## 4.38 Debugging and Tracing Programs

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
  Start the tracer. trace/0 itself cannot be seen in the tracer. Note that the Prolog top-level treats trace/0 special; it means `trace the next goal'.
tracing
  True if 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.
guitracer
  Installs hooks (see prolog_trace_interception/4) into the system that redirects tracing information to a GUI front-end providing structured access to variable-bindings, graphical overview of the stack and highlighting of relevant source-code.
noguitracer
  Reverts back to the textual 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 port names (`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 choice-points of Goal after successful completion. See once/1. Later implementations may have the same semantics as call/1.
debug
  Start debugger. In debug mode, Prolog stops at spy- and trace-points, disables last-call optimisation and aggressive destruction of choice points to make debugging information accessible. Implemented by the Prolog flag debug.
nodebug
  Stop debugger. Implemented by the Prolog flag debug. See also debug/0.
debugging
  Print debug status and spy points on current output stream. See also the Prolog flag debug.
spy(+Pred)
  Put a spy point on all predicates meeting the predicate specification Pred. See section 4.4.
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)
  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`.
style_check(+Spec)
  Set style checking options. Spec is either `+<option>`, `-<option>`, `?(<option>)`63In older versions ```?``' was a prefix operator. Inversions after 5.5.13, explicit brackets are needed. 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. 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.