[swift-evolution] [Pitch] Parse expressions after 'as' and 'is' instead of just types

[Adrian Zubarev]

+1 Missed this one for a long time :) 

-- 
Adrian Zubarev 

Am 17. Mai 2016 um 16:25:10, David Hart via swift-evolution (swift-evolution at swift.org(mailto:swift-evolution at swift.org)) schrieb:

> 
> +1 Seems like it could enable very powerful behaviour going forward
> 
> > On 16 May 2016, at 23:06, Joe Groff via swift-evolution <swift-evolution at 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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