[commit: ghc] master: Add Alternative instance for ZipList (fix #13520). (486b8db)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/486b8db05fefd1cfa916928c74958f8099b9f9f8/ghc

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

commit 486b8db05fefd1cfa916928c74958f8099b9f9f8
Author: Edvard Hübinette <edvard at hubinette.me>
Date:   Tue Apr 4 21:48:37 2017 -0400

    Add Alternative instance for ZipList (fix #13520).
    
    Reviewers: austin, hvr, bgamari, RyanGlScott
    
    Reviewed By: RyanGlScott
    
    Subscribers: adamse, RyanGlScott, rwbarton, thomie
    
    Differential Revision: https://phabricator.haskell.org/D3416


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

486b8db05fefd1cfa916928c74958f8099b9f9f8
 libraries/base/Control/Applicative.hs | 7 ++++++-
 libraries/base/changelog.md           | 5 +++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/libraries/base/Control/Applicative.hs b/libraries/base/Control/Applicative.hs
index 9045bcd..559cced 100644
--- a/libraries/base/Control/Applicative.hs
+++ b/libraries/base/Control/Applicative.hs
@@ -59,7 +59,7 @@ import Data.Functor.Const (Const(..))
 
 import GHC.Base
 import GHC.Generics
-import GHC.List (repeat, zipWith)
+import GHC.List (repeat, zipWith, drop)
 import GHC.Read (Read)
 import GHC.Show (Show)
 
@@ -121,6 +121,11 @@ instance Applicative ZipList where
     pure x = ZipList (repeat x)
     liftA2 f (ZipList xs) (ZipList ys) = ZipList (zipWith f xs ys)
 
+-- | @since 4.11.0.0
+instance Alternative ZipList where
+   empty = ZipList []
+   ZipList xs <|> ZipList ys = ZipList (xs ++ drop (length xs) ys)
+
 -- extra functions
 
 -- | One or none.
diff --git a/libraries/base/changelog.md b/libraries/base/changelog.md
index f2ea265..854a9b8 100644
--- a/libraries/base/changelog.md
+++ b/libraries/base/changelog.md
@@ -1,5 +1,10 @@
 # Changelog for [`base` package](http://hackage.haskell.org/package/base)
 
+## 4.11.0.0 *TBA*
+  * Bundled with GHC *TBA*
+
+  * Add `Alternative` instance for `ZipList` (#13520)
+
 ## 4.10.0.0 *April 2017*
   * Bundled with GHC *TBA*