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

[Xiaodi Wu]

On Thu, Apr 28, 2016 at 12:25 PM, Erica Sadun <erica at ericasadun.com> wrote:

> >
> > On Apr 28, 2016, at 11:05 AM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> >
> > How is retroactive modeling accommodated in this scheme? Say I want to
> conform three types I don't own to a protocol of my design and supply a
> default implementation for a protocol requirement. How would I go about it?
>
> extension NotMyType: ProtocolIdesigned {...}
>
> extension NotMyProtocol {
>      required func method1() {...}
>      override required func method2() {...} // assumes NMP has already
> provided default
>      func newThing() {...}
> }
>
> The only tricky bit is when NotMyProtocol already has a default required
> func implementation and an override required func implementation (although
> that should be rare). In such case, you probably have to create a new
> protocol DerivedProtocol: NotMyProtocol and work from there. The override
> in the DerivedProtocol will then take priority over the override in
> NotMyProtocol.
>

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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