On May 15, 2011, at 1:54 PM, Dmitry A. Soshnikov wrote: >> See last reply for more on joining. It occurs to me you thought scope chain >> varying in the context of a pure hash-rocket such as #->42 means that >> function cannot be joined, but since it is pure, it need not entrain its >> scope chain as an internal [[Scope]] property. >> >> More, since # freezes, there's no need for it to be duplicated, since there >> is no mutation side channel. >> >> So http://wiki.ecmascript.org/doku.php?id=strawman:arrow_function_syntax >> proposes freezing and joining, only if the user specifies via the # prefix. > > Yep, I got it, thanks, though, as mentioned seems it can be managed then at > engine level without explicit specifying by the user, no?
Do you mean that users shouldn't have to write # prefixes to get joining? That is hard. SpiderMonkey implements joining for lambdas that do not close over outer lexical variables provided those lambdas are used as the initializer of a property in an object literal, or as the full RHS of assignment to a property of an Object instance. This optimization wins but it requires read and write barriers (which we already have for getters and setters in general). It's complex. The # prefix means freeze too, and that is required for joining. ES3 left it to implementations to join but that was a mistake that (if implementations had done so; none did after early SpiderMonkey bugs were fixed) creates a mutation side channel. See also the ES3 global regexp singleton per lexical regular expression bug. /be _______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
