prolog_debug:debug/1

Documentation
- Reference manual
  - The SWI-Prolog library
    - library(debug): Print debug messages and test assertions
- Packages

Availability::- use_module(library(debug)).(can be autoloaded)

[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.