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EXPRESS-I

ISO 10303-12

Overview

EXPRESS-I is an instance definition language for creating and exchanging populated data models conforming to EXPRESS schemas. It provides a lexical syntax for defining concrete instances of entities defined in an EXPRESS schema, making it essential for conformance testing and example data exchange.

EXPRESS-I enables users to define concrete data instances that can be validated against an EXPRESS schema, supporting: conformance testing of STEP implementations, exchange of reference test data, and documentation of populated data models.

Key features

  • Instance definitions — create concrete entity instances conforming to EXPRESS schemas

  • Population data — define complete datasets of interrelated entity instances

  • Schema conformance — validate that instances comply with all EXPRESS constraints (WHERE rules, GLOBAL rules, type constraints)

  • Conformance testing — define test cases for STEP implementation validation

  • Reference data exchange — share example data sets across organizations

Syntax

EXPRESS-I uses a structured instance notation:

Example EXPRESS-I instance
SCHEMA_INSTANCE example_model;

  -- Instance of a point entity
  #1 = POINT(0.0, 1.5, 2.0);
  #2 = POINT(1.0, 0.0, 0.5);

  -- Instance of a line referencing two points
  #3 = LINE(#1, #2);

END_SCHEMA_INSTANCE;

History

EXPRESS-I was developed to address the need for exchanging and validating concrete data instances against EXPRESS schemas. It fills the gap between schema definition (EXPRESS) and data exchange (STEP physical files), providing a human-readable format for instance specification.

Timeline

1994

EXPRESS-I defined in ISO 10303-12 as part of the STEP standard suite

Present

Used primarily for conformance testing and validation reference data

Key people

EXPRESS-I was developed by the ISO TC 184/SC 4 committee as part of the STEP standards family, with contributions from the same team involved in EXPRESS development including Douglas Schenck, Peter Wilson, and other ISO TC 184/SC 4 experts.

Standardization

EXPRESS-I is defined by ISO 10303-12:

  • ISO 10303-12 — EXPRESS-I: EXPRESS Test Suite Application Protocol Model and Abstract Test Suite

EXPRESS-I complements ISO 10303-21 (STEP Part 21 physical file format) by providing an alternative instance notation more suitable for conformance testing specifications.

Applications

  • Conformance testing — defining test cases for STEP implementation validation

  • Reference data — sharing populated example datasets

  • Documentation — illustrating schema usage with concrete data examples

  • Validation — verifying that data implementations satisfy EXPRESS schema constraints

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