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 <konrad.hin...@fastmail.net>
wrote:
> "Richard O'Keefe" <rao...@gmail.com> 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 <pharo-users@lists.pharo.org> 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.
>
