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EXPRESSLanguage Foundation
EXPRESS

EXPRESS

ISO 10303-11:2004

Overview

EXPRESS is a formally defined information modelling language designed for specifying data schemas, entities, types, constraints, and rules. It is the core language of the EXPRESS family and serves as the foundation for data exchange standards within ISO 10303 (STEP).

EXPRESS is a machine-readable language that combines data modelling with algorithmic expressiveness. Unlike general-purpose programming languages, EXPRESS is purpose-built for defining the structure and constraints of information models used in industrial data exchange.

Key features

  • Entity declarations — define object types with attributes and constraints

  • Inheritance hierarchies — SUBTYPE/SUPertype relationships with complex constraints (ONEOF, ANDOR, AND)

  • WHERE rules — local constraints on entity instances

  • GLOBAL rules — schema-level constraints spanning multiple entities

  • FUNCTION and PROCEDURE — algorithmic constructs for constraint evaluation

  • ENUMERATION and SELECT types — extensible type definitions

  • Schema partitioning — modular organization of large information models

Building blocks

An EXPRESS schema is composed of several fundamental building blocks that work together to define an information model.

Datatypes

EXPRESS provides a layered type system built on a set of simple (atomic) datatypes:

  • NUMBER — abstract numeric type (INTEGER or REAL)

  • INTEGER — whole numbers

  • REAL — floating-point numbers

  • STRING — character sequences

  • BOOLEAN — TRUE or FALSE

  • LOGICAL — TRUE, FALSE, or UNKNOWN

  • BINARY — bit sequences

On top of these, EXPRESS provides aggregation types:

  • ARRAY — fixed-size, ordered collection with optional uniqueness constraint

  • LIST — variable-size, ordered collection

  • SET — variable-size, unordered collection with uniqueness constraint

  • BAG — variable-size, unordered collection (duplicates allowed)

User-defined types
TYPE mass = REAL;
END_TYPE;

TYPE colour = ENUMERATION OF (red, green, blue);
END_TYPE;

TYPE shape = SELECT (box, cylinder, sphere);
END_TYPE;

Entity attributes

Entities are the core modelling construct in EXPRESS. Each entity declares a set of attributes that define the properties of the object being modelled.

ENTITY person;
  name : STRING;
  birth_date : date;
  height : OPTIONAL REAL;
DERIVE
  age : INTEGER := years_since(birth_date);
INVERSE
  employer : SET[0:?] OF employment FOR employee;
END_ENTITY;

Attributes can be:

  • Explicit — mandatory or OPTIONAL values provided directly

  • Derived (DERIVE) — computed from other attributes using expressions

  • Inverse (INVERSE) — bidirectional relationships maintained automatically

Supertypes and subtypes

EXPRESS supports single and multiple inheritance through supertype/subtype relationships. Subtypes inherit all attributes of their supertypes and may add new attributes and constraints.

ENTITY vehicle;
  registration : STRING;
END_ENTITY;

ENTITY car
  SUBTYPE OF (vehicle);
  no_of_doors : INTEGER;
END_ENTITY;

ENTITY truck
  SUBTYPE OF (vehicle);
  payload : mass;
END_ENTITY;

Complex supertype constraints control which combinations of subtypes are valid:

  • ONEOF — exactly one subtype must be instantiated

  • ANDOR — any combination of subtypes may coexist

  • AND — all listed subtypes must coexist

Constraints and rules

EXPRESS provides three levels of constraint specification:

WHERE rules (local constraints)
ENTITY positive_integer;
  value : INTEGER;
WHERE
  is_positive : value > 0;
END_ENTITY;
GLOBAL rules (schema-level constraints)
RULE unique_registration FOR (vehicle);
  LOCAL
    regs : SET OF STRING := [];
  END_LOCAL;
  WHERE
    no_duplicates : SIZEOF(regs - QUERY(v <* vehicle | v.registration IN regs)) = 0;
END_RULE;
UNIQUE constraints
ENTITY employee;
  employee_id : STRING;
  social_security : STRING;
UNIQUE
  id_unique : employee_id;
  ss_unique : social_security;
END_ENTITY;

Functions and procedures

EXPRESS includes algorithmic constructs for constraint evaluation and data manipulation:

FUNCTION distance(p1 : point; p2 : point) : REAL;
  LOCAL
    dx, dy, dz : REAL;
  END_LOCAL;
  dx := p1.x - p2.x;
  dy := p1.y - p2.y;
  dz := p1.z - p2.z;
  RETURN (SQRT(dx*dx + dy*dy + dz*dz));
END_FUNCTION;

