[swift-users] Weird protocol behaviour.

[Rien]

> On 23 Dec 2016, at 09:43, Mikhail Seriukov <zloisop at gmail.com> wrote:
> 
> No it does not.
> You have made a type out of the parameter. It’s no longer a protocol.
> IMO the failure here is to understand the difference between a type and a protocol.
> A type (even if empty) is always a combination of storage with functions (that are assumed to work on the data in storage)
> A protocol is just a definition of functions without the accompanying data.
> 
> I see your point. 
> But actually when I write it as  `let x = X() as P` I really mean that I want `x` to be `AnyObject` but conforming to P, not just protocol itself.
> Is it even possible to downcast it this way?

Yes, but only for a destination that needs a protocol. Not for a destination that needs a type.

I.e. in the example

protocol P {}
struct X:P {}

func foo<A: P>(_ x:A) {}

func bar(_ x:P) {}

func bar() {
    var p: P
    //let x = X() // this compiles
    let x = X() as P // this does not compile. Why?
    p = x
    bar(p)
}

Regards,
Rien.

> 
> 2016-12-23 14:51 GMT+07:00 Marinus van der Lugt <rien at starbase55.com>:
> 
>> On 22 Dec 2016, at 22:43, Howard Lovatt <howard.lovatt at gmail.com> wrote:
>> 
>> The following variation works:
>> 
>> protocol P {}
>> 
>> class P1:P {}
>> 
>> class X:P1 {}
>> 
>> func foo<A:P>(_ x:A) {}
>> 
>> func bar() {
>>     //let x = X() // this compiles
>>     let x = X() as P1 // this does not compile. Why?
>>     foo(x)
>> }
>> 
>> Which adds credence to the bug theory.
> 
> 
> No it does not.
> You have made a type out of the parameter. It’s no longer a protocol.
> IMO the failure here is to understand the difference between a type and a protocol.
> A type (even if empty) is always a combination of storage with functions (that are assumed to work on the data in storage)
> A protocol is just a definition of functions without the accompanying data.
> 
> Rien.
> 
> 
> 
>> 
>> Note two changes: 1. two levels of inheritance and 2. change to classes. If you do two levels using protocols it doesn't work if you use either classes or structs.
>> 
>> 
>>   -- Howard.
>> 
>> On 23 December 2016 at 07:29, Kevin Nattinger <swift at nattinger.net> wrote:
>> I recall seeing a request on the -evolution list for something like `T := X` to indicate it could be X itself or anything inheriting / implementing it, so it’s certainly known behavior, if not desired. IMO it’s a bug and `:` should be fixed to include the root type, whether or not that requires a discussion on -evolution.
>> 
>>> On Dec 22, 2016, at 2:17 PM, Howard Lovatt via swift-users <swift-users at swift.org> wrote:
>>> 
>>> I suspect a compiler bug since A is a P. The equivalent in Java works:
>>> 
>>> interface P {}
>>> class X implements P {}
>>>  
>>> <A extends P> void foo(A x) {}
>>>  
>>> void bar() {
>>>     final P x = new X();
>>>     foo(x);
>>> }
>>> 
>>> -- Howard. 
>>> 
>>> On 23 Dec 2016, at 3:19 am, Rien via swift-users <swift-users at swift.org> wrote:
>>> 
>>>> IMO the error message says it all:
>>>> 
>>>> Playground execution failed: error: MyPlayground8.playground:9:5: error: cannot invoke 'foo' with an argument list of type '(P)'
>>>>    foo(x)
>>>>    ^
>>>> 
>>>> MyPlayground8.playground:9:5: note: expected an argument list of type '(A)'
>>>>    foo(x)
>>>>    ^
>>>> 
>>>> I.e. you are passing in a protocol while the function is specified for a type.
>>>> Said other way: On which data do you expect the protocol to operate?
>>>> 
>>>> Regards,
>>>> Rien
>>>> 
>>>> Site: http://balancingrock.nl
>>>> Blog: http://swiftrien.blogspot.com
>>>> Github: http://github.com/Swiftrien
>>>> Project: http://swiftfire.nl
>>>> 
>>>> 
>>>> 
>>>> 
>>>>> On 22 Dec 2016, at 17:05, Mikhail Seriukov via swift-users <swift-users at swift.org> wrote:
>>>>> 
>>>>> Hello community! I' wondering if somebody can explain this to me.
>>>>> Please take look at the snippet.
>>>>> 
>>>>> protocol P {}
>>>>> struct X:P {}
>>>>> 
>>>>> func foo<A:P>(_ x:A) {}
>>>>> 
>>>>> func bar() {
>>>>>    //let x = X() // this compiles
>>>>>    let x = X() as P // this does not compile. Why?
>>>>>    foo(x)
>>>>> }
>>>>> 
>>>>> I expect the both cases to work though. But only first works? And I do not understand why.
>>>>> My coworkers said that it is a compiler bug, but I'm not shure it is.
>>>>> Thanks for the help.
>>>>> _______________________________________________
>>>>> swift-users mailing list
>>>>> swift-users at swift.org
>>>>> https://lists.swift.org/mailman/listinfo/swift-users
>>>> 
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>> 
>> 
> 
>