Just to clarify: in your example, did you mean for DerivedA and DerivedB to
inherit from Base?
···
On Mon, May 16, 2016 at 4:18 PM Matthew Johnson via swift-evolution < swift-evolution@swift.org> wrote:
+1
Sent from my iPhone
> On May 16, 2016, at 4:06 PM, Joe Groff via swift-evolution < > swift-evolution@swift.org> wrote:
>
> Currently, we parse a type after 'as[?!]' and 'is'. This is mostly what
you'd expect, but does lead to problems when an 'as' expression appears as
part of a comparison:
>
> 20 as Int64 < y as Int64 // error, '>' expected to close generic
parameter list Int64<y>
>
> Looking to the future, many people have also expressed interest in the
ability to do dynamic type checks against metatype values, not only static
types, as in:
>
> class Base {}
> class DerivedA {}
> class DerivedB {}
>
> var x: Base.Type = DerivedA
>
> DerivedA() as? x // succeeds
> DerivedB() as? x // fails
>
> If we accept
https://github.com/apple/swift-evolution/blob/master/proposals/0090-remove-dot-self.md,
dropping the '.self' requirement to refer to type objects, then I think we
should also change 'is' and 'as' to parse the expression grammar on their
right-hand side, leaving it up to the normal expression disambiguation rule
to handle angle brackets. This solves the '20 as Int64 < x' problem, and
prepares us to support dynamic is/as queries in the future. (To be clear,
designing dynamic queries should be its own discussion.) What do you all
think?
>
> -Joe
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Dan Appel