SWI-Prolog 8.4.0 Reference Manual: Section A.35

## A.35 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.
[det]trap(+Exception)
[det]notrap(+Exception)
  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.
[failure]exception_hook(+ExIn, -ExOut, +Frame, +Catcher)
  Trap exceptions and consider whether or not to start the tracer.