[wg11] EXPRESS Section 11 question

[Ed Barkmeyer]

Alan R Williams wrote:

> Section 11 of EXPRESS (interface specification) ...  states that
> 
> "An entity which is either explicitly REFERENCE'd or implicitly
> interfaced shall only be instantiated to play the role described by an
> attribute of an instantiation of an entity in the schema"
> 
> I have no problem agreeing with the explicitly REFERENCE'd entities
> restriction.  However, IMO there is a problem wrt implicitly
> interfaced objects as shown in Example 80.
> 
> SCHEMA s1;
> 
>   TYPE t1 = REAL;
>   END_TYPE;
> 
>   ENTITY e1;
>     a : t1;
>   END_ENTITY;
> 
>   ENTITY e2;
>     a1 : e1;
>   END_ENTITY;
> 
> END_SCHEMA;
> 
> SCHEMA s2;
>   REFERENCE FROM s1 (e2);
> 
>   ENTITY e3;
>     a3 : e2;
>   END_ENTITY;
> END_SCHEMA;
> 
>>From the viewpoint of s2,
> 
>   s1.e1 is implicitly interfaced but there is no way it can play a
> 	role described by an attribute of an entity in s2.

Agreed.
It is my position that the text of Part 11 is slightly in error, and the 
clause should be corrected to read:
"An entity which is either explicitly REFERENCE'd or implicitly
interfaced shall only be instantiated to play the role described by an
attribute of an instantiation of an entity in **, or interfaced into, ** 
the interfacing schema."

BTW, it has been pointed out before that this feature was ill-designed. 
Combining the "dependent entity" concept with the interfacing concept 
makes a completely orthogonal notion available only on interfacing. 
There is no reason why an ENTITY declared in the schema couldn't have 
the same "dependent instantiation" property, but there is no way to 
declare it for entities that aren't interfaced.  And therefore, in the 
long form, there is no way to convey that property at all.

-Ed

-- 
Edward J. Barkmeyer                        Email: edbark at nist.gov
National Institute of Standards & Technology
Manufacturing Systems Integration Division
100 Bureau Drive, Stop 8264                Tel: +1 301-975-3528
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