1. How to Run Prolog

The examples in this Prolog Tutorial were developed using either Quintus Prolog running on Digital Equipment Corporation MicroVAXes (ancient history) or using SWI Prolog on either Sun Sparks (long ago), in Windows on a PC (a while ago), or (recently) under the OS X operating system on a Mac.

Other important prolog systems (Borland, XSB, LPA, Minerva ...) have been used for development and testing over the past 25 years. A new section of this tutorial is planned to describe prolog systems in a general way, but that section is not available at this time.

SWI-Prolog's website has lots of information about SWI-Prolog, a download area, and documentation. The upkeep for SWI-Prolog is excellent. The link ...

SWI-Prolog Home Page   (current as of August 2006)

The examples in this tutorial use a simplified form of interaction with a typical Prolog interpreter. The sample programs should execute similarly on any system using an Edinburgh-style Prolog interpreter or interactive compiler.

To start an interactive SWI-Prolog session under Unix, open a terminal window and type the approprite command (indicated in the installation instructions). For example, on our Mac this is ...

$ /opt/local/bin/swipl

{We never type that last line: we employ Unix source files to start SWI-Prolog using additional command line arguments and/or switches for special purposes. The reader could explore this possibility soon after learning more prolog basics.}

Under Windows, SWI-Prolog installs a start icon that can be double-clicked to initiate the interpreter. The interpreter then starts in its own command window.

A startup message or banner may appear, and that will soon be followed by a goal prompt looking similar to the following

?- _
Interactive goals in Prolog are entered by the user following the '?- ' prompt.

Many Prologs have command-line help information. SWI Prolog has extensive help information. This help is indexed and guides the user. To learn more about it, try

?- help(help).
Notice that all of the displayed symbols need to be typed in, followed by a carriage return.

To illustrate some particular interactions with prolog, consider the following sample session. Each file referred to is assumed to be a local file in the user's account, which was either created by the user, obtained by copying directly from some other public source, or obtained by saving a text file while using a web browser. The way to achieve the latter is either to follow a URL to the source file and then save, or to select text in a Prolog Tutorial web page, copy it, paste into a text editor window and then save to file. The comments /* ... */ next to goals are referred to in the notes following the session.

?- ['2_2.pl'].           /* 1. Load a program from a local file*/  
?- listing(factorial/2). /* 2. List program to the screen*/  
factorial(A,B) :-  
           A > 0, 
           C is A-1,
           B is A*D. 
?- factorial(10,What).     /* 3. Compute factorial of 10 */
?- ['2_7.pl'].            /* 4. Load another program */

?- listing(takeout).
takeout(A,[B|C],[B|D]) :-

?- takeout(X,[1,2,3,4],Y).  /* 5. Take X out of list [1,2,3,4] */
X=1  Y=[2,3,4] ;              Prolog waits ... User types ';' and Enter
X=2  Y=[1,3,4] ;               again ...  
X=3  Y=[1,2,4] ;               again ...
X=4  Y=[1,2,3] ;               again ...
no                             Means: No more answers.

?- takeout(X,[1,2,3,4],_), X>3.  /* 6.  Conjunction of goals */
X=4 ;

?- halt.                         /* 7. Return to OS */
The comments appearing at the right at various spots in a sample session were added with a text processor. They also serve as reference signposts for the notes which appear below. We discuss several points now, while other details will be deferred to later sections.


1. A Prolog goal is terminated with a period "." In this case the goal was to load a program file. This "bracket" style notation dates back to the first Prolog implementations. Several files can be chain loaded by listing the filenames sequentially within the brackets, separated by commas. In this case, the file's name is 2_1.pl (programs corresponding to Section 7.1 of this tutorial), which contains two prolog programs for calculating the factorial of a positive integer. The actual program in the file is discussed in Section 2.1. The program file was located in the current directory. If it had not been, then the path to it would have to have been specified in the usual way.

2. The built-in predicate 'listing' will list the program in memory -- in this case, the factorial program. The appearance of this listing is a little different than the appearance of the source code in the file, which we will see in Section 2.1. Actually, Quintus Prolog compiles programs unless predicates are declared to be dynamic. Compiled predicates do not have an interactive source listing that can be supplied by a 'listing' goal. So, in order to illustrate this Prolog interpreter feature, the predicates were declared as dynamic in the source code before this sample run.

3. The goal here, 'factorial(10,What)', essentially says "the factorial of 10 is What?". The word 'What' begins with an upper-case letter, denoting a logical variable. Prolog satisfies the goal by finding the value of the variable 'What'.

4. Both "programs" now reside in memory, from the two source files 2_2.pl and 2_7.pl. The 2_7.pl file has many list processing definitions in it. (See Section 2.7.)

5. In the program just loaded is a definition of the logical predicate 'takeout'. The goal 'takeout(X,[1,2,3,4],Y)' asks that X be taken out of list [1,2,3,4] leaving remainder list Y, in all possible ways. There are four ways to do this, as shown in the response. The 'takeout' predicate is discussed in Section 2.7. Note, however, how Prolog is prodded to produce all of the possible answers: After producing each answer, Prolog waits with a cursor at the end of the answer. If the user types a semicolon ';' , Prolog will look for a next answer, and so on. If the user just hits Enter, then Prolog stops looking for answers.

6. A compound or conjunctive goal asks that two individual goals be satisfied. Note the arithmetic goal (built-in relation), 'X>3'. Prolog will attempt to satisfy these goals in the left-to-right order, just as they would be read. In this case, there is only one answer. Note the use of an anonymous variable '_' in the goal, for which no binding is reported ("don't-care variable").

7. The 'halt' goal always succeeds and returns the user to the operating system.

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