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Chapters 14 and 15 respectively presented two models of application organisation: The functional/modular model and the object model. These two models address, each in its own way, the needs of application development:

The development of a modular application begins by dividing it into logical units: modules. This is followed by the actualization of their specification by writing their signature, and finally by implementation. During the implementation of a module, it may be necessary to modify its signature or that of its parameters; it is then necessary to modify their sources. This is unsatisfactory if the same module is already used by another application. Nevertheless, this process offers a strict and reassuring framework for the programmer.

In the object model, the analysis of a problem results in the description of the relations between classes. If, later on, a class does not provide the required functionality, it is always possible to extend it by subclassing. This process permits the reuse of large hierarchies of classes without modifying their sources, and thus not modifying the behavior of an application that uses them, either. Unfortunately, this technique leads to code bloat, and poses difficulties of duplication with multiple inheritance.

Many problems necessitate recursive data types and operations which manipulate values of these types. It often happens that the problem evolves, sometimes in the course of implementation, sometimes during maintenance, requiring an extension of the types and operations. Neither of these two models permits extension in both ways. In the functional/modular model, types are not extensible, but one can create new functions (operations) on the types. In the object model, one can extend the objects, but not the methods (by creating a new subclass on an abstract class which implements its methods.) In this respect, the two models are duals.

The advantage of uniting these two models in the same language is to be able to choose the most appropriate model for the resolution of the problem in question, and to mix them in order to overcome the limitations of each model.

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