I think we should preserve double-lifting via one trap in the
meta-handler. No has-before-invoke. Allen?
/be
Tom Van Cutsem <mailto:[email protected]>
September 10, 2013 3:58 AM
+1 for refactoring to use [[Invoke]] for most of these.
One reservation about the use of [[Invoke]] to call proxy traps (i.e.
functions on handler objects): the alternative pattern of first
probing with HasProperty destroys the "double lifting" pattern, which
depends on the proxy only ever performing 1 type of operation (a
property access) on its handler.
(double lifting is a pattern whereby the handler object is itself a
proxy. It allows you to write very generic proxies that only need to
implement a single "get" trap. See
<http://soft.vub.ac.be/~tvcutsem/invokedynamic/assets/proxies.pdf
<http://soft.vub.ac.be/%7Etvcutsem/invokedynamic/assets/proxies.pdf>>
section 4.5 for details.)
If we refactor to use [[HasProperty]] + [[Invoke]] to call traps, the
double lifting pattern requires implementing both the has() trap and
the get() trap. Not fatal, but I just want to point it out.
Regards,
Tom
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Jason Orendorff <mailto:[email protected]>
September 9, 2013 4:23 PM
A lot more can be found by searching for [[Call]]:
- the call to @@toPrimitive from ToPrimitive (in 7.1.1)
- the call to @@hasInstance from instanceofOperator (in 12.8.1)
- the call to .toISOString from Date.prototype.toJSON (in 20.3.4.37)
- the call to .join from Array.prototype.toString (in 22.1.3.27)
- the call to .set from the Map constructor (in 23.1.1.1)
- the call to .add from the Set constructor (in 23.2.1.1)
and at that point I stopped looking.
-j
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Allen Wirfs-Brock <mailto:[email protected]>
September 9, 2013 2:50 PM
On Sep 9, 2013, at 2:37 PM, Jason Orendorff wrote:
There are some places in the spec where we might use [[Invoke]], but don't:
- implicit .toString() and .valueOf() calls
(except Object.prototype.toLocaleString)
- implicit .toJSON() calls
- calls to Proxy handler methods
This seems unfortunate. Proxies with default handlers will work in
most places, thanks to the default [[Invoke]] trap, but not in ("" +
proxyObject) etc.
Is this still likely to change?
I'll take a look at it. My first reaction is that this sounds like a good idea.
The apparent reason [[Invoke]] is not used in these places is that
they all do a [[Get]] first and check whether the property has a
callable value.
Actually, I don't think even considered those uses when I added [[Invoke]].
Allen
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Jason Orendorff <mailto:[email protected]>
September 9, 2013 2:37 PM
There are some places in the spec where we might use [[Invoke]], but
don't:
- implicit .toString() and .valueOf() calls
(except Object.prototype.toLocaleString)
- implicit .toJSON() calls
- calls to Proxy handler methods
This seems unfortunate. Proxies with default handlers will work in
most places, thanks to the default [[Invoke]] trap, but not in ("" +
proxyObject) etc.
Is this still likely to change?
The apparent reason [[Invoke]] is not used in these places is that
they all do a [[Get]] first and check whether the property has a
callable value. But in the algorithm for yield*, in 14.4.1.2, we have
a similar check-before-calling situation, implemented using a
different idiom:
(step 4.d.viii in the last algorithm of 14.4.1.2)
Having a [[GetMethod]] hook instead of an [[Invoke]] hook might make
it easier to use consistently everywhere.
-j
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