Introduction to Prolog
Prolog
• "Programming with Logic"
• Declarative
• Very different from other
(procedural) programming
languages
• Good for knowledge-rich tasks
History of Prolog
First Prolog
interpreter by
Colmerauer and
Roussel
1972
1977 1980
1980s/1990s
2005
History of Prolog
implementation of
DEC10 compiler by
Warren
1972
1977 1980
1980s/1990s
2005
History of Prolog
Definite Clause Grammars
implementation by Pereira
and Warren
1972
1977 1980
1980s/1990s
2005
History of Prolog
Prolog grows in popularity
especially in Europe and
Japan
1972
1977 1980
1980s/1990s
2005
History of Prolog
Prolog used to program
natural language interface
in International Space
Station by NASA
1972
1977 1980
1980s/1990s
2005
Consequences
• Think declaratively, not
procedurally
– Challenging
– Requires a different mindset
• High-level language
– Not as efficient as, say, C
– Good for rapid prototyping
– Useful in many AI applications
Deficiencies of Prolog
 Resolution Order Control
 The Prolog resolution technique is
sequential, it starts with the first predicate
in the database and continues until it has
examined all of the facts and rules. This can
be very inefficient and hinder optimization.
 By contrast, pure logic examines the
prepositions in a non-deterministic manner,
leaving out facts or rules which are less likely
to be called upon.
Deficiencies of Prolog.
Closed-World Assumption
 The Prolog system knows nothing about the
world other than what is in its database, and
anything that cannot be proven true from
inference of that database is assumed to be
false.
 Thus, Prolog is actually a true/fail rather
than a true/false system. This means that a
programmer must account for every
possibility while writing a Prolog program.
Deficiencies of Prolog.
Intrinsic Limitations
 Despite the versatility and power of logic
programming, there are some sets of
problems that it cannot solve without
resorting to procedural programming
concepts, such as sorting lists.
 The programmer must still define a
procedure for the system to follow in
these cases.
Application Area of Prolog
 implementation of RDBMS
 In DBS, the query language SQL is used to
execute queries.
 SQL is a nonprocedural language, which
makes it similar to logic programming
languages such as Prolog.
 The table which stores information in the
database system can be described as a set
of structures in a logic programming
language, and the relationships between
the tables can be described by rules of
inference.
Application Area of Prolog
 Expert Systems.
Expert systems, also called knowledge-based
systems, are programs that behave as an expert
for some small set of problems.
They are capable of solving problems that require
some knowledge built into the system.
They are designed to offer solutions to problems
for which the set of initial information is
inconsistent and/or incomplete.
Application Area of Prolog
 Natural Language Processing.
A form of logical programming can be
used to describe the language syntax just
as a context-free grammar would.
The semantic of a natural language can
be made clear by modeling the language
with logic programming.
Basic idea of Prolog
Describe the situation of interest
Ask a question
Prolog logically deduces new facts
about the situation we described
Prolog gives us its deductions back
as answers
Using Prolog
SWI Prolog
• Freely available Prolog interpreter
• Works with
– Linux,
– Windows, or
– Mac OS
• There are many more Prolog
interpreters
• Not all are ISO compliant
Using Prolog
First, write your program (away from
computer!).
Then, type it into a file, with a .pl
extension.
Any text editor will do.
Then, start prolog system
You will be presented with the Prolog
prompt
|?-
Using Prolog
Then, `consult' your file (omitting the .pl):
|?- consult(YourFileName). Or
|?- [YourFileName].
The entire content of your file is then stored
in the memory of the Prolog interpreter.
You can see what is consulted by typing
| ?- listing.
Then you can ask questions (queries) of
your database.
Using Prolog
If you edit your program file (e.g. to correct
something), be sure to consult it again
afterwards!
To exit from Prolog, type
|?- halt. or press Control/D
Using Prolog
The Prolog comment characters:
– Single line comments:%
% This is a comment
This not a comment, but an error
– Multiple line comments: /*
/* This is a multi-line comment
which must be closed with a */
Using Prolog
Prolog & English
Knowledge Base 1
woman(mia).
woman(jody).
woman(yolanda).
playsAirGuitar(jody).
party.
Knowledge Base 1
woman(mia).
woman(jody).
woman(yolanda).
playsAirGuitar(jody).
party.
?-
Knowledge Base 1
woman(mia).
woman(jody).
woman(yolanda).
playsAirGuitar(jody).
party.
?- woman(mia).
