Add Functor, Applicative, Monad instances for First, Last

Summary:
This was proposed in 2011 [1] with no serious objections although wasn't
implemented until it was again mentioned in 2014 [2].

[1] http://www.haskell.org/pipermail/libraries/2011-January/015552.html
[2] http://www.haskell.org/pipermail/libraries/2014-June/023228.html

Test Plan: None

Reviewers: austin

Reviewed By: austin

Subscribers: hvr, phaskell, simonmar, relrod, carter, ekmett

Differential Revision: https://phabricator.haskell.org/D81

2 files changed, 24 insertions, 1 deletions

diff --git a/libraries/base/Control/Applicative.hs b/libraries/base/Control/Applicative.hs
index 4e77479e15..81ce513a58 100644
--- a/libraries/base/Control/Applicative.hs
+++ b/libraries/base/Control/Applicative.hs
@@ -54,7 +54,7 @@ import Control.Monad (liftM, ap, MonadPlus(..))
 import Control.Monad.ST.Safe (ST)
 import qualified Control.Monad.ST.Lazy.Safe as Lazy (ST)
 import Data.Functor ((<$>), (<$))
-import Data.Monoid (Monoid(..))
+import Data.Monoid (Monoid(..), First(..), Last(..))
 import Data.Proxy
 
 import Text.ParserCombinators.ReadP (ReadP)
@@ -281,6 +281,15 @@ instance (ArrowZero a, ArrowPlus a) => Alternative (WrappedArrow a b) where
     empty = WrapArrow zeroArrow
     WrapArrow u <|> WrapArrow v = WrapArrow (u <+> v)
 
+-- Added in base-4.8.0.0
+instance Applicative First where
+        pure x = First (Just x)
+        First x <*> First y = First (x <*> y)
+
+instance Applicative Last where
+        pure x = Last (Just x)
+        Last x <*> Last y = Last (x <*> y)
+
 -- | Lists, but with an 'Applicative' functor based on zipping, so that
 --
 -- @f '<$>' 'ZipList' xs1 '<*>' ... '<*>' 'ZipList' xsn = 'ZipList' (zipWithn f xs1 ... xsn)@
diff --git a/libraries/base/Data/Monoid.hs b/libraries/base/Data/Monoid.hs
index 58899541b6..2100518e3a 100644
--- a/libraries/base/Data/Monoid.hs
+++ b/libraries/base/Data/Monoid.hs
@@ -252,6 +252,13 @@ instance Monoid (First a) where
         r@(First (Just _)) `mappend` _ = r
         First Nothing `mappend` r = r
 
+instance Functor First where
+        fmap f (First x) = First (fmap f x)
+
+instance Monad First where
+        return x = First (Just x)
+        First x >>= m = First (x >>= getFirst . m)
+
 -- | Maybe monoid returning the rightmost non-Nothing value.
 newtype Last a = Last { getLast :: Maybe a }
         deriving (Eq, Ord, Read, Show, Generic, Generic1)
@@ -261,6 +268,13 @@ instance Monoid (Last a) where
         _ `mappend` r@(Last (Just _)) = r
         r `mappend` Last Nothing = r
 
+instance Functor Last where
+        fmap f (Last x) = Last (fmap f x)
+
+instance Monad Last where
+        return x = Last (Just x)
+        Last x >>= m = Last (x >>= getLast . m)
+
 {-
 {--------------------------------------------------------------------
   Testing