Re: [Harmony proxies] if a trap is missing
Le 07/12/2011 21:21, Tom Van Cutsem a écrit :
2011/12/6 David Bruant bruan...@gmail.com mailto:bruan...@gmail.com
All traps are optional. If missing (more precisely, if
handler[trapName] returns undefined), the proxy defaults to forwarding
the intercepted operation to its target object. [1]
Proxy({}, {get:undefined}).someProperty;
In this case, the description says that the operation is forwarded. I
would prefer a TypeError to be thrown in this case.
To do this, the proxy would need to test whether the trap actually
exists, by performing an in test on the handler.
This breaks the double lifting pattern [1], which depends on the only
interaction between the proxy and the handler being property access.
True.
Aside from that main issue, I think it would be odd for
Proxy({},{}).foo to just work while Proxy({},{get:undefined}).foo
would fail with a TypeError.
On one hand I like when the language throw errors (I'm aware I'm not a
majority with this point of view)
On the other hand, while writing handlers, it can be a very good
practice to explicitely state {trap:undefined} to be sure that the
intention of the programmer was to have the default behavior.
So yeah, undefined-as-a-value and property non-existence having the same
behavior is a good idea indeed.
David
[1] See http://wiki.ecmascript.org/doku.php?id=harmony:reflect_api,
last code snippet under Forwarding.
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[Harmony proxies] if a trap is missing
Hi,
All traps are optional. If missing (more precisely, if
handler[trapName] returns undefined), the proxy defaults to forwarding
the intercepted operation to its target object. [1]
Proxy({}, {get:undefined}).someProperty;
In this case, the description says that the operation is forwarded. I
would prefer a TypeError to be thrown in this case.
Opinions?
David
[1] http://wiki.ecmascript.org/doku.php?id=harmony:direct_proxies
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Re: [Harmony proxies] if a trap is missing
Imagine you have a proxy set dynamically, meaning that you don't have
the traps written in the code, you just pass variables. Then you need
the undefined value to behave the way it does or you can't default to
forwarding.
But I think there is a need to disable a trap. Meaning that instead
of forwarding, it would throw an error. So I'd say:
var p =Proxy({prop:1}, {get:undefined,set:null});
p.prop;// 1
p.prop = 2;// error: no set trap
On Tue, Dec 6, 2011 at 11:13 PM, David Bruant bruan...@gmail.com wrote:
Hi,
All traps are optional. If missing (more precisely, if
handler[trapName] returns undefined), the proxy defaults to forwarding
the intercepted operation to its target object. [1]
Proxy({}, {get:undefined}).someProperty;
In this case, the description says that the operation is forwarded. I
would prefer a TypeError to be thrown in this case.
Opinions?
David
[1] http://wiki.ecmascript.org/doku.php?id=harmony:direct_proxies
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Re: [Harmony proxies] if a trap is missing
Le 07/12/2011 19:23, Xavier MONTILLET a écrit :
Imagine you have a proxy set dynamically, meaning that you don't have
the traps written in the code, you just pass variables. Then you need
the undefined value to behave the way it does
You don't need undefined, you can delete the property.
or you can't default to forwarding.
Even if you couldn't delete the property, you can implement the default
behavior in JavaScript. Something along the lines of:
function trap(...args){
return Reflect.trap(...args);
}
The default handlers will be also provided in a module if I recall
correctly.
But I think there is a need to disable a trap. Meaning that instead
of forwarding, it would throw an error. So I'd say:
var p =Proxy({prop:1}, {get:undefined,set:null});
p.prop;// 1
p.prop = 2;// error: no set trap
I've proposed once that in some API, null and undefined had different
semantics and someone (Brendan, maybe?) said that there is no precedent
of this. I would assume the same stands here (and is another argument to
trigger default trap on property non-existence rather than undefined)
If someone ever considers changing traps dynamically, the trouble will
be hard enough by itself and I think the language should do as much as
possible to help catching errors early.
-
function h(g, s){
return {get: g, set:s};
}
var p = Proxy({}, h(function(...args){console.log(args);}));
p.a = 1;
-
I would claim that in this (simplified-maybe-not-fully-realistic) case
the missing second argument in the h function is very likely to be a bug
and I would be thankful to the language to warn me about it.
David
On Tue, Dec 6, 2011 at 11:13 PM, David Bruant bruan...@gmail.com wrote:
Hi,
All traps are optional. If missing (more precisely, if
handler[trapName] returns undefined), the proxy defaults to forwarding
the intercepted operation to its target object. [1]
Proxy({}, {get:undefined}).someProperty;
In this case, the description says that the operation is forwarded. I
would prefer a TypeError to be thrown in this case.
Opinions?
David
[1] http://wiki.ecmascript.org/doku.php?id=harmony:direct_proxies
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Re: [Harmony proxies] if a trap is missing
2011/12/6 David Bruant bruan...@gmail.com
All traps are optional. If missing (more precisely, if
handler[trapName] returns undefined), the proxy defaults to forwarding
the intercepted operation to its target object. [1]
Proxy({}, {get:undefined}).someProperty;
In this case, the description says that the operation is forwarded. I
would prefer a TypeError to be thrown in this case.
To do this, the proxy would need to test whether the trap actually exists,
by performing an in test on the handler.
This breaks the double lifting pattern [1], which depends on the only
interaction between the proxy and the handler being property access.
Aside from that main issue, I think it would be odd for Proxy({},{}).foo to
just work while Proxy({},{get:undefined}).foo would fail with a TypeError.
Cheers,
Tom
[1] See http://wiki.ecmascript.org/doku.php?id=harmony:reflect_api, last
code snippet under Forwarding.
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