[wg11] Part 11: redeclared attributes

[Phil Spiby]

Georgi et al,

Both are legal, beyond that it is a matter of taste and clarification for
the reader.

If in entity C we used:
SELF\B.attribute_a : Z; 

Then it is clearer (at least for me!) that Z must be a specialisation of Y,
which itself must be a specialisation of X.

Now if Y and Z are both subtypes of X and related by ANDOR, then the above
statement would be wrong!, since Z is not a specialisation of Y!
However, if in entity C we used:
SELF\A.attribute_a : Z;

Then we get into the interesting situation, both redeclarations are valid,
but because C is a subtype of B, both redeclarations must apply, therefore
instances of C MUST use complex instances of X&Y&Z.

Phil

PS Isn't EXPRESS a wonderful language in which we can get into these
difficulties?

-----Original Message-----
From: wg11-bounces at steptools.com [mailto:wg11-bounces at steptools.com] On
Behalf Of Georgi Kuzmanov
Sent: 21 April 2004 12:06
To: Jochen Haenisch
Cc: wg11 at steptools.com
Subject: RE: [wg11] Part 11: redeclared attributes


Jochen, Heidi

Having a redeclaration reference [the entity containing] another
redeclaration (rather than the original declaration) of an attribute was
made "officially" legal by TC2 in 1999.  The original 1994 version of the
standard does not allow this possibility, but does not explicitly forbid it
either.

With the risk of being accused of splitting hairs, I would favour
referencing the original declaration over referencing a redeclaration, even
though the alternative approach is perfectly legal.

Quoting the second sentence from P11 (1994) 9.2.3.4: "The attribute remains
in the supertype but the allowed domain of values for that attribute is
governed by the redeclaration given in the subtype". 
Logically this means a redeclaration of an attribute is not its repeat
declaration with a new type, but rather a constraint on its value domain.
Like domain rules, all applicable redeclarations of an attribute for a
complex entity instance are combined to constrain its value domain.

Following Heidi's example, attribute_a stays in A even though its domain is
constrained (by the redeclaration in B) in complex entity instances which
are instances of both A and B.  So if C redeclares attribute_a refering to
the redeclaration in B rather than the original declaration in A, it is, in
a way, using B as a "proxy" to point indirectly to the an attribute which
still belongs to A (and A as a supertype of B is always guaranteed to be
present in any valid instance of C because of inheritance rules/P11 Annex
B).

This is why having a redeclaration refer to another redeclaration seems to
me a bit like using the label of a domain rule to refer to the attribute
whose value it constrains (apologies for the lame analogy, but I hope the
meaning comes across).

The only situation I can imagine in which a redeclaration might need to
refer to another redeclaration is where some strange interaction of
explicit/implicit inter-schema interfacing and attribute renaming would
prevent the direct naming of the originally declaring supertype.  I cannot
think of such an example at the moment.

On a separate note, I think attribute renaming in e2 may have been mis-named
and "attribute aliasing" would be a more accurate description
-  according to e2 9.2.3.4 NOTE 1 "The old identifier remains available in
this entity data type and in any subtypes that are declared for this entity
data type".

Regards,
Georgi

On Tue, 2004-04-20 at 22:24, Jochen Haenisch wrote:
> Yes, Heidi, this is valid Express (as long as Z is a specialization of 
> Y). P11 (e2) says in 9.2.3.4:
> "d) The group_qualifier in the qualified_attribute syntax shall 
> identify either the entity data type in which the attribute was 
> originally declared, or an entity data type that
> redeclares the attribute from another supertype.
> "
> And 'group_qualifier' is '\' entity_ref .
> 
> I hope the experts agree :-)
> 
> Best regards,
> Jochen
> 
> > -----Original Message-----
> > From:	Heidi L Preston [SMTP:hpreston at ebmail.gdeb.com]
> > Sent:	20. april 2004 22:32
> > To:	wg11 at steptools.com
> > Subject:	[wg11] Part 11: redeclared attributes
> > 
> > All-
> > 
> > I was hoping someone could entertain a Part 11 question.  Is the 
> > following EXPRESS legal?:
> > 
> > ENTITY A;
> > attribute_a : X;
> > END_ENTITY;
> > 
> > ENTITY B
> > SUBTYPE OF (A);
> > SELF\A.attribute_a : Y;
> > END_ENTITY;
> > 
> > ENTITY C
> > SUBTYPE OF (B);
> > SELF\A.attribute_a : Z;
> > END_ENTITY;
> > 
> > Questionable here is the redeclare in C.  It appears that it should 
> > be a redeclare of B (not A).  Is the above construct allowed 
> > according to Part 11?
> > 
> > Thanks,
> > Heidi
> > 
> > 
> > 
> > 
> > 
> > ********************************************************************
> > ******
> > *
> > Heidi Preston
> > Electric Boat Corporation
> > 75 Eastern Point Road
> > Groton, CT 06340
> > (860) 433-8649
> > 
> > 
> > _______________________________________________
> > wg11 mailing list
> > wg11 at steptools.com http://lists.steptools.com/mailman/listinfo/wg11
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