This section is a reference to the debugger interaction predicates. A more use-oriented overview of the debugger is in section 2.9.
If you have installed XPCE, you can use the graphical front-end of the tracer. This front-end is installed using the predicate guitracer/0.
trace(Pred, +all).
call,
redo, exit, fail). The atom all 
refers to all ports. If the port is preceded by a -+The predicate trace/2 activates debug mode (see debug/0). Each time a port (of the 4-port model) is passed that has a trace-point set the goal is printed as with trace/0. Unlike trace/0 however, the execution is continued without asking for further information. Examples:
| ?- trace(hello). | Trace all ports of hello with any arity in any module. | 
| ?- trace(foo/2, +fail). | Trace failures of foo/2 in any module. | 
| ?- trace(bar/1, -all). | Stop tracing bar/1. | 
The predicate debugging/0 shows all currently defined trace-points.
call, redo, exit, fail 
and
unify. The special shorthand all refers to all 
ports,
full refers to all ports except for the unify port 
(default).
half refers to the call, redo and fail 
port.full.trace (meaning error) and fail. 
If the unknown flag 
is set to
warning, unknown/2 
reports the value as trace.?:- style_check(-atom).
Currently available options are:
| Name | Default | Description | 
| singleton | on | read_clause/1 (used by consult/1) warns on variables only appearing once in a term (clause) which have a name not starting with an underscore. See section 2.15.1.5 for details on variable handling and warnings. | 
| atom | on | read/1 and derivatives will produce an error message on quoted atoms or strings longer than 5 lines. | 
| dollar | off | Accept dollar as a lower case character, thus avoiding the need for quoting atoms with dollar signs. System maintenance use only. | 
| discontiguous | on | Warn if the clauses for a predicate are not together in the same source file. | 
| string | off | Backward compatibility. See the Prolog flag double_quotes (current_prolog_flag/2). | 
| charset | off | Warn on atoms and variables holding non-ASCII characters that are not quoted. See also section 2.15.1.1. |