[squeak-dev] [Simulation errors] ObjectTracer on: true

Tobias Pape Das.Linux at gmx.de
Thu Mar 26 18:16:30 UTC 2020

> On 26.03.2020, at 19:02, Thiede, Christoph <Christoph.Thiede at student.hpi.uni-potsdam.de> wrote:
> Hi all,
> ah, you mean because the following does not work either:
> True basicNew ifFalse: [0] ifTrue: [1] "NonBooleanReceiver"
> Again, this is a bit sad :-( Wouldn't it be possible to compare true and false by class rather than identity - both on image + VM side?

That's possible but a different context. That would be Value classes, more or less anti-Singletons.

This is my view:

Values are used to model abstractions; however, besides simple abstractions like integers there are
compound abstractions. That is, they consist of not exactly one component.
However, there is one aspect to compound values that can be important for programming
systems. There is no concept of identity among values as there is among objects;
the question whether this 17 printed here is the same as this 17 one is not meaningful. 
Both represent the abstraction over the integer seventeen. The same holds for compound values 
and their components.

Further, the kind of abstraction itself plays a role in the relation between
two values. The complex number 3+i4 has components that
are equal to the components of (3,4), a point in two-dimensional space, and
the rational number 3/4. However, these three entities do not represent the
same abstraction. From an object oriented point of view, one could say that the
“class” of two entities has to match.

A compound value is a composition of other values and can be the 
components of other values, for example, matrices of complex numbers.

 - * -

There can be compound values that have no components whatsoever, their arity is zero.
For example, given that members of vector spaces are indeed values, the sole
member of the zero-dimensional vector space certainly is a value, too. However,
since the space has no dimensions, its member vector  has no components. 

As a consequence, the only information such Values can convey is  their
“grouping”, that is, what kind of abstraction they represent.

Under certain circumstances, such values can be used to convey the information of singleton-like
entities that are void of information beyond their existence, for example, the
abstract idea of an empty list or set, such as nil.

Best regards

> Best,
> Christoph
> Von: Squeak-dev <squeak-dev-bounces at lists.squeakfoundation.org> im Auftrag von Eliot Miranda <eliot.miranda at gmail.com>
> Gesendet: Donnerstag, 26. März 2020 13:07:44
> An: The general-purpose Squeak developers list
> Betreff: Re: [squeak-dev] [Simulation errors] ObjectTracer on: true
> Hi Christoph,
>> On Mar 26, 2020, at 1:01 AM, Thiede, Christoph <Christoph.Thiede at student.hpi.uni-potsdam.de> wrote:
>> Hi Tobias,
>> > true false nil are special. :)
>> > The ObjectTracer on whatever cannot be ever true.
>> I know they don't work like usual objects because many boolean/nil selectors are inlined ATM.
>> However, this only affects specific selectors.
>> The following worked - if ObjectTracer would not cause a recursion:
>> (ObjectTracer on: true) xor: (ObjectTracer on: false)
>> IMHO, everything in Smalltalk should be an object and should also behave like an object, especially in terms of message dispatching. Is there any other reason to deviate from these rules besides performance?
> That’s not the issue.  The issue is whether nil true and false are singletons or not.  They are.  Alas this is not sufficiently explicit, but it is true of Smalltalk.  The inlining of ifTrue: et all is only practicable because they are singletons, not the other way around.
> So if they are singletons (as U assert they are) then the rest of the system has to behave accordingly and not create other instances.  So the big lies in ObjectTracer not treating nil true and false as singletons.  (IMO)
>> Best,
>> Christoph
>> PS: You cannot suppress the debuggers spawn by an ObjectTracer ... Should we maybe use some kind of notification for this? IWLT do the following:
>> [(ObjectTracer on: true) xor: (ObjectTracer on: false)] on: WarningOrSo do: #resume.
>> Von: Squeak-dev <squeak-dev-bounces at lists.squeakfoundation.org> im Auftrag von Tobias Pape <Das.Linux at gmx.de>
>> Gesendet: Mittwoch, 25. März 2020 21:57:43
>> An: The general-purpose Squeak developers list
>> Betreff: Re: [squeak-dev] [Simulation errors] ObjectTracer on: true
>> > On 25.03.2020, at 21:35, Thiede, Christoph <Christoph.Thiede at student.hpi.uni-potsdam.de> wrote:
>> > 
>> > Hi all!
>> > 
>> > I rarely discovered that much errors with one single command :-)
>> > 
>> > Steps to reproduce
>> > 
>> > Print it:
>> > 
>> > (ObjectTracer on: true) ifTrue: [1]
>> > Expected behavior
>> > 
>> > It is printed: 1
>> > 
>> No :D
>> true false nil are special. :)
>> The ObjectTracer on whatever cannot be ever true.
>> -t
>> > Actual behavior
>> > 
>> > The following tracer notifications appear (proceed them all):
>> > 
>> >        • (3x) #printStringLimitedTo:, #longPrintOn:limitedTo:indent: and again #printStringLimitedTo:, caused by SmalltalkImage >> #logSqueakError:inContext:
>> >        • #mustBeBoolean
>> >        • (3 more print message called by #logSqueakError:inContext: again, see the next error)
>> >        • Error: Where's the jump?
>> > Caused by #mustBeBooleanIn:, #skipBackBeforeJump
>> > If you still keep proceeding:
>> > 
>> >        • (3 more print message called by #logSqueakError:inContext: again, see the next error)
>> >        • Finally, the NonBooleanReceiver error from 2.
>> >        • VM crashes.
>> > Summarizing the bugs
>> > 
>> > ObjectTracer traces too much
>> > 
>> > The amount of messages printed by the ObjectTracer is an unintended side effect of the way it signals calls via the debugger. The debugger logs this error, including the stack trace. How can we avoid this? Should we suppress all further notifications from one ObjectTracer instance during the first one is debugged?
>> > 
>> > #mustBeBoolean is sent to boolean proxy
>> > 
>> > This is caused by the simulation of the inlined #ifTrue: call.
>> > I don't know whether this is worth to be fixed before Scorch?
>> > If yes, I think we would need to disable inlining for this particular selector or make inlining opt-out-able as proposed here (this would probably be way too slow).
>> > 
>> > #mustBoBoolean depends on caller chain
>> > 
>> > It turns out that #mustBeBoolean relies on being called from a context that just did a jump. This makes it impossible to forward this message as usual via a transparent proxy/decorator.
>> > 
>> > We use this pattern in other methods as well. How can we enable transparent wrappers not to collide with method contexts depending on their sender?
>> > 
>> > Best,
>> > Christoph

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