Ahmed Sallam Slides based on original lecture slides Dr. Temur Kutsia

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Chapter 1: Introduction

"What" vs "How"

Declarative vs Procedural Programming

Procedural programming

• The programmer has to specify how to get the output for

the range of required inputs.

• The programmer must know the appropriate algorithm.

Declarative programming

• Requires a more descriptive style.

• The programmer must know what relationships hold

between various entities.

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Example: List Concatenation

In procedural style:

list procedure cat(list a,list b)

{

list t = list u = copylist(a);

while (t.tail != nil) t = t.tail;

t.tail = b;

return u;

}

In declarative style:

cat([], L, L).

cat([H|T], L, [H|Z]) :- cat(T, L, Z).

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Logic Programming

A declarative style programming paradigm.

Computation through logical deduction.

Uses the language of logic to express data and programs.

Most of current logic programming languages use first

order logic (FOL).

Prolog – the most popular logic programming language.

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Historical Facts

1970-ies:

• Bob Kowalski. "Predicate Logic as a Programming Language".

IFIP Congress, Stockholm

• Alain Colmerauer and his group. Interpreter of the first logic programming language Prolog.

Marseille

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Prolog

• Prolog is the main subject of this course

• Used in Artificial Intelligence, Natural Language

Processing, Automated Reasoning, XML Querying...

• Exists in many dialects (Sicstus Prolog, SWI Prolog,

Prolog IV, BinProlog, Ciao Prolog, Prolog LPA, Visual

Prolog, YAP Prolog, Strawberry Prolog)...

• (Almost) all the dialects agree on the “core" part (ISO

Standard for Prolog)

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Prolog in Industrial Applications

Clarissa:

• A fully voice-operated procedure browser.

• Developed at NASA.

• Used on the International Space Station.

• Enables astronauts to give full attention to the task while

they navigate through complex procedures using spoken

commands.

• Implemented in SICStus Prolog.

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Prolog in Industrial Applications

Some other solutions:

• FleetWatch – fully integrated operations control and schedules

planning solution. Used by 21 international airlines, among

them Comair (USA), Italian branch of Lauda Air, Malev

(Hungary), DHL Europe, Asiana (South Korea), Hainan (China),

Royal Jordanian, Kuwait Airways, Cimber Air (Denmark), etc.

• SCORE – a long term airport capacity management system for

coordinated airports. Successfully Used at IATA Scheduling

Conferences. Users in more than 20 countries.

• ARGOS – a Decision Support System (DSS) for enhancing

Crisis Management for incidents with Chemical, Biological,

Radiological, and Nuclear (CBRN) releases. Introduction

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Contents

Basics of PROLOG

Facts

Questions

Variables

Conjunction

Rules

Introduction

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PROLOG

Used to solve problems involving

• objects, and

• relationships between objects.

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Relationships

Example

John owns the book

• The relationship: ownership

• The objects: book, John

Directional:

• John owns the book

• Not: The book owns John

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PROLOG

Program can be thought of as a storehouse of facts and rules.

Conversational Language: The user can ask questions about

the set of facts and rules in the PROLOG program.

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PROLOG

Sisters Example:

A rule defining sisters and the facts about the people involved.

The user would ask:

Are these two people sisters?

The system would answer

yes (true) or no (false)

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Programming in PROLOG

Declaring Facts about objects and their relationships.

Defining Rules about objects and their relationships.

Asking Questions about objects and their relationships.

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Contents

Basics of PROLOG

Facts

Questions

Variables

Conjunction

Rules

Introduction

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Parts of Fact

0

Introduction

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Order of Objects

Introduction

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Examples of Facts

Example

Gold is valuable. v a l u a b l e ( g o l d ) Jane is a female. f e m a l e ( j an e ) John owns some gold.

ow ns( john ,go ld ) John is the father of Mary.

f a t h e r ( j o h n , m a r y )

Are these expressions really facts? Is there anything missing?

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Interpretation of Names

The name refers to an object.

Semantic Meaning: Given by the programmer.

Syntactic Meaning: a set of characters, as PROLOG sees it.

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Interpretation of Names

Name refers to an object.

Name gold can refer to:

a particular lump of gold, or

the chemical element Gold having atomic number 79.

va luab le ( g o ld ) can mean:

that particular lump of gold, named gold, is valuable, or

the chemical element Gold, named gold, is valuable.

The programmer decides (in her usage) the meaning.

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Fact Terminology

Introduction

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Database

Definition In PROLOG, database is a collection of facts.

PROLOG draws its knowledge from these facts.

The programmer is responsible for their accuracy.

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Contents

Basics of PROLOG

Facts

Questions

Variables

Conjunction

Rules

Introduction

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Questions

The database contains the facts from which the questions are

answered.

A Question can look exactly like a fact:

owns(mary,book) .

