Re: Proposal: Max and Min for Monoid (ticket # 1952)

Ross Paterson wrote:

Funny, I was thinking of proposing a Max type that adjoined a synthetic identity, as we did in the finger tree paper:

data Max a = NoMax | Max a deriving (Eq, Ord, Read, Show)

instance Ord a => Monoid (Max a) where mempty = NoMax NoMax `mappend` b = b a `mappend` NoMax = a Max x `mappend` Max y = Max (x `max` y)

and similarly for Min. One could even define

getMax :: Bounded a => Max a -> a getMax NoMax = minBound getMax (Max x) = x

I was thinking you could get that with some version of the (Maybe (Max a)) Monoid, but you are right: your version doesn't require (a) to be Bounded, thus works with Integers etc. Isaac