Re: How to escape implicit 'with (this)' of a method body
On Jul 31, 2008, at 5:24 AM, Dave Herman wrote: We should take this problem seriously. ... Dynamic scope is very bad. Specifically: - Classes are supposed to provide integrity, but dynamic scope makes the internals of code brittle; any variable reference inside the implementation could be subverted by the seemingly innocuous insertion of a property. - Dynamic dispatch has a reasonably understandable cost model, but only if it's confined to explicit property references. With dynamic scope, any variable reference could potentially be very expensive. - Generally, code within a `with' block is brittle and hard to understand, and as Tucker says, the implicit `this.' means that all code inside class methods is within a `with' block... this means that all code inside class methods is brittle! - In the past, this has been enough for many programmers to deprecate all use of `with' -- we should certainly hope to avoid the same happening for classes. I'm not sure of the benefits on the whole of implicit 'this' for class methods, but isn't it plausible to apply it only to static properties and not dynamically inserted ones, so all references continue to be bound at compile time and this sort of brittleness does not come up? Regards, Maciej ___ Es4-discuss mailing list Es4-discuss@mozilla.org https://mail.mozilla.org/listinfo/es4-discuss
Re: How to escape implicit 'with (this)' of a method body
On Fri, Aug 1, 2008 at 1:03 PM, Maciej Stachowiak [EMAIL PROTECTED] wrote:
On Jul 31, 2008, at 5:24 AM, Dave Herman wrote:
We should take this problem seriously. ...
Dynamic scope is very bad.
Specifically:
- Classes are supposed to provide integrity, but dynamic scope makes
the
internals of code brittle; any variable reference inside the
implementation could be subverted by the seemingly innocuous insertion
of a property.
- Dynamic dispatch has a reasonably understandable cost model, but
only
if it's confined to explicit property references. With dynamic scope,
any variable reference could potentially be very expensive.
- Generally, code within a `with' block is brittle and hard to
understand, and as Tucker says, the implicit `this.' means that all
code
inside class methods is within a `with' block... this means that all
code inside class methods is brittle!
- In the past, this has been enough for many programmers to deprecate
all use of `with' -- we should certainly hope to avoid the same
happening for classes.
I'm not sure of the benefits on the whole of implicit 'this' for class
methods, but isn't it plausible to apply it only to static properties
and not dynamically inserted ones,
What is dynamically inserted? I guess would mean properties added to
an instance of a non-sealed class.
so all references continue to be
bound at compile time and this sort of brittleness does not come up?
I think I remember discussion that 'this' in a static context was not valid.
If 'this' in a static context points to the class itself, it allows
for the possibility of the class having a static method, with a
private constructor and a public getInstance method with code
something like:
class E { static function f(){ return new this; } }
E.f()
[object E]
Works in the RI.
But I there was apparently a reason that that was not good, so that is a bug.
http://bugs.ecmascript.org/ticket/74
Garrett
Regards,
Maciej
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Re: How to escape implicit 'with (this)' of a method body
On Aug 1, 2008, at 2:43 PM, Garrett Smith wrote: What is dynamically inserted? I guess would mean properties added to an instance of a non-sealed class. Right. Those should not be addressable by unqualified names in method scope -- you have to use this. so all references continue to be bound at compile time and this sort of brittleness does not come up? I think I remember discussion that 'this' in a static context was not valid. Maciej meant static in the compile-time or lexical sense, not static in the class singleton object property sense. /be ___ Es4-discuss mailing list Es4-discuss@mozilla.org https://mail.mozilla.org/listinfo/es4-discuss
Re: How to escape implicit 'with (this)' of a method body
Here's my take at an example of brittleness:
var bar = 42;
class foo {
function zot () { return bar; }
}
...
class subfoo extends foo {
var bar = 'oops!';
}
(new foo).zot() = 42
(new myfoo).zot() = ?
In AS3, the reference to bar in the zot function would be bound to
this.bar so, even in the subclass, there is no ambiguity and both
cases would output 42. I assume that ES4 would follow this behaviour.
The fragility is more likely to be in the opposite situation, where a
method in a class intends to access a global variable, but the
superclass has declared it too.
Peter
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Re: How to escape implicit 'with (this)' of a method body
On 2008-07-31, at 12:34EDT, Peter Hall wrote:
Here's my take at an example of brittleness:
var bar = 42;
class foo {
function zot () { return bar; }
}
...
class subfoo extends foo {
var bar = 'oops!';
}
(new foo).zot() = 42
(new myfoo).zot() = ?
In AS3, the reference to bar in the zot function would be bound to
this.bar
I don't follow. There is no `this.bar` in the class where zot is
defined.
so, even in the subclass, there is no ambiguity and both
cases would output 42. I assume that ES4 would follow this behaviour.
The fragility is more likely to be in the opposite situation, where a
method in a class intends to access a global variable, but the
superclass has declared it too.
That was my original example, which would also exhibit fragility if
the superclass is developed/evolves independently. In either case,
the fragility stems from the implicit (unreformed) `with this` in
method bodies.
