Lazy functional languages are declarative and allow the programmer to write programs where operational issues such as the evaluation order are left implicit. It is desirable to maintain a declarative view also during debugging so as to avoid burdening the programmer with operational details, for example concerning the actual evaluation order which tends to be difficult to follow. Conventional debugging techniques focus on the operational behaviour of a program and thus do not constitute a suitable foundation for a general-purpose debugger for lazy functional languages. Yet, the only readily available, general-purpose debugging tools for this class of languages are simple, operational tracers.
This thesis presents a technique for debugging lazy functional programs declaratively and an efficient implementation of a declarative debugger for a large subset of Haskell. As far as we know, this is the first implementation of such a debugger which is sufficiently efficient to be useful in practice. Our approach is to construct a declarative trace which hides the operational details, and then use this as the input to a declarative (in our case algorithmic) debugger.
The main contributions of this thesis are: