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

[Chris Lattner]

> 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 <mailto:swift-evolution at swift.org>> wrote:
>> 
>> On Sat, Dec 9, 2017 at 11:20 Steven Brunwasser <sbrunwasser at gmail.com <mailto: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 <https://gist.github.com/lattner/b016e1cf86c43732c8d82f90e5ae5438#make-this-be-a-attribute-on-a-type-instead-of-a-protocol-conformance>

-Chris


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