Declarative diagnosis of wrong answers in constraint functional-logic programming

Debugging tools are a practical need for diagnosing the causes of erroneous com-putations. Declarative programming paradigms involving complex operational details, such as constraint solving and lazy evaluation, do not fit well to tradi-tional debugging techniques relying on the inspection of low-level computation traces. As a solution to this problem, declarative diagnosis uses Computation Trees (shortly, CTs) in place of traces. CTs are built a posteriori to represent the structure of a computation whose top level outcome is regarded as an error symptom by the user. Each node in a CT represents the computation of some observable result, depending on the results of its children nodes. Declarative diagnosis explores a CT looking for a so-called buggy node which computes an incorrect result from children whose results are correct; such a node must point to an incorrect program fragment. The search for a buggy node can be imple-mented with the help of an external oracle (usually the user with some semi-automatic support) who has a reliable declarative knowledge of the expected program semantics, the so-called intended interpretation