> On 30 Dec 2016, at 12:14, Mikhail Seriukov via swift-users
> <swift-users@swift.org> wrote:
>
> Ok,
> But I think I still do not get it.
> What does really happen when we write this?
>> let x = X() as P
>>
'X()' creates a value.
'as P’ constrains the value such that the only things we know about it is that
the value will conform to the protocol P
‘let x =‘ assigns the value to a constant, and the only thing we know about
that constant is that we can call an operation of protocol P on it.
Rien.
> As I said, I expect x to be Any<P> after that. If it is, then it should be ok
> IMO.
> But if it is not then what is the actual type of x?
>
> So the real question is how the type checker works here?
>
>
> 2016-12-25 22:13 GMT+07:00 Slava Pestov <spes...@apple.com>:
>
>> On Dec 22, 2016, at 4:43 PM, Howard Lovatt via swift-users
>> <swift-users@swift.org> 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.
>
> It’s an intentional limitation. Protocols do not conform to themselves.
> Lifting the restriction would be difficult to do efficiently given our
> representation of generics and protocols at runtime.
>
> Slava
>
>>
>> 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 <sw...@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@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@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@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@swift.org
>>>>> https://lists.swift.org/mailman/listinfo/swift-users
>>>>
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