A Declarative Debugger for a Logical-Functional Language

Logic and functional programming languages have many advantages and there is a growing trend to develop languages which incorporate both these paradigms. One of the disadvantages of such languages is that the execution mechanisms are so complex that traditional debugging methods are diicult to use. Declarative debugging techniques have been successful applied to Prolog and more recently functional languages. They allow programs to be debugged using only knowledge of the declarative semantics, hiding the details of the execution from the programmer. In this paper we describe a declarative debugger for a combined logic and functional programming language, NUE-Prolog. The debugger combines both existing and new algorithms. The main new algorithm is for declarative debugging of functions which for some inputs are not well-deened (typically resulting in a runtime error). The algorithms can easily be adapted to other logic and functional languages.