On Nov 11, 2007, at 4:26 PM, Brendan Eich wrote:
> Part of the thinking in
> resolving #103 in favor of type expression on right of 'is' was to
> future-proof against a world where type and value expressions are
> combined somehow. I don't believe that world will come to pass,
> though. If it should, sooner is better. Restricting 'is' as #103-as-
> resolved did isn't making anyone too happy right now :-/.
Of course, Lars cut the Gordian knot in email just a bit later today:
type T
var v
x is T // if T is type then the compiler knows that,
// otherwise next case applies
x is v // reflect::typeOf(x).isSubtypeOf(v)
// fails if v not a Type object
x is { ... } // interpret as record type
x is [ ... ] // interpret as array type
x is ( ... ) // interpret as union type
x is function ... // interpret as function type
x is null // interpret as null type
x is undefined // interpret as undefined type
x is like T // T must be a type expression anyhow
This allows arbitrary value expressions on the right of 'is', just so
long as they don't look like type expressions in their leftmost
token. Everyone wins. Yay?
/be
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