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Re: How to escape implicit 'with (this)' of a method body
On 2008-07-29, at 01:19EDT, Brendan Eich wrote: On Jul 28, 2008, at 10:05 PM, Jon Zeppieri wrote: The original code used without (this), not with, which I took to mean avoid instance properties shadowing globals. Indeed. Perhaps I was being too clever in my pseudo-code. If you read the original as with, then there is no such problem. But if you construct a problematic case using 'with' and dynamic properties, then I concede that 'global' could be shadowed. This is a reason to avoid 'with'. In the ES4 proposals last sent out, you could always use __ES4__::global if you really wanted to avoid conflicts -- unless someone perversely added '__ES4__' as a dynamic instance property. There's no solution to this problem other than reserving at least one name, and we can't do that compatibly. We could reserve __ES4__ in version-selected ES4 mode, but that seems unnecessary. I guess this is considered a small penalty to pay in exchange for adding the magical instance scope to methods (which O-O programmers seem to expect these days). Something we'd regret more if we had multi-methods, perhaps... ___ Es4-discuss mailing list Es4-discuss@mozilla.org https://mail.mozilla.org/listinfo/es4-discuss
How to escape implicit 'with (this)' of a method body
/function foo () { return 'global'; }
class bar {
function foo () { return 'local'; }
function zot () {
// How can I call the global foo from here?
without (this) { foo(); }
}
}/
You could use window[foo](); or whatever the global object is named in
the environment
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Re: How to escape implicit 'with (this)' of a method body
On Jul 28, 2008, at 3:22 PM, Michael Haufe wrote:
function foo () { return 'global'; }
class bar {
function foo () { return 'local'; }
function zot () {
// How can I call the global foo from here?
without (this) { foo(); }
}
}
It's the same as if you lambda-coded the above (here shown in JS1.8
[Firefox 3], note the expression closures):
function bar() {
function foo() 'local';
function zot() global.foo();
}
function foo() 'global';
This example uses ES4's global synonym for the global object, but you
could capture this in a global var at top level:
var global = this;
print(new bar().zot()); // print 'global'
in ES3 or JS1.8 to get the same effect.
You could use window[foo](); or whatever the global object is
named in the environment
No need to quote and bracket, of course -- window.foo() is fine too.
/be
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Re: How to escape implicit 'with (this)' of a method body
On Jul 28, 2008, at 9:46 PM, Brendan Eich wrote:
On Jul 28, 2008, at 3:22 PM, Michael Haufe wrote:
function foo () { return 'global'; }
class bar {
function foo () { return 'local'; }
function zot () {
// How can I call the global foo from here?
without (this) { foo(); }
}
}
It's the same as if you lambda-coded the above (here shown in JS1.8
[Firefox 3], note the expression closures):
function bar() {
function foo() 'local';
function zot() global.foo();
+ return {foo: foo, zot: zot};
}
function foo() 'global';
This example uses ES4's global synonym for the global object, but
you could capture this in a global var at top level:
var global = this;
print(new bar().zot()); // print 'global'
in ES3 or JS1.8 to get the same effect.
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Re: How to escape implicit 'with (this)' of a method body
2008/7/29 Brendan Eich [EMAIL PROTECTED]:
On Jul 28, 2008, at 3:22 PM, Michael Haufe wrote:
function foo () { return 'global'; }
class bar {
function foo () { return 'local'; }
function zot () {
// How can I call the global foo from here?
without (this) { foo(); }
}
}
It's the same as if you lambda-coded the above (here shown in JS1.8 [Firefox
3], note the expression closures):
function bar() {
function foo() 'local';
function zot() global.foo();
}
function foo() 'global';
This example uses ES4's global synonym for the global object, but you could
capture this in a global var at top level:
var global = this;
print(new bar().zot()); // print 'global'
in ES3 or JS1.8 to get the same effect.
You could use window[foo](); or whatever the global object is named in the
environment
No need to quote and bracket, of course -- window.foo() is fine too.
/be
Isn't the 'with' statement in the original example significant? In
the general case, assuming that you don't know what properties 'this'
has (as it may have dynamic properties in addition to the fixtures
determined by its class), you have no way of knowing whether 'global'
or 'window' refers to the global object or to some arbitrary property
of 'this.'
-Jon
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Re: How to escape implicit 'with (this)' of a method body
On Jul 28, 2008, at 10:05 PM, Jon Zeppieri wrote: Isn't the 'with' statement in the original example significant? In the general case, assuming that you don't know what properties 'this' has (as it may have dynamic properties in addition to the fixtures determined by its class), you have no way of knowing whether 'global' or 'window' refers to the global object or to some arbitrary property of 'this.' The original code used without (this), not with, which I took to mean avoid instance properties shadowing globals. If you read the original as with, then there is no such problem. But if you construct a problematic case using 'with' and dynamic properties, then I concede that 'global' could be shadowed. This is a reason to avoid 'with'. In the ES4 proposals last sent out, you could always use __ES4__::global if you really wanted to avoid conflicts -- unless someone perversely added '__ES4__' as a dynamic instance property. There's no solution to this problem other than reserving at least one name, and we can't do that compatibly. We could reserve __ES4__ in version-selected ES4 mode, but that seems unnecessary. /be ___ Es4-discuss mailing list Es4-discuss@mozilla.org https://mail.mozilla.org/listinfo/es4-discuss
