At the last meeting, Mark Miller committed to explore creating a proposal involving double-dispatch and value types. Upon reflection, I remembered that ECMA 334 defines "struct":
http://en.csharp-online.net/ECMA-334:_18._Structs Without delving into syntax, though I do note that "struct" is not a reserved word in ECMAScript, this type of concept does seem to address the basic requirements that Mark was going to pursue, and additionally addresses some of the secondary characteristics (such as not necessarily requiring heap allocation) that Brendan alluded to in the discussion. Independent of any discussion of predefined numeric types, I believe that there is a class of performance critical applications that could benefit from an introduction of "structs" into ES. Additionally, such an addition could make ES more attractive as a compilation target (I say while glancing meaningfully in the direction of Allen). Since implementations will be free to (but not required to) copy the value instead of the reference, it is important that the triple equals operator compares the values for struct types. Double equals can implement different semantics. Taking Rational as an example, the following makes sense to me. Anybody differ? Rational(2,1) == 2 ==> true Rational(1,2) == Rational(2,4) ==> true Rational(2,1) === 2 ==> false Rational(1,2) === Rational(2,4) ==> false Given the above, the answer to the below is less clear: foo = {} foo[Rational(4,2)]='a' foo[Rational(2,1)]='b' foo[2]='c' foo[Rational(2,1)] ==> 'b' ??? foo[Rational(4,2)] ==> 'a' ??? The motivation for the above is the question of whether or not there will be the possibility of any user visible cohorts. Double dispatch also provides an opportunity to mitigate the usability problem that Brendan often laments. While the Rational package can provide a default implementation for a set of operator overloads (I'll note that every double precision binary floating number has an exact representation in terms of a rational number, something that is not true in reverse), users that desire different behavior can simply replace these operators. This could even be done globally (a big red switch, as Brendan seems to prefer). The syntax question remains open, particularly for literals. - Sam Ruby _______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
