identity...... Was: The wisdom of the object mentors

I'm not really interested in RM vs OO...to me they are to special
cases...but you have intrigued me with some stuff on identity....which
is something I've only recently managed to mildly formalise in my head.

I've dropped the database cross post, in order not to become involved
in the rather harsh attacks on Mr Martin.

Dmitry A. Kazakov wrote:
On 1 Jun 2006 02:58:43 -0700, Erwin wrote:

It is strange to hear talks about identity from RM side. I thought RM
overcame that disease. There is no identity. Files are same, neither is
real. Paths aren't same. Identity is a relation, isn't it? Now if you'd
consider objects like (path,file), these could have identity defined as
id((path,file))=path.

In the sense that "identity" refers to "the quality of being
identical", there is of course nothing strange in the RM crowd using
that term.

Note that "being identical" requires two things, for else equality
between them cannot be measured/observed/tested.

Note further that the OO crowd always uses the term with respect to one
single thing ("the identity of an object").

This is not true. As for identity of objects there are many of. For
example, polymorphic objects have the identity of its specific type.

not with this....for me an object has 1 identity.

Can you give me an example.

The
identity of objects in its usual sense is not required in OO. It is quite
possible to have unidentifiable objects.

OK, I actually agree with this in a way.....but probably not in the
same way as you.

Can you give me an example though.


The OO crowd thus never uses that term in the "relational" sense of the
word.

I don't see much difference in respect of identity. The standpoint is that
there is no data, only behavior. Data is expressed in observed behavior. So
there is no immanent identity of values. There is only "=" relation defined
on them (if any). The object is same as long as it exposes same behavior.
Any question about what is inside is illegal.


not sure about this......I think it depends on context....but can't
really properly comment unless I understand the above.

.