Add `Monoid` instance for `IO`

See original proposal at
https://mail.haskell.org/pipermail/libraries/2014-November/024310.html
for more details

Reviewed By: hvr, austin

Differential Revision: https://phabricator.haskell.org/D988

GHC Trac Issues: #10523

libraries/base/GHC/Base.hs

@@ -312,6 +312,10 @@ instance Monoid a => Monad ((,) a) where
     return x = (mempty, x)
     (u, a) >>= k = case k a of (v, b) -> (u `mappend` v, b)
 
+instance Monoid a => Monoid (IO a) where
+    mempty = pure mempty
+    mappend = liftA2 mappend
+
 {- | The 'Functor' class is used for types that can be mapped over.
 Instances of 'Functor' should satisfy the following laws:
 

libraries/base/changelog.md

@@ -35,6 +35,8 @@
 
   * `Data.Version.Version` now has a `Generic` instance
 
+  * `IO` now has a `Monoid` instance
+
 ## 4.8.1.0  *TBA*
 
   * Bundled with GHC 7.10.2