On Thu, Jun 8, 2017 at 11:32 AM, Tab Atkins Jr. <[email protected]> wrote:
> Heya! As part of designing the CSS Typed OM > <https://drafts.css-houdini.org/css-typed-om/>, we've ended up with at > three (so far) places where we want an interface that represents a > list of values: > > * CSSUnparsedValue > <https://drafts.css-houdini.org/css-typed-om/#unparsedvalue-objects>, > representing alternating unparsed strings and variable references in a > property > * CSSNumericArray > <https://drafts.css-houdini.org/css-typed-om/#cssnumericarray>, > representing the n-ary arguments of sum/product/min/max operations > * CSSTransformValue > <https://drafts.css-houdini.org/css-typed-om/#csstransformvalue>, > representing a list of transform functions > > The most natural way to represent these is as an Array, so authors can > get all the Array methods, use `val[i]` syntax, etc. However, doing > so means we lose the ability to type-check sets to the values. > > In general, type-checking is an important part of APIs defined in > WebIDL. Any set to a property on an object is automatically > type-checked, any arguments to methods are automatically type-checked, > etc. By building this into WebIDL, it removes the need for a lot of > annoying boilerplate on the part of spec authors, and more > importantly, removes the possibility that spec authors will forget to > typecheck at all, or will typecheck in incorrect or weird bespoke > ways. Every API responds in exactly the same way when you pass the > wrong type of object, and that's a Good Thing for both users and > implementors. > > And so, it would be great to have the same ability to typecheck these > Array-likes in the Typed OM. > > Naively, this requires a Proxy, so we can intercept uses of the [] > syntax. However, we don't need all the rest of the Proxy > functionality, just this one intercept - a setter function, just for > `obj[foo]` rather than `obj.foo`. Further, Typed Arrays already have > *precisely* the functionality I'd like to use - they intercept setting > using [], and convert it into the appropriate type of number. AWB > also proposed adding exactly this hook in the past (I think it was > called "Array reformation" or something?). > > Thoughts? > > Note that if we don't get some variant of this functionality, these > APIs will instead do one of: > > * just using Proxies (already defined in WebIDL) > When you say "Proxies" here, I believe you're referring to the "indexed properties" feature of WebIDL ( https://heycam.github.io/webidl/#idl-indexed-properties). This seems like the right mechanism to use, from a WebIDL spec, to get the behavior you desire. In Chromium/V8, this doesn't actually use Proxies under the hood (we have something called "indexed property handlers", see the API at https://cs.chromium.org/chromium/src/v8/include/v8.h?rcl=ff98ddca4a1770c2868d44f1cdfe1d4656363f30&l=5781), but it's definitely implementable using Proxies. - Adam > * using .get()/.set() functions with integer arguments to badly emulate > arrays > * just relying on iterator/constructor, so users have to convert the > object to an Array, fiddle with it, then construct a brand new object > > And whichever I end up with, I'll be advocating that as the Standard > WebIDL Way to do array-likes, so we can finally have some consistency. > > ~TJ > _______________________________________________ > es-discuss mailing list > [email protected] > https://mail.mozilla.org/listinfo/es-discuss >
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