Prolog Programming : Basics

Prolog Programming
BY: MITUL K. DESAI

What is Prolog?
 Prolog stands for programming in logic (PROgrammation en LOgique).
 Prolog is the most widely used language to have been inspired by logic
programming research.
 Prolog is the only successful example of the family of logic programming
languages.
 A Prolog program is a theory written in a subset of first-order logic, called
Horn clause logic.
 Prolog is declarative. A Prolog programmer concentrates on what the program
needs to do, not on how to do it.

A Little History
 Prolog was invented by Alain Colmerauer, a professor of computer science at
the university of Aix-Marseille in France, in 1972.
 The first application of Prolog was in natural language processing.
 Its theoretical underpinning are due to Donald Loveland of Duke university
through Robert Kowalski (formerly) of the university of Edinburgh

Main Advantages
 Ease of representing knowledge.
 Natural support of pattern-matching.
 Natural support of meta-programming.
 Meaning of programs is independent of how they are executed.
 Simple connection between programs and computed answers and
specifications.
 No need to distinguish programs from databases.

Anatomy of Prolog Program
 Prolog programs are made up of facts and rules.
 A fact asserts some property of an object, or relation between two or more
objects.
e.g. parent(jane,alan).
Can be read as “Jane is the parent of Alan.”
 Rules allow us to infer that a property or relationship holds based on
preconditions.
e.g. parent(X,Y) :- mother(X,Y).
= “Person X is the parent of person Y if X is Y‟s mother.”

Predicate Definitions
 Both facts and rules are predicate definitions.
 ‘Predicate’is the name given to the word occurring before the bracket in a fact
or rule:
parent (jane,alan).
 By defining a predicate you are specifying which information needs to be
known for the property denoted by the predicate to be true.
Predicate name

Clauses
 Predicate definitions consist of clauses.
= An individual definition (whether it be a fact or rule).
e.g. mother(jane, alan). = Fact
parent(P1,P2):- mother(P1,P2). = Rule
 A clause consists of a head
 And sometimes a body.
-- Facts don‟t have a body because they are always true.
head body

Arguments
 A predicate head consists of a predicate name and sometimes some arguments
contained within brackets and separated by commas.
mother(jane, alan).
 A body can be made up of any number of subgoals (calls to other predicates) and
terms.
 Arguments also consist of terms, which can be:
-- Constants e.g. jane,
-- Variables e.g. Person1, or
-- Compound terms
Predicate name Arguments

Terms: Constants
Constants can either be:
 Numbers:
 integers are the usual form (e.g. 1, 0, -1, etc), but
 floating-point numbers can also be used (e.g. 3.0E7)
 Symbolic constants:
 always start with a lower case alphabetic character and contain any mixture of
letters, digits, and underscores (but no spaces, punctuation, or an initial capital).
 e.g. abc, big_long_constant, x4_3t.
 String constants:
 are anything between single quotes e.g. „Like this‟.

Terms: Variables
 Variables always start with an upper case alphabetic character or an underscore.
 Other than the first character they can be made up of any mixture of letters, digits, and
underscores.
e.g. X, ABC, _89two5, _very_long_variable
 There are no “types” for variables (or constants) – a variable can take any value.
 All Prolog variables have a “local” scope:
--- they only keep the same value within a clause; the same variable used outside
of a clause does not inherit the value (this would be a “global” scope).

Naming Tips
 Use real English when naming predicates, constants, and variables.
e.g. “John wants to help Somebody.”
Could be: wants(john, to_help, Somebody).
Not: x87g(j,_789).
 Use a Verb Subject Object structure:
wants(john, to_help).
 BUT do not assume Prolog Understands the meaning of your chosen
names!
-- You create meaning by specifying the body (i.e. preconditions) of a clause.

Using Predicate Definitions
Command line programming is tedious
e.g. | ?- write(„What is your name?‟), nl, read(X),
write(„Hello „), write(X).
We can define predicates to automate commands:
| ?- greetings.
What is your name?
|: tim.
Hello tim
X = tim ?
yes
Prolog Code Terminal
greetings:-
write(‘What is your name?’),
nl,
read(X),
write(‘Hello ‘),
write(X).

Running prolog program 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
executable, 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 .p1 is associated with the program swipl-win.exe.
 Opening a .p1 file will cause swipl-win.exe to start, change directory to the directory in which
the file to open resides, and load this file.

Executing a query
After loading a program, one can ask Prolog queries about the program.
?- likes (sam, x) .
X = dahl ;
X = tandoori ;
……
X = chips ;
?-

Prolog Execution
Most Prolog clauses have both a declarative reading and a procedural reading.
Whenever possible, the declarative reading is to be preferred.
mother (X, Y) :- parent (X, Y), female (X) .
Declarative reading: x is the mother of y
if x is parent of y and x is female

Prolog Execution
Procedural reading :
To show that x is the mother of y, first show that x is a parent of y, then show that x is female.
Clauses:
parent (john, bill) .
parent (jane, bill) .
female(jane) .
Query:
| ?- mother (M, bill) .

Prolog Execution
 The clause of mother /2 will be located, and the unification X=M, Y=bill will occur.
 Then parent (M, bill) will be attempted, resulting in the unification M=john.
 Next, female (john) will be attempted, but will fail.
 Prolog will backtrack to parent (M, bill) and look for another solution for this; it will
succeed and unify M=jane.
 Finally, it will attempt female (jane), and succeed; so the inquiry will succeed,
having performed the unification M=jane.

Prolog Programming : Basics

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Prolog Programming : Basics