Genericity, extensibility and type : safety in the VISITOR pattern

A software component is, in a general sense, a piece of software that can be safely reused and flexibly adapted by some other piece of software. The safety can be ensured by a type system that guarantees the right usage of the component; the flexibility stems from the fact that components are parametrizable over different aspects affecting their behaviours. Component-oriented programming (COP), a programming style where software would be built out of several independent components, has for a long time eluded the software industry. Several reasons have been raised over time, but one that is consistently pointed out is the inadequacy of existing programming languages for the development of software components. Generic Programming (GP) usually manifests itself as a kind of parametrization. By abstracting from the differences of what would otherwise be separate but otherwise similar specific programs, one can develop a single unified generic program. Instantiating the parameter in various ways retrieves the various specific programs (and ideally some new ones too). Instances of GP include the generics (parametrization by types) mechanism as found in recent versions Java and C# and Datatype-Generic Programming (DGP) (parametrization by shape). Both mechanisms allow novel ways to parametrize programs that can largely increase the flexibility of programs. Software components and GP, and in particular DGP, are clearly related: GP and DGP provide novel ways to parametrize software, while software components benefit from parametrization in order to be flexible. However, DGP and COP have mostly been studied in isolation, with the former being a research topic among some functional programming communities and the latter being mostly studied within the object-oriented communities. In this thesis we will argue for the importance of the parametrization mechanisms provided by GP, and in particular DGP, in COP. We will defend that many design patterns can be captured as software components when using such kinds of parametrization. As evidence for this we will, using DGP techniques, develop a component library for the V pattern that is generic (i.e. can be used on several concrete visitors); extensible (i.e. concrete visitors may be extended); and