8 Patterns and Variables

In this section, we get a bit more formal with the BETA way of expressing object-oriented concepts. This section treats topics from chapter 3 of the BETA Book but with a slightly different approach.

Often object-oriented concepts are summarized as follows:

• 1) Objects are meant to represent the things that we see or talk about in the real world.
• 2) Objects have properties. These are often divided between state (attributes) and potential actions (services, methods or scripts).
• 3) Objects which have the same attributes and behave the same way are said to belong to the same class. A class definition specifies the attributes and actions common to all objects of the same class. The objects described by a class are said to be instances of that class.

• 4) Often it is useful to introduce the notion of class hierarchy to reflect various levels of similarity. The specification mechanism which allows incremental description of the similarities at each level is called inheritance.

• 5) Additionally, some languages model the fact that individual objects can operate in parallel. These active objects are sometimes called agents, processes or actors.

Actually, in this section, we will only talk about points 2 and 3. Point 1 is included for completeness and point 4 is not covered in this tutorial (see the BETA language introduction [MIA 94-26] or the BETA book [MMN 93]). Point 5 will be covered later.

In BETA, objects are described by a syntactic construct called an object-descriptor (or descriptor for short). This has the form:

(# ... #)

and is used to specify the local attributes and actions of an object (or class of objects).

An unusual feature of the BETA object-descriptor (compared to other object-oriented languages) is that, with enter and exit lists, it introduces the notion of value for objects. The different facets of an object are defined by the various (optional) parts of the object description:

(# <declarations> enter <input list> do <imperatives> exit <output list> #)

This single concept of descriptor has been used to replace many diverse concepts that we are familiar with from traditional languages. In particular, it is used for procedures, functions, types, classes, macros and local blocks. These different roles are achieved by using the descriptor in different contexts, by combining it with other operators or by selective use of the internal parts.