Re: Proposal: Max and Min for Monoid

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 wrote:

Felipe Lessa writes:

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On Thu, Sep 23, 2010 at 12:58 PM, Jake McArthur wrote:

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-- | 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

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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 _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries