A.39 library(prolog_debug): 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.

[multifile]prolog:debug_control_hook(+Action)

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.

[det]spy(:Spec)

[det]nospy(:Spec)

[det]nospyall

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.

[det]debugging

Report current status of the debugger.

[multifile]debugging_hook

Multifile hook that is called as forall(debugging_hook, true) and that may be used to extend the information printed from other debugging libraries.

[det]trap(+Formal)

[det]notrap(+Formal)

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).

?- trap(_).
?- run.

The multifile hook trap_alias/2 allow for defining short hands for commonly used traps. Currently this defines

det

Trap determinism exceptions raised as a result of the det/1 directive.

=>

Trap rule existence error exceptions.

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.

[multifile]trap_alias(+Alias, -Error)

Define short hands for commonly used exceptions.