SWI-Prolog 9.2.9 Reference Manual: Section A.39

## 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.