Chilton::INF::Data Exchange

Express

EXPRESS was a textual conceptual schema language based on the entity-attribute-relationship model developed in CAD*I with both generalization and constraint-specification constructs. It was used to specify the normative part of all the information models in CAD*I and later STEP. It needed to be human readable and computer-processable.

EXPRESS had the following declarative constructs:

Schema to define a topic of interest (such as the geometry of the product)
Type and Entity both types and entities were used to describe the data and the relationships between data.
Constant
Function, Procedure, Rule used to define constraints on the data.

A simple example might be:

ENTITY point ;
  x : REAL ;
  y : REAL ;
END-ENTITY
ENTITY line ;
 p0 : point ;
 p1 : point ;
 END-ENTITY

The first entity point is used as an attribute type in the second entity line.

Inheritance allowed constraints on a parent to be inherited by the child. Multiple inheritances were allowed. The file reader and parser implemented at RAL originally for the CAD*I project were successfully converted to deal with STEP exchange files. A back-end to the file reader was written which provided an interface to the Nastran Finite Element program.

The Express compiler was originally written for CAD*I and then upgraded for STEP. Many utilities such as pretty printers were also added. The software was distributed to a number of academic and industrial organisations, both in the UK and overseas. A neutral database for electromagnetic and semiconductor modelling data was created based on an EXPRESS schema and a mapping from EXPRESS to SQL. Genera1ised subroutines to access the database were developed.

Many UK universities, both in the UK and world-wide, and industry were supplied with the Express compiler and/or the physical file reader RAL retained the Intellectual Property Rights for the software.