Traits: Composable Units of Behaviour

Abstract

Abstract. Despite the undisputed prominence of inheritance as the fundamental reuse mechanism in object-oriented programming languages, the main variantssingle inheritance, multiple inheritance, and mixin inheritance -all suffer from conceptual and practical problems. In the first part of this paper, we identify and illustrate these problems. We then present traits, a simple compositional model for structuring object-oriented programs. A trait is essentially a group of pure methods that serves as a building block for classes and is a primitive unit of code reuse. In this model, classes are composed from a set of traits by specifying glue code that connects the traits together and accesses the necessary state. We demonstrate how traits overcome the problems arising from the different variants of inheritance, we discuss how traits can be implemented effectively, and we summarize our experience applying traits to refactor an existing class hierarchy.

Bridging Agent Theory and Object Orientation: Importing Social Roles in Object Oriented Languages

“…On the other hand, these languages do not provide the notion of role casting we introduce in powerJava. Roles as double face interfaces have some similarities with Traits [23] and Mixins. However, they are distinguished because roles are used to extend instances and not classes.…”

Gradual typing for Smalltalk

“…Note that a named protocol can serve to give a type to a trait [19]. However, traits come with a specific implementation, while named protocols are pure interface specifications.…”

Magda: A New Language for Modularity

“…Traits [46,22] have been designed as an alternative mechanism of code reuse. Initially they were introduced in a untyped setting as an extension of Smalltalk [22,46].…”

“…In its classical, single-inheritance version, this mechanism is simple, however often not sufficient. In order to overcome its limits, other proposals were developed, like multiple inheritance, mixins [43,17,15,8,24,14,7] and traits [46,22]. Multiple inheritance is perceived as too complicated and too dangerous [20].…”