[swift-evolution] [Proposal] Allow protocols to require functions with default parameter values.

[肇鑫]

Hi Dave,

If you have read the whole list, you can find this proposal is trying to
simplify a Protocol like:

protocol A {

    func printSomething()

    func printSomething(something:String)

}


to


protocol B {

    func printSomething(something:String = default) // default is a
keyword, not a certain value, meaning the implementation must have a
default parameter value

}


So that we can solve the previous talking bug. Protocol B can use both
printSomething() and printSomething("john")


Currently, if we only have


protocol C {

    func printSomething(something:String)

}


We can not call printSomething() with protocol c.


zhaoxin

On Sun, Jan 17, 2016 at 9:51 AM, <davesweeris at mac.com> wrote:

>
> On Jan 16, 2016, at 17:28, 肇鑫 <owenzx at gmail.com> wrote:
>
> Hi Dave,
>
> Like you said, you approach does not solve the bug.
>
> And this is the bug: If your protocol `Foo` has a function `foo(X)`, and
>> your type has a function `foo(Y)` *where Y can be turned into X through
>> the use of default parameters*, then it’s impossible for your type to
>> conform to `Foo`, even though you could use it as a `Foo` in your code. In
>> practice, I think this only matters if you’re using “macros” like
>> __LINE__ or something as your default parameter, but it’s still annoying
>> (to me, anyway).
>>
>>
> ​as this proposal does​.
>
>
> No, I was saying that (at least at a glance) I didn’t think the original
> proposal solved the problem I described, and that I thought my “counter”
> proposal would have all the functionality (albeit from the “other
> direction”) that Vatsal was proposing *and* fix the glitch.
>
> …Maybe we’re saying the same thing.
>
>
>
> I still think if we can call instance.function(), we should also call
> (instance as protocol).function(). If we can't, there must be something not
> right here.
>
> zhaoxin
>
>
> You can, as long as your protocol isn’t restricted to only being used as a
> generic constraint (no Self or typealiases):
>
> protocol Foo {
>     func foo() -> String
> }
>
> extension Int : Foo {
>     func foo() -> String { return "lovely" }
> }
>
> func bar(x: Any) -> String {
>     if x is Foo { return "Here, have some \((x as! Foo).foo()) foo." }
>     else        { return "NO FOO FOR YOU!!!" }
> }
>
> print(bar(0))   //Here, have some lovely foo.
> print(bar(""))  //NO FOO FOR YOU!!!
>
> - Dave Sweeris
>
>
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