Looks good to me.
-Edward
On Sat, Jun 28, 2014 at 1:58 PM, Ben Gamari
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 --- libraries/base/Control/Applicative.hs | 11 ++++++++++- libraries/base/Data/Monoid.hs | 14 ++++++++++++++ 2 files changed, 24 insertions(+), 1 deletion(-)
diff --git a/libraries/base/Control/Applicative.hs b/libraries/base/Control/Applicative.hs index 4e77479..81ce513 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 b71176b..bb87243 100644 --- a/libraries/base/Data/Monoid.hs +++ b/libraries/base/Data/Monoid.hs @@ -251,6 +251,13 @@ 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) @@ -260,6 +267,13 @@ 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 -- 1.9.1