I encountered exactly this issue in my use of the Max & Min monoids in Reactive. I wanted to allow already-bounded types to use their own minBound for Max and maxBound for Min, and also allow unbounded types to be used. So, rather than entangling bound addition with Max & Min, I defined AddBounds type wrapper, which is *orthogonal* to Max & Min. If your type is already Bounded, then use my simple Max & Min wrappers directly. If not (as in Reactive's use), compose Max or Min with AddBounds. I'm sorry I didn't think to mention this useful composition the Max & Min trac ticket. - Conal On Thu, Sep 23, 2010 at 10:47 AM, Gregory Collins <greg@gregorycollins.net>wrote:
Felipe Lessa <felipe.lessa@gmail.com> writes:
On Thu, Sep 23, 2010 at 12:58 PM, Jake McArthur <jake.mcarthur@gmail.com> wrote:
-- | Ordered monoid under 'max'. newtype Max a = Max { getMax :: a } deriving (Eq, Ord, Read, Show, Bounded)
instance (Ord a, Bounded a) => Monoid (Max a) where mempty = Max minBound Max a `mappend` Max b = Max (a `max` b)
Why should we prefer this monoid over
data Max a = Minimum | Max a deriving (Eq, Ord, Read, Show)
instance Ord a => Monoid (Max a) where mempty = Minimum Minimum `mappend` x = x x `mappend` Minimum = x Max a `mappend` Max b = Max (a `max` b)
You're right, the original wouldn't fly because there are unbounded types (like Integer) that you'd like to be able to use with Max/Min.
Rather than your proposal I would suggest that Max/Min mirror the existing First/Last, namely:
newtype Max a = Max { getMax :: Maybe a } deriving (Eq, Ord, Read, Show)
instance (Ord a) => Monoid (Max a) where mempty = Max Nothing mappend = max
G -- Gregory Collins <greg@gregorycollins.net> _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries