Debugging messages
Author: Daniel Diaz
Keywords: GNU Prolog, manual, Prolog, compiler, constraints, finite domains

### 5.4 Debugging messages

We here described which information is displayed by the debugger when it shows a goal. The basic format is as follows:

  S N M Port: Goal ?

S is a spy-point indicator: if there is a spy-point on the current goal the + symbol is displayed else a space is displayed. N is the invocation number. This unique number can be used to correlate the trace messages for the various ports, since it is unique for every invocation. M is an index number which represents the number of direct ancestors of the goal (i.e. the current depth of the goal). Port specifies the particular port (call, exit, fail, redo, exception). Goal is the current goal (it is then possible to inspect its current instantiation) which is displayed using write_term/3 with quoted(true) and max_depth(D) options (section 8.14.6). Initially D (the print depth) is set to 10 but can be redefined using the < debugger command (section 5.5). The ? symbol is displayed when the debugger is waiting a command from the user. (i.e. Port is a leashed port). If the port is unleashed, this symbol is not displayed and the debugger continues the execution displaying the next goal.

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