[Harmony proxies] Thoughts on function proxies

David Bruant bruant at enseirb-matmeca.fr
Fri Mar 18 03:46:30 PDT 2011

Le 16/03/2011 09:11, Tom Van Cutsem a écrit :
> Hi,
> I have no objections to the use cases you're proposing, but in the
> interest of keeping the Proxy.createFunction method somewhat simple,
> why isn't the following sufficient:
> // [[Construct]]-less functions:
> var f = Proxy.createFunction(handler, callTrap, function() { throw
> "can't construct"; });
> // [[Call]]-less functions:
> var f = Proxy.createFunction(handler, function() { throw "can't call";
> }, constructTrap);
It is sufficient, you are completely right :-)
And as noted by Brendan, it is actually indeed better that the user can
customize the thrown message.

> Distinguishing between |null| and |undefined| to trigger one default
> behavior or the other feels a bit subtle to me. Are there precedents
> for this?
> As for the clarifying note on constructors: agreed. It doesn't hurt to
> be explicit about this.
> Cheers,
> Tom
> 2011/3/15 David Bruant <bruant at enseirb-matmeca.fr
> <mailto:bruant at enseirb-matmeca.fr>>
>     Le 15/03/2011 00:52, David Bruant a écrit :
>>     Hi,
>>     I've been thinking about Function proxy use cases lately. Things
>>     that the spec do and that could be convenient to emulate as well
>>     or just things that could be useful.
>>     * [[Construct]]-less functions.
>>     All Array.prototype methods are like that as well as all global
>>     object methods (parseInt...) as well as a most of built-in
>>     methods. In some way, if you want to define function which
>>     wouldn't make sense as constructors (Math functions are a
>>     caricature of that), [[Construct]]-less functions would be a good
>>     way to raise error if someone, by accident, write "new blabla()"
>>     or even "new blabla".
>>     It has been suggested here:
>>     https://mail.mozilla.org/pipermail/es-discuss/2011-March/013019.html
>>     I think that it was a bit too hacky. I would suggest more of a
>>     syntax like:
>>     var pf = Proxy.createFunction({}, myCall, null);
>>     * [[Call]]-less constructors
>>     Already possible throughpf = Proxy.createFunction({}, null,
>>     myConstructor);
>     Spoke too quickly. This throws a TypeError both in proposal and
>     FF4 implementation.
>     I would argue that this can be useful if I want to define "just a
>     constructor". In most cases, people use function as functions or
>     constructor, I have rarely seen both (except for Array or other
>     built-in constructors which are, as said, built-in). [[Call]]-less
>     functions would be a way to prevent what is intended to be a
>     constructor to be called as a function.
>>     * "new A" behaves exactly like "A". This can be found for Arrays
>>     for instance.
>>     Already possible through pf = Proxy.createFunction({}, myCall,
>>     undefined);
>>     Finally, I'd like to add a note on constructors. On the proposal
>>     is written: "If no constructTrap is provided, new proxy(...args)
>>     is reified as calling the proxy’s callTrap with |this| bound to a
>>     new object delegating to proxy.prototype".
>>     In order to clarify, I think that instead of "proxy.prototype" it
>>     would be better to say "the result of calling the get trap on the
>>     proxy with argument 'prototype'". The get trap is the equivalent
>>     tothe [[Get]] internal method used in ES5.1 13.2.2step 5.
>>     Or maybe saying that the [[Construct]]algorithm (13.2.2) will be
>>     used.
>>     David
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