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.
- 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.
- debugging is det
- Report current status of the debugger.
- trap(+Formal) is det
- 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.
In many cases debugging an exception that is caught is as simple as below (assuming run/0 starts your program).
?- gtrap(_). ?- run.
- exception_hook(+ExIn, -ExOut, +Frame, +Catcher) is failure
- Trap exceptions and consider whether or not to start the tracer.
- Make sure our handler is the first of the hook predicate.
The following predicates are exported, but not or incorrectly documented.