[swift-evolution] [Proposal] mapValues

davesweeris at mac.com davesweeris at mac.com
Wed Apr 13 02:31:09 CDT 2016

“map” already "maps the values” so I think something like “transform” might be a little clearer, if we don’t want to just overload “map”. Regardless of what it’s called, though, I’m +1 on the functionality.

- Dave Sweeris

> On Apr 13, 2016, at 1:41 AM, Jonathan Hull via swift-evolution <swift-evolution at swift.org> wrote:
> I would really like to see something like the following added to the standard library:
> extension Dictionary {
>     func mapValues<U>(transform:(Key,Value)->U)->[Key:U] {
>         var output:[Key:U] = [:]
>         for (k,v) in self {
>             output[k] = transform(k,v)
>         }
>         return output
>     }
> }
> It comes up enough that I have had to add it to pretty much every one of my projects.  I also don’t feel comfortable adding it to my frameworks, since I figure a lot of people are also adding something like this to their projects, and I don’t want to cause a conflict with their version.  Prime candidate for the standard library.
> I like calling it ‘mapValues' as opposed to providing an override for map, since it makes the specific behavior more clear.  I would expect ‘map' to possibly map the keys as well (though there are issues where the new keys overlap).  I suppose you could just have a bunch of overrides for map if the compiler becomes good enough at differentiating return types: (Value)->(Value), (Key,Value)->Value, (Key, Value)->(Key,Value)
> Thanks,
> Jon
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