[commit: packages/base] master: Add `instance Monoid a => Monoid (Const a b)` (6f3a194)

[git at git.haskell.org]

Repository : ssh://git at git.haskell.org/base

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/6f3a194accd2a1a75a3f86c1b4dca30521488110/base

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

commit 6f3a194accd2a1a75a3f86c1b4dca30521488110
Author: Herbert Valerio Riedel <hvr at gnu.org>
Date:   Sat Oct 5 15:40:28 2013 +0200

    Add `instance Monoid a => Monoid (Const a b)`
    
    For more details, see original proposal at
    
     http://permalink.gmane.org/gmane.comp.lang.haskell.libraries/20369
    
    Signed-off-by: Herbert Valerio Riedel <hvr at gnu.org>


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

6f3a194accd2a1a75a3f86c1b4dca30521488110
 Control/Applicative.hs |    5 +++++
 1 file changed, 5 insertions(+)

diff --git a/Control/Applicative.hs b/Control/Applicative.hs
index 78e2b4b..dbfa272 100644
--- a/Control/Applicative.hs
+++ b/Control/Applicative.hs
@@ -230,6 +230,11 @@ newtype Const a b = Const { getConst :: a }
 instance Functor (Const m) where
     fmap _ (Const v) = Const v
 
+-- Added in base-4.7.0.0
+instance Monoid a => Monoid (Const a b) where
+    mempty = Const mempty
+    mappend (Const a) (Const b) = Const (mappend a b)
+
 instance Monoid m => Applicative (Const m) where
     pure _ = Const mempty
     Const f <*> Const v = Const (f `mappend` v)