PROLOG?

1. PROLOG
P R I N C I P L E S O F P R O G R A M M I N G L A N G U A G E S

2. PRESENTED BY:
H A S S A A N A H M A D ( 0 7 )
A Z E E M A S H R A F ( 1 4 )
M U H A M M A D U S A M A I Q B A L ( 1 5 )
S Y E D B A Q A R A B B A S ( 1 9 )

3. 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.

4. CONT..
• Prolog is a declarative programming language.
• In the case of prolog, it will tell you whether a prolog sentence is true or not.
• if it contains variables, what the values of the variables need to be.
• Prolog is declarative. A Prolog programmer concentrates on what the program
needs to do, not on how to do it.

5. 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

6. 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.”

7. DATA TYPES
• Prolog single data type is the term. Terms are either atoms, numbers or
variables.
• An Atom is a general-purpose name with no inherent meaning.
Examples of atoms include
– X
– Red
– Taco
• Numbers can be floats or integers.
• Variables are denoted by a string consisting of letters, numbers and
underscore characters, and beginning with an upper-case letter or
underscore.

8. WHY LEARN PROLOG?
• Learning prolog will certainly not always be as fruitful as learning C++,
Java or Python.
• Prolog is most useful in the areas related to Artificial Intelligence
research, such as problem solving, (path) planning or natural language
interpretation.
• As prolog uses certain programming techniques that are considered
difficult or advanced
• prolog programmer will certainly have an in-depth understanding of
such concepts as recursion, searching problem trees etc.

9. CONT..
• Very complex programs have been written in prolog.
• Most of all prolog is a complex, powerful, and elegant language that
can inspire great satisfaction in its users.
• learning prolog is not easy because Prolog doesn't work as naturally as
procedural languages and therefore requires a specific way of thinking.

10. INTRODUCTION TO LOGIC
• Since Prolog is based heavily on formal logic, it's useful to have some
experience with it.
• There are two types of logical languages:
– propositional logic
– first-order logic.

11. PROPOSITIONAL LOGIC
• Propositional logic has two basic elements:
– terms
– Connectives
• Terms are represented by letters (usually upper-case), and represent
the values true and false.
• Connectives, like the word suggests, connect two terms.
• Connectives are usually represented by symbols rather than words.

13. FIRST ORDER LOGIC
• First Order Logic (also known as predicate logic) expands on
propositional logic, by using predicates, variables and objects.
• In first order logic the atomic sentences are predicates.
• Predicates have a name (starting with a capital) which is followed
by several variables (starting with a lowercase letter).

14. CONT..
• The following are examples of predicates:
– Predicate(variable1, variable2)
– BrotherOf(sibling, brother)
– MotherOf(child, mother)
– HasWheels(thing)
• these variables can be instantiated with objects.

15. CONT..
• Objects are elements represented by words that start with a capital
letter.
• Such a collection of sentences that are true is called a knowledge base.
• Such a knowledge base could, for instance contain the following
sentences:
– HasWheels(Car)
– MotherOf(Charles, Elizabeth)
• The sentences tell us that the HasWheels predicate is true

16. 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.

17. 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.
• Clauses with empty bodies are called facts.
• Clauses with bodies are called rules.

18. ARGUMENTS
• A predicate head consists of a predicate name and sometimes some
arguments
contained within brackets and separated by commas.
mother(jane, alan).
Predicate name Arguments
• A body can be made up of any number of subgoals (calls to other
predicates) and terms.

19. CONT..
• Arguments also consist of terms, which can be:
– Constants e.g. jane,
– Variables e.g. Person1, or
– Compound terms

20. TERMS

21. 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. 3.9)
• 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:

22. 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 refers as “global” scope.

23. 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

24. RUNNING
PROLOG
O N W I N D O W S .

25. After SWI-Prolog has been installed on a Windows system, the following
important new things are available to the user:
• A folder (in program files) called swipl containing the executable, libraries, etc.,
of the system.
• No files are installed outside this directory.
• 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.

26. EXECUTING A QUERY
After loading a program, one can ask Prolog queries about the program.
?- likes (sam, x) .
X = biryani ;
X = tandoori ;
……
X = chips ;
?-

27. 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

28. CONT..
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:
oparent (john, bill) .
oparent (jane, bill) .
ofemale(jane) .
Query:
o| ?- mother (M, bill) .

29. CONT..
• The clause of mother will be checked, 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.

30. 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.

31. LIMITATIONS
• Although Prolog is widely used in research and education, Prolog and other logic
programming languages have not had a significant impact on the computer
industry in general.
• Most applications are small by industrial standards, with few exceeding 100,000
lines of code.
• Programming in the large is considered to be complicated because not all Prolog
compilers support modules
• because there are compatibility problems between the module systems of the
major Prolog compilers.
• Prolog is not purely declarative because of constructs like the cut operator,
• a procedural reading of a Prolog program is needed to understand it.

32. EXTENSIONS
• Various implementations have been developed from Prolog to extend logic
programming capabilities
• these include:
– types,
– modes,
– constraint logic programming (CLP),
– object-oriented logic programming (OOLP),
– linear logic (LLP),
– functional and higher-order logic programming capabilities,
– interoperability with knowledge bases

33. TYPES
• Prolog is an untyped language.
• Attempts to introduce types date back to the 1980s and as of 2008
there are still attempts to extend Prolog with types.

34. MODES
• The syntax of Prolog does not specify which arguments of a predicate
are inputs and which are outputs.
• Modes provide valuable information when reasoning about Prolog
programs and can also be used to accelerate execution.

35. EXAMPLE # 1:

36. EXAMPLE # 2:

37. GRAPHICS
• Prolog systems that provide a graphics library are:
– SWI-Prolog
– Visual Prolog WIN-PROLOG
– B-Prolog
• SWI also support server-side web programming with support for web
protocols, HTML and XML.

38. THANK
YOU