[swift-evolution] [swift-evolution-announce] [Review] SE-0096: Converting dynamicType from a property to an operator

[Dave Abrahams]

on Wed May 25 2016, Joe Groff <jgroff-AT-apple.com> wrote:

>> On May 25, 2016, at 12:28 PM, Dave Abrahams <dabrahams at apple.com> wrote:
>> 
>> 
>> on Wed May 25 2016, Joe Groff <jgroff-AT-apple.com> wrote:
>> 
>
>>>> On May 25, 2016, at 11:42 AM, Erica Sadun <erica at ericasadun.com> wrote:
>>>> 
>>>> 
>>>>> On May 25, 2016, at 12:26 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>>>>> I don't understand why the proposal says we can't implement this in the
>>> 
>>>>> library today.  
>>>>> 
>>>>> $ swift
>>>>> Welcome to Apple Swift version 2.2 (swiftlang-703.0.18.8 clang-703.0.30). Type :help for assistance.
>>>>>  1> func dynamicType_<T>(_ x: T) -> T.Type { return x.dynamicType }
>>>>>  2> dynamicType_(42)
>>>>> $R0: Int.Type = Int
>>>>>  3> class B {}
>>>>>  4. class C : B {}
>>>>>  5. dynamicType_(C() as B)
>>>>> $R1: B.Type = __lldb_expr_5.C
>>>>>  6>
>>> 
>>> Now try it with a protocol type, or Any:
>>> 
>>> (swift) var x: Any = 1738
>>> // x : Any = 1738
>>> (swift) dynamicType_(x)
>>> // r0 : Any.Protocol = protocol<>
>>> 
>>> `dynamicType` is really two operations: For normal concrete types, it
>>> produces concrete metatypes, and for existentials, it produces
>>> existential metatypes. There's no way to express the latter for an
>>> arbitrary unknown protocol type in the language today.
>> 
>> Can't we detect in the runtime library that we've got an existential and
>> do the right thing?
>
> Not within the constraints of the type system. P.Protocol and P.Type
> are different types, and the former isn't a model of the latter (since
> P has no methods of its own so can't satisfy P's static requirements).

I mean at runtime, in C++ code.

-- 
Dave