I don't want to maintain a whole package of newtype-wrapped everything for this, but I gather that Ben Gamari already has one people can use if they like. Now that Semigroup is in base, I think Kris's idea of using newtype wrappers *permanently* might be a good idea. That way we can have newtype UnionCombine k v = UnionCombine (Map k v) UnionCombine xs <> UnionCombine ys = UnionCombine $ unionWith (<>) xs ys newtype IntersectionCombine k v = IntersectionCombine (Map k v) IntersectionCombine xs <> IntersectionCombine ys = IntersectionCombine $ intersectionWith (<>) xs ys We can add such newtypes (by whatever names people prefer) immediately, and kick the question of whether to ever reinstate the instances down the road. On Tue, Feb 13, 2018 at 3:16 PM, Kris Nuttycombe wrote:

Given that we're talking about a years-long process, what about using the newtype approach as an intermediate step, so that we don't get additional proliferation of third-party newtype wrappers in the interim? Deprecate the current instances and add instances using newtype wrappers at the same type, such that a newtype'd version of the current functionality is available to folks as a migration path, as well as a newtype for which the instances provide the correct behavior. Then, in the future, the newtypes for the corrected behavior will eventually become superfluous, and for those who rely on the newtypes that represent the current behavior, they won't have to change anything.

On Tue, Feb 13, 2018 at 1:07 PM, Daniel Cartwright wrote:

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I am strongly in favour of this as well. I do not like the newtype approach, I think backwards-*incompatibility* with four to five year-old (or two to three year-old) code is OK.

On Tue, Feb 13, 2018 at 3:04 PM, Daniel Cartwright wrote:

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I am strongly in favour of this as well. I do not like the newtype approach, I think backwards-*incompatibility* with four to five year-old (or two to three year-old) code is OK.

On Tue, Feb 13, 2018 at 2:59 PM, David Feuer wrote:

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I don't think the old proposal was really the right way.

On Tue, Feb 13, 2018 at 2:54 PM, Mario Blažević wrote:

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+1. But whatever happened to your proposal from last May? I don't think there were any objections to it. Would the two proposals be combined, or have you decided to drop the previous one?

https://mail.haskell.org/pipermail/libraries/2017-May/028036.html

On 2017-05-25 12:55 PM, David Feuer wrote:

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A lot of people have wrappers around Data.Map and Data.IntMap to give them more useful (Semigroup and) Monoid instances. I'd like to add such wrappers to containers. What we need to be able to do that are *names* for the new modules. I can't think of any, so I'm reaching out to the list. Please suggest names! Another question is whether we should take the opportunity of new modules to modernize and streamline the API a bit. I'd like, at least, to separate "safe" from "unsafe" functions, putting the unsafe ones in .Unsafe modules.

On 2018-02-13 02:33 PM, David Feuer wrote:

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Many people have recognized for years that the Semigroup and Monoid instances for Data.Map, Data.IntMap, and Data.HashMap are not so great. In particular, when the same key is present in both maps, they simply use the value from the first argument, ignoring the other one. This somewhat counter-intuitive behavior can lead to bugs. See, for example, the discussion in Tim Humphries's blog post[*]. I would like do do the following:

1. Deprecate the Semigroup and Monoid instances for Data.Map.Map, Data.IntMap.IntMap, and Data.HashMap.HashMap in the next releases of containers and unordered-containers.

2. Remove the deprecated instances.

3. After another several years (four or five, perhaps?), make a major release of each package in which the instances are replaced with the following:

instance (Ord k, Semigroup v) => Semigroup (Map k v) where (<>) = Data.Map.Strict.unionWith (<>) instance (Ord k, Semigroup v) => Monoid (Map k v) where mempty = Data.Map.Strict.empty

instance Semigroup v => Semigroup (IntMap v) where (<>) = Data.IntMap.Strict.unionWith (<>) instance Semigroup v => Monoid (IntMap v) where mempty = Data.IntMap.Strict.empty

instance (Eq k, Hashable k, Semigroup v) => Semigroup (HashMap k v) where (<>) = Data.HashMap.Strict.unionWith (<>) instance (Eq k, Hashable k, Semigroup v) => Monoid(HashMap k v) where mempty = Data.HashMap.Strict.empty

Why do I want the strict versions? That choice may seem a bit surprising, since the data structures are lazy. But the lazy versions really like to leak memory, making them unsuitable for most practical purposes.

The big risk:

Someone using old code or old tutorial documentation could get subtly wrong behavior without noticing. That is why I have specified an extended period between removing the current instances and adding the desired ones.

Alternatives:

1. Remove the instances but don't add the new ones. I fear this may lead others to write their own orphan instances, which may not even all do the same thing.

2. Write separate modules with newtype-wrapped versions of the data structures implementing the desired instances. Unfortunately, this would be annoying to maintain, and also annoying to use--packages using the unwrapped and wrapped versions will use different types. Manually wrapping and unwrapping to make the different types work with each other will introduce lots of potential for mistakes and confusion.

Discussion period: three weeks.

[*] http://teh.id.au/posts/2017/03/03/map-merge/index.html _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

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