The Meander Language and System

Parallel programming is characterized by a growing set of parallel architectures, paradigms and programming languages. The question how to support a programmer best in designing and implementing a parallel program is still an important topic of research. This is especially true when trying to utilize distributed memory machines (e.g. Transputers or workstation clusters). For many application areas, explicit parallel programming using some sort of message passing is still essential for gaining efficiency. Unfortunately, explicit parallel programming is more complex than programming in a sequential paradigm: the user has to perform more steps when developing a parallel application (algorithm and/or data partitioning, coordination of complex systems, mapping); more important, parallelism is hard to understand because there is no longer a linearly ordered flow of control which implies problems like determinacy, proper termination and poor testability. We argue that parallel languages which are entirely based on textual representations are not the best choice for describing parallelism. The main drawbacks stem from the fact that textual representations are always written down in some sequential order which hides the parallel structure of a program. The inadequacy of purely textual programming becomes evident when communicating a parallel program to a colleague or reading a textbook on parallel programming: even people studying for years in topics of parallelism resort to drawing when explaining a specific parallel situation, mechanism or whatever. Moreover, the evolving set of tools supporting parallel programming (debugging, visualization) is especially useful because of exploiting graphical methods. We propose the usage of graphical languages to specify parallel programs where parts of a graph may be connected or not, depending on the existence of logical relationships (e.g. causal ordering, communication). Although graphical methods are central in our approach, we are far from using graphics for all parts of a parallel program: purely sequential parts without communication and parallel control should be formulated in the old-fashioned way. Hence, we use a hybrid approach integrating textual and graphical representations into one language. Such a hybrid language provides the ideal basis for an integrated programming environment: The same graph which is used for drawing a sketch of the planned process system is used for coding and for the visualization of program behaviour.