2.1 Getting started quickly

2.1.1 Starting SWI-Prolog

2.1.1.1 Starting SWI-Prolog on Unix

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 https://raw.githubusercontent.com/SWI-Prolog/swipl-devel/master/demo/likes.pllikes.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 ?-

After this point, Unix and Windows users unite, so if you are using Unix please continue at section 2.1.2.

2.1.1.2 Starting SWI-Prolog on Windows

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.

2.1.2 Adding rules from the console

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 conveniant 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.

2.1.3 Executing a query

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 spacebar^{6On 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 the 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.^{7The SWI-Prolog top level differs in several ways from traditional Prolog top level. The current top level was designed in cooperation with Ulrich Neumerkel.}

2.1.4 Examining and modifying your program

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 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 is 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.^{8This lists several hook predicates that are defined by default and is typically not very informative.}

?- listing(mild).
mild(dahl).
mild(tandoori).
mild(kurma).

true.

2.1.5 Stopping Prolog

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.
$