The difference is in which mode one is in

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Questions

In the interactive question mode (indicated by the question

mark and dash): ? -

Question: ? - owns(mary,book).

Meanins: If mary is interpreted as a person called Mary, and book is

interpreted as some particular book, then

?- owns(mary,book).

means: Does Mary own the book?

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Database Search

Example

Facts in the database:

l i k e s ( j o e , f i s h ) .

l i k e s ( j o e , m a r y ) .

l i k es ( m ar y , book ) .

l i k e s ( j o h n , b o o k ) .

Questions: ?- l i k es ( j oe , m oney) . no

?- l i k e s ( j o e , m a r y ) . yes

?- k i n g ( j o h n , f r a n c e ) . n o

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Knowledge

The questions are always answered with respect to the

database.

Example

Facts in the database:

human(socrates).

h u m a n ( a r i s t o t l e ) .

a t hen ian ( soc r a tes ) .

Question:

Is Socrates Greek?

?- g reek (soc ra tes )

The answer with respect to this database is No.

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Questions

Up until now questions just reflect exactly the database.

Does Mary like the book?

?- l i k es ( m ar y , book ) . More Interesting Question:

What objects does Mary like?

Variables.

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Contents

Basics of PROLOG

Facts

Questions

Variables

Conjunction

Rules

Introduction

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Variables

Tiresome to ask about every object:

l i k e s ( j o h n , t h i s ) .

l i k e s ( j o h n , t h a t ) .

Better to ask: What does John like?

or

Does John like X?

(i.e. use variables)

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Question With Variables

Does John like X?

? - l i k e s ( j o h n , X ) .

or

? - l i kes( john,Someth ingThatJohnL ikes ) .

X and SomethingThatJohnLikes are variables.

Variable begins with a capital letter.

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PROLOG Answer

Database:

l i k e s ( j o h n , f l o w e r s ) .

Question:

? - l i k e s ( j o h n , X ) .

PROLOG answers:

X=f lowers

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Many Answers

Database:

l i k e s ( j o h n , f l o w e r s ) .

l i k e s ( j o h n , m a r y ) .

l i k e s ( p a u l , m a r y ) .

Question:

? - l i k e s ( j o h n , X ) .

PROLOG answers:

X=f lowers

and the user acknowledges

X=mary

and the user acknowledges

no

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Placemarker

The first match is found: X=f lowers .

The user acknowledges.

From that place on the next match is found (the search continues).

From the place of the last instantiation no more match was found.

Thus answer: no.

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Contents

Basics of PROLOG

Facts

Questions

Variables

Conjunction

Rules

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Conjunctions

More Complicated Relationships:

Does Mary like John and does John like Mary?

Both Conditions must be fulfilled.

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Conjunctions

Database:

l i k e s ( m a r y , f o o d ) .

l i k e s ( m a r y , c o l a ) .

l i k e s ( j o h n , c o l a ) .

l i k e s ( j o h n , m a r y ) .

Comma means Conjunction:

? - l i k e s ( j o h n , m a r y ) , l i k e s ( m a r y , j o h n ) .

Answer: no

A match for l i k es ( j ohn , m a r y )

but none for l i k es ( m ar y , j ohn )

Introduction

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Conjunctions with Variables

Is there anything that both mary and john like?

Find out what Mary likes and then see if John likes it.

? - l i k e s ( m a r y , X ) , l i k e s ( j o h n , X ) .

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Backtracking

Find match for the first goal.

Then see if matches the second.

If not, find another match for the first.

See if this matches the second.

…etc.

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Match First

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,

Cola).

,

Cola).

Match Second

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,

Cola). ,

Cola).

Backtrack

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,

Cola). ,

Cola).

Cola

Cola

Success

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,

Cola). ,

Cola).

Cola

Cola

Contents

Basics of PROLOG

Facts

Questions

Variables

Conjunction

Rules

Introduction

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Rules

• How to express that John likes all people?

• Listing all people?

• likes(john, alfred).

• likes(john, bertrand).

• likes(john, charles).

• likes(john, david).

• etc.

• Not feasible. More compact way: Using rules.

John likes any object provided it is a person.

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Rule Examples

Rules state Dependence:

I use an umbrella if there is rain.

Rules Define:

X is a bird if X is an animal and X has feathers.

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Formulating Rules

John likes anyone who likes cola.

John likes any Something if it likes cola

John likes X if X likes cola

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Rule Syntax

Introduction

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, Cola).

Variable Scope

The occurrences of X within a rule refer to the same object:

l i k e s ( j o h n , X ) : - l i k e s ( X , c o la ) , l i k e s ( X , f o o d ) .

l i k e s ( j o h n , mary):-

l i k e s ( m a r y , c o la ) , l i k e s ( m a r y , f o o d ) .

l i k e s ( j o h n , adam):- l i k es ( adam , c o l a ) , l i k es ( adam,

f o o d ) .

Introduction

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Royal Parents

Example

The parents of X are Y and Z.

