+1
-----Original Message-----
From: "Joe Groff via swift-evolution" <swift-evolution@swift.org>
Sent: 16/05/2016 06:06 PM
To: "swift-evolution" <swift-evolution@swift.org>
Subject: [swift-evolution] [Pitch] Parse expressions after 'as' and 'is'instead
of just types
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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