[Vm-dev] Re: [Pharo-dev] Proxy DNU vs CannotInterpret. (Ghost
stepharo at free.fr
Thu Mar 24 06:43:11 UTC 2016
In pharo 60 we will revisit and shrink protoObject.
Le 15/2/16 19:34, Eliot Miranda a écrit :
> Hi Stef,
> On Mon, Feb 15, 2016 at 8:18 AM, stepharo <stepharo at free.fr
> <mailto:stepharo at free.fr>> wrote:
> Hi eliot
> I think that the difference is really what guillermo explained.
> With DNU you can only trapped methods that are not defined while
> if you want to implement true proxy you do not want holes.
> I think that also with DNU proxy there were some limits realyed to
> classe but I do not remember.
> As far as the VM goes, there is /one/ minor issue with special
> selector #== and #class (*). But the real issue is that ProtoObject
> implements far too much protocol. If one creates a class that
> inherits from nil all it needs to do is implement doersNotUnderstand:
> and it will catch every message (*). For this to work the debugger
> and basic inspectors must use the mirror primitives otherwise there
> will be infinite recursion of MNUs.
> Now the issue with special selector #== and #class is that these
> bytecodes are specified as no lookup. So if one uses bytecodes 182
> (#==) or 199 (#class) these operate without sending messages and will
> compare the object or answer the object's class without sending
> In the VW VM I added a flag so one could turn this behaviour off, and
> that's certainly easy to do in our VM. Another approach is to have the
> bytecode compiler not send these.
> But given our VM it is certainly possible to use doesNotUnderstand:
> proxies, using a much slimmer class than ProtoObject. In fact a good
> project would be to try and shrink ProtoObject until it provides the
> minimum, which would be a #doesNotUnderstand: method that raises an
> error that reminds the programmer that they need to implement their
> own doesNotUnderstand: handler in subclasses of ProtoObject.
> Le 15/2/16 16:06, Eliot Miranda a écrit :
>> Hi Denis,
>> On Feb 15, 2016, at 6:53 AM, Denis Kudriashov
>> <dionisiydk at gmail.com <mailto:dionisiydk at gmail.com>> wrote:
>>> 2016-02-15 15:32 GMT+01:00 Mariano Martinez Peck
>>> <marianopeck at gmail.com <mailto:marianopeck at gmail.com>>:
>>> As far as I can remember (this was about 4 years ago) The
>>> use of #cannotInterpret: was the only way to be able to
>>> intercept everything without breaking the system.
>>> Subclassing from ProtoObject make your proxy to understand
>>> some messages. Subclassing from nil was technically
>>> possible, but I found out the system would simply crash as
>>> it didn't know at all how to handle other subclasses of nil.
>>> Even with the debugging support I added as you can read in
>>> the paper. Maybe things have changed.
>>> If subclassing from nil does not break the system and you
>>> can still inspect, debug proxies, then sure, you can give a
>>> try to Ghost using #dnu rather than #cannotInterpret:. In
>>> fact, I would like to have a dnu-based approeach that is as
>>> reliable as it was #cannotInterpret: in the sense of how
>>> much I can intercept. With #cannotInterpret I could trap
>>> EVERYTHING (everything but #==) and then decide what to do.
>>> Subclassing from nil would get you there too if you make it
>>> Actually I already implemented it many yeas ago for Mocketry.
>>> But subclassing from nil works correctly only in VW. I got
>>> problems when porting it to Squeak (at that time).
>>> Now I want to give DNU another chance. It will simplify logic
>>> very much. It will remove strange hierarchy of proxies which is
>>> needed for trick but raises many questions.
>> There is no reason in principle why Squeak/Pharo should not be
>> able to use MNU proxies in the same way as VW. The key is to use
>> the mirror primitives in basic inspectors that are used to
>> inspect the proxies and in the debugger to simulate code.
>> Without the mirror primitives many sends to a proxy are likely
>> to create an infinite recursion. As yet we don't have a robust
>> low space handler that catches infinite recursion so debugging
>> can be frustrating. But we can work on this. IMO the MNU
>> approach is to be preferred.
> best, Eliot
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