question regarding bound specs

[Lothar Klein]

Ed,

A few corrections:
SDAI as it is today has no concept to run an Express-function 
directly. However the user may access a derived attribute of an
entity instance.

For cases when one or several newly created entity instances have
to be returned for derived attributes when invoking the
operation "Get Attribute" (clause 10.10.1) it is totally unclear
if these are persistent instances or temporarily ones.
If these would be persistent entity instances, then the owning 
SDAI-model has to be specified somehow.
If these would be temporarily entity instances, then the
lifetime and validity of them has to be controlled somehow.
As of today it is up to the implementations to solve the problem
in one or the other way. Only in a new edition of SDAI
we could fix the exact logic.

Lothar

Ed Barkmeyer wrote:
> 
> All,
> 
> I want to make it very clear here that what I said and what David
> Price said are not at odds.  David is 100% correct when he says:
> 
> > I'd take a look at Part 22 and the SDAI dictionary. There you find a concept
> > where bounds are indeed evaluated at run time. For example, a bound can
> > contain a query which can only be evaluated at run time. This applies to
> > more than just aggregate bounds also.
> 
> This is because Part 22 defines the behavior of a particular kind of
> *implementation*!  In particular, Part 22 defines a runtime environment,
> a set of procedure invocations in that environment, and the required
> behaviors of those procedures relative to the specifications in the
> EXPRESS model.  The concepts of "compile time" and "run time" are
> meaningful for Part 22, and the procedural interpretation of EXPRESS
> FUNCTIONs is part of the Part 22 specification for the runtime behavior.
> 
> EXPRESS itself, Part 11, does *not* define a runtime environment.
> In EXPRESS, the definition of a FUNCTION is the effect the algorithm
> would produce, using the EXPRESS computational model, whether it is
> implemented or not.  Part 22 defines the effect of a function as the
> execution of the EXPRESS statements using the computational model of
> the programming language and the mapping provided by Part 2x.  While
> an attempt is made to render the intent of the EXPRESS model as
> accurately as possible into the programming language, those
> computational models are *not* identical, and some of the Part 22
> results will not necessarily be identical to the EXPRESS interpretation.
> 
> It is entirely possible that the rendering of Liam's model into C++
> via Part 23, for example, will cause the second call to HIBOUND to
> return 5.  (I don't know that it does.)  In any case, the result of
> that call is an artifact of the Part 23 mapping and the computational
> semantics of C++.
> 
> In the same way, it is unlikely that Part 2x implementations will
> produce the "correct" results for some of the strange operation
> overload cases that can arise from the TC2 change to EXPRESS that
> allows A + B where A and B are both instances of SELECT types.
> And floating-point approximation is well-known to produce anomalous
> results for WHERE clauses that test for equality between REAL values.
> 
> In sum, the effect of the Part 22 mapping is *not* the definition
> of the EXPRESS construct.  It will usually be consistent, but not
> always, particularly in "outlying cases".
> 
> -Ed
> 
> "The problem with having 'statements' in EXPRESS is that encourages
>  people to write FORTRAN into models."
>   -- Peter Wilson
> 
> --
> 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."

-- 
// Lothar Klein, LKSoftWare GmbH
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