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Can a class be not equal to itself?

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khinsen

Can a class be not equal to itself?

Hi everyone,

It's been a while since I have encountered mysterious behavior in Pharo,
but today it happened. I set up a dictionary with classes as keys, and
got "Key not found" errors for keys that were definitely in my
dictionary. A quick debugging session showed the underlying issue: I had
two references to a class which look the same when inspected, but turn
out to be not identical (==) and thus not equal (=). How can this happen?

Konrad.

Tudor Girba-2

Re: Can a class be not equal to itself?

Hi Konrad,

Could it be that you have overridden = on the class side of your class?

Otherwise, I do not see how this can happen.

Cheers,
Tudor

> On Dec 31, 2020, at 12:19 PM, Konrad Hinsen <[hidden email]> wrote:
>
> Hi everyone,
>
>
> It's been a while since I have encountered mysterious behavior in Pharo, but today it happened. I set up a dictionary with classes as keys, and got "Key not found" errors for keys that were definitely in my dictionary. A quick debugging session showed the underlying issue: I had two references to a class which look the same when inspected, but turn out to be not identical (==) and thus not equal (=). How can this happen?
>
>
> Konrad.

--
feenk.com

"It's not how it is, it is how we see it."

khinsen

Re: Can a class be not equal to itself?

In reply to this post by khinsen

On 31/12/2020 12:19, Konrad Hinsen wrote:

> It's been a while since I have encountered mysterious behavior in
> Pharo, but today it happened. I set up a dictionary with classes as
> keys, and got "Key not found" errors for keys that were definitely in
> my dictionary. A quick debugging session showed the underlying issue:
> I had two references to a class which look the same when inspected,
> but turn out to be not identical (==) and thus not equal (=). How can
> this happen?

Update, after a few more debugging sessions: my dictionary is the result
of a deepCopy operation, meaning the class has been copied. And the copy
of a class is indeed a distinct but otherwise indistinguishable class.
'Object copy = Object' is 'false'.

Konrad.

Richard O'Keefe

Re: Can a class be not equal to itself?

#deepCopy is one of those things that is best avoided,

because it violates the key principle of OOP that an

object is in charge of its own data and behaviour.

It can not so much break invariants as crush them and

bury them in an unmarked grave, just like #shallowCopy.

On Fri, 1 Jan 2021 at 03:25, Konrad Hinsen <[hidden email]> wrote:

On 31/12/2020 12:19, Konrad Hinsen wrote:

> It's been a while since I have encountered mysterious behavior in
> Pharo, but today it happened. I set up a dictionary with classes as
> keys, and got "Key not found" errors for keys that were definitely in
> my dictionary. A quick debugging session showed the underlying issue:
> I had two references to a class which look the same when inspected,
> but turn out to be not identical (==) and thus not equal (=). How can
> this happen?

Update, after a few more debugging sessions: my dictionary is the result
of a deepCopy operation, meaning the class has been copied. And the copy
of a class is indeed a distinct but otherwise indistinguishable class.
'Object copy = Object' is 'false'.

Konrad.

Richard O'Keefe

Re: Can a class be not equal to itself?

What I said just now is true, but it doesn't tell the

whole story.  It turns out that Squeak, VisualWorks,

GNU Smalltalk, and Pharo *deliberately* make

aClass copy return a new class with the same superclass

and methods (but no subclasses).  Dolphin and VisualAge

do not.  So if you don't want to make copies of classes,

you must avoid ALL of #deepCopy, #shallowCopy, AND #copy.

On Fri, 1 Jan 2021 at 13:59, Richard O'Keefe <[hidden email]> wrote:

#deepCopy is one of those things that is best avoided,
because it violates the key principle of OOP that an
object is in charge of its own data and behaviour.
It can not so much break invariants as crush them and
bury them in an unmarked grave, just like #shallowCopy.

On Fri, 1 Jan 2021 at 03:25, Konrad Hinsen <[hidden email]> wrote:
On 31/12/2020 12:19, Konrad Hinsen wrote:

> It's been a while since I have encountered mysterious behavior in
> Pharo, but today it happened. I set up a dictionary with classes as
> keys, and got "Key not found" errors for keys that were definitely in
> my dictionary. A quick debugging session showed the underlying issue:
> I had two references to a class which look the same when inspected,
> but turn out to be not identical (==) and thus not equal (=). How can
> this happen?

Update, after a few more debugging sessions: my dictionary is the result
of a deepCopy operation, meaning the class has been copied. And the copy
of a class is indeed a distinct but otherwise indistinguishable class.
'Object copy = Object' is 'false'.

Konrad.

Pharo Smalltalk Users mailing list

Re: Can a class be not equal to itself?

It never occurred to me that it would ever be the case!

I've always thought classes were singleton and that SomeClass copy would always return the sole instance of that class!

I wonder what are the implications of returning a copy, say, when you add an instVar to the "original" class ? What happens to the reshaping of the instances created by the copy?

