Debugging programs written in lazy functional languages is difficult, and there are currently no realistic, general purpose debugging tools available. The basic problem is that computations in general do not take place in the order one might expect. Furthermore, lazy functional languages to a large extent free programmers from concerns regarding operational issues such as evaluation order, i.e. they are `declarative'. Debugging should therefore take place at the same, high level of abstraction. Thus, we propose to base debugging on what we call Evaluation Dependence Trees (EDT), which reflect the declarative semantics of the programs rather than operational concerns such as evaluation order. This in turn naturally suggests a two level debugger architecture where the lower level generates the EDT and the upper level allows the user to investigate it. The main advantage of this is flexibility: components realizing the two levels may be chosen independently to suit the debugging problem at hand.

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