Path: utzoo!attcan!uunet!husc6!bloom-beacon!clyde!concour!marcap
From: marcap@concour.CS.Concordia.CA (Marc Pawlowsky)
Newsgroups: comp.lang.eiffel
Subject: PEEVES
Message-ID: <882@clyde.Concordia.CA>
Date: 8 Jun 89 18:34:13 GMT
Sender: news@Clyde.Concordia.CA
Reply-To: marcap@concour.CS.Concordia.CA (Marc Pawlowsky)
Organization: Concordia University, Montreal Quebec
Lines: 41

>Please repost this to comp.lang.eiffel for me.  Our news posting
>software has been broken for a long time and I don't know when it will
>be fixed.

Please reply to author not me.

Subject: pet peeves re := and deepCopy
Summary: neither problem is real
Reply-To: deutsch@parcplace.com (L. Peter Deutsch)
Organization: ParcPlace Systems, Mountain View, CA

1) Re the "inconsistency" of :=.  You are confusing the notions of
identity, primitiveness, and equality.  For immutable objects (such as
numbers and characters), regardless of whether they enjoy special
implementation, an argument can be made that "identity" is not a useful
concept, since two immutable objects with the same contents will behave
the same under all circumstances.  If you decide that in your language
every object has a hidden, immutable piece of state called its
"identity", well and good, but then you have to pay a very heavy
performance price (you have to leave enough room in every object, even
integers, for this unique "identity").

I believe a better approach is to take the point of view that the
"identity" of an immutable object is a function of its contents, i.e.,
that immutable objects with the same (client-visible) contents really
cannot be distinguished.  This has nothing to do with whether the object
is "primitive".  As a consequence, whether := copies an immutable object
or not becomes entirely a matter of implementation with no semantic
consequences.

2) deepCopy is not a semantically useful notion.  Where do you stop?  Do
you copy classes as well as contents?  If an object has a reference to
some widely shared data structure, do you copy that entire structure?
What about references to objects like Symbols (in Smalltalk), where
there is an invariant requiring unique representation of Symbols with
the same contents?

I believe providing a deepCopy operation is a mistake.  If it were up to
me, it would be removed from the Smalltalk-80 system.

	L. Peter Deutsch