[swift-users] Why can I not filter or map a dictionary to another dictionary?

[Rick Mann]

> On Oct 26, 2016, at 22:23 , Dave Abrahams via swift-users <swift-users at swift.org> wrote:
> 
> 
> on Wed Oct 26 2016, Rick Mann <swift-users-AT-swift.org> wrote:
> 
>> It seems fairly natural to want to do this:
>> 
>> let bigDictionary = ...
>> let smallerDictionary = bigDictionary.filter { key, value in <some test returning Bool> }
> 
>> Similarly, it seems natural to want to map this way.
> 
> It's reasonable for filter, but maybe not for map.  Map follows certain
> laws, and one of them is that you get the same number of elements out as
> you put in.  But you can easily map all keys to the same key, and the
> law would be violated.

Sure map would produce a dictionary of the same number of entries. I didn't mean to imply map would also filter. But I'm basically transforming a JSON dictionary into a new version of that dictionary with fewer elements. I'd like to first filter it, then map it, and have the result be a dictionary.


-- 
Rick Mann
rmann at latencyzero.com