[commit: ghc] master: Add more MonadZip instances (3f3782d)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/3f3782df63f7c55382a25687a8e5c7f64202fa0a/ghc

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

commit 3f3782df63f7c55382a25687a8e5c7f64202fa0a
Author: Oleg Grenrus <oleg.grenrus at iki.fi>
Date:   Tue Mar 17 11:03:44 2015 -0500

    Add more MonadZip instances
    
    Summary: Add MonadZip Alt and MonadFix Alt instances
    
    Reviewers: ekmett, dfeuer, hvr, austin
    
    Reviewed By: austin
    
    Subscribers: thomie
    
    Differential Revision: https://phabricator.haskell.org/D716
    
    GHC Trac Issues: #10107


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

3f3782df63f7c55382a25687a8e5c7f64202fa0a
 libraries/base/Control/Monad/Fix.hs |  6 +++++-
 libraries/base/Control/Monad/Zip.hs | 24 +++++++++++++++++++++++-
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/libraries/base/Control/Monad/Fix.hs b/libraries/base/Control/Monad/Fix.hs
index ef8eeee..ae37911 100644
--- a/libraries/base/Control/Monad/Fix.hs
+++ b/libraries/base/Control/Monad/Fix.hs
@@ -26,7 +26,8 @@ module Control.Monad.Fix (
 import Data.Either
 import Data.Function ( fix )
 import Data.Maybe
-import Data.Monoid ( Dual(..), Sum(..), Product(..), First(..), Last(..) )
+import Data.Monoid ( Dual(..), Sum(..), Product(..)
+                   , First(..), Last(..), Alt(..) )
 import GHC.Base ( Monad, error, (.) )
 import GHC.List ( head, tail )
 import GHC.ST
@@ -99,3 +100,6 @@ instance MonadFix First where
 
 instance MonadFix Last where
     mfix f   = Last (mfix (getLast . f))
+
+instance MonadFix f => MonadFix (Alt f) where
+    mfix f   = Alt (mfix (getAlt . f))
diff --git a/libraries/base/Control/Monad/Zip.hs b/libraries/base/Control/Monad/Zip.hs
index df096b1..1f63cab 100644
--- a/libraries/base/Control/Monad/Zip.hs
+++ b/libraries/base/Control/Monad/Zip.hs
@@ -17,7 +17,8 @@
 
 module Control.Monad.Zip where
 
-import Control.Monad (liftM)
+import Control.Monad (liftM, liftM2)
+import Data.Monoid
 
 -- | `MonadZip` type class. Minimal definition: `mzip` or `mzipWith`
 --
@@ -53,3 +54,24 @@ instance MonadZip [] where
     mzipWith = zipWith
     munzip   = unzip
 
+instance MonadZip Dual where
+    -- Cannot use coerce, it's unsafe
+    mzipWith = liftM2
+
+instance MonadZip Sum where
+    mzipWith = liftM2
+
+instance MonadZip Product where
+    mzipWith = liftM2
+
+instance MonadZip Maybe where
+    mzipWith = liftM2
+
+instance MonadZip First where
+    mzipWith = liftM2
+
+instance MonadZip Last where
+    mzipWith = liftM2
+
+instance MonadZip f => MonadZip (Alt f) where
+    mzipWith f (Alt ma) (Alt mb) = Alt (mzipWith f ma mb)