170 THE STATIC STRUCTURE:CLASSES $7.3 Modules and types Programming languages and other notations used in software development (design languages,specification languages,graphical notations for analysis)always include both some module facility and some type system. A module is a unit of software decomposition.Various forms of module,such as See chapter 3. routines and packages,were studied in an earlier chapter.Regardless of the exact choice of module structure,we may call the notion of module a syntactic concept,since the decomposition into modules only affects the form of software texts,not what the software can do;it is indeed possible in principle to write any Ada program as a single package,or any Pascal program as a single main program.Such an approach is not recommended,of course,and any competent software developer will use the module facilities of the language at hand to decompose his software into manageable pieces.But if we take an existing program,for example in Pascal,we can always merge all the modules into a single one,and still get a working system with equivalent semantics.(The presence of recursive routines makes the conversion process less trivial,but does not fundamentally affect this discussion.)So the practice of decomposing into modules is dictated by sound engineering and project management principles rather than intrinsic necessity. Types,at first sight,are a quite different concept.A type is the static description of certain dynamic objects:the various data elements that will be processed during the execution ofa software system.The set of types usually includes predefined types such as INTEGER and CHARACTER as well as developer-defined types:record types (also known as structure types),pointer types,set types(as in Pascal),array types and others. The notion of type is a semantic concept,since every type directly influences the execution of a software system by defining the form of the objects that the system will create and manipulate at run time. The class as module and type In non-O-0 approaches,the module and type concepts remain distinct.The most remarkable property of the notion of class is that it subsumes these two concepts,merging them into a single linguistic construct.A class is a module,or unit of software decomposition;but it is also a type (or,in cases involving genericity,a type pattern) Much of the power of the object-oriented method derives from this identification. Inheritance,in particular,can only be understood fully if we look at it as providing both module extension and type specialization. What is not clear yet is how it is possible in practice to unify two concepts which appear at first so distant.The discussion and examples in the rest of this chapter will answer this question.170 THE STATIC STRUCTURE: CLASSES §7.3 Modules and types Programming languages and other notations used in software development (design languages, specification languages, graphical notations for analysis) always include both some module facility and some type system. A module is a unit of software decomposition. Various forms of module, such as routines and packages, were studied in an earlier chapter. Regardless of the exact choice of module structure, we may call the notion of module a syntactic concept, since the decomposition into modules only affects the form of software texts, not what the software can do; it is indeed possible in principle to write any Ada program as a single package, or any Pascal program as a single main program. Such an approach is not recommended, of course, and any competent software developer will use the module facilities of the language at hand to decompose his software into manageable pieces. But if we take an existing program, for example in Pascal, we can always merge all the modules into a single one, and still get a working system with equivalent semantics. (The presence of recursive routines makes the conversion process less trivial, but does not fundamentally affect this discussion.) So the practice of decomposing into modules is dictated by sound engineering and project management principles rather than intrinsic necessity. Types, at first sight, are a quite different concept. A type is the static description of certain dynamic objects: the various data elements that will be processed during the execution of a software system. The set of types usually includes predefined types such as INTEGER and CHARACTER as well as developer-defined types: record types (also known as structure types), pointer types, set types (as in Pascal), array types and others. The notion of type is a semantic concept, since every type directly influences the execution of a software system by defining the form of the objects that the system will create and manipulate at run time. The class as module and type In non-O-O approaches, the module and type concepts remain distinct. The most remarkable property of the notion of class is that it subsumes these two concepts, merging them into a single linguistic construct. A class is a module, or unit of software decomposition; but it is also a type (or, in cases involving genericity, a type pattern). Much of the power of the object-oriented method derives from this identification. Inheritance, in particular, can only be understood fully if we look at it as providing both module extension and type specialization. What is not clear yet is how it is possible in practice to unify two concepts which appear at first so distant. The discussion and examples in the rest of this chapter will answer this question. See chapter 3