Traits: Composing Classes from Behavioral Building Blocks

Abstract

Inheritance is well-known and accepted as a fundamental mechanism for reuse in object-oriented languages. Unfortunately, the main variants —- single inheritance, multiple inheritance, and mixin inheritance —- all suffer from conceptual and practical problems related to software reuse and robustness with respect to changes. In a rst part of this thesis, we identify and illustrate these problems. To overcome these problems, we then present traits, a simple compositional model that extends single inheritance. A trait is essentially a (parameterized) set of methods; it serves as a behavioral building block for classes and is the primitive unit of code reuse. We develop a formal model of traits that establishes how traits can be composed to form other traits or classes, and we describe how we implemented traits in Squeak Smalltalk by bootstrapping a new language kernel. We present our experimental validation in which we apply traits to refactor parts of the Smalltalk kernel and library, and we develop a programming methodology around the usage of traits and the trait browser, the tool that we implemented to take full advantage of the availability of traits in the Squeak programming environment.