First-Class Compositions

A considerable amount of research, especially within the OO and AOSD communities, has focused on understanding the potential and limitations of various composition techniques. This has led to a large number of proposals for alternative composition techniques, including many variations of message dispatch, inheritance, and aspect mechanisms. This paper makes the case that there is no single perfect composition technique that suits every situation, since different techniques incur different tradeoffs. The proper composition technique to use depends on the particular design problem and its requirements (such as the required adaptability, reusability, understandability and robustness). However, most programming languages limit the available composition techniques to a very few. To address this, we propose a novel composition model, called

Co-op

. The model provides

dedicated abstractions

that can be used to express a

wide variety of object composition techniques

(“composition operators”). Examples include various forms of inheritance, delegation, and aspects. The proposed model

unifies

objects (with encapsulated state and a message interface) and composition operators; composition operators are specified as first-class citizens. Multiple composition operators can be combined within the same application, and composition operators can even be used to compose new composition operators from existing ones. This opens new possibilities for developing domain-specific composition operators, taxonomies of composition operators, and for reuse and refinement of composition operators. To validate and experiment with the proposed model, we have designed and implemented a simple language,

Co-op/I

, that we also use in this paper to show concrete examples.