[swift-evolution] Proposal: Introduce User-defined "Dynamic Member Lookup" Types

[Steven Brunwasser]

> On Dec 10, 2017, at 13:01, Chris Lattner via swift-evolution <swift-evolution at swift.org> wrote:
> 
>>> On Dec 10, 2017, at 6:00 AM, Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> On Dec 9, 2017, at 10:32 AM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>>> On Sat, Dec 9, 2017 at 11:20 Steven Brunwasser <sbrunwasser at gmail.com> wrote:
>>>> Just wanted to give my 2¢
>>>> 
>>>> ¢
>>>> I don’t like empty protocols—they feel like an abuse of the feature.
>>> 
>>> As has been discussed here before, protocols aren’t about bags of syntax but rather about semantics. Empty protocols are explicitly a demonstration of this settled principle and are very much consistent with the direction of Swift.
>> 
>> 
>> I also think it should be an attribute.
>> 
>> The last time I said this, I pointed out that this was a protocol which:
>> 
>> 1. Has no formal members,
>> 2. But imposes informal requirements enforced by the compiler,
>> 3. Permits and uses arbitrary overloads, and
>> 4. Cannot be usefully used in a generic context or as a type constraint,
>> 
>> None of which are true of ordinary protocols. Since then, we have added:
>> 
>> 5. Can only be conformed to in the main declaration.
>> 
>> This is looking less like a protocol by the day. The square-peg grooves in the round hole are getting deeper and more splintery with every revision.
> 
> Hi Brent,
> 
> This approach definitely could work.  I added it to the alternatives section with a breakdown of why I don’t think it’s the right direction:
> https://gist.github.com/lattner/b016e1cf86c43732c8d82f90e5ae5438#make-this-be-a-attribute-on-a-type-instead-of-a-protocol-conformance

I think the protocol vs attribute debate is more about what it means to be an attribute. I personally see attributes as eventually becoming a more extensible meta-programming tool, but that might not be the community’s consensus.

Assuming that attributes are for meta-programming, this would make more sense as an attribute, since its a tool for dynamically resolving undefined symbols at compile-time.

- Steve
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