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:

machine% 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].
% likes compiled, 0.00 sec, 17 clauses
true.

?-

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 with 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 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 a full stop (.) if the user uses the return key or Prolog knowns there are no more answers. If Prolog cannot find (more) answers, it writes false. Finally, Prolog can answer 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 toplevel differs in several ways from traditional Prolog toplevel. The current toplevel was designed in cooperation with Ulrich Neumerkel.}