Knowledge Base 1
woman(mia).
woman(jody).
woman(yolanda).
playsAirGuitar(jody).
party.
?- woman(mia).
yes
?-
Knowledge Base 1
woman(mia).
woman(jody).
woman(yolanda).
playsAirGuitar(jody).
party.
?-playsAirGuitar(jody).
Knowledge Base 1
woman(mia).
woman(jody).
woman(yolanda).
playsAirGuitar(jody).
party.
?-playsAirGuitar(jody).
Yes
?-
Knowledge Base 1
woman(mia).
woman(jody).
woman(yolanda).
playsAirGuitar(jody).
party.
?- playsAirGuitar(mia).
No
?
Knowledge Base 1
woman(mia).
woman(jody).
woman(yolanda).
playsAirGuitar(jody).
party.
?- tattoed(jody).
Knowledge Base 1
woman(mia).
woman(jody).
woman(yolanda).
playsAirGuitar(jody).
party.
?- tattoed(jody).
no
?-
Knowledge Base 1
woman(mia).
woman(jody).
woman(yolanda).
playsAirGuitar(jody).
party.
?- tattoed(jody).
ERROR: predicate tattoed/1 not defined.
?-
Knowledge Base 1
woman(mia).
woman(jody).
woman(yolanda).
playsAirGuitar(jody).
party.
?-party.
Knowledge Base 1
woman(mia).
woman(jody).
woman(yolanda).
playsAirGuitar(jody).
party.
?-party.
Yes
?-
Knowledge Base 1
woman(mia).
woman(jody).
woman(yolanda).
playsAirGuitar(jody).
party.
?-rockConcert.
No
?-
Knowledge Base 2
happy(yolanda).
listens2music(mia).
listens2music(yolanda):- happy(yolanda).
playsAirGuitar(mia):- listens2music(mia).
playsAirGuitar(yolanda):listens2music(yolanda).
Knowledge Base 2
happy(yolanda).
listens2music(mia).
listens2music(yolanda):- happy(yolanda).
playsAirGuitar(mia):- listens2music(mia).
playsAirGuitar(yolanda):- listens2music(yolanda).
?- playsAirGuitar(mia).
yes
?-
Knowledge Base 2
happy(yolanda).
listens2music(mia).
listens2music(yolanda):- happy(yolanda).
playsAirGuitar(mia):- listens2music(mia).
playsAirGuitar(yolanda):- listens2music(yolanda).
?- playsAirGuitar(mia).
yes
?-playsAirGuitar(yolanda).
yes
Knowledge Base 2
fact
happy(yolanda).
fact
listens2music(mia).
listens2music(yolanda):- happy(yolanda).
playsAirGuitar(mia):- listens2music(mia).
playsAirGuitar(yolanda):- listens2music(yolanda).
Knowledge Base 2
fact
rule
happy(yolanda).
rule
fact
listens2music(mia).
listens2music(yolanda):- happy(yolanda).
playsAirGuitar(mia):- listens2music(mia).
playsAirGuitar(yolanda):- listens2music(yolanda).
rule
Basic Elements of Prolog
 Prolog program is a database of facts and
rules.
 Some are always true (facts):
father( john, jim).
 Some are dependent on others being true
(rules):
parent( Person1, Person2 ) :father( Person1, Person2 ).
 To run a program, we ask questions (query
about the database
Knowledge Base 2
Predicate
happy(yolanda).
name
Predicate: is the name given to the word
occurring before the bracket in a fact or rule.
Both facts and rules are predicate definitions.
By defining a predicate you are specifying
which information needs to be known for the
property denoted by the predicate to be
true.
Knowledge Base 2
Predicate
name
father( john, jim).
Arguments
A predicate head consists of a predicate
name and sometimes some arguments
contained within brackets and separated
by commas.
Knowledge Base 2
start:sum,nl.
sum:write('X= '),read(X),
write('Y= '),read(Y),
S is X+Y,
write('Sum is '),write(S).
A predicate may have no arguments
Knowledge Base 2
happy(yolanda).
listens2music(mia).
listens2music(yolanda):- happy(yolanda).
playsAirGuitar(mia):- listens2music(mia).
playsAirGuitar(yolanda):- listens2music(yolanda).
head
body
Clause
A clause consists of a head (fact) and
sometimes a body (rule).
Clauses
playsAirGuitar(vincent):- listens2music(vincent), happy(vincent).
goal
goal
Head
body
Clause
 A body can be made up of any number of
subgoals (calls to other predicates)
Clauses
happy(yolanda).
listens2music(mia).
listens2music(yolanda):- happy(yolanda).
playsAirGuitar(mia):- listens2music(mia).
playsAirGuitar(yolanda):- listens2music(yolanda).
