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Predicate debug/3
Availability::- use_module(library(debug)).(can be autoloaded)
Source[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.
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LogicalCaptain said (2020-07-05T15:31:44):0 upvotes 0 0 downvotes
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Topics

Please read important note on debug topics in the comment section of Section debug

Possible improvement

There should be a debug/2 for when one is not interested in passing format parameters at all.

Output

The debug output goes to STDERR (when on a TTY, they are printed in blue)

debug/3 also tries to prepend the output with `%` but only succeeds on the first line!

This should (possibly) be corrected, but in fact it's only meant for one-liners.

However, one can intercept the call and patch in one's own output generator (but I don't know how yet):

?- debug(toplevel).
Warning: toplevel: no matching debug topic (yet)
true.

?- debug(toplevel,"This is multiline: ~q\n~q\n~q",[alpha,bravo,charlie]).
% This is multiline: alpha
bravo
charlie
true.
Alan Baljeu said (2013-07-18T16:26:11):0 upvotes 0 0 downvotes
Picture of user Alan Baljeu.
debug/1 is not documented. Also, how is a topic disabled? Where should one find the debug output?