1.2 Modular, reusable, component-oriented programs
Dylan is an object-oriented language. Programs create and use objects, and they use classes to categorize and abstract attributes of objects. Classes play a number of key roles:
They are data types, embodying subtype-supertype relationships between objects.
They are the vehicle for abstraction of common attributes of objects.
They organize sharing of attributes: Subclasses inherit the attributes of superclasses.
They are the principal basis for specializing behavior of objects.
Objects contain data in slots, which are like structure members or fields in other languages. But the behavior of objects resides in generic functions and methods. A generic function is a function that embodies an operation common to different classes of the objects that are its arguments. A method is a function that acts as a specific implementation of a generic operation for objects of a particular class. A program calls a generic function, and Dylan determines the most appropriate method to invoke based on the arguments to the generic function. A program controls method selection, or dispatch, by means of class relationships, rather than via explicit conditional statements.
Abstraction of common attributes and methods in superclasses lets you reuse code, rather then reimplement it, for subclasses. By defining a subclass, you can add specialized data or behavior while having the subclass inherit attributes of superclasses, which may be defined in another component or library, or in Dylan itself.
Generic functions constitute abstract interfaces for specific operations. You can usually change the implementation of an operation or a data representation without changing the interface to the operation. In this way, you can change an implementation without changing the functions or objects that use the implementation. These functions or objects may be defined in another component or library.
Dylan provides large-scale variable namespaces, called modules. A module can include or use other modules, but only the variables explicitly exported from those modules are visible to it. Modules provide public and private global variables. Because functions and classes, as well as data, are variable values, modules define external interfaces for collections of classes and generic functions.