> On Oct 27, 2016, at 10:29 , Dave Abrahams <dabrah...@apple.com> wrote:
>
>
> 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) }
Fair point. I suppose I'd be okay with unpredictable results, or an exception,
if possible.
>
> ? 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.
I guess for now I write it in a clunky way.
--
Rick Mann
rm...@latencyzero.com
_______________________________________________
swift-users mailing list
swift-users@swift.org
https://lists.swift.org/mailman/listinfo/swift-users