 There are five clauses in this kb:
 two facts, (fact is a rule with an empty body)
 three rules.
 The end of a clause is marked with ‘.’
Clauses
happy(yolanda).
listens2music(mia).
listens2music(yolanda):- happy(yolanda).
playsAirGuitar(mia):- listens2music(mia).
playsAirGuitar(yolanda):- listens2music(yolanda).
 There are three predicates (procedure)
in this kb:



happy
listens2music
playsAirGuitar
(1 clause/rule)
(2 clauses/rules)
(2 clauses/rules)
Knowledge Base 3
happy(vincent).
listens2music(butch).
playsAirGuitar(vincent):listens2music(vincent), happy(vincent).
playsAirGuitar(butch):- happy(butch).
playsAirGuitar(butch):- listens2music(butch).
 The comma “," expresses conjunction
in Prolog
Q. How many facts & rules in the kb?
Knowledge Base 3
happy(vincent).
listens2music(butch).
playsAirGuitar(vincent):listens2music(vincent), happy(vincent).
playsAirGuitar(butch):- happy(butch).
playsAirGuitar(butch):- listens2music(butch).
?- playsAirGuitar(vincent).
no
?-
Knowledge Base 3
happy(vincent).
listens2music(butch).
playsAirGuitar(vincent):listens2music(vincent), happy(vincent).
playsAirGuitar(butch):- happy(butch).
playsAirGuitar(butch):- listens2music(butch).
?- playsAirGuitar(butch).
yes
?-
Expressing Conjunction
happy(vincent).
listens2music(butch).
playsAirGuitar(vincent):listens2music(vincent), happy(vincent).
playsAirGuitar(butch):- happy(butch).
playsAirGuitar(butch):- listens2music(butch).
Expressing Disjunction
happy(vincent).
Two rules with
listens2music(butch).
the same head
playsAirGuitar(vincent):- listens2music(vincent),
Ξ or
happy(vincent).
playsAirGuitar(butch):- happy(butch).
playsAirGuitar(butch):- listens2music(butch).
happy(vincent).
listens2music(butch).
playsAirGuitar(vincent):- listens2music(vincent),
happy(vincent).
playsAirGuitar(butch):- happy(butch);
listens2music(butch).
Prolog and Logic
• Clearly Prolog has something to do
with logic
• Operators
– Implication :– Conjunction , (the comma means and)
– Disjunction ; (Prolog symbol for or)
• Use of modus ponens
Modus Ponens
The rule of logic stating that if a conditional
statement (“if p then q ”) is accepted, and the
antecedent ( p ) holds, then the consequent ( q ) may
be inferred. Example:
 If you have a current password, then you can log on
to the network:
"You have a current password“
Therefore:
"You can log on to the network"
 This has the form:
p→q
p
∴q
Modus Ponens
 If you have a current password, then you
can log on to the network:
“You can't log into the network”
"You don't have a current password"
 This has the form:
¬q
p→q
∴ ¬p
Therefore:
Knowledge Base 4
woman(mia).
woman(jody).
woman(yolanda).
loves(vincent, mia).
loves(marsellus, mia).
loves(pumpkin, honey_bunny).
loves(honey_bunny, pumpkin).
Q. How many facts & rules in the kb?
Prolog Variables
woman(mia).
woman(jody).
woman(yolanda).
loves(vincent, mia).
loves(marsellus, mia).
loves(pumpkin, honey_bunny).
loves(honey_bunny, pumpkin).
?- woman(X).
Prolog Variables
woman(mia).
woman(jody).
woman(yolanda).
loves(vincent, mia).
loves(marsellus, mia).
loves(pumpkin, honey_bunny).
loves(honey_bunny, pumpkin).
?- woman(X).
X=mia
Prolog Variables
woman(mia).
woman(jody).
woman(yolanda).
loves(vincent, mia).
loves(marsellus, mia).
loves(pumpkin, honey_bunny).
loves(honey_bunny, pumpkin).
?- woman(X).
X=mia;
Prolog Variables
woman(mia).
woman(jody).
woman(yolanda).
loves(vincent, mia).
loves(marsellus, mia).
loves(pumpkin, honey_bunny).
loves(honey_bunny, pumpkin).
?- woman(X).
