Nested Mixin-Methods in Agora

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.