On 2020-12-31 20:25, Richard O'Keefe wrote:

Squeak, VisualWorks,

GNU Smalltalk, and Pharo *deliberately* make

aClass copy

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khinsen

Re: Can a class be not equal to itself?

In reply to this post by Richard O'Keefe

"Richard O'Keefe" <[hidden email]> writes:

> #deepCopy is one of those things that is best avoided,
> because it violates the key principle of OOP that an

In the abstract, I agree. In practice, I don't see what else I could do
for my use case. I have an object whose internal state is updated
by incoming messages. I need to take immutable snapshots of that
object's state at some points. And that I do by sending "deepCopy"
and then "beRecursivelyReadOnlyObject".

Benoit St-Jean via Pharo-users <[hidden email]> writes:

> It never occurred to me that it would ever be the case!
> I've always thought classes were singleton and that SomeClass copy would
> always return the sole instance of that class!

Me too. But after taking a closer look at how copy and deepCopy work, it
seems that maintaining singletons under copy operations is impossible.
There is no way to tell the copying machinery to return the original
object instead of making a new one. Unlike e.g. Python, where this is
possible and even quite common.

Konrad.

Richard O'Keefe

Re: Can a class be not equal to itself?

Well, when you talk about "THE copying machinery" you have to be a bit more specific.  There is no such thing as #deepCopy or #shallowCopy in the ANSI Smalltalk standard.  Many Smalltalks have

Object>>copy

  ^self shallowCopy postCopy

and encourage you to override #postCopy.

But nothing stops you doing

MyClass

  methods for: 'copying'

    copy

      ^self

    deepcopy

      ^self

    postCopy

      ^self

    shallowCopy

      ^self

You will note that in Pharo, AtomicCollection, Behavior, Boolean,

Character, Float, SmallFloat64, Form, ColorForm, Morph, Paragraph,

Point, SmallInteger, String, UndefinedObject, and perhaps others

override #deepCopy.  Also, Boolean, Character, Float, SmallFloat64,

SmallInteger, Symbol, UndefinedObject, and perhaps others

override #shallowCopy.

So it *is* possible to have singletons in Smalltalk.

Classes can be copied in Pharo because someone thought it

would be a good idea.

On to the next point.

'I have an object whose internal state is updated

by incoming messages. I need to take immutable snapshots of that

object's state at some points. And that I do by sending "deepCopy"

and then "beRecursivelyReadOnlyObject".'

You do not know which objects will be copied by #deepCopy and

which won't, or if you do, you cannot be sure that that will not

change in a future release, or indeed if you load a package.

You do not know whether some of these objects will break if made

read-only.  

#deepCopy is *serious* "Hic sunt dracones" territory.

It's marked on the Hunt-Lenox Globe, just beside the

Vesuvian introitus ad infernum.

Seriously, the OOP way to do this is

MyClass>>immutableSnapshot

  "Answer an immutable copy of myself."

...

and then whatever it takes to make precisely that happen.

On Fri, 1 Jan 2021 at 23:35, Konrad Hinsen <[hidden email]> wrote:

"Richard O'Keefe" <[hidden email]> writes:

> #deepCopy is one of those things that is best avoided,
> because it violates the key principle of OOP that an

In the abstract, I agree. In practice, I don't see what else I could do
for my use case. I have an object whose internal state is updated
by incoming messages. I need to take immutable snapshots of that
object's state at some points. And that I do by sending "deepCopy"
and then "beRecursivelyReadOnlyObject".

Benoit St-Jean via Pharo-users <[hidden email]> writes:

> It never occurred to me that it would ever be the case!
> I've always thought classes were singleton and that SomeClass copy would
> always return the sole instance of that class!

Me too. But after taking a closer look at how copy and deepCopy work, it
seems that maintaining singletons under copy operations is impossible.
There is no way to tell the copying machinery to return the original
object instead of making a new one. Unlike e.g. Python, where this is
possible and even quite common.

Konrad.

ducasse

Re: Can a class be not equal to itself?

Hi Richard

Indeed you are correct.

We should continue to use and specialize postCopy.

deepCopy and its friends such as veryDeepInnerInner …. do not smell good.

Ideally I would like to remove all deepCopy and have a deepCopier that has a clear protocol and ask the objects.

So may be at the end we will rewrite deepCopy but with tests and a clear semantics.

On 2 Jan 2021, at 02:32, Richard O'Keefe <[hidden email]> wrote:

Well, when you talk about "THE copying machinery" you have to be a bit more specific. There is no such thing as #deepCopy or #shallowCopy in the ANSI Smalltalk standard. Many Smalltalks have
Object>>copy
^self shallowCopy postCopy
and encourage you to override #postCopy.
But nothing stops you doing
MyClass
methods for: 'copying'
    copy
      ^self
    deepcopy
      ^self
    postCopy
      ^self
    shallowCopy
      ^self
You will note that in Pharo, AtomicCollection, Behavior, Boolean,
Character, Float, SmallFloat64, Form, ColorForm, Morph, Paragraph,
Point, SmallInteger, String, UndefinedObject, and perhaps others
override #deepCopy. Also, Boolean, Character, Float, SmallFloat64,
SmallInteger, Symbol, UndefinedObject, and perhaps others
override #shallowCopy.

