Proposal: add full complement of support for decreasing things in Data.Map

David Feuer david.feuer at
Thu May 19 14:44:51 UTC 2016

Yes, but no. Once you do that, the whole map is Down. Aside from the
inconvenience factor, such a map can't be combined with one in the usual
order using union, intersection, etc. We could add conversion functions,
but they'd copy the whole structure and really only be special cases of the
hypothetical mapKeysDescending.
On May 19, 2016 10:40 AM, "Nickolay Kudasov" <nickolay.kudasov at>

> Hi David,
> Can't this be done simply by using Data.Ord.Down wrapper for the keys?
> Kind regards,
> Nick
> On Thu, 19 May 2016 at 17:00 David Feuer <david.feuer at> wrote:
>> Data.Map offers functions to convert from ascending lists to maps, and
>> also offers a function to map an increasing function over the keys of a
>> map. Equivalents for descending lists and decreasing functions are missing.
>> I think we should add them. Any objections?
>> _______________________________________________
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>> Libraries at
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