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Predicate debug/0
Availability::- use_module(library(edinburgh)).(can be autoloaded)
Sourcedebug
Start debugger. In debug mode, Prolog stops at spy and break points, disables last-call optimisation and aggressive destruction of choice points to make debugging information accessible. Implemented by the Prolog flag debug.

Note that the min_free parameter of all stacks is enlarged to 8 K cells if debugging is switched off in order to avoid excessive GC. GC complicates tracing because it renames the _<NNN> variables and replaces unreachable variables with the atom <garbage_collected>. Calling nodebug/0 does not reset the initial free-margin because several parts of the top level and debugger disable debugging of system code regions. See also set_prolog_stack/2.

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Eric Zinda said (2020-11-03T03:34:09):0 upvotes 0 0 downvotes
Picture of user Eric Zinda.

"In debug mode, Prolog stops at spy and trace points..." It actually only stops at spy points. Trace points set via trace/1 or trace/2 never stop for user input, they just output traces. And: they will output traces regardless of the state of the debug flag (or the tracing flag for that matter):

is_a(rock1, rock).
is_a(rock2, rock).
color(rock1, red).

noun(X, Type) :- is_a(X, Type).
?- trace(is_a/2).
%         is_a/2: [all]
true.

?- debugging.
% Debug mode is off
true.

?- tracing.
false.

?- noun(X, rock).
 T Call: is_a(_17058, rock)
 T Exit: is_a(rock1, rock)
X = rock1 ;
 T Redo: is_a(rock1, rock)
 T Exit: is_a(rock2, rock)
X = rock2.