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prolog_debug.pl -- User level debugging tools
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This library provides tools to control the Prolog debuggers. Traditionally this code was built-in. Because these tools are only required in (interactive) debugging sessions they have been moved into the library.

Source prolog:debug_control_hook(+Action)[multifile]
Allow user-hooks in the Prolog debugger interaction. See the calls below for the provided hooks. We use a single predicate with action argument to avoid an uncontrolled poliferation of hooks.
Source spy :Spec is det
Source nospy :Spec is det
Source nospyall is det
Set/clear spy-points. A successfully set or cleared spy-point is reported using print_message/2, level informational, with one of the following terms, where Spec is of the form M:Head.
  • spy(Spec)
  • nospy(Spec)
See also
- spy/1 and nospy/1 call the hook prolog:debug_control_hook/1 to allow for alternative specifications of the thing to debug.
Source debugging is det
Report current status of the debugger.
Source trap(+Formal) is det
Source notrap(+Formal) is det
Install a trap on error(Formal, Context) exceptions that unify. The tracer is started when a matching exception is raised. This predicate enables debug mode using debug/0 to get more context about the exception. Even with debug mode disabled exceptions are still trapped and thus one may call nodebug/0 to run in normal mode after installing a trap. Exceptions are trapped in any thread. Debug mode is only enabled in the calling thread. To enable debug mode in all threads use tdebug/0.

Calling debugging/0 lists the enabled traps. The predicate notrap/1 removes matching (unifying) traps.

In many cases debugging an exception that is caught is as simple as below (assuming run/0 starts your program).

?- gtrap(_).
?- run.
See also
- gtrap/1 to trap using the graphical debugger.
- Edit exceptions menu in PceEmacs and the graphical debugger that provide a graphical frontend to trap exceptions.
Source exception_hook(+ExIn, -ExOut, +Frame, +Catcher) is failure
Trap exceptions and consider whether or not to start the tracer.