SWI-Prolog 9.2.7 Reference Manual: Section A.14

## A.14 library(debug): Print debug messages and test assertions

This library is a replacement for format/3 for printing debug messages. Messages are assigned a topic. By dynamically enabling or disabling topics the user can select desired messages. Calls to debug/3 and assertion/1 are removed when the code is compiled for optimization unless the Prolog flag optimise_debug is set to `true`.

Using the predicate assertion/1 you can make assumptions about your program explicit, trapping the debugger if the condition does not hold.

Output and actions by these predicates can be configured using hooks to fit your environment. With XPCE, you can use the call below to start a graphical monitoring tool.

    ?- prolog_ide(debug_monitor).

[semidet]debugging(+Topic)
[nondet]debugging(-Topic)
[nondet]debugging(?Topic, ?Bool)
  Examine debug topics. The form `debugging(+Topic)` may be used to perform more complex debugging tasks. A typical usage skeleton is:

            (   debugging(mytopic)
            ->  <perform debugging actions>
            ;   true
            ),
            ...

  The other two calls are intended to examine existing and enabled debugging tokens and are typically not used in user programs.

[det]debug(+Topic)
[det]nodebug(+Topic)
  Add/remove a topic from being printed. `nodebug(_)` removes all topics. Gives a warning if the topic is not defined unless it is used from a directive. The latter allows placing debug topics at the start of a (load-)file without warnings.

  For debug/1, Topic can be a term `Topic > Out`, where Out is either a stream or stream-alias or a filename (an atom). This redirects debug information on this topic to the given output. On Linux systems redirection can be used to make the message appear, even if the `user_error` stream is redefined using

      ?- debug(Topic > '/proc/self/fd/2').

  A platform independent way to get debug messages in the current console (for example, a `swipl-win` window, or login using `ssh` to Prolog running an SSH server from the `libssh` pack) is to use:

      ?- stream_property(S, alias(user_error)),
         debug(Topic > S).

  Do not forget to disable the debugging using nodebug/1 before quitting the console if Prolog must remain running.

[det]list_debug_topics
[det]list_debug_topics(+Options)
  List currently known topics for debug/3 and their setting. Options is either an atom or string, which is a shorthand for `[search(String)]` or a normal option list. Defined options are:
  search(String)
    Only show topics that match String. Match is case insensitive on the printed representation of the term.
  active(+Boolean)
    Only print topics that are active (`true`) or inactive (`false`).
  output(+To)
    Only print topics whose target location matches To. This option implicitly restricts the output to active topics.
[det]debug_message_context(+What)
  Specify additional context for debug messages.
  deprecated
    New code should use the Prolog flag message_context. This predicates adds or deletes topics from this list.
[det]debug(+Topic, +Format, :Args)
  Format a message if debug topic is enabled. Similar to format/3 to `user_error`, but only prints if Topic is activated through debug/1. Args is a meta-argument to deal with goal for the @-command. Output is first handed to the hook prolog:debug_print_hook/3. If this fails, Format+Args is translated to text using the message-translation (see print_message/2) for the term `debug(Format, Args)` and then printed to every matching destination (controlled by debug/1) using print_message_lines/3.

  The message is preceded by’% ’and terminated with a newline.

  See also
    format/3.
[semidet,multifile]prolog:debug_print_hook(+Topic, +Format, +Args)
  Hook called by debug/3. This hook is used by the graphical frontend that can be activated using prolog_ide/1:

      ?- prolog_ide(debug_monitor).

[det]assertion(:Goal)
  Acts similar to C `assert()` macro. It has no effect if Goal succeeds. If Goal fails or throws an exception, the following steps are taken:

  - call prolog:assertion_failed/2. If prolog:assertion_failed/2 fails, then:

    - If this is an interactive toplevel thread, print a message, the stack-trace, and finally trap the debugger.
    - Otherwise, throw `error(assertion_error(Reason, G),_)` where Reason is one of `fail` or the exception raised.
[semidet,multifile]prolog:assertion_failed(+Reason, +Goal)
  This hook is called if the Goal of assertion/1 fails. Reason is unified with either `fail` if Goal simply failed or an exception call otherwise. If this hook fails, the default behaviour is activated. If the hooks throws an exception it will be propagated into the caller of assertion/1.