a >>= f = f a
 
 instance MonadFix Max where
@@ -476,8 +474,7 @@ instance Applicative First where
   First f <*> First x = First (f x)
 
 instance Monad First where
-  return = First
-  _ >> a = a
+  (>>) = (*>)
   First a >>= f = f a
 
 instance MonadFix First where
@@ -523,8 +520,7 @@ instance Applicative Last where
   Last f <*> Last x = Last (f x)
 
 instance Monad Last where
-  return = Last
-  _ >> a = a
+  (>>) = (*>)
   Last a >>= f = f a
 
 instance MonadFix Last where
@@ -584,14 +580,13 @@ instance Applicative Option where
   pure a = Option (Just a)
   Option a <*> Option b = Option (a <*> b)
 
-instance Monad Option where
-  return = pure
+  Option Nothing  *>  _ = Option Nothing
+  _               *>  b = b
 
+instance Monad Option where
   Option (Just a) >>= k = k a
   _               >>= _ = Option Nothing
-
-  Option Nothing  >>  _ = Option Nothing
-  _               >>  b = b
+  (>>) = (*>)
 
 instance Alternative Option where
   empty = Option Nothing