KnowledgeWorks and Prolog User Guide > A Common Prolog > A.7 Debugging

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A.7.2 Spy Points

Spy points are the most important debugging facility in Common Prolog. They are used in the same way trace is used in Lisp. After executing (spy foo) , all events associated with satisfying foo goals will be traced and the user will enter a debugging command loop at every port (see Interactive Debugging below). A user can also specify (spy (foo 3)) , (spy (foo bar)) , or (spy ((foo 3) bar)) to place spy points on foo goals with arity 3, on all predicates for foo and bar , or on foo with arity 3 and all predicates for bar respectively. Spy points are turned off with (nospy < spypoints >) . If no spy points are mentioned, nospy will turn off all spy points.


KnowledgeWorks and Prolog User Guide (Macintosh version) - 22 Dec 2009

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