A Debugging Model for Lazy Functional Logic Languages

In this paper we present a debugging model for a lazy functional logic programming language whose semantics is based on lazy narrowing under a particular control regime: Demand Driven (dpt). dpt makes quite hard to predict the sequence of evaluations in computing head normal forms for total function applications. The order in which rules are applied in such computations can be different from its textual definition order. Moreover, demanded arguments are evaluated by computing the head normal form of itself and of those positions inside the argument which are also demanded. However, demanded positions for one and the same argument may not be evaluated sequentially. The sophisticated operational semantics of dpt makes necessary to develop a good debugging model which reflects the computational behaviour of programs, showing the different lazy unifications for demanded positions and the set of possible rules for applying depending on the former lazy unifications. We present a debugging m..