Y is the mother.

Z is the father.

Database:

m a l e ( a l b e r t ) .

male(edward).

f e m a l e ( a l i c e ) .

f e m a l e ( v i c t o r i a ) .

p a r e n t s ( e d w a r d , v i c t o r i a , a l b e r t ) .

p a r e n t s ( a l i c e , v i c t o r i a , a l b e r t ) . Introduction

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Sisters

Example

X is a sister of Y if:

X is female,

X has parents M and F,

Y has parents M and F.

Rule:

s i s t e r ( X , Y ) : - f ema le (X ) ,

pa rents (X ,M,F ) ,

pa rents (Y ,M,F ) .

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Sisters Question

Question:

? - s i s t e r ( a l i c e , e d w a r d ) .

The question (goal) matches the head of the rule, if one

replaces X with a l i c e and Y with edward.

The instance of the body becomes new

goal: female(a l i c e ) ,

pa ren ts ( a l i c e , M , F ) ,

pa ren ts ( edward, M, F ) .

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7 s i s t e r ( a l i c e , e d wa r d )

X0=a l i c e , Y0=edward

3 f e m a l e ( a l i c e ) ,

p a r e n t s ( a l i c e ,M 0 , F0 ) ,

parents(edward,M0,F0).

6 p a r e n t s (a l i c e , M 0 , F0 ) ,

parents(edward,M0,F0).

M0 = v i c t o r i a , F0=alber t

5 p a r e n t s ( e d w a r d , v i c t o r i a , a l b e r t ) .

•

Is Alice Edward’s Sister?

( 1 ) m a l e ( a l b e r t ) .

(2) male(edward).

( 3 ) f e m a l e ( a l i c e ) .

( 4 ) f e m a l e ( v i c t o r i a ) .

(5) parents (edward ,

v i c t o r i a , a l b e r t ) .

( 6 ) p a r e n t s ( a l i c e ,

v i c t o r i a , a l b e r t ) .

( 7 ) s i s t e r ( X , Y ) : -

f em a le (X) ,

parents (X,M,F ) ,

pa ren t s (Y ,M,F ) .

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Is Alice Edward’s Sister?

7 s i s t e r ( a l i c e , e d wa r d )

X0=a l i c e , Y0=edward

3 f e m a l e ( a l i c e ) ,

p a r e n t s ( a l i c e ,M 0 , F0 ) ,

parents(edward,M0,F0).

6 p a r e n t s (a l i c e , M 0 , F0 ) ,

parents(edward,M0,F0).

M0 = v i c t o r i a , F0=alber t

5 p a r e n t s ( e d w a r d , v i c t o r i a , a l b e r t ) .

•

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( 1 ) m a l e ( a l b e r t ) .

(2) male(edward).

( 3 ) f e m a l e ( a l i c e ) .

( 4 ) f e m a l e ( v i c t o r i a ) .

(5) parents (edward ,

v i c t o r i a , a l b e r t ) .

( 6 ) p a r e n t s ( a l i c e ,

v i c t o r i a , a l b e r t ) .

( 7 ) s i s t e r ( X , Y ) : -

f em a le (X) ,

parents (X,M,F ) ,

pa ren t s (Y ,M,F ) .

Who’s Sister Is Alice?

7 s i s t e r ( a l i c e , X )

X0=a l i ce , Y0=X

3 f e m a l e ( a l i c e ) ,

p a r e n t s ( a l i c e ,M 0 , F0 ) ,

parents (X,M0,F0) .

6 p a r e n t s (a l i c e , M 0 , F0 ) ,

parents (X,M0,F0) .

M0=vic tor ia F0=alber t

5 p a r e n t s ( X , v i c t o r i a , a l b e r t ) .

X=edward

•

Answer: X = edward. Introduction

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( 1 ) m a l e ( a l b e r t ) .

(2) male(edward).

( 3 ) f e m a l e ( a l i c e ) .

( 4 ) f e m a l e ( v i c t o r i a ) .

(5) parents (edward ,

v i c t o r i a , a l b e r t ) .

( 6 ) p a r e n t s ( a l i c e ,

v i c t o r i a , a l b e r t ) .

( 7 ) s i s t e r ( X , Y ) : -

f em a le (X) ,

parents (X,M,F ) ,

pa ren t s (Y ,M,F ) .

Useful Links

• SWI-Prolog:

• http://www.swi-prolog.org/

• SWI-Prolog Editor (Windows, SWI-x86 only)

http://lakk.bildung.hessen.de/netzwerk/faecher/informatik/swiprolog/ind

exe.html

• Prolog mode for (X)Emacs:

http://turing.ubishops.ca/home/bruda/emacs-prolog/

• Prolog newsgroup:

http://groups.google.com/groups?group=comp.lang.prolog

• Links from SWI-Prolog Web page:

http://www.swi-prolog.org/Links.html

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