> On May 25, 2016, at 12:28 PM, Dave Abrahams <[email protected]> wrote: > > > on Wed May 25 2016, Joe Groff <jgroff-AT-apple.com> wrote: > >>> On May 25, 2016, at 11:42 AM, Erica Sadun <[email protected]> wrote: >>> >>> >>>> On May 25, 2016, at 12:26 PM, Dave Abrahams via swift-evolution >>>> <[email protected]> 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). -Joe _______________________________________________ swift-evolution mailing list [email protected] https://lists.swift.org/mailman/listinfo/swift-evolution
