[swift-evolution] Smart KeyPaths

[Matthew Johnson]

> On Mar 22, 2017, at 11:41 AM, Ricardo Parada <rparada at mac.com> wrote:
> 
> Agree.
> 
> Another question.  If `Bag` does have a static thing called `myStaticThingy` would you refer to it as:
> 
> Bag.Type#myStaticThingy

I would expect that to work eventually but I’m not sure if it would be included in the initial proposal or not.

> 
> ?
> 
> 
>> On Mar 22, 2017, at 12:37 PM, Matthew Johnson <matthew at anandabits.com <mailto:matthew at anandabits.com>> wrote:
>> 
>>> 
>>> On Mar 22, 2017, at 11:16 AM, Ricardo Parada <rparada at mac.com <mailto:rparada at mac.com>> wrote:
>>> 
>>> I see three possibilities:
>>> 
>>> 1) # + «space» +«path» like this:
>>> 
>>> let path = # Bag.things[0].name
>>> bag[path] 
>>> bag[# Bag.things[0].name]
>>> bag[# .things[0].name]  // Root is inferred as Bag
>>> bag.things[0][# Thing.name]
>>> bag.things[0][# .name] // Root is inferred as Thing
>>> 
>>> 2) # + «path» like this::
>>> 
>>> let path = #Bag.things[0].name
>>> bag[path] 
>>> bag[#Bag.things[0].name]
>>> bag[# .things[0].name]  // Root is inferred as Bag
>>> bag.things[0][#Thing.name]
>>> bag.things[0][#.name]  // Root is inferred as Thing
>>> 
>>> 3) «Root» + # + «path» like this: 
>>> 
>>> let path = Bag#things[0].name
>>> bag[path] 
>>> bag[Bag#things[0].name]
>>> bag[#things[0].name]
>>> bag.things[0][#name]
>>> bag.things[0][Thing#name]
>> 
>> I prefer the third option.  I don’t think we should go with the first option.  Allowing (or requiring) a space seems likely to be confusing.  Both the first and the second still have potential to be confusing.  If `Bag` has a static property named `things` people could be confused about what `#Bag.things` refers to.  If we’re going to use a special sigil it should be positioned such that it eliminates potential for that kind of confusion.
>> 
>>> 
>>> 
>>> 
>>>> On Mar 22, 2017, at 11:47 AM, Matthew Johnson <matthew at anandabits.com <mailto:matthew at anandabits.com>> wrote:
>>>> 
>>>> 
>>>>> On Mar 22, 2017, at 10:36 AM, Vladimir.S via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>> 
>>>>> On 22.03.2017 17:37, Ricardo Parada wrote:
>>>>>> 
>>>>>> 
>>>>>>> On Mar 22, 2017, at 9:30 AM, Vladimir.S <svabox at gmail.com <mailto:svabox at gmail.com>> wrote:
>>>>>>> 
>>>>>>> let path = @Bag.things[0].name
>>>>>>> 
>>>>>>> bag at path
>>>>>>> bag at .things[0].name
>>>>>>> bag at Bag.things <mailto:bag at Bag.things>[0].name
>>>>>>> bag.things[0]@.name
>>>>>>> bag.things[0]@Thing.name
>>>>>> 
>>>>>> It sounds like the @ character is serving two different purposes which confused me at first.
>>>>>> 
>>>>>> If I understood correctly, you are using it to get the key path but also to apply the key path to the bag struct and get the corresponding value.
>>>>>> 
>>>>> 
>>>>> Yes. And the initial proposal suggest the following syntax accordingly:
>>>>> 
>>>>> let path = Bag.things[0].name
>>>>> bag[path]
>>>>> bag[.things[0].name]
>>>>> bag[Bag.things[0].name]
>>>>> bag.things[0][.name]
>>>>> bag.things[0][Thing.name]
>>>> 
>>>> # makes a lot more sense than @ as a sigil.  It follows from #selector and #keyPath.  These are the most similar language features right now where the compiler produces special values.  I think it’s also worth noticing that values produced by #selector and #keyPath are used in normal ways.  There is no magic syntax for their use, just a typed value.  If we’re going to make a change we should use # instead of `.` for accessing these special values but we should stick with subscript for use.
>>>> 
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