X=mia;
X=jody;
X=yolanda;
no
Prolog Variables
woman(mia).
woman(jody).
woman(yolanda).
loves(vincent, mia).
loves(marsellus, mia).
loves(pumpkin, honey_bunny).
loves(honey_bunny, pumpkin).
?- loves(marsellus,X), woman(X).
X=mia
yes
?-
Prolog Variables
woman(mia).
woman(jody).
woman(yolanda).
loves(vincent, mia).
loves(marsellus, mia).
loves(pumpkin, honey_bunny).
loves(honey_bunny, pumpkin).
?- loves(pumpkin,X), woman(X).
no
?-
Knowledge Base 5
loves(vincent,mia).
loves(marsellus,mia).
loves(pumpkin, honey_bunny).
loves(honey_bunny, pumpkin).
jealous(X,Y):- loves(X,Z), loves(Y,Z).
Q. How many facts & rules in the kb?
four facts - one rule – 3 variables.
Knowledge Base 5
loves(vincent,mia).
loves(marsellus,mia).
loves(pumpkin, honey_bunny).
loves(honey_bunny, pumpkin).
jealous(X,Y):- loves(X,Z), loves(Y,Z).
?- jealous(marsellus,W).
W=vincent
?-
Prolog Syntax
What exactly are facts, rules and queries
built out of?
There are three types of term:
1. Constants. These can either be atoms (such as
vincent) or numbers (such as 24).
2. Variables.
3. Complex terms. These have the form:
functor(term_1,...,term_n).
Atoms
A sequence of characters of upper-case
letters, lower-case letters, digits, or
underscore, starting with a lowercase
letter
Examples: butch, big_kahuna_burger,
playGuitar
An arbitrary sequence of characters enclosed
in single quotes
Examples: 'Vincent', 'Five dollar shake', '@$'
A sequence of special characters
Examples:
@= and ====> and ; and :-
Numbers
Integers: 12, -34, 22342
Floats: 34573.3234
Variables
A sequence of characters of upper-case
letters, lower-case letters, digits, or
underscore, starting with either an
uppercase letter or an underscore
Examples:
X, Y, Variable, Vincent, _tag
The variable _ is special. Is called the
anonymous variable
Anonymous Variable
A variable that stands in for some unknown object
Stands for some objects about which we don’t care
Several anonymous variables in the same clause
need not be given consistent interpretation
written as _ in Prolog for example
?- composer(X, _, _).
X = beethoven;
X = mozart;
We are interested in the names of composers
but not their birth and death years
Complex Terms
Atoms, numbers and variables are building
blocks for complex terms
Complex terms are often called structures.
Complex terms are built of a functor directly
followed by a sequence of arguments
Arguments are put in round brackets,
separated by commas
The functor must be an atom and cannot be
a variable.
Examples of complex terms
Examples as seen before:
– playsAirGuitar(jody)
– loves(vincent, mia)
– jealous(marsellus, W)
Complex terms inside complex terms:
– hide(X,father(father(father(butch))))
Arity
 The number of arguments a complex
term has is called its arity
 Examples:
- woman(mia) is a term with arity 1
- loves(vincent,mia)
has arity 2
- father(father(butch))
arity 1
Arity is important
In Prolog you can define two
predicates with the same functor but
with different arity. Prolog treat this
as two different predicates:
love(vincent, mia).
love(vincent, marcellus, mia).
Arity is important
In Prolog documentation arity of a
predicate is usually indicated with the
suffix "/" followed by a number to
indicate the arity
This knowledge base defines:
–happy/1
–listens2music/1
–playsAirGuitar/1
Example of Arity
happy(yolanda).
listens2music(mia).
listens2music(yolanda):- happy(yolanda).
playsAirGuitar(mia):- listens2music(mia).
playsAirGuitar(yolanda):listens2music(yolanda).
• This knowledge base defines:
– happy/1
– listens2music/1
– playsAirGuitar/1
quiz
1- Which of the following sequences of characters
are atoms, which are variables, and which are
neither?
atom
1. vINCENT
variable
2. Footmassage
atom
3. variable23
atom
4. variable2000
atom
5. big_kahuna_burger
atom
6. ’big kahuna burger’
neither
7. big kahuna burger
atom
8. ’Jules’
variable
9. _Jules
atom
10. ’_Jules’
quiz
2- Which of the following sequences of characters
are atoms, which are variables, which are complex
terms, and which are not terms at all? Give the
functor and arity of each complex term.
