on Thu Oct 27 2016, Rick Mann <rmann-AT-latencyzero.com> wrote:
>> On Oct 26, 2016, at 22:23 , Dave Abrahams via swift-users
>> <swift-users@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.
I think you're missing my point. What result do you expect from
{1:2, 3:4}.map { _ in (1, 1) }
? How many elements does it have? Note that you can't have the same key
twice in a Dictionary.
> 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.
What we need are Dictionary initializers that operate on Sequences, per
https://github.com/apple/swift-evolution/blob/master/proposals/0100-add-sequence-based-init-and-merge-to-dictionary.md
Dictionary(merging: d.lazy.filter {... }.map {...})
That would get you the result you're looking for in a principled way,
without creating an intermediate Array.
--
-Dave
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