prolog_debug.pl -- User level debugging tools

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.

 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.

 spy(:Spec) is det

 nospy(:Spec) is det

 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.

 debugging is det

Report current status of the debugger.

 trap(+Exception) is det

 notrap(+Exception) is det

Install a trap on error(Formal, Context) exceptions that unify with Exception. 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.

The predicate notrap/1 removes matching (unifying) traps.

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.

 exception_hook(+ExIn, -ExOut, +Frame, +Catcher) is failure

Trap exceptions and consider whether or not to start the tracer.

Undocumented predicates

The following predicates are exported, but not or incorrectly documented.

 notrap(Arg1)

 nospy(Arg1)

 nospyall