1. loves(Vincent,mia)
2. ’loves(Vincent,mia)’
3. Butch(boxer)
4. boxer(Butch)
5. and(big(burger),kahuna(burger))
6. and(big(X),kahuna(X))
7. _and(big(X),kahuna(X))
8. (Butch kills Vincent)
9. kills(Butch,Vincent)
10. kills(Butch,Vincent
complex term, functor: loves, arity: 2
atom
neither
complex term, functor: boxer, arity: 1
complex term, functor: and, arity: 2
complex term, functor: and, arity: 2
neither
neither
complex term, functor: kills, arity: 2
neither
quiz
3- How many facts, rules, clauses, and predicates
are there in the following knowledge base? What
are the heads of the rules, and what are the goals
they contain?
facts: 3
woman(vincent).
rules: 4
woman(mia).
clauses: 7
man(jules).
predicates: 5
person(X) :- man(X); woman(X).
loves(X,Y) :- knows(Y,X).
father(Y,Z) :- man(Y), son(Z,Y).
father(Y,Z) :- man(Y).
quiz
4- How many facts, rules, clauses, and predicates
are there in the following knowledge base? What
are the heads of the rules, and what are the goals
they contain?
woman(vincent).
woman(mia).
man(jules).
person(X) :- man(X); woman(X).
loves(X,Y) :- knows(Y,X).
father(Y,Z) :- man(Y), son(Z,Y).
father(X,Y,Z) :- man(Y).
quiz
5. Here are simple rules:
Formulate the following
queries?
has(jack,apples).
has(ann,plums).
has(dan,money).
fruit(apples).
fruit(plums).
- what has Jack?
- does Jack have something?
- who has apples and who has plums?
- does someone have apples and plums?
- has Dan fruits?
quiz
Answer
?- has(jack,X).
% what has Jack?
?- has(jack,_). % does Jack have something?
?- has(X,apples),has(Y,plums).
% who has apples and who has plums?
?- has(X,apples),has(X,plums).
% does someone have apples and plums?
?- has(dan,X),fruit(X). %has Dan fruits?
quiz
6. Here are simple facts.
likes(mary,food).
likes(mary,wine).
likes(john,wine).
likes(john,mary).
Formulate the following queries?
1. John likes anything that Mary likes
2. John likes anyone who likes wine
3. John likes anyone who likes themselves
quiz
Answer
likes(mary,food).
likes(mary,wine).
likes(john,wine).
likes(john,mary).
likes(john,X),likes(mary,X).
% John likes anything that Mary likes
Likes(john,X),likes(X,wine).
% John likes anyone who likes wine
Likes(john,X),likes(X,X).
% John likes anyone who likes themselves
quiz
7- Bible tree
Write the appropriate clauses (facts) which
represents the following family tree?
abraham
ismael
isaac
esau
iacob
quiz
Answer
parent(abraham,ismael).
parent(abraham,isaac).
parent(isaac,esau).
parent(isaac,iacob).
grandfather(B,N):- parent(B,P),parent(P,N).
% B is grandfather of N
brother(F1,F2):- parent(P,F1),parent(P,F2),\F1=F2.
% F1 brother of F2
descendent(X,Y):- parent(X,Y).
% X descendant of Y
descendent(X,Y):- parent(X,Z),descendent(Z,Y).
quiz
8- Family tree
Write the appropriate clauses (facts)
which represents the following family
tree?
James I
Elizabeth
Charles I
Sophia
Catherine
Charles II
James II
George I
quiz
Formulate the following queries:
Was George I the parent of Charles I?
Who was Charles I's parent?
Who were the children of Charles I?
Express the following rules:
- M is the mother of X if she is a parent of X and is female
- F is the father of X if he is a parent of X and is male
- X is a sibling of Y if they both have the same parent.
Furthermore add rules defining:
"sister", "brother", "aunt", "uncle", "grandparent", "cousin"
quiz
Answer
male(james1).
male(charles1).
male(charles2).
male(james2).
male(george1).
female(catherine).
female(elizabeth).
female(sophia).
parent(charles1, james1).
parent(elizabeth, james1).
parent(charles2, charles1).
parent(catherine,
charles1).
parent(james2, charles1).
parent(sophia, elizabeth).
parent(george1, sophia).
quiz
Answer
?- parent(charles1, george1).
% Was George I the parent of Charles I?
?- parent(charles1,X).
% Who was Charles I's parent?
?- parent(X,charles1).
% Who were the children of Charles I?
quiz
Answer
mother(X,M):- parent(X,M), female(M).
% M is the mother of X if she is a parent of X and is
female
father(X,F):- parent(X,M), male(M).
