EXPRESS DAm -- A request from the Part 108 folk

[Ed Barkmeyer]

EXPRESS experts:

At the Myrtle Beach meeting, I was contacted by Mike Pratt of the Part 108
team.  There is a modeling concept that they can only weakly document in
Express, so weakly in fact that the most important WHERE rules in the Part 108
model can only be written as "informal propositions".  (I have been told by
several WG3 persons, however, that this is a typical state-of-affairs and we
should not be overly concerned. ;-)  Mike has promised to send WG11 an excerpt
from the Part 108 schema and text that illustrates the problem.  (I told Mike
that we cannot officially go beyond the NB comments in editing the DAm unless we
get a formal request from the Part 108 team.)

The issues are two:

1. reference to an Express-defined attribute of an entity instance IN THE DATA
(the Part 21/28 file);

2. reference to the value of a variable (i.e. dynamically chosen)
Express-defined attribute IN THE SCHEMA.


Issue 2 first.

It is my understanding that the Part 108 excerpt would look something like:

(* attribute_identifier is a string value that identifies a modeled Express 
   attribute, in the form:  <schema>.<entity>.<attribute>
 *)
TYPE attribute_identifier = STRING; END_TYPE;

ENTITY referenced_element;
  ref_object: representation_item;
  ref_role:   attribute_identifier;
DERIVE
  ref_value: NUMBER := ???ValueOf???(ref_object, ref_role) 
END_ENTITY;

ENTITY parameter;
  model_reference:  referenced_element;
  parameter_value:  NUMBER;
WHERE
  parameter_value_must_match_referenced_attribute:
    parameter_value = referenced_element.ref_value;
END_ENTITY;

For the Express DAm, can we define the function denoted by ???ValueOf??? in the
above?  That is, can we define a function that is equivalent to the attribute
selection operation (.) when the second operand is a constant, but allows the
attribute-name (role name) to be variable?

Note: The ref_object would probably have to be "narrowed" to the particular
subtype of representation_item implied by the "completely qualified role name". 
That can be defined to be the behavior of the ???ValueOf??? operation.  But it
implies that we might also need a function for the group selection operation (\)
as well, e.g.:
 FUNCTION GroupOf(inst: GENERIC_ENTITY:et, gtype: STRING): GENERIC_ENTITY:gt;
which returns inst as a value of type gtype, where gtype/gt is a subtype of et.
Of course, this would force us to define the data type of the value returned by
the group operator, which is not in fact the gtype entity type, but rather a
data set comprising the values of the attributes declared in the declaration of
gtype (and no others)!  A partial entity instance is a record/struct object that
doesn't have an Express data type!  (This, and value comparison, are the places
in which it is necessary to make a distinction between the entity instance and a
record/struct of (some of) its attribute values.  The designers of Express
decided there were no such places and discarded the vocabulary needed to define
these operations.) 

Also relative to the above and Jochen's SEDS, should we think about importing
into the DAm the Express Ed3 concept of typenames and rolenames as first-class
datatypes different from STRING?  Then we could avoid creating the ValueOf and
GroupOf functions and just use .(<expression>) and \(<expression>), where the
values of the <expression>s are of type rolename and typename respectively.  The
problem with this idea is that it might invalidate existing schemas that use
string-literals, and more rarely but less remediably STRING variables, to denote
values of these types.

Now Issue 1.

To the best of my knowledge, values of the typename and rolename types do not
appear in existing STEP or PLIB data sets.  They only appear in schemas.  But
they will appear in future data sets conforming to Part 108!  If we formalize
the typename and rolename types in Express, then we will have to change Part 21
to write the rule for their representation.  But if we don't formalize them in
Express, we will have no peg on which to hang that rule in Part 21.  And if WG11
does not act on this, the syntax for the representation of Express role names in
Part 21 files will be defined in Part 108!

-Ed

P.S. This is by way of early warning.  We may not see an official request from
WG12 (for Part 108) until just before the Stockholm meeting. 

-- 
Edward J. Barkmeyer                       Email: edbark at nist.gov
National Institute of Standards & Technology
Manufacturing Systems Integration Division
100 Bureau Drive, Mail Stop 8260          Tel: +1 301-975-3528
Gaithersburg, MD 20899-8260               FAX: +1 301-975-4482

"The opinions expressed above do not reflect consensus of NIST,
and have not been reviewed by any Government authority."