Overview

The openEHR Basic Expression Object Model (BEOM) defines the semantics of computable expressions and statements that are used in openEHR archetypes, and might be used in healthcare and life sciences computing where rules and expressions are needed.

The openEHR Basic Expression Language (BEL) specified here is an abstract syntax counterpart to the BEOM, and may be considered the default syntax. Other syntaxes or syntax variants are possible, and other serialisation methods are possible, such as object graph serialisation into XML, JSON, YAML etc. The BEL is provided firstly as a way of specifying and explaining the semantics of the BEOM, and secondly, as one way of actually authoring expressions and rules in textual form. This approach is the same as with any programming language, where the usual form for learning and programming is the abstract language form, while the computational form is an abstract syntax tree (AST) or similar.

Accordingly, the BEOM should be considered the normative definition (rather than the syntax) of the openEHR Basic Expression formalism, and indeed not all implementations need support the language: they might for example only serialse in JSON or use purely graphical visualisation.

Key features of BEL include:

  • strong typing;

  • standard logical operators including universal quantification;

  • standard arithmetic and relational comparison operators, enabling the use of numerics and date/time types;

  • parentheses for overriding operator precedence;

  • built-in functions like current_date, standard functions such as max() defined on primitive types, as well as the ability to use external functions;

  • support for simple statements, such as variable declaration and assignment.

For reasons of comprehensibility and explanation, the Expression Language is defined first in this specification.

Design Background

The openEHR Basic Expression Language is based on a limited first-order predicate logic language with some additions to support a few basic types of statements, enabling the authoring of executable texts, i.e. programs. One type of statement is the assertion, which has similarities with OMG’s OCL (Object Constraint Language), and to the Object-Z language [1]. See [2], [3], [4] for an explanation of predicate logic in information modelling. It is not exactly the same as any of these languages because:

  • it has a different meta-model;

  • the syntax is designed to be comprehensible to developers familiar with modern mainstream object-oriented and functional languages such as Java, C#, Python, TypeScript etc.

The semantic requirements are for expressions including arithmetic, boolean, and relational operators, functions, quantifier operators, operator precedence, parentheses, constant values, and certain kinds of variables. However, there is no support in the core specification for procedural semantics or most of the other complexities of full-blown programming languages.

Execution Model

The assumed execution model of the Expression language is that a BEL text is executed by an execution engine. Expressions within statements are evaluated by an evaluator against a data context, which determines the truth values of the expression(s). Initially the data context is assumed to be that of openEHR templated archetypes, within a typical component that uses archetypes.

General Structure

The Expression Language may be used to write statements containing typed expressions. Statements may be declarations, assignments or assertions. These contain expressions that in turn contain symbols which represent typed variables and constants. Variables are either bound or local, and are defined using $ names, e.g. $heart_rate as used in shell script and some other programming languages. A bound variable is mapped to a field in the data context. In the current version of the language, this is achieved by assignment statements that either assign a path in the data context to a symbolic variable (reading) or the reverse (writing back). In this sense, 'assignment' can be understood as associating a data context path with a named variable. == References bibliography::[]

References

  • [1] G. Smith, The Object Z Specification Language. Kluwer Academic Publishers, 2000. [Online]. Available: http://www.itee.uq.edu.au/~smith/objectz.html

  • [2] J. F. Sowa, Knowledge Representation: Logical, philosophical and Computational Foundations. California: Brooks/Cole, 2000.

  • [3] J. L. Hein, Discrete Structures, Logic and Computability, Second. Jones, 2002.

  • [4] H. Kilov and J. Ross, Information Modelling - an object-oriented approach. Prentice Hall, 1994.