Would you mind posting to public-script-coord? This sounds like a good addition to WebIDL.
-----Original Message----- From: Tab Atkins Jr. [mailto:[email protected]] Sent: Wednesday, May 29, 2013 3:24 PM To: public-webapps Subject: [webidl] Add a [Maplike] tag? I want to convert the interface at <http://dev.w3.org/csswg/css-variables/#the-cssvariablesdeclaration-interface> from an "object map"* to an actual Map, so that it uses a (soon-to-be) familiar API and interoperates with other code that works on Maps. I recently talked about this on es-discuss a bit (see the threads "Overriding Map/etc with get/set hooks?" and "Non-generic traps for non-generic objects"), and the conversation seems to have ended with the conclusion "do it in WebIDL instead". So! I propose that we have some way to tag an interface as "map-like". This means that it exposes the same methods and properties as Map and has Map on the prototype chain, but specifies potentially custom behavior for getting/setting/deleting values. This could be based on the gist at <https://gist.github.com/jorendorff/5662673>, in terms of defining the operations. The spec author would have to define similar things to today's "object maps" - a list of map tuples (for the iterator), a mapgetter, a mapsetter, and a mapdeleter. For my specific example, I'd define: * the list of map tuples are the custom properties defined on the associated CSSStyleRule, * the mapgetter coerces the argument to string, mangles it appropriately, then calls getPropertyValue() on the associated CSSStyleRule, * the mapsetter coerces both arguments to strings, mangles the first appropriately, then calls setProperty() on the associated CSSStyleRule, * the mapdeleter coerces+mangles the argument, then calls removeProperty() on the associated CSSStyleRule. There are a lot of other interfaces that could benefit from being changed to Maps, so I think there's an obvious strong argument for this being used in the future. For example, we're having a conversation about the interface for @font-feature-values, and it would likely benefit from this as well. While we're at it, do the same thing for "set-like", which is the same but for the Set class. ~TJ * Why change from an "object map"? This kind of interface is subject to accidental or malicious corruption when things are added to any of its prototypes (including Object.prototype), where it *looks* like there's a value on it, but there really isn't, and the only way to actually set that key on the actual object map is to use Object.defineProperty(). It's also harder to extend in the future; if you ever want to add a method or property to it, you have to be careful about existing usage possible using your desired name as a key, and add a restriction disallowing that key from being used to the appropriate spec. Maps gets around all these issues nicely.
