1. How to Run PrologThe examples in this primer were all developed
using either Quintus Prolog running on Digital Equipment Corporation MicroVAXes
or using SWI Prolog on either Sun Sparks or in Windows on a PC. SWI (in Holland)
has Prolog information and a download area. See ...
SWI-Prolog Home
Page (current as of 06/28/2004) SWI-Prolog's home
page has lots of information about SWI-Prolog, a download area, and
documentation.
[Note added 06/28/2004. In Chapter 6 we use XSB Prolog. See
Chapter 6 for references to XSB.]
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 compiler.
To start the prolog interpreter type 'prolog'
$ prolog after the VMS prompt '$', for Quintus
Prolog, or type 'pl'
% pl for SWI Prolog, after the Unix prompt '%'.
In Windows, SWI-Prolog installs a start icon that can be double clicked to
initiate the interpreter. The interpreter then starts in its own window.
A startup message or banner may appear, and will soon be followed by a goal
prompt similar to the following
?- _ Goals in Prolog are entered by the
user following the '?- ' prompt.
Many Prologs have on-line help information. Quintus and SWI Prolog have
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_1.pl']. /* 1. Load a program from a local file*/
yes
?- listing(factorial/2). /* 2. List program to the screen*/
factorial(0,1).
factorial(A,B) :-
A > 0,
C is A-1,
factorial(C,D),
B is A*D.
yes
?- factorial(10,What). /* 3. Compute factorial of 10 */
What=3628800
?- ['2_7.pl']. /* 4. Load another program */
?- listing(takeout).
takeout(A,[A|B],B).
takeout(A,[B|C],[B|D}) :-
takeout(A,C,D).
yes
?- 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 ;
no
?- halt. /* 7. Return to OS */
The
italicized 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 after the session. We will discuss some of these points now, while others
will be referred to in later discussions.
Notes:
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. One can start
Quintus Prolog and automatically load the program file fact.pro as follows:
$ prolog fact.pro 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_1.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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