[swift-evolution] [Pitch] Add `mapValues` method to Dictionary

[Patrick Smith]

This seems like a great way to do mapValues, as you get the ability to lazily map or flatMap the pairs. I’m pretty sure a key/value pair constructor is coming to Dictionary.


> On 22 May 2016, at 5:01 AM, Haravikk via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
>> On 21 May 2016, at 15:47, Honza Dvorsky <jan.dvorsky at me.com <mailto:jan.dvorsky at me.com>> wrote:
>> 
>> While I agree that it'd be nice to add a Map abstraction into which we could move a lot of the Dictionary-ness, my original pitch is *just* about adding the specific implementation of `mapValues` in its regular, non-lazy form. My example was about only keeping a subset of the information in memory in a Dictionary to allow for quick and frequent access (lazy goes against that). I think it'd be better to get that in first, or at least evaluate that separately from a comprehensive refactoring of the Dictionary, which would just accumulate more opinions and slow this specific step down.
> 
> Sorry, my point was that I think it’s better to wait until we can also do the lazy equivalent and have both done together, otherwise we end up with one map function that can work both lazily and one that never does. Sure that will require a refactoring into a protocol, but it seems to me that it’s better to do that as the first step, then add the feature after that. In the mean time extensions have you well covered for convenience.
> 
> Another alternative to this feature might be to add a key/value pair constructor to Dictionary (it technically already has one, but it’s variadic only) so you could do something like this:
> 
> 	let myTransformedDictionary = Dictionary(myIntegerDictionary.lazy.map { ($0, $1 + 5) })
> 
> Since this would be a useful initialiser both now and in future. I dunno, it’s just my opinion, but I find it a bit weird to get half the implementation now, and could lead to misunderstandings with people trying to do myMap.lazy.mapValues (won’t be recognised) and wondering why there isn’t one.
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