Re: A more useful Monoid instance for Data.Map

Hi all,

-----Original message----- From: Ben Gamari Sent: 12 Mar 2013, 11:06

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We should take advantage of this opportunity to begin an easy cleanup and accept that there are still other issues that will need to be dealt with once the tools for doing so exist.

IIRC, the instances we are talking about are Current: instance (Ord k) => Monoid (Map k v) where mempty = empty mappend = union mconcat = unions Proposed: instance (Ord k, Monoid v) => Monoid (Map k v) where mempty = empty mappend map0 map1 = unionWith mappend map0 map1 mconcat maps = foldr (unionWith mappend) empty maps or even instance (Ord k, Semigroup v) => Monoid (Map k v) where mempty = empty mappend map0 map1 = unionWith <> map0 map1 mconcat maps = foldr (unionWith <>) empty maps I am not convinced that the proposed instance is universally the best one. I like the current instance -- I sometimes use it on Map k v where v is a "primitive" type like Int or Double for which no Monoid instance exists. Personally, I am also not sure that fromList [(1, "A")] `mappend` fromList [(1, "B") should really be fromList [(1, "AB")] The price for any change is very high: a) silent semantic change for replacing the instance is a no go, consider for example for Map k String or Map k ByteString. b) removing and then adding the instance means we will have two major version bumps. That places a burden on every user of the library to at least upgrade upper bounds. And as Edward Kmett wrote earlier today, the users that want to use the Monoid instance must revert to some kind of conditional compilation. I am not saying we should never do any breaking changes, but in this case, the benefit seems rather small to me. So -1 on changing the Monoid instance from me. Cheers, Milan Straka