Add various instances to newtypes in Data.Monoid

Summary:
Add Functor instances for Dual, Sum and Product
Add Foldable instances for Dual, Sum and Product
Add Traversable instances for Dual, Sum and Product
Add Foldable and Traversable instances for First and Last
Add Applicative, Monad instances to Dual, Sum, Product
Add MonadFix to Data.Monoid wrappers
Derive Data for Identity
Add Data instances to Data.Monoid wrappers
Add Data (Alt f a) instance

Reviewers: ekmett, dfeuer, hvr, austin

Reviewed By: dfeuer, austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D673

GHC Trac Issues: #10107

1 files changed, 18 insertions, 0 deletions

diff --git a/libraries/base/Control/Monad/Fix.hs b/libraries/base/Control/Monad/Fix.hs
index 76faeaf655..ef8eeee776 100644
--- a/libraries/base/Control/Monad/Fix.hs
+++ b/libraries/base/Control/Monad/Fix.hs
@@ -26,6 +26,7 @@ module Control.Monad.Fix (
 import Data.Either
 import Data.Function ( fix )
 import Data.Maybe
+import Data.Monoid ( Dual(..), Sum(..), Product(..), First(..), Last(..) )
 import GHC.Base ( Monad, error, (.) )
 import GHC.List ( head, tail )
 import GHC.ST
@@ -81,3 +82,20 @@ instance MonadFix (Either e) where
 
 instance MonadFix (ST s) where
         mfix = fixST
+
+-- Instances of Data.Monoid wrappers
+
+instance MonadFix Dual where
+    mfix f   = Dual (fix (getDual . f))
+
+instance MonadFix Sum where
+    mfix f   = Sum (fix (getSum . f))
+
+instance MonadFix Product where
+    mfix f   = Product (fix (getProduct . f))
+
+instance MonadFix First where
+    mfix f   = First (mfix (getFirst . f))
+
+instance MonadFix Last where
+    mfix f   = Last (mfix (getLast . f))