So it *is* possible to have singletons in Smalltalk.
Classes can be copied in Pharo because someone thought it
would be a good idea.

On to the next point.
'I have an object whose internal state is updated
by incoming messages. I need to take immutable snapshots of that
object's state at some points. And that I do by sending "deepCopy"
and then "beRecursivelyReadOnlyObject".'

You do not know which objects will be copied by #deepCopy and
which won't, or if you do, you cannot be sure that that will not
change in a future release, or indeed if you load a package.
You do not know whether some of these objects will break if made
read-only.

#deepCopy is *serious* "Hic sunt dracones" territory.
It's marked on the Hunt-Lenox Globe, just beside the
Vesuvian introitus ad infernum.

Seriously, the OOP way to do this is
MyClass>>immutableSnapshot
"Answer an immutable copy of myself."
...
and then whatever it takes to make precisely that happen.

On Fri, 1 Jan 2021 at 23:35, Konrad Hinsen <[hidden email]> wrote:
"Richard O'Keefe" <[hidden email]> writes:

> #deepCopy is one of those things that is best avoided,
> because it violates the key principle of OOP that an

In the abstract, I agree. In practice, I don't see what else I could do
for my use case. I have an object whose internal state is updated
by incoming messages. I need to take immutable snapshots of that
object's state at some points. And that I do by sending "deepCopy"
and then "beRecursivelyReadOnlyObject".

Benoit St-Jean via Pharo-users <[hidden email]> writes:

> It never occurred to me that it would ever be the case!
> I've always thought classes were singleton and that SomeClass copy would
> always return the sole instance of that class!

Me too. But after taking a closer look at how copy and deepCopy work, it
seems that maintaining singletons under copy operations is impossible.
There is no way to tell the copying machinery to return the original
object instead of making a new one. Unlike e.g. Python, where this is
possible and even quite common.

Konrad.

khinsen

Re: Can a class be not equal to itself?

In reply to this post by Richard O'Keefe

"Richard O'Keefe" <[hidden email]> writes:

> Well, when you talk about "THE copying machinery" you have to be a bit more
> specific. There is no such thing as #deepCopy or #shallowCopy in the ANSI
> Smalltalk standard. Many Smalltalks have

I am referring only to Pharo, which is the only Smalltalk I have ever
used.

> You will note that in Pharo, AtomicCollection, Behavior, Boolean,
> Character, Float, SmallFloat64, Form, ColorForm, Morph, Paragraph,
> Point, SmallInteger, String, UndefinedObject, and perhaps others
> override #deepCopy.

Interesting. I didn't even consider this possibility because the comment
in Object>>#deepCopy says "should never be overridden".

> #deepCopy is *serious* "Hic sunt dracones" territory.
> It's marked on the Hunt-Lenox Globe, just beside the
> Vesuvian introitus ad infernum.

:-) That's more or less what I discovered in my recent debugging
experiment.

> Seriously, the OOP way to do this is
> MyClass>>immutableSnapshot
> "Answer an immutable copy of myself."
> ...
> and then whatever it takes to make precisely that happen.

In other words, implement my own variant of deepCopy. That's probably
the safest way to go, but it also means there is no extensible
copying infrastructure that everybody can build on.

Konrad.

ducasse

Re: Can a class be not equal to itself?

Hi konrad

in fact I introduced postCopy/copy long time ago and did not get the force
to clean the deepCopy and veryDeepInner mess.
This is something that one day we will have to do.

S

> On 2 Jan 2021, at 12:17, Konrad Hinsen <[hidden email]> wrote:
>
> "Richard O'Keefe" <[hidden email]> writes:
>
>> Well, when you talk about "THE copying machinery" you have to be a bit more
>> specific. There is no such thing as #deepCopy or #shallowCopy in the ANSI
>> Smalltalk standard. Many Smalltalks have
>
> I am referring only to Pharo, which is the only Smalltalk I have ever
> used.
>
>> You will note that in Pharo, AtomicCollection, Behavior, Boolean,
>> Character, Float, SmallFloat64, Form, ColorForm, Morph, Paragraph,
>> Point, SmallInteger, String, UndefinedObject, and perhaps others
>> override #deepCopy.
>
> Interesting. I didn't even consider this possibility because the comment
> in Object>>#deepCopy says "should never be overridden".
>
>> #deepCopy is *serious* "Hic sunt dracones" territory.
>> It's marked on the Hunt-Lenox Globe, just beside the
>> Vesuvian introitus ad infernum.
>
> :-) That's more or less what I discovered in my recent debugging
> experiment.
>
>> Seriously, the OOP way to do this is
>> MyClass>>immutableSnapshot
>> "Answer an immutable copy of myself."
>> ...
>> and then whatever it takes to make precisely that happen.
>
> In other words, implement my own variant of deepCopy. That's probably
> the safest way to go, but it also means there is no extensible
> copying infrastructure that everybody can build on.
>
> Konrad.