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.hinsen@fastmail.net> wrote:

"Richard O'Keefe" <raoknz@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.