Syntax

EXPRESS uses a Pascal-like lexical syntax. A schema definition typically includes:

  • SCHEMA declarations containing entities, types, functions, and rules

  • ENTITY declarations with ATTRIBUTE fields, WHERE clauses, and SUBTYPE constraints

  • TYPE declarations for user-defined types including ENUMERATION, SELECT, and DEFINED types

  • FUNCTION and PROCEDURE for reusable algorithmic logic

  • RULE declarations for global constraints

Complete schema example
SCHEMA Geometry;

TYPE point_list = LIST[2:?] OF point;
END_TYPE;

ENTITY point;
  x : REAL;
  y : REAL;
  z : REAL;
WHERE
  not_at_origin : (x <> 0.0) OR (y <> 0.0) OR (z <> 0.0);
END_ENTITY;

ENTITY line;
  start : point;
  end : point;
WHERE
  not_zero_length : distance(start, end) > 0.0;
END_ENTITY;

FUNCTION distance(p1 : point; p2 : point) : REAL;
  LOCAL
    dx, dy, dz : REAL;
  END_LOCAL;
  dx := p1.x - p2.x;
  dy := p1.y - p2.y;
  dz := p1.z - p2.z;
  RETURN (SQRT(dx*dx + dy*dy + dz*dz));
END_FUNCTION;

END_SCHEMA;

History

The EXPRESS language originated from the PDDI (Product Data Definition Interface) program, created in 1982 by the US Air Force at McDonnell Aircraft (now merged into Boeing). The language’s name reflects its purpose — to facilitate “expressiveness” in modelling information.

Timeline

1982

EXPRESS conceived during the PDDI program at McDonnell Aircraft, initiated by the US Air Force

1984

Language entered standardization process at the National Bureau of Standards (now NIST)

1986

NBS proposed EXPRESS as an International Standard at ISO

1994

First edition published as ISO 10303-11:1994, by Douglas Schenck and Prof. Bernd Wenzel

1994

First authoritative reference manual published: “Information Modelling: The EXPRESS Way” by Douglas Schenck and Peter Wilson

2004

Second edition published as ISO 10303-11:2004, with David Loffredo and Peter Wilson as project leaders

Key people

Douglas Schenck

Original inventor of EXPRESS at McDonnell Aircraft during the PDDI program. Project leader of ISO 10303-11:1994 (first edition) together with Prof. Bernd Wenzel. Author of the foundational reference work on the language.

Prof. Bernd Wenzel

Co-project leader of ISO 10303-11:1994 (first edition) together with Douglas Schenck.

Peter Wilson

Assisted Douglas Schenck with the first edition of ISO 10303-11:1994. Co-author of the authoritative EXPRESS reference. Designer of EXPRESS-G (the graphical notation) and EXPRESS-X (the schema mapping language). Led the second edition of ISO 10303-11:2004.

David Loffredo

Co-project leader for ISO 10303-11:2004 (Edition 2) together with Peter Wilson. Long-time contributor to EXPRESS standardization and STEP implementations.

Allison Barnard Feeney

Key contributor to the standardization of EXPRESS within ISO 10303 at NIST.

Standardization

EXPRESS is defined by ISO 10303-11, part of the STEP (Standard for the Exchange of Product model data) family of standards.

  • ISO 10303-11:1994 — First edition of the EXPRESS language reference

  • ISO 10303-11:2004 — Second edition (current), with enhanced EXPRESS-X support and clarifications

EXPRESS is used as the schema definition language for all Application Protocols (APs) within ISO 10303, making it the backbone of industrial data exchange standards used in aerospace, automotive, construction, and digital twin applications.

Applications

EXPRESS is used across multiple industries:

  • Aerospace — Airbus, Boeing, and suppliers exchange product data using STEP/EXPRESS schemas

  • Automotive — Automotive manufacturers use EXPRESS-defined APs for design and manufacturing data exchange

  • Construction and AEC — Building information models reference EXPRESS-defined schemas (e.g., IFC, which is derived from STEP)

  • Digital twins — EXPRESS schemas define the information models for digital representations of physical assets

  • Smart manufacturing — Industry 4.0 data exchange leverages EXPRESS-defined standards

Learn more

  • Tutorial: Learn EXPRESS — comprehensive introductory course by Peter Wilson

  • Standards & BNF — download the formal EXPRESS grammar

  • EXPRESS Course — structured course covering ISO 10303 and EXPRESS topics

  • Information Modelling: The EXPRESS Way — Douglas Schenck and Peter Wilson (Oxford University Press, 1994)