I/O Mode Analysis in Concurrent Logic Programming

This paper brieey reviews concurrent logic programming and the I/O mode system designed for the concurrent logic language Flat GHC. The mode system plays fundamental r^ oles both in programming and implementation in almost the same way as type systems do but in diierent respects. It provides us with the information on how data are generated and consumed and thus the view of \data as resources". It statically detects bugs resulting from ill-formed dataaow and advocates the \programming as wiring" paradigm. Well-modedness guarantees the safety of uniication, the basic operation in concurrent logic programming. Information on the numbers of access paths to data can be obtained by slightly extending the framework, which can be used for compile-time garbage collection and the destructive update of structures. Concurrent logic programming was born around 1980 from the study of the parallel execution of logic programs, and became an important paradigm for concurrent programming in its own right. Relational Language 3] was the rst to appear in the form of a complete programming language. Guarded Horn Clauses (GHC) was designed in the Fifth Generation Computer Project in 1984 11], after thorough examination of its predecessors Concurrent Prolog 9] and an early version of PARLOG 4]. Because of its simplicity, GHC was soon accepted in the Project as the base of KL1, the full-edged kernel language for the Parallel Inference Machine. GHC was soon subsetted to Flat GHC with a simpler guard construct, and the design of KL1 started based on Flat GHC. While GHC was designed as a concurrent language that did not address how programs should be executed, KL1 was designed as a parallel language in which programmers could specify what processor should execute what processes and at what priorities 13]. For the detailed history of the kernel language design in the Fifth Generation Computer Project, the readers are referred to 14]. 1 Several more concurrent logic programming languages were proposed in the past decade, but the diierence between all those languages is rather small as 1 An eecient KL1-to-C compiler system for Unix-based general-purpose computers, named KLIC, can be obtained via anonymous ftp from ftp.icot.or.jp.