A Propagation Tracer for GNU-Prolog: From Formal Definition to Efficient Implementation

Tracers give some insight of program executions: with an execution trace a programmer can debug and tune programs. Traces can also be used by analysis tools, for example to produce statistics or build graphical views of program behaviors. Constraint propagation tracers are especially needed because constraint propagation problems are particularly hard to debug. Yet, there is no satisfactory tracer for CLP(FD) systems. Some do not provide enough information, others are very inefficient. The tracer formally described in this article provides more complete information than existing propagation tracers. Benchmarks show that its implementation is efficient. Its formal specification is useful both to implement the tracer and to understand the produced trace. It is designed to cover many debugging needs.