3.39 Debugging and Tracing Programs

trace

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'.

tracing

Succeeds when the tracer is currently switched on. tracing/0 itself can not be seen in the tracer.

notrace

Stop the tracer. notrace/0 itself cannot be seen in the tracer.

trace(+Pred)

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.

notrace(+Goal)

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.

debug

Start debugger (stop at spy points).

nodebug

Stop debugger (do not trace, nor stop at spy points).

debugging

Print debug status and spy points on current output stream.

spy(+Pred)

Put a spy point on all predicates meeting the predicate specification Pred. See section 3.3.

nospy(+Pred)

Remove spy point from all predicates meeting the predicate specification Pred.

nospyall

Remove all spy points from the entire program.

leash(?Ports)

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.

visible(+Ports)

Set the ports shown by the debugger. See leash/1 for a description of the port specification. Default is full.

unknown(-Old, +New)

Unify Old with the current value of the unknown system flag. On success New will be used to specify the new value. New should be instantiated to either fail or trace and determines the interpreters action when an undefined predicate which is not declared dynamic is encountered (see dynamic/1). fail implies the predicate just fails silently. trace implies the tracer is started. Default is trace. The unknown flag is local to each module and unknown/2 is module transparent. Using it as a directive in a module file will only change the unknown flag for that module. Using the :/2 construct the behaviour on trapping an undefined predicate can be changed for any module. Note that if the unknown flag for a module equals fail the system will not call exception/3 and will not try to resolve the predicate via the dynamic library system. The system will still try to import the predicate from the public module.

style_check(+Spec)

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:

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.
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	Read and derivatives transform " ... " into a prolog string instead of a list of ASCII characters.