% F is the father of X if he is a parent of X and is male
sibling(X,Y):- parent(X,Z), parent(Y,Z).
% X is a sibling of Y if they both have the same parent.
quiz
Answer
Furthermore add rules defining:
"sister", "brother", "aunt", "uncle", "grandparent", "cousin"
sister(Y,X):- female(X), parent(X,M), parent(Y,M).
% X is a sister of Y
brother(Y,X):- male(X), parent(X,M), parent(Y,M).
% X is a brother of Y
parent
female
X
M
sister
parent
Y
parent
male
X
M
brother
parent
Y
quiz
Answer
Furthermore add rules defining:
"sister", "brother", "aunt", "uncle", "grandparent", "cousin"
aunt(Y,X):- female(X), sister(X,M), mother(Y,M).
aunt(Y,X):- female(X), brother(X,M), father(Y,M).
% X is an aunt of Y
M
father
brother
female
X
M
aunt
Y
mother
sister
female
X
aunt
Y
quiz
Answer
Furthermore add rules defining:
"sister", "brother", "aunt", "uncle", "grandparent", "cousin"
uncle(Y,X):- male(X), sister(X,M), mother(Y,M).
uncle(Y,X):- male(X), brother(X,M), father(Y,M).
% X is an uncle of Y
M
father
brother
male
X
M
uncle
Y
mother
sister
male
X
uncle
Y
quiz
Answer
Furthermore add rules defining:
"sister", "brother", "aunt", "uncle", "grandparent", "cousin"
grandparent(Y,X):- male(X), father(X,M), parent(M,Y).
grandparent(Y,X):- female(X), mother(X), parent(X,M).
% X is a grandparent of Y
M
parent
father
male
M
X
Y
grandparent
parent
mother
female
X
Y
grandparent
quiz
9- Here are six English words:
(abalone, abandon, anagram, connect,
elegant, enhance)
They are to be arranged
in a crossword puzzle
like fashion in the given
grid given.
Write prolog program to
solve this crossword
word(abalone,a,b,a,l,o,n,e).
word(abandon,a,b,a,n,d,o,n).
word(enhance,e,n,h,a,n,c,e).
word(anagram,a,n,a,g,r,a,m).
word(connect,c,o,n,n,e,c,t).
word(elegant,e,l,e,g,a,n,t).
crossword(H1,H2,H3,V1,V2,V3) :word(H1,_,A,_,B,_,C,_),
word(H2,_,D,_,E,_,F,_),
word(H3,_,G,_,H,_,I,_),
word(V1,_,A,_,D,_,G,_),
word(V2,_,B,_,E,_,H,_),
word(V3,_,C,_,F,_,I,_).
quiz
?crossword(H1,H2,H3,V1,V2,V3).
RECALL
Basic Elements of Prolog
The Prolog comment characters:
– Single line comments:%
% This is a comment
This not a comment, but an error
– Multiple line comments: /*
/* This is a multi-line comment
which must be closed with a */
© Patrick Blackburn, Johan Bos & Kristina Striegnitz
Basic Elements of Prolog
happy(yolanda).
listens2music(mia).
listens2music(yolanda):- happy(yolanda).
playsAirGuitar(mia):- listens2music(mia).
playsAirGuitar(yolanda):listens2music(yolanda).
Basic Elements of Prolog
 Prolog program is a database of facts and
rules.
 Some are always true (facts):
father( john, jim).
 Some are dependent on others being true
(rules):
parent( Person1, Person2 ) :father( Person1, Person2 ).
 To run a program, we ask questions (query
about the database
Basic Elements of Prolog
playsAirGuitar(vincent):- listens2music(vincent), happy(vincent).
goal
goal
Head
body
Clause
Basic Elements of Prolog
fact
rule
happy(yolanda).
rule
fact
listens2music(mia).
listens2music(yolanda):- happy(yolanda).
playsAirGuitar(mia):- listens2music(mia).
playsAirGuitar(yolanda):- listens2music(yolanda).
 Fact  clause without body
rule
Basic Elements of Prolog
Predicate Name
Argument
happy(yolanda).
listens2music(mia).
listens2music(yolanda):- happy(yolanda).
playsAirGuitar(mia):- listens2music(mia).
Summary
• 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.
• We can set our program goals by typing a command that unifies
with a clause head.
• A goal unifies with clause heads in order (top down).
• Unification leads to the instantiation of variables to values.
• If any variables in the initial goal become instantiated this is
reported back to the user.