[swift-evolution] [Pitch] Requiring proactive overrides for default protocol implementations.

[Matthew Johnson]

Sent from my iPad

> On Apr 28, 2016, at 1:09 PM, Erica Sadun via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Sorry, that's not my question, which doesn't involve protocols I don't own. Let me restate. Given three types I don't own as follows:
> 
> ```
> struct A : Frobnicate {
>     override required func frobnicate() { print("A") } // "A", or delete to get default
> }
> struct B: Frobnicate {
>     override required func frobnicate() { print("B") } // "B", or delete to get default
> }
> struct C: Frobnicate  { }
> ```
> 
We can't add the keywords if the structs are defined in a module we import but don't own.  We are only declaring the conformance retroactively.  The ability to do this is a crucial aspect of generic programming.  It isn't yet clear how your proposal handles retroactive modeling.

> -- E
> 
> 
>> On Apr 28, 2016, at 11:48 AM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>> 
>> Sorry, that's not my question, which doesn't involve protocols I don't own. Let me restate. Given three types I don't own as follows:
>> 
>> ```
>> struct A {
>>     func frobnicate() { print("A") }
>> }
>> struct B {
>>     func frobnicate() { print("B") }
>> }
>> struct C { }
>> ```
>> 
>> I want to conform them to a protocol of my own design, Frobnicatable, and supply a default `frobnicate()`:
>> 
>> ```
>> protocol Frobnicatable {
>>     func frobnicate()
>> }
>> extension Frobnicatable {
>>     func frobnicate() { print("Default") }
>> }
>> extension A: Frobnicatable { }
>> extension B: Frobnicatable { }
>> extension C: Frobnicatable { }
>> 
>> let c = C()
>> c.frobnicate() // "Default"
>> ```
>> 
>> (Yes, I realize there are issues regarding static and dynamic dispatch that limit the utility of this particular example--let's leave those aside for now.)
>> Where would I affix keywords such as `required` and `override` to make this work after implementation of your proposal?
> 
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