Re: Proposal: Max and Min for Monoid

On Thu, 2010-09-23 at 13:14 -0300, Felipe Lessa wrote:

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

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[1] http://hackage.haskell.org/trac/ghc/ticket/1951

I think this isn't the right link. It should be

http://hackage.haskell.org/trac/ghc/ticket/1952

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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)

Or should we have both variants? Or should we have something like

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data AddBounds a = Minimum | This a | Maximum deriving (Eq, Ord, Read, Show)

instance Bounded (AddBounds a) where minBound = Minimum maxBound = Maximum

Cheers! =)

-- Felipe.

The original version: - Uses newtype which would be more efficient - For many practical uses the Min/Max have concrete, 'natural' values - It is analogy to Sum and Product from Data.Monoid - It is simpler to use fromMax :: Max a -> a then fromMax :: Max a -> Maybe a - It shows that the bounded types form a monoid rather then wrappes arbitrary type into monoid Reagrds