swish_debug.pl -- Print debug messages and test assertions

This library is a SWISH specific replacement for library(debug) that allows for debug/3 messages in SWISH code as well as libraries loaded into SWISH. Unlike library(debug), this library maintains the debug state per thread (Pengine).

This library allows for debug/3 calls inside user programs stored in SWISH as well as libraries preloaded into SWISH. Libraries that wish to enable debugging by SWISH users must include this file rather than library(debug). This may be done dynamically using the following snippet.

:- if(exists_source(swish(lib/swish_debug))).
:- use_module(swish(lib/swish_debug)).
:- else.
:- use_module(library(debug)).
:- endif.

swish_debugging(+Topic) is semidet

swish_debugging(-Topic) is nondet

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.

debug(+Topic) is det

nodebug(+Topic) is det

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.

debug_topic(+Topic) is det[private]

Declare a topic for debugging. This can be used to find all topics available for debugging.

list_debug_topics is det

list_debug_topics(+Options) is det

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.

swish_debug(+Topic, +Format, :Args) is det

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.

Re-exported predicates

The following predicates are exported from this file while their implementation is defined in imported modules or non-module files loaded by this module.