Intelligent Architecture

s for Electronic Commerce

A (Brief) Prolog Tutorial

Intelligent Architectures for Electronic CommerceTimothy J Norman and Wamberto Vasconcelos

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Overview

1. Introduction2. Syntax (the shape of Prolog programs)3. Semantics (the meaning of Prolog

programs)4. Pragmatics (how to write Prolog

programs)5. Practicalities6. Reading List

Intelligent Architectures for Electronic CommerceTimothy J Norman and Wamberto Vasconcelos

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1. Introduction

• Prolog:– PROgramming (in) LOGic– logic + control = program– results/behaviour obtained as a proof

• This tutorial:– quick look over essential issues– not exhaustive, not authoritative– won’t be enough to write very clever agents

in Prolog, but hopefully will help!– is idiosyncratic…

Intelligent Architectures for Electronic CommerceTimothy J Norman and Wamberto Vasconcelos

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2. Syntax

• Prolog programs are made up of– facts– clauses

• And these are made up of– atoms– variables– terms

Intelligent Architectures for Electronic CommerceTimothy J Norman and Wamberto Vasconcelos

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2.1 Atoms

• Atoms are the most essential components.

• Numbers are atoms.• Strings starting with small letter are

atoms.• Strings within single quotes are atoms.• Examples:

freddie 123 car sam umbrella ‘123’

Intelligent Architectures for Electronic CommerceTimothy J Norman and Wamberto Vasconcelos

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2.2 Variables

• Variables are:– any string starting with capital letter, or– any string starting with “_” (underscore)

• Variables are untyped.• Variables are defined at run-time (no

need to declare).• Examples

A Var1 Freddie Xs

Intelligent Architectures for Electronic CommerceTimothy J Norman and Wamberto Vasconcelos

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2.3 Terms

• “Structure” with atoms and variables.• General format:

atomNotNumber ( AtomOrVar, …, AtomOrVar )

• Examples:p(23,rover) owns(sam,X,23)

• Terms can be nested:odd(p(23,rover),owns(sam,X,23))

• Terms are used to build data structures.

Intelligent Architectures for Electronic CommerceTimothy J Norman and Wamberto Vasconcelos

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2.4 Facts

• Facts are terms asserted as true.• Examples

owns(freddie,car).owns(sam,X).

• Period “.” indicates end of fact.• No intrinsic meaning.

Intelligent Architectures for Electronic CommerceTimothy J Norman and Wamberto Vasconcelos

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2.5 Clauses

• General format:Term :- Term,…,Term.

• Examples

rich(X):- owns(X,house),owns(X,car).siblings(X,Y):- mother(X,Z),mother(Y,Z).

• Period “.” indicates end of clause.• Clauses are also known as rules.• Terms of clauses also called goals or

literals.

head

body

Intelligent Architectures for Electronic CommerceTimothy J Norman and Wamberto Vasconcelos

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2.5 Editing Programs

• Create a file with extension .pl• Type in your program (facts and rules). • Do not forget the “.” !!• Save your file.• Load your file in a Prolog session.• Always make sure you save and re-load

files.

Intelligent Architectures for Electronic CommerceTimothy J Norman and Wamberto Vasconcelos

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3. Semantics

• Meaning of program defined by its inferences

• Scenario: – given a program , try to prove if goal P is true

⊢ P– goal P is also called a query – queries may also be a conjunction of goals:

⊢ P1 , P2 , …, Pn

• Essential concepts:– unification– resolution

Intelligent Architectures for Electronic CommerceTimothy J Norman and Wamberto Vasconcelos

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3.1 Unification

• Process of making two or more terms equal.

• Variables are assigned values (computation!)

• Examples:– rich(X)unifies with rich(tom)if X = tom– rich(tom)doesn’t unify with rich(abe)

• Not always possible!• Unification problem:find values for variables to make terms equal

Intelligent Architectures for Electronic CommerceTimothy J Norman and Wamberto Vasconcelos

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3.1 Unification (Cont’d)

• Unification also obtains a substitution – set of pairs Variable /Term

= {A/a, B/f(1)}– apply it by replacing Variable by Term:

p(A,B) = p(a,f(1))• unify(TermLeft,TermRight, ) iff

TermLeft = TermRight

Intelligent Architectures for Electronic CommerceTimothy J Norman and Wamberto Vasconcelos

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3.1 Unification (Cont’d)

• More examples:– unify(p(a,q(X,X),t),p(Y,q(a,a),B), ) ?– unify(q(b,c(d)), X, ) ?– unify(s(f(1,2),X),s(f(X,C),1), ) ?

Intelligent Architectures for Electronic CommerceTimothy J Norman and Wamberto Vasconcelos

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3.2 Resolution

• process to prove if ⊢ P • resolution(,P ,) iff ⊢ P :

1. resolution(,P ,) if Q and unify(P,Q,)

2. resolution(,P ,) if (Q :- R1,…, Rn) and unify(P,Q,’) and

resolution(, (R1,…, Rn) ’,)

3. resolution(, (R1,…, Rn) ,) if

resolution(,R1,1) and

resolution(,(R2,…, Rn)1,)

Intelligent Architectures for Electronic CommerceTimothy J Norman and Wamberto Vasconcelos

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3.2 Resolution (Cont’d)

• Prolog: clauses/facts are chosen top-down.

• Prolog tries different clauses and facts.• example:

mother(john,barbara). father(john,philip).mother(mary,barbara). father(mary,philip).parent(X,Y):- mother(X,Y).parent(X,Y):- father(X,Y).siblings(X,Y):- parent(X,P),parent(Y,P).

?- siblings(john,mary).

Intelligent Architectures for Electronic CommerceTimothy J Norman and Wamberto Vasconcelos

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4. Pragmatics

• recursive data structures• lists: dynamic allocation• sequence of elements within “[” and

“]”• examples:

[1,2,rtu] [hello,(p(X):-q(X)),[4]]

• generic way to break list: head and tail– [1,2,rtu] = [Head|Tail]Head = 1 Tail = [2,rtu]

Intelligent Architectures for Electronic CommerceTimothy J Norman and Wamberto Vasconcelos

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4. Pragmatics (Cont’d)

• example: creation of list

create([X|Xs]):- read(X), \+ X = stop, create(Xs).create([]).

Intelligent Architectures for Electronic CommerceTimothy J Norman and Wamberto Vasconcelos

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4. Pragmatics (Cont’d)

• generic agent

loop(MntlSt):- final_condition(MntlSt).loop(MntlSt):- senseEnvironment(Percepts), react(Percepts,MntlSt,NewMntlSt), loop(MntlSt).

Intelligent Architectures for Electronic CommerceTimothy J Norman and Wamberto Vasconcelos

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5. Practicalities

• CIAO Prolog: free, efficient.• Emacs: integrated editor.• Dowload and install them.• Loop: edit-load-run (be careful!)• Debugging is possible and useful• CIAO Prolog talks to JAVA (and hence

JADE)

Intelligent Architectures for Electronic CommerceTimothy J Norman and Wamberto Vasconcelos

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6. Reading List

• Bratko, I (2001). Prolog Programming for Artificial Intelligence, 3rd Edition, Addison-Wesley.

• Stirling, L & Shapiro, E (1997). The Art of Prolog, 2nd Edition, MIT Press.