Once the ‘dd’ command has been given, the declarative debugger asks the user a series of questions about the truth of various assertions in the intended interpretation. The first question in this series will be about the validity of the event for which the ‘dd’ command was given. The answer to this question will nearly always be “no”, since the user has just implied the assertion is false by giving the ‘dd’ command. Later questions will be about other events in the execution of the program, not all of them necessarily of the same kind as the first.
The user is expected to act as an “oracle” and provide answers to these questions based on their knowledge of the intended interpretation. The debugger provides some help here: previous answers are remembered and used where possible, so questions are not repeated unnecessarily. Commands are available to provide answers, as well as to browse the arguments more closely or to change the order in which the questions are asked. See the next section for details of the commands that are available.
When seeking to determine the validity of the assertion corresponding to an EXIT event, the declarative debugger prints the exit atom followed by the question ‘Valid?’ for the user to answer. The atom is printed using the same mechanism that the debugger uses to print values, which means some arguments may be abbreviated if they are too large.
When seeking to determine the validity of the assertion corresponding to a FAIL event, the declarative debugger prints the call atom, prefixed by ‘Call’, followed by each of the exit atoms (indented, and on multiple lines if need be), and prints the question ‘Complete?’ (or ‘Unsatisfiable?’ if there are no solutions) for the user to answer. Note that the user is not required to provide any missing instance in the case that the answer is no. (A limitation of the current implementation is that it is difficult to browse a specific exit atom. This will hopefully be addressed in the near future.)
When seeking to determine the validity of the assertion corresponding to an EXCP event, the declarative debugger prints the call atom followed by the exception that was thrown, and prints the question ‘Expected?’ for the user to answer.
In addition to asserting whether a call behaved correctly or not the user may also assert that a call should never have occurred in the first place, because its inputs violated some precondition of the call. For example if an unsorted list is passed to a predicate that is only designed to work with sorted lists. Such calls should be deemed inadmissible by the user. This tells the declarative debugger that either the call was given the wrong input by its caller or whatever generated the input is incorrect.
In some circumstances the declarative debugger provides a default answer to the question. If this is the case, the default answer will be shown in square brackets immediately after the question, and simply pressing return is equivalent to giving that answer.