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.
| 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 (/
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. |
| 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 |
| 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. |
