1. Chapter 2: Prolog (Introduction and Basic Concepts)
DM552: Part 2 Programing Logic
Chapter 2: Prolog (Introduction and Basic Concepts)
Dr Youcef Djenouri

2. Introduction (1/2)
(imperative, functional, object-oriented, declarative)
Prolog is the logic declarative language.
Alain Colmeraeur & Philippe Roussel,
With help from theorem proving folks such as Robert Kowalski
Original project: Type in French statements & question
Computer needed NLP and deductive reasoning

3. Introduction (2/2) Assists thinking in terms of objects and entities
Not good for number crunching
Useful applications of Prolog in
Expert Systems (Knowledge Representation and Reasoning)
Natural Language Processing
Relational Databases

4. Basic Concepts (1/10) 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 the mother of Y.”

5. Basic Concepts (2/10) Predicate Interpretation valuable(gold)
Gold is valuable.
owns(john,gold)
John owns gold.
father(john,mary)
John is the father of Mary
gives (john,book,mary)
John gives the book to Mary

6. Basic Concepts (3/10)
Names of relationship and objects must begin with a lower-case letter.
Variables must begin with a higher-case letter.
Relationship is written first (typically the predicate of the sentence).
Objects are written separated by commas and are enclosed by a pair of parenthesis.
The full stop character ‘.’ must come at the end of fact and rules.

7. Basic Concepts (4/10) 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

8. Basic Concepts (5/10) 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

9. Basic Concepts (6/10) The symbol ‘,’ use as conjunction operator.
Example
Facts:
likes(mary,food).
likes(mary,tea).
likes(john,tea).
likes(john,mary)
likes(mary,X),likes(john,X).

10. Basic Concepts (7/10) And: «, » OR: A new line, Not: \+ Equality: ==
Anonymous variable: _

11. Basic Concepts (8/10)
A Prolog program consists of predicate definitions.
A predicate denotes a property or relationship between objects.
Definitions consist of clauses.
A clause has a head and a body (Rule) or just a head (Fact).
A head consists of a predicate name and arguments.
A clause body consists of a conjunction of terms.
Terms can be constants, variables, or compound terms.

12. Atom at the beginning is called functor
Basic Concepts (9/10)
mother_of(cathy,maria) An atom alone is a structure too.
arguments
atom
Closing paren.
Opening parenthesis
Atom at the beginning is called functor

13. How prolog reacts depends on the implementation
Basic Concepts (10/10)
Clauses from the same predicate must be put in a group:
How prolog reacts depends on the implementation
mother(…).
father(…).
mother(…).
father(…).

14. Prolog reasoning If we have this fact and rule rainy(london).
rainy(bangkok).
dull(X):- rainy(X).
We can ask (or query) prolog on its command prompt
?- dull(C). (is there a C that makes this predicate true?)
It will automatically try to substitute atoms in its fact into its rule such that our question gives the answer true
in this example, we begin with dull(X), so the program first chooses an atom for X, that is london (our first atom in this example)
The program looks to see if there is rainy(london). There is!
So the substitution gives the result “true”
The Prolog will answer
C= london
To find an alternative answer, type “;” and “Enter”
It’ll give C= bangkok
If it cannot find any more answer, it will answer “no”

15. How to ask question First, write a prolog program in a .pl file.
Then load the file, using a prolog interpreter. Or use the consult command:
?- consult(‘file.pl’).
To exit, use command: halt.

16. Example (1\8)
/* Clause 1 */ located_in(atlanta,georgia). /* Clause 2 */ located_in(houston,texas). /* Clause 3 */ located_in(austin,texas). /* Clause 4 */ located_in(toronto,ontario). /* Clause 5 */ located_in(X,usa) :- located_in(X,georgia). /* Clause 6 */ located_in(X,usa) :- located_in(X,texas). /* Clause 7 */ located_in(X,canada) :- located_in(X,ontario). /* Clause 8 */ located_in(X,north_america) :- located_in(X,usa). /* Clause 9 */ located_in(X,north_america) :- located_in(X,canada).

17. Example (2/8) To ask whether atlanta is in georgia:
?- located_in(atlanta,georgia).
This query matches clause 1. So prolog replies “yes”.
?- located_in(atlanta,usa).
This query can be solve by calling clause 5, and then clause 1. So prolog replies “yes”.

18. Example (3/8)
?-located_in(atlanta,texas). this query gets “no” as its answer because this fact cannot be deduced from the knowledge base. The query succeeds if it gets a “yes” and fails if it gets a “no”.

19. Example (4/8) ?- located_in(X, texas).
This is a query for prolog to find X that make the above query true.
This query can have multiple solutions: both houston and austin are in texas.
What prolog does is: find one solution and asks you whether to look for another.
->

20. Cannot find any more solution
Example (5/8)
The process will look like
X = houston
More (y/n)? y
X = austin no
Some implementations let you type semicolon without asking any question.
Cannot find any more solution

21. Example (6/8) To get all the cities in texas:
?-located_in(X,texas), write(X), nl, fail.
New line
Rejects the current solution. Forcing prolog to go back and substitutes other alternatives for X.

22. Example (7/8)
?- located_in(austin,X). ?- located_in(X, texas). ?- located_in(X,Y).
Gets the names of regions that contain austin.
Gets the names of the cities that are in texas.
Gets all the pairs that of located_in that it can find or deduce.

23. Example (8/8) ?-located_in(X,X).
Forces the two arguments to have the same value, which will result in a fail.

24. Classwork
parent(michael, cathy). parent(melody, cathy). parent(charles_gordon, michael). parent(hazel, michael). male(michael). male(charles_gordon). female(cathy). female(melody). female(hazel). father(X,Y):- parent(X,Y), male(X). mother(X,Y):- parent(X,Y), female(X).