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

[Nate Cook]

> On May 24, 2016, at 7:43 AM, Matthew Johnson via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Sent from my iPad
> 
> On May 24, 2016, at 12:59 AM, Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org> wrote:
> 
>>> I have a small remark though, wouldn’t it be better to let transform be of type (Key, Value) throws -> T instead of (Value) throws -> T? You can just ignore the key (with _) if you don’t need it, but I think it might come in handy in some cases.
>> 
>> The problem is, that closes the door to writing many simple maps in functional style. For instance, this:
>> 
>>   dictionaryOfNumbers.mapValues(abs)
>> 
>> Would have to become this:
>> 
>>   dictionaryOfNumbers.mapValues { _, v in abs(v) }
>> 
>> (It *might* be possible to do it with `$1`, but I'm not sure; there are some limitations around that.)
>> 
>> A value-value map is just simpler and cleaner, while almost always giving you what you need.
> 
> +1.  
> 
> I don't think I have ever mapped keys.  Incidentally, that doesn't have the usual semantics of a map operation as you can produce duplicate keys.

I think this conversation suffers from not knowing whether Dictionary is going to receive a sequence-based initializer. There's been a proposal for that in the PR queue for quite a while. Could someone in the core team please either merge or close that proposal?

https://github.com/apple/swift-evolution/pull/125

If there's no sequence-based init, mapValues should probably produce a new Dictionary. If there is, having mapValues produce a dictionary would be redundant. I'd rather see a MapDictionary type that wraps a dictionary and only evaluates the transformation on access, rather than eagerly allocating storage and performing all the transformations.

You can take a look at a rudimentary MapDictionary in this gist: https://gist.github.com/natecook1000/7cfc5c04f18237066f7aca6e9658f276

Best,
Nate

>> -- 
>> Brent Royal-Gordon
>> Architechies
>> 
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