Dimensions and Dichotomy in Metamodeling

Syntax: An abstract description of the entities that form a model of the respective language. Static Semantics (context conditions, constraints) : Well-formedness conditions between the syntactic entities, such as absence of circular inheritance. Dynamic Semantics (denotation): The (operational) behavior of the entities of the speci cation, such as I/O, reaction to stimuli, e ect of executing an operation, etc. The description of dynamic semantics in a metamodel, in particular, is not treated precisely in other approaches. The dimensions of metamodeling are depicted in Figure 3. Here, the concepts of intensional and extensional entities and its relation to abstract syntax, static and dynamic semantics is summarized. Intensional Entities Extensional Entities abstract Syntax st at ic S em an tic sSyntax st at ic S em an tic s dy na m ic S em an tic s Figure 3: The Dimensions of Metamodeling A metaentity has an internal state that is protected by object-oriented encapsulation principles. This ensures that each metaentity has its own meaning, as independent as possible from other metaentities. We call this principle, localization principle (see [  Ove98]). In classical approaches for the de nition of semantics of languages, e.g. [Mey88], this localization is not ensured. The advantages are twofold: rst, we achieve a better understandability and readability. Second, and maybe more important, we achieve an increased exibility with respect to extensions and changes. This has proved very important for the development of metamodels. In general, a metaclass might be related to other metaclasses by an association relationship. An extensional entity is connected to its intensional entity by a special instantiation association. The division between intensional and extensional entities partitions the metamodel in two parts that are only connected by these special associations. In Section 4.2 we formulate guidelines concerning these associations. The generalization-specialization relationship is important for de ning abstract metaclasses as interfaces for metaentities. These interfaces have to be ful lled by the specialized metaentities. In Section 3, we illustrate the structuring techniques as well as the methodological aspects of our approach. In Figure 4, the di erent roles of abstract syntax, static and dynamic semantics for intensional and extensional objects are summarized. The most remarkable di erence is observable in the dynamic semantics. While dynamic semantics on the extensional level means run-time behavior, dynamic semantics on intensional level describes system evolution in the development process.