Now we discuss some of the potential uses of the EXPRESS-I language. In Object-Oriented terms, an EXPRESS entity would be called a class, and an instance of a class is termed an object. One object may reference another object. EXPRESS, though, distinguishes between entities and types (i.e., the enumeration, select and the defined data type) as entities may be subtyped whereas types cannot be. EXPRESS-I treats entity instances as objects in the OO sense. It also allows types to be treated as objects in that they can be instantiated and referenced. Alternatively, type values may be embedded in entity instances. The simplest use of EXPRESS-I is as a paper exercise in displaying examples of EXPRESS declarations that are populated with data. The language allows the display of entity instances as referenceable objects. Types instances may also be displayed as referenceable objects, or they may appear as unreferenceable values within other objects' values. Examples in this book show both forms of type instantiation. Values of explicit entity attributes are required. The values of derived or inverse attributes need not be displayed, except as exemplars, because as noted, these are essentially calculable from the values of the explicit attributes. Examples of EXPRESS schemas can also be displayed, as well as individual objects. The EXPRESS-I model construct is provided to enable the display of multiple schemas. Typically, a model would be used when two or more EXPRESS schemas interact with each other. Note that EXPRESS itself does not support such a construct. Note: We do not discuss models in this book. The test case construct is provided to assist in the formal specification of test cases against the implementation of an EXPRESS schema. EXPRESS itself does not provide an equivalent construct. For a test case, a base set of EXPRESS-I objects must be defined which will be the objects (and their supporting data) to be tested. The values of these objects may be in the form of parameters, whose formal definition are given in an enclosing context. A series of test cases may then be defined on the context, by providing actual parameter values.
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