The EXPRESS-I Language
The EXPRESS-I Language
Now we turn to the question: ‘Once I have created an abstract declaration in EXPRESS, what would an instance of that thing look like?’ EXPRESS-I allows you to create instances of EXPRESS things that have values in place of references to datatypes. The main reason for doing this is to study some realistic examples of things that otherwise might be difficult to understand. After all, it is one thing to describe a tree and quite another to actually see one. Some of the design goals of EXPRESS-I are based on these requirements: • Major information modeling projects are large and complex. Managing them without appropriate tools based on formal languages and methods is a risky proposition. Informal specification techniques eliminate the possibility of employing computer automation in checking for inconsistencies in presentation or specification. • The language should focus on the display of the realization of the properties of entities, which are the things of interest. The definition of entities is in terms of data and behavior. Data represents the properties by which an entity is realized and behavior is represented by constraints. • The language should seek to avoid, as far as possible, specific implementation views. That is, EXPRESS-I models do not suggest the structure of databases, object bases, or of information bases in general. • The language should provide a means for displaying small populated models of EXPRESS schemas as examples for design reviews. • The language should provide a means for supporting the specification of test suites for information model processors. EXPRESS-I represents entity instances in terms of the values of its attributes (attributes are the traits or characteristics considered important for use and understanding). These values have a representation which might be considered simple (an integer value) or something more complex (an entity value). A geometric point might be defined in terms of three real numbers named x, y and z, and the actual values associated with those attributes might be 1.0, 2.5 and 7.9. The EXPRESS-I instance language provides a means of displaying instantiations of EXPRESS data elements. The language is designed principally for human readability and for ease of generating EXPRESS-I element instances from definitions in an EXPRESS schema.
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