Inheritance is well-known and accepted as a mechanism for reuse in object-oriented languages. Unfortunately, due to the coarse granularity of inheritance, it may be difficult to decompose an application into an optimal class hierarchy that maximizes software reuse. Existing schemes based on single inheritance, multiple inheritance, or mixins, all pose numerous problems for reuse. To overcome these problems we propose traits, pure units of reuse consisting only of methods. We develop a formal model of traits that establishes how traits can be composed, either to form other traits, or to form classes. We also outline an experimental validation in which we apply traits to refactor a non-trivial application into composable units.

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Adaptive Object Models are a sophisticated way of building object-oriented systems that let non-programmers customize the behavior of the system and that are most useful for businesses that are rapidly changing. Although systems based on an Adaptive Object Model are often much smaller than competitors, they can be difficult to build and to learn. We believe that the problems with Adaptive Object Models are due in part to a mismatch between their design and the languages that are used to build them. This paper describes how to avoid this mismatch by using implicit and explicit metaclasses.