Attached is a git patch for base which makes the proposed changes.
From 824bdca994b3fcceff21fcb68e1b18f1d4f03bd5 Mon Sep 17 00:00:00 2001 From: Bas van Dijk <v.dijk....@gmail.com> Date: Fri, 16 Dec 2011 15:16:14 +0100 Subject: [PATCH] Give the Maybe Monoid the expected failure-and-prioritized-choice semantics instead of the lift-a-semigroup-to-a-monoid semantics. The old semantics didn't even achieve the latter since it required a Monoid instance on a, rather than a semigroup Also DEPRECATE First in favor of Maybe and Last in favor of Dual.
--- Data/Monoid.hs | 46 ++++++++++++++++++++-------------------------- 1 files changed, 20 insertions(+), 26 deletions(-) diff --git a/Data/Monoid.hs b/Data/Monoid.hs index 228e254..d1d9564 100644 --- a/Data/Monoid.hs +++ b/Data/Monoid.hs @@ -186,14 +186,14 @@ instance Num a => Monoid (Product a) where -- -- @ -- findLast :: Foldable t => (a -> Bool) -> t a -> Maybe a --- findLast pred = getLast . foldMap (\x -> if pred x --- then Last (Just x) --- else Last Nothing) +-- findLast pred = getDual . foldMap (\x -> if pred x +-- then Dual (Just x) +-- else Dual Nothing) -- @ -- -- Much of Data.Map's interface can be implemented with -- Data.Map.alter. Some of the rest can be implemented with a new --- @alterA@ function and either 'First' or 'Last': +-- @alterA@ function and either 'Maybe' or 'Dual Maybe': -- -- > alterA :: (Applicative f, Ord k) => -- > (Maybe a -> f (Maybe a)) -> k -> Map k a -> f (Map k a) @@ -204,28 +204,21 @@ instance Num a => Monoid (Product a) where -- insertLookupWithKey :: Ord k => (k -> v -> v -> v) -> k -> v -- -> Map k v -> (Maybe v, Map k v) -- insertLookupWithKey combine key value = --- Arrow.first getFirst . alterA doChange key +-- alterA doChange key -- where --- doChange Nothing = (First Nothing, Just value) --- doChange (Just oldValue) = --- (First (Just oldValue), --- Just (combine key value oldValue)) +-- doChange m@Nothing = (m, Just value) +-- doChange m@(Just oldValue) = (m, Just (combine key value oldValue)) -- @ --- | Lift a semigroup into 'Maybe' forming a 'Monoid' according to --- <http://en.wikipedia.org/wiki/Monoid>: \"Any semigroup @S@ may be --- turned into a monoid simply by adjoining an element @e@ not in @S@ --- and defining @e*e = e@ and @e*s = s = s*e@ for all @s ∈ S@.\" Since --- there is no \"Semigroup\" typeclass providing just 'mappend', we --- use 'Monoid' instead. -instance Monoid a => Monoid (Maybe a) where +instance Monoid (Maybe a) where mempty = Nothing - Nothing `mappend` m = m - m `mappend` Nothing = m - Just m1 `mappend` Just m2 = Just (m1 `mappend` m2) - + Nothing `mappend` r = r + l `mappend` _ = l -- | Maybe monoid returning the leftmost non-Nothing value. +-- +-- /DEPRECATED in favor of 'Maybe'!/ +{-# DEPRECATED First "Use Maybe instead" #-} newtype First a = First { getFirst :: Maybe a } #ifndef __HADDOCK__ deriving (Eq, Ord, Read, Show) @@ -237,11 +230,13 @@ instance Show a => Show (First a) #endif instance Monoid (First a) where - mempty = First Nothing - r@(First (Just _)) `mappend` _ = r - First Nothing `mappend` r = r + mempty = First mempty + First l `mappend` First r = First (l `mappend` r) -- | Maybe monoid returning the rightmost non-Nothing value. +-- +-- /DEPRECATED in favor of 'Dual'!/ +{-# DEPRECATED Last "Use Dual instead" #-} newtype Last a = Last { getLast :: Maybe a } #ifndef __HADDOCK__ deriving (Eq, Ord, Read, Show) @@ -253,9 +248,8 @@ instance Show a => Show (Last a) #endif instance Monoid (Last a) where - mempty = Last Nothing - _ `mappend` r@(Last (Just _)) = r - r `mappend` Last Nothing = r + mempty = Last mempty + Last x `mappend` Last y = Last (y `mappend` x) {- {-------------------------------------------------------------------- -- 1.7.5.4
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