On Sat, Aug 30, 2008 at 2:02 PM, Erik Arvidsson
<[EMAIL PROTECTED]> wrote:
> On Fri, Aug 29, 2008 at 22:51, Brendan Eich <[EMAIL PROTECTED]> wrote:
>> #c wins by maximizing TC purity, utility, and safety.
>>
>> /be
>
> d) Lexical scoping
>
> I thought one of the points with changing *this* in strict mode was to
> make it use lexical scoping where it "makes sense"?
I agree with all the goals you state. There was an earlier proposal,
made by Crock and agreed to before March by the overall committee
(both the 3.1 and 4 factions). It was even somewhat orthogonal to
strict-mode, and would have made the non-strict language more
intuitive as well. However, at the March meeting I presented something
like the following example which demonstrates a fatal ambiguity:
function PointMaker() {
this.count = 0;
that = this;
function Point(x, y) {
this.x = x;
this.y = y;
that.count++;
}
Point.prototype.getX = function() { return this.x; }
Point.prototype.setX = function(x) { this.x = x; }
this.Point = Point;
}
// Each PointMaker is a factory for new points that counts how many
points it made:
var pointMaker = new PointMaker();
foo(new (pointMaker.Point)(3,5));
// How many has it made so far?
print(pointMaker.count); // 1
If foo() only uses the point it's been given in the expected ways, all
is cool. But what if foo() says:
function foo(pt) {
var setX = pt.setX;
setX(44);
}
or equivalently
function foo(pt) {
(true && pt.setX)(44);
}
Under Crock's proposed rule, this will bring about the effect of
pointMaker.x = 44;
even though one would have thought foo() had no access path whatsoever
to pointMaker. In other words, a lexical-this-defaulting rule as Crock
proposed, and as I gather you seem to be proposing, would cause the
'this' in setX to be bound to the lexically enclosing 'this' when setX
is called as a function. This violates it's authors intent in the same
way that the non-strict global-this-capture does. The only difference
is that inappropriate access is being provided to pointMaker rather
than the global object.
If a lexical this-capture rule can be made to work without these kinds
of problems, it would indeed be better than having to explain
".bind(this)" to ES programmers. Suggestions?
--
Cheers,
--MarkM
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