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3.42 Debugging and Tracing Programs

Start the tracer. trace/0 itself cannot be seen in the tracer. Note that the Prolog toplevel treats trace/0 special; it means `trace the next goal'.
Succeeds when the tracer is currently switched on. tracing/0 itself can not be seen in the tracer.
Stop the tracer. notrace/0 itself cannot be seen in the tracer.
Equivalent to trace(Pred, +all).
trace(+Pred, +Ports)
Put a trace-point on all predicates satisfying the predicate specification Pred. Ports is a list of portnames (call, redo, exit, fail). The atom all refers to all ports. If the port is preceded by a - sign the trace-point is cleared for the port. If it is preceded by a + the trace-point is set.

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 Goal, but suspend the debugger while Goal is executing. The current implementation cuts the choicepoints of Goal after successful completion. See once/1. Later implementations may have the same semantics as call/1.
Start debugger. In debug mode, Prolog stops at spy- and trace-points, disables tail-recursion optimisation and aggressive destruction of choice-points to make debugging information accessible. Implemented by the Prolog flag debug.
Stop debugger. Implementated by the prolog flag debug. See also debug/0.
Print debug status and spy points on current output stream. See also the prolog flag debug.
Put a spy point on all predicates meeting the predicate specification Pred. See section 3.4.
Remove spy point from all predicates meeting the predicate specification Pred.
Remove all spy points from the entire program.
Set/query leashing (ports which allow for user interaction). Ports is one of +Name, -Name, ?Name or a list of these. +Name enables leashing on that port, -Name disables it and ?Name succeeds or fails according to the current setting. Recognised ports are: 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.
Set the ports shown by the debugger. See leash/1 for a description of the port specification. Default is full.
unknown(-Old, +New)
Edinburgh-prolog compatibility predicate, interfacing to the ISO prolog flag unknown. Values are trace (meaning error) and fail. If the unknown flag is set to warning, unknown/2 reports the value as trace.
Set style checking options. Spec is either +<option>, -<option>, ?<option> or a list of such options. +<option> sets a style checking option, -<option> clears it and ?<option> succeeds or fails according to the current setting. consult/1 and derivatives resets the style checking options to their value before loading the file. If---for example---a file containing long atoms should be loaded the user can start the file with:

:- style_check(-atom).

Currently available options are:

singleton onread_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.
atom onread/1 and derivatives will produce an error message on quoted atoms or strings longer than 5 lines.
dollar offAccept dollar as a lower case character, thus avoiding the need for quoting atoms with dollar signs. System maintenance use only.
discontiguous onWarn if the clauses for a predicate are not together in the same source file.
string offBackward compatibility. See the prolog-flag double_quotes (current_prolog_flag/2).