[commit: packages/base] master: Add Foldable/Traversable instances for 'Const m' (dbd9181)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/dbd9181e06e4e06ca30a52f8dd30e38151e6b52d/base

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

commit dbd9181e06e4e06ca30a52f8dd30e38151e6b52d
Author: Austin Seipp <austin at well-typed.com>
Date:   Tue Sep 24 22:11:27 2013 -0500

    Add Foldable/Traversable instances for 'Const m'
    
    These were proposed a while ago but never went anywhere.
    
    Authored-by: Edward Kmett <ekmett at gmail.com>
    Signed-off-by: Austin Seipp <austin at well-typed.com>


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

dbd9181e06e4e06ca30a52f8dd30e38151e6b52d
 Data/Foldable.hs    |    3 +++
 Data/Traversable.hs |    3 +++
 2 files changed, 6 insertions(+)

diff --git a/Data/Foldable.hs b/Data/Foldable.hs
index e61972d..0f0d5bf 100644
--- a/Data/Foldable.hs
+++ b/Data/Foldable.hs
@@ -201,6 +201,9 @@ instance Foldable Proxy where
     foldr1 _ _ = error "foldr1: Proxy"
     {-# INLINE foldr1 #-}
 
+instance Foldable (Const m) where
+    foldMap _ _ = mempty
+
 -- | Monadic fold over the elements of a structure,
 -- associating to the right, i.e. from right to left.
 foldrM :: (Foldable t, Monad m) => (a -> b -> m b) -> b -> t a -> m b
diff --git a/Data/Traversable.hs b/Data/Traversable.hs
index 5fa91e8..e69d2b3 100644
--- a/Data/Traversable.hs
+++ b/Data/Traversable.hs
@@ -199,6 +199,9 @@ instance Traversable Proxy where
     sequence _ = return Proxy
     {-# INLINE sequence #-}
 
+instance Traversable (Const m) where
+    traverse _ (Const m) = pure $ Const m
+
 -- general functions
 
 -- | 'for' is 'traverse' with its arguments flipped.