. Predicate classes are a new linguistic construct designed to complement normal classes in objectoriented languages. Like a normal class, a predicate class has a set of superclasses, methods, and instance variables. However, unlike a normal class, an object is automatically an instance of a predicate class whenever it satisfies a predicate expression associated with the predicate class. The predicate expression can test the value or state of the object, thus supporting a form of implicit property-based classification that augments the explicit type-based classification provided by normal classes. By associating methods with predicate classes, method lookup can depend not only on the dynamic class of an argument but also on its dynamic value or state. If an object is modified, the property-based classification of an object can change over time, implementing shifts in major behavior modes of the object. A version of predicate classes has been designed and implemented in the context of t...

In object-oriented programming languages the class concept is heavily overworked. To simplify things, there is a tendency to unbundle the different roles they play by trying to create more modular inheritance operators [Bracha&Lindstrom92]. This offers the advantage that classes can be composed in a modular way. Prototype-based languages on the other hand, also provide a simpler view on object-oriented programming, but there modular composition is totally neglected.

Mixin-based inheritance is an inheritance technique that has been shown to subsume a variety of different inheritance mechanisms. It is based directly upon an incremental modification model of inheritance. This paper addresses the question of how mixins can be seen as named attributes of classes the same way that objects, methods, and also classes in their own right, are seen as named attributes of classes. The general idea is to let a class itself have control over how it is extended. This results in a powerful abstraction mechanism to control the construction of inheritance hierarchies in two ways. Firstly, by being able to constrain the inheritance hierarchy; secondly, by being able to extend a class in a way that is specific for that class. Nested mixins are a direct consequence of having mixins as attributes. The scope rules for nested mixins are discussed, and shown to preserve the encapsulation of objects.