2.10.4 Trace Mode Commands When Paused

You can do way more than just press spacebar when the tracer is paused at a port. All actions are single-character commands which are executed without waiting for a return (unless the command line option --no-tty is active). Pressing ? or h when paused will print out a list of these commands as well.

2.10.4.1 Control Flow Commands

Abort	a	Abort Prolog execution (see abort/0)
Break	b	Enter a Prolog break environment (see break/0)
Creep	c	Continue execution, stop at next port. (Also return, space)
Exit	e	Terminate Prolog (see halt/0)
Fail	f	Force failure of the current goal
Find	/	Search for a port (see below for the description of this command (section 2.10.4.1))
Ignore	i	Ignore the current goal, pretending it succeeded
Leap	l	Continue execution, stop at next spy point
No debug	n	Continue execution in’no debug' mode
Repeat find	.	Repeat the last find command (see’Find' (section 2.10.4.1))
Retry	r	Undo all actions (except for database and I/O actions) back to the call port of the current goal and resume execution at the call port
Skip	s	Continue execution, stop at the next port of this goal (thus skipping all calls to children of this goal)
Spy	+	Set a spy point (see spy/1) on the current predicate. Spy points are described later in the overview (section 2.10.6).
No spy	-	Remove the spy point (see nospy/1) from the current predicate. Spy points are described later in the overview (section 2.10.6).
Up	u	Continue execution, stop at the next port of the parent goal (thus skipping this goal and all calls to children of this goal). This option is useful to stop tracing a failure driven loop.

Find (/) Description and Examples

The Find (/) command continues execution until a port matching a find pattern is found. After the /, the user can enter a line to specify the port to search for. This line consists of a set of letters indicating the port type, followed by an optional term, that should unify with the goal run by the port. If no term is specified it is taken as a variable, searching for any port of the specified type. If an atom is given, any goal whose functor has a name equal to that atom matches. Examples:

/f	Search for any fail port
/fe solve	Search for a fail or exit port of any goal with name solve
/c solve(a, _)	Search for a call to solve/2 whose first argument is a variable or the atom a
/a member(_, _)	Search for any port on member/2. This is equivalent to setting a spy point on member/2.

2.10.4.2 Informational Commands

Alternatives	A	Show all goals that have alternatives
Goals	g	Show the list of parent goals (the execution stack). Note that due to tail recursion optimization a number of parent goals might not exist any more.
Help	h	Show available options (also ?)
Listing	L	List the current predicate with listing/1

2.10.4.3 Formatting Commands

Context	C	Toggle’Show Context'. If on, the context module of the goal is displayed between square brackets (see modules section (section 6)). Default is off.
Display	d	Set the max_depth(Depth) option of debugger_write_options (section 2.12), limiting the depth to which terms are printed. See also the w and p options.
Print	p	Set the Prolog flag debugger_write_options to [quoted(true), portray(true), max_depth(10), priority(699)]. This is the default.
Write	w	Set the Prolog flag debugger_write_options to [quoted(true), attributes(write), priority(699)], bypassing portray/1, etc.