On Aug 8, 2012, at 9:32 AM, Rick Waldron wrote: > > > On Mon, Aug 6, 2012 at 6:44 PM, Allen Wirfs-Brock <[email protected]> > wrote: > Based upon discussions last week in the "July 25, 2012 - TC39 Meeting Notes" > thread, I've created a new strawman proposal for a := operator. See > http://wiki.ecmascript.org/doku.php?id=strawman:define_properties_operator > > := is a convient way to copy properties from one object to another or to > extend an object with new properties. It combines supports for many of the > same use cases as the previously proposed "object extension literals" and the > JSFixed Object.extend proposal. > > The most important characteristic of := is that it uses [[DefineOwnProperty]] > semantics rather than [[Put]] semantics to define properties on its target > object so it doesn't run into issues with assignment to accessor properties > or over-riding inherited readonly properties. It is also smart about dealing > with methods that reference super. > > Some basic examples: > > target := src; //define all own properties of src onto target > > //add a method and an accessor to an existing prototype > Point.prototype := { > plus(aPoint) {return new this.comstructor(this.x+aPoint.x,this.y+aPoint.y}, > get rho() {return Math.sqrt(this.x*this.x+this.y*this.y} > }; > > > Have at it, > > Allen, > > Based on the semantics, specifically 4.I - 4.III, this doesn't work: > > var div = document.createElement("div"); > > // Create a new div that will be hidden > div := { > innerHTML: "<p>P child element that contains a Text node</p>", > id: "my-div", > hidden: true > }; > > > ...This is a pretty major use case that's satisfied by today's "extend" > implementations.
The above is an assignment use case, not a definitional use case. For this you want something that uses [[Put]] semantics like http://wiki.ecmascript.org/doku.php?id=strawman:batch_assignment_operator. There really is a difference between [[Put]] and [[DefineOwnProperty]] semantics and when we tried to combine them in http://wiki.ecmascript.org/doku.php?id=strawman:object_extension_literals it din't work very well. A key point about := is that devs need to be intentional about whether they mean [[Put]] or [[DefineOwnProperty]]. They need to think about the differences and what they are actually trying to do with a specific piece of code. Part of the motivation of using two similar looking operators is to encourage devs to think "does this need to be a + or a := ?" Currently, the convenience of = relative to the inconvenience of Object.defineProperty encourages devs to use = when they really intend [[DefineOwnProperty]] semantics. In some cases this does something other than was actually intended. From your response, I infer we may also need to be concerned that the convenience of := for "setting" multiple property values may encourage dev to use := when they really intend =/[[Put]] semantics. Allen > > I took a crack at implementing your semantics as "Object.extend" to feel out > any issues or "gotchas" and came up with a few. I've posted the impl as well > as a few relevant tests in this gist: > > https://gist.github.com/3296375 > > (NOTE: The super binding is probably wrong or inadequate) > > > I have a feeling that this may be overly complex, but I think that > LeftHandSideExpression accessors should be enforced unless > AssignmentExpression is defining new accessors of the same name. So, if the > AssignmentExpression contains a _data_ property of the same name as an > existing property in the LeftHandSideExpression that is an accessor, the > value of the AssignmentExpression's data property should be assigned, not > defined. > > I hope that makes sense. > > > Rick > > > > > > > > Allen > > _______________________________________________ > es-discuss mailing list > [email protected] > https://mail.mozilla.org/listinfo/es-discuss > >
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