[wg11] SEDS for EXPRESS (10303-11:2004)

[Lothar Klein]

Ed,

I do not agree with several details of your proposal.
See attached an alternative Express-G diagram.
I can ensure you that this concept is working.

> A SingleEntityValue represents a collection of values for the explicit
> attributes contained in a single EXPRESS entity declaration, and the
> SingleEntityType represents the "data type" of that collection, i.e. the
> data type of the output of the "implicit" partial entity constructor and
> the group (\) operator for that entity data type.

ok

> A PartialEntityValue, per Part 11 (after TC2), represents one or more
> SingleEntityValues packaged together.  This represents the output of the
> "entity instance constructor" (||) operator, as well as the operators
> above.  This may not correspond to any actual entity data type, because
> it is missing the values of some required supertype.  (Part 11 says 
> this.)  A PartialEntityType is the data type of one of those collections.

You introduce the new term "PartialEntityValue" and before we had only
"partial complex entity value". You would be semantically more precise
to speak of "complex entity values" which may be either "complete" or
"partial".

> "Without loss of generality", a PartialEntityValue (-Type) consisting of
> exactly one SingleEntityValue (-Type) is "equivalent to" that 
> SingleEntityValue (-Type).

This is the bend in your data model and you better avoid it.
If you say a partial or complete complex entity value is a SET of one
or more single entity values you can avoid this glitch.

> ...
> Now some PartialEntityValues contain all the attributes needed to 
> represent an EntityInstance of a given EntityType, i.e. they have 
> SingleEntityValues for an EntityType and all of its supertypes.

You better reserve the word "partial" for those which are really
incomplete.

> ...
> I would also be surprised
> if this model breaks any existing EXPRESS or SDAI tools, although it may
> bring to light some dubious practices on both the modeling side and the
> tooling side.

The sdai_dictionary_schema has a simplified view of what is an
"entity_definition" and partial complex entity types are not covered
at all.
There is the sdai_parameter_data_scheme which talks on values, but
unfortunately it also does not talk on entity_values, only
entity_instances.

Lothar
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