Re: A more useful Monoid instance for Data.Map

On Tue, 12 Mar 2013, Milan Straka wrote:

Hi all,

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

<snip>

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.

First of all, I do not know, why people require the current semantics of "mappend" for Data.Map and why they do not just use "Map.union" in order to explicitly tell that they drop duplicate elements intentionally. The best semantics for "mappend" should be driven by the question what is best for working with monoids (e.g. as writer type in the Writer monad). The original proposal gave the good argument that by choosing a monoid type for the Map element you can choose how elements for duplicate keys are merged. My experience with Map.union and Map.fromList is that I had bugs that were very hard to debug because I did not kept the case of duplicate keys in mind. Now I have added warnings to HLint for every use of Map.union and Map.fromList. Either duplicate keys are allowed, then I should explicitely state how to combine them, or keys must be distinct, then I should use Map.unionWith (error "duplicate keys"). Unfortunately, my HLint checks are bypassed if the 'Map.union' is disguised as a 'mappend'. I think that changing 'mappend' in the proposed way has the advantage to make programmers think about duplicate keys. The answer could be to replace 'mappend' by 'Map.union' or choose an element Monoid for combining the elements. Additionally Semigroup/Monoid types like First and Last are more explicit than the current behaviour of "mappend" where you cannot guess whether the left or the right element is kept when keys clash.