Diagnosing Non-Well-Moded Concurrent Logic Programs

Strong moding and constraint-based mode analysis are expected to play fundamental roles in debugging concurrent logic/constraint programs as well as in establishing the consistency of communication protocols and in optimization. Mode analysis of Moded Flat GHC is a constraint satisfaction problem with many simple mode constraints, and can he solved efficiently by unification over feature graphs. In practice, however, it is important to be able to analyze non-well-moded programs (programs whose mode constraints are inconsistent) and present plausible “reasons” of inconsistency to the programmers in the absence of mode declarations.
This paper discusses the application of strong moding to systematic and efficient static program debugging. The basic idea, which turned out to work well at least for small programs, is to find a minimal inconsistent subset from an inconsisteilt set of mode constraints and indicate the symbol( occurrence)s in the program text that imposed those constraints. A bug can he pinpointed better by finding more than one overlapping minimal subset. These ideas can be readily extended to finding multiple hugs at once. For large programs, stratification of predicates narrows search space and produces more intuitive explanations. Stratification plays a fundamental role in introducing mode polymorphism as well.