A Distributed Logic Programming Language and its Implementation on Transputer Networks

Currently there is a trend towards the development of programming tools and mechanisms for the support of heterogeneous multi-agent systems on paralell computer architectures. This paper presents a contribution to this area, as far as logic programming on a distributed execution environment is concerned. We discuss the main issues on the design and implementation of the logic programming language Delta Prolog [2] [3] [6] [7] [11], extending Prolog with constructs for concurrency and communication. The work described is one of the research components of a project on the development of mechanisms for parallel logic programming support on parallel architectures, currently running in this University [8]. 1. Delta Prolog: the language In this section a simple example is used to review the main language constructs, along the following relevant dimensions: specifications of sequential and concurrent composition of Prolog goals; communication and synchronization between processes; local non-determinism (i.e. within each Prolog process) and global (or external) non-determinism (allowing the choice among multiple communication alternatives). 1.1 _-Prolog programs A _-Prolog program is a sequence of clauses of the form: H :G1,...,Gn. (n_0). The comma is the sequential composition operator. Declaratively, the truth of goals in _-Prolog is orderdependant, so that H is true if G1,...,Gn are true in succession. Whereas H is a Prolog goal, each Gi may be either a Prolog or a _-Prolog goal. A _-Prolog goal is either a split goal (for parallelism), an event goal (for inter-process communication) or a choice goal (for external non-determinism). A _-Prolog program without _-Prolog goals is and executes like a Prolog program, so _-Prolog is an extension to Prolog. The programming model relies on the programmer to specify the sequentiality constraints and the desirable parallelism existing in each problem (using the comma and the split operators), and the corresponding communication schemes (through event and choice goals). The paper assumes some knowledge of logic programming languages, such as Prolog. An informal introduction to the language constructs follows. 1.2 Illustration of the main language constructs Published in the Procs. of 12th Technical Meeting of Occam User Group, Exeter, IOS Amsterdam, April 1990.