Algorithmic debugging for lazy functional languages

Abstract

AbstractLazy functional languages have non-strict semantics and are purely declarative, i.e. they support the notion of referential transparency and are devoid of side-effects. Traditional debugging techniques are, however, not suited for lazy functional languages, since computations generally do not take place in the order one might expect. Sincealgorithmic debuggingallows the user to concentrate on the declarative aspects of program semantics, and will semi-automatically find functions containing bugs, we propose to use this technique for debugging lazy functional programs. Because of the non-strict semantics of lazy functional languages, arguments to functions are in general partially evaluated expressions. The user is, however, usually more concerned with the values that these expressions represent. We address this problem by providing the user with astrictifiedview of the execution trace whenever possible. In this paper, we present an algorithmic debugger for a lazy functional language based on strictification and some experience in using it. A number of problems with the current implementation of the debugger (e.g. too large trace size and too many questions asked) are also discussed and some techniques for overcoming these problems, at least partially, are suggested. The key techniques areimmediate strictificationandpiecemeal tracing.