By default, SWI-Prolog is installed as‘swipl’. The
command line arguments of SWI-Prolog itself and its utility programs are
documented using standard Unix man pages. SWI-Prolog is normally
operated as an interactive application simply by starting the program:
$ swipl
Welcome to SWI-Prolog ...
...
1 ?-
After starting Prolog, one normally loads a program into it using
consult/1,
which may be abbreviated by putting the name of the program file between
square brackets. The following goal loads the file
likes.pl
containing clauses for the predicates likes/2 :
?- [likes].
true.
?-
Alternatively, the source file may be given as command line
arguments:
$ swipl likes.pl
Welcome to SWI-Prolog ...
...
1 ?-
Both the above assume likes.pl is in your working
directory. If you use the command line version swipl the
working directory is the same as the shell from which you started
SWI-Prolog. If you started the GUI version (swipl-win) this
depends largely on the OS. You can use pwd/0
and cd/0 to find and
change the working directory. The utility ls/0
lists the contents of the working directory.
?- pwd.
% /home/janw/src/swipl-devel/linux/
true.
?- cd('~/tmp').
true.
?- pwd.
% /home/janw/tmp/
true.
The file likes.pl is also installed in a subdirectory demo
insides SWI-Prolog's installation directory and may be loaded regardless
of the working directory using the command below. See
absolute_file_name/3
and file_search_path/2
for details on how SWI-Prolog specifies file locations.
?- [swi(demo/likes)].
true.
After this point, Unix and Windows users unite, so if you are using
Unix please continue at section
2.1.2.
After SWI-Prolog has been installed on a Windows system, the
following important new things are available to the user:
- A folder (called directory in the remainder of this
document) called
swipl containing the executables,
libraries, etc., of the system. No files are installed outside this
directory.
- A program swipl-win.exe, providing a window for interaction
with Prolog. The program swipl.exe is a version of SWI-Prolog
that runs in a console window.
- The file extension
.pl is associated with the program
swipl-win.exe. Opening a .pl file will cause
swipl-win.exe to start, change directory to the directory in
which the file to open resides, and load this file.
The normal way to start the likes.pl file mentioned in
section 2.1.1.1 is
by simply double-clicking this file in the Windows explorer.
Although we strongly advice to put your program in a file, optionally
edit it and use make/0
to reload it (see section
2.1.4), it is possible to manage facts and rules from the terminal.
The most convenient way to add a few clauses is by consulting the pseudo
file
user. The input is ended using the system end-of-file
character.
?- [user].
|: hello :- format('Hello world~n').
|: ^D
true.
?- hello.
Hello world
true.
The predicates assertz/1
and retract/1
are alternatives to add and remove rules and facts.
After loading a program, one can ask Prolog queries about the
program. The query below asks Prolog what food‘sam’likes.
The system responds with X = <value> if it
can prove the goal for a certain
X. The user can type the semi-colon (;) or spacebar7On
most installations, single-character commands are executed without
waiting for the RETURN key.
if (s)he wants another solution. Use the return
key if you do not want to see more answers. Prolog completes the output
with a full stop (.) if the user uses the return
key or Prolog
knows there are no more answers. If Prolog cannot find (more)
answers, it writes false. Finally, Prolog answers using an error
message to indicate the query or program contains an error.
?- likes(sam, X).
X = dahl ;
X = tandoori ;
...
X = chips.
?-
Note that the answer written by Prolog is a valid Prolog program
that, when executed, produces the same set of answers as the original
program.8The SWI-Prolog top level
differs in several ways from traditional Prolog top level. The current
top level was designed in cooperation with Ulrich Neumerkel.
If properly configured, the predicate edit/1
starts the built-in or user configured editor on the argument. The
argument can be anything that can be linked to a location: a file name,
predicate name, module name, etc. If the argument resolves to only one
location the editor is started on this location, otherwise the user is
presented a choice.
If a graphical user interface is available, the editor normally
creates a new window and the system prompts for the next command. The
user may edit the source file, save it and run make/0
to update any modified source file. If the editor cannot be opened in a
window, it opens in the same console and leaving the editor runs make/0
to reload any source files that have been modified.
?- edit(likes).
true.
?- make.
% /home/jan/src/pl-devel/linux/likes compiled 0.00 sec, 0 clauses
?- likes(sam, X).
...
The program can also be decompiled using listing/1
as below. The argument of listing/1
is just a predicate name, a predicate
indicator of the form Name/Arity, e.g., ?-
listing(mild/1).
or a head, e.g., ?- listing(likes(sam, _)).,
listing all matching clauses. The predicate listing/0,
i.e., without arguments lists the entire program.9This
lists several hook predicates that are defined by default and
is typically not very informative.
?- listing(mild).
mild(dahl).
mild(tandoori).
mild(kurma).
true.
The interactive toplevel can be stopped in two ways: enter the system
end-of-file character (typically Control-D) or by executing the
halt/0
predicate:
?- halt.
$
0 
