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

[Dave Abrahams]

on Tue May 24 2016, Nate Cook <swift-evolution at swift.org> wrote:

>> 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

Done.  I was just waiting until we could assign it SE-0100.
Congratulations! ;-)

> 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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-- 
Dave