Logical representations have been widely used in Computer Science and Artificial Intelligence (AI). In recent years, particularly with the advent of languages such as Prolog, the direct execution of such representations has been shown to be both feasible and useful. Logic-based languages have been used, not only for applications such as the animation of logical specifications, program verification, the characterization of database queries and knowledge representation, but also as high-level programming languages in their own right. However, as the problems tackled have become more complex, the requirement for more powerful logical representations has been growing. In particular, since the concept of time is of central importance to an increasingly wide range of applications, including the representation of time-dependent data, modeling reactive systems, and the specification and verification of concurrent and distributed systems, many logics incorporating temporal notions are being developed and applied.
It is not surprising, therefore, that executable temporal logics have been proposed in order to provide system developers with access to these, more powerful, logical techniques. Just as the development of sophisticated, and relatively efficient, theorem-proving techniques for first-order logic led to executable forms, such as Prolog, so the development of executable methods for temporal logics has often been based on temporal theoremproving techniques. However, each particular executable temporal logic combines not only a logical perspective, but also an operational model, drawn from its intended application areas. Thus a wide range of languages have appeared, exhibiting a variety of characteristics and execution mechanisms. Consequently, such languages have a variety of application areas, such as temporal databases, temporal planning, animation of temporal specifications, hardware simulation, and distributed AI.
This issue of the Journal of Symbolic Computation is dedicated to topics in Executable Temporal Logics (ETL). Although this area is relatively new, it has seen an increase in both applications and systems in the last few years and now it can stand on its own as an independent research area. The breadth and diversity of the papers published in this special issue are an indication that the area is close to reaching maturity. We trust that the papers presented in this special issue will foster further research spanning from theoretical to practical issues in ETL and iron out their connection to related areas such as constraint programming, deductive databases, temporal reasoning, theorem proving, executable specifications and so on. In the near future, as the implemented systems mature and more efficient implementations emerge, we are going to see more and more languages and systems applied to a broader range of applications with success.
The concept of time is especially important in reactive systems. Reactive systems