A more useful Monoid instance for Data.Map

Christian Sattler sattler.christian at gmail.com
Sat Jan 5 20:07:02 CET 2013

Hi all,

After repeated frustration over the wrong Monoid (Data.Map.Map k v)
instance I finally went ahead and did a practical test concerning its
current usage.

After removing the Monoid instance for Map and IntMap, each reverse
dependency of containers was separately compiled under a standard setup of
GHC 7.6.1 in order to avoid shared dependency problems. Out of 1440 reverse
dependencies, I could get 545 to compile. However, only the following
packages fail because of Monoid instance issues:

- caledon
- data-default
- dom-lt
- EnumMap
- i18n
- semigroups
- unamb-custom
- vacuum
- stringtable-atom

EnumMap has containers <0.3, semigroups declares <0.6, unamb-custom appears
to be a private abandoned clone with uploads only on 24/12/08,
stringtable-atom fails to build because of a previous API change for
updateMax, and the rest only use the instance internally for saying mempty
instead of Data.Map.empty.

Under these circumstances, fixing the Monoid instance mistake for
containers does not seem to introduce any semantic breakage at all.
I have CCed the maintainers of the lastly mentioned packages.

Let's do it!

2012/4/28 Daniel Peebles <pumpkingod at gmail.com>

> I don't actually think there are any rules/optimizations for fmap of
> newtype constructors or extractors in general. Luckily, unsafeCoerce is
> explicitly specified to be safe, in this kind of situation (assuming the
> Map is actually parametric in its value type, which it is)! ;)
> On Sat, Apr 28, 2012 at 12:31 PM, Evan Laforge <qdunkan at gmail.com> wrote:
>> On Fri, Apr 27, 2012 at 7:00 PM, Daniel Peebles <pumpkingod at gmail.com>
>> wrote:
>> > Why not be explicit about the replacement strategy by injecting your
>> values
>> > into First/Last? My point is that in terms of functionality, using
>> mappend
>> > on the values is strictly more general than the current instance. It
>> seems
>> > unfortunate to be stuck with the current instance for historical
>> reasons,
>> > but I guess that's how a lot of this stuff works :/
>> Yeah, I suppose it would be a bit more regular that way.  I'm always
>> reluctant to map newtypes over things other than lists because I don't
>> trust there to be a RULES that will eliminate it, but I suppose for
>> Map there must be.  I guess I wouldn't mind updating my code if the
>> definition changed.  It's hard to change a general purpose method
>> though, simply because searching for it in your code will turn up so
>> many false positives.
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