Getting started with Prolog Language
Prolog categorizes everything into:
- Atoms - Any sequence of characters that do not start with an uppercase alphabet. Eg -
- Numbers - There is no special syntax for numbers, no declaration is required. Eg
- Variables - A string which starts with an uppercase character or underscore (
- Complex Terms - They are made from a functor and a sequence of arguments. Name of a complex term is always an atom, while arguments can either be atoms or variables. Eg
A logic database contains a set of facts and rules.
A complex term with only atoms as arguments is called a fact, while a complex term with variables as arguments is called a rule.
Example of facts in Prolog:
father_child(fred, susan). mother_child(hillary, joe).
Example of a rule in Prolog:
child_of(X,Y):- father_child(Y,X) ; mother_child(Y,X).
Note that the
; here is like the
or operator in other languages.
Prolog is a declarative language and you can read this database as follows:
fred is the father of susan
hillary is the mother of joe.
Xis a child of
Yis a father of
Yis a mother of
In fact, a finite set of facts and or rules constitutes as a logic program.
The use of such a program is demonstrated by doing queries. Queries lets you retrieve information from a logic program.
To load the database into the interpreter (assuming that you’ve saved the database into the directory you are running the interpreter in) you simply enter:
nameofdatabase with the actual file name (note that here we exclude the
.pl extension to the filename).
Example of queries in the interpreter for the program above and the results:
?- child_of(susan,fred). true ?- child_of(joe,hillary). true ?- child_of(fred,susan). false ?- child_of(susan,hillary). false ?- child_of(susan,X). X = fred ?- child_of(X,Y). X = susan, Y = fred ; X = joe, Y = hillary.
The queries above and their answers can be read as follows:
is susan a child of fred? - true
is joe a child of hillary? - true
is fred a child of susan? - false
is susan a child of hillary? - false
who is susan a child of? - fred
This is how we program logic in Prolog. A logic program is more formally: a set of axioms, or rules, defining relations (aka predicates) between objects. An alternative way of interpreting the database above in a more formal logic way is:
father_childholds between fred and susan
mother_childholds between hillary and joe
Yif the relation
X, or the relation
Hello, World in the interactive interpreter
To print “Hello, World!” in the Prolog interpreter (here we are using
swipl, the shell for SWI Prolog):
$ swipl <...banner...> ?- write('Hello, World!'), nl.
?- is the system prompt: it indicates that the system is ready for the user to enter a sequence of goals (i.e. a query) that must be terminated with a
. (full stop).
Here the query
write('Hello World!'), nl has two goals:
'Hello World!'has to be displayed and (
- a new line (
nl) must follow.
/1 is used to indicate that the predicate takes one argument) and
nl/0 are built-in predicates (the definition is provided in advance by the Prolog system). Built-in predicates provide facilities that cannot be obtained by pure Prolog definition or to save the programmer from having to define them.
yes meaning that the query has succeeded. In some systems
true is printed instead of
Hello, World from a file
Open a new file called
hello_world.pl and insert the following text:
:- initialization hello_world, halt. hello_world :- write('Hello, World!'), nl.
initialization directive specifies that the goal
hello_world, halt should be called when the file is loaded.
halt exits the program.
This file can then be executed by your Prolog executable. The exact flags depend on the Prolog system. If you are using SWI Prolog:
$ swipl -q -l hello_world.pl
This will produce output
Hello, World!. The
-q flag suppresses the banner that usually displays when you call run
-l specifies a file to load.
Installation or Setup
Windows and Mac:
- Download SWI-Prolog at the official website
- Simply install by following the installer instructions.
Add the PPA
ppa:swi-prolog/stableto your system’s software sources (developers may choose for
Open a terminal (Ctrl+Alt+T) and type:
sudo add-apt-repository ppa:swi-prolog/stable
Afterwards, update the package information:
sudo apt-get update
Now install SWI-Prolog through the package manager:
sudo apt-get install swi-prolog
You can now start SWI-Prolog through the command-line with command
append(, Bs, Bs). append([A|As], Bs, [A|Cs]) :- append(As, Bs, Cs).
append/3 is one of the most well-known Prolog relations. It defines a relation between three arguments and is true if the third argument is a list that denotes the concatenation of the lists that are specified in the first and second arguments.
Notably, and as is typical for good Prolog code,
append/3 can be used in several directions: It can be used to:
append two fully or partially instantiated lists:
?- A = [1, 2, 3], B=[4, 5, 6], append(A, B, Y) Output: A = [1, 2, 3], B = [4, 5, 6], Y = [1, 2, 3, 4, 5, 6].
check whether the relation is true for three fully instantiated lists:
?- A = [1, 2, 3], B = [4, 5], C = [1, 2, 3, 4, 5, 6], append(A, B, C) Output: false
generate all possible ways to append two lists to a given list:
?- append(A, B, [1, 2, 3, 4]). Output: A = , B = [1, 2, 3, 4] ; A = , B = [2, 3, 4] ; A = [1, 2], B = [3, 4] ; A = [1, 2, 3], B =  ; A = [1, 2, 3, 4], B =  ; false.
CLP(FD) constraints are provided by all serious Prolog implementations. They allow us to reason about integers in a pure way.
?- X #= 1 + 2. X = 3. ?- 5 #= Y + 2. Y = 3.