On Apr 3, 2011, at 11:12 AM, Allen Wirfs-Brock wrote: > On Apr 2, 2011, at 5:11 PM, Brendan Eich wrote: >>> Is multiple inheritance a use case that TC39 intends to address in a >>> generic manner? >> >> No. > > Inheritance-based instanceof testing for the purpose of dynamic > classification of objects is most appropriate in statically typed > subclasing===subtyping languages. Multiple inheritance (at least for > interfaces) is almost essential in such languages. That isn't is the case > for dynamically typed subclassing!==subtyping languages. There is lots of > experience with such languages that shows that dynamic classification via > class or superclass inclusion testing is a poor practice.
Thanks for filling in after my terse "No." The countervailing force here is the DOM being implemented in C++ (or C in the old days) in browsers. Such implementations use nominal subtyping and it leaked right through into JS. This is not just an accident of history: implementors want fast is-a testing in the host programming language. Nevertheless, we are working on self-hosting the DOM, which should help dispel concerns about performance and scalability. I don't see a lot of instanceof checks in modern JS, even DOM-based JS, so I believe the "nominal subtyping for performance" argument is overstated. However, there are currently C++-implemented internal-to-the-DOM-and-browser code paths that do such is-a tests (e.g., event dispatch and bubbling). These need to be self-hosted, and the whole system benchmarked. We'll keep this list apprised of results. /be _______________________________________________ es-discuss mailing list es-discuss@mozilla.org https://mail.mozilla.org/listinfo/es-discuss
