On Aug 11, 2014, at 5:55 PM, Tab Atkins Jr. wrote: > Restarting this thread, because it's still an issue a year+ on. > > In the Font Loading spec <http://dev.w3.org/csswg/css-font-loading/>, > I have a FontFaceSet interface > <http://dev.w3.org/csswg/css-font-loading/#FontFaceSet-interface>, > which is a Set that contains FontFace objects, plus a few other > methods and attributes. > > Except that it's not a Set, because I can't subclass Sets reliably. > Instead, I have to duplicate the entire Set interface *as it exists > today*, and have it delegate to a hidden internal Set object. This > means that when TC39 extends the Set interface, FontFaceSet will have > to be manually updated and bugfixed to contain its own version of the > new methods. It also means that when authors define new methods on > Set.prototype, they don't apply to FontFaceSet objects unless they > manually copy them to FontFaceSet.prototype as well. > > This is obviously bad. But I can't just directly subclass Set. If I > do, then authors can do Set.prototype.add(ffset, "hahaha, I'm a > string, not a font face!"), and then the Set is corrupted. I could, > of course, defensively write all the FFS operations to check each > entry for being a FontFace object first, but that's not quite enough, > because *authors* also have to do this when iterating a FFS. > > It's accepted and idiomatic for all sorts of methods in the web > platform to do typechecks on their arguments, and throw errors when > they don't match what's expected. It's impossible for me to robustly > do this for the Set and Map methods, though. Can we come up with > something that allows me to enforce these kinds of checks, like I'm > currently doing with my Set-lookalike, and like we do everywhere else > in the entire web platform? > > ~TJ >
You can imaging the specification of Set being enhanced to include a private slot whose value was a function that was used to validate values that were being added to the set. Such a function could be (optionally) set when a set is created and would be called by the built-in add method. Somebody might want to develop a proposal for ES7. I think it could be done in a manner that was backwards compatible with the E#S6 Set spec. You could also do something similar with Map and a function that validates both keys and values. Allen _______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
