Logic Programming and Prolog

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### Logic Programming and Prolog

Stuart Howard

CS 550

Montana State University

December 5, 2003

Presentation Outline
• Definition
• History and background
• Logical calculus
• Resolution theorem proving
• Prolog syntax and example program
• Strengths and weaknesses
• Summary and conclusions

Logic programming

• based on symbolic logic
• write declarations and infer the results
• logic programming languages are declarative languages
What does declarative mean?
• Logic programmers write declarations describing relationships between entities
• the ‘what’ vs. the ‘how’
• what does the answer look like? (specification)

vs.

• how do I exactly compute the answer?(implementation)
Let’s sort a list nonprocedurally

permute(oldList, newList) Ù sorted(newList) => sort(oldList, newList)

• j such that 1 ≤ j < n, list(j) ≤ list(j + 1) => sorted(list)

example from [Sebesta]

Prolog

Prolog = Programmation en Logique

• Early 70’s
• Alain Colmerauer and Phillipe Roussel
• Robert Kowalski
Origins of Prolog

Q-systems – designed for machine natural language translation (Colmerauer)

• used to construct automatic English-French translations
• used in Canada to translate weather forecasts from English to French
First large Prolog program

man-machine communication system –1972

TOUT PSYCHIATRE EST UNE PERSONNE.

CHAQUE PERSONNE QU'IL ANALYSE, EST MALADE.

*JACQUES EST UN PSYCHIATRE A *MARSEILLE.

EST-CE QUE *JACQUES EST UNE PERSONNE?

OU EST *JACQUES?

OUI.

A MARSEILLE.

JE NE SAIS PAS.

Prolog example

Shaun, the bull, and the tractor.

Logical calculus
• language
• inference rules
• semantics
Propositional calculus

the language…

• atoms: T, F, {w | w is a string and begins

with a capital letter}…H, On_A_B

• connectives: Ù, Ú, Ø, →
• rules for forming sentences(aka. well-formed formulas, wff’s)
Rules for wff’s
• any atom is a wff
• If w1 and w2 are wff’s, so are:w1 Ù w2conjunctionw1 Ú w2disjunctionw1 →w2implication

Ø w1negation

• there are no other wff’s
more terms…
• literal: atom or negated atom (P, ØP)
• w1 →w2(antecedent) (consequent)
Propositional calculus

the inference rules…(6 of them anyway)

• w2 from w1 and w1 →w2 (modus ponens)
• w1 Ù w2 from w1 and w2 (Ù introduction)
• w2 Ù w1 from w1 Ù w2 (commutativity of Ù)
• w1 from w1 Ù w2 (Ù elimination)
• w1 Ú w2 from w1 or w2 (Ú introduction)
• w1 from Ø(Øw1) (Ø elimination)
Propositional calculus

the semantics…

• associate atoms with propositions about the world

Shaun_strong with “Shaun is strong”|_________________| denotation |__________________________________________|

interpretation

Propositional calculus

remember…?

• literal: an atom or its negation

a new term!

• clause: a set of literals with disjunction implied {P, Q, ØR} ≡ (P ÚQ ÚØR)
Propositional calculus

a new inference rule…resolution

S1 S2from{a} S1 and{Øa} S2

P ÚR from P ÚQ and R ÚØQ

{ } from{a} and {Øa}

Why not?
• Soundness - OK
• Completeness – not OK
limitations…
• Propositional calculus limits us to referring to ‘hard-coded’ propositions about the world.
• What if we also want to refer to objects and propositions by name?
Predicate Calculus

Objects: constants (names) or variables

Functions/relations: constants (names)

weird(MichaelJackson)

cute(Russell)

smart(x)

hate(Michael, Microsoft)

Predicate Calculus
• Quantifiers
• Universal
• Existential
• Express properties of entire collections of objects
Predicate Calculus
• Universal quantifiers make statements about every object, "x

A cat is a mammal

"x Cat(x) ÞMammal(x)

Cat(Spot) Þ Mammal(Spot) Ù

Cat(Rebecca) Þ Mammal(Rebecca) Ù

Cat(Felix) Þ Mammal(Felix) Ù

Cat(Richard) Þ Mammal(Richard) Ù

Cat(John) Þ Mammal(John) Ù

… [Comp313A]

Predicate Calculus
• Existential Quantifiers make statements about some objects, \$x

Spot has a sister who is a cat

\$x Sister(x, Spot) Ù Cat(x)

(Sister(Spot, Spot) Ù Cat(Spot)) Ú

(Sister(Rebecca, Spot) Ù Cat(Rebecca)) Ú

(Sister(Felix, Spot) Ù Cat(Felix)) Ú

(Sister(Richard, Spot) Ù Cat(Richard)) Ú

(Sister(John, Spot) Ù Cat(John)) Ú

… [Comp313A]

Predicate Calculus

"is a conjunction over the universe of objects

\$is a disjunction over the universe of objects

Predicate Calculus Example

The George W. Bush Family

mother(Barbara, George Jr.)

father(George Jr., Jenna)

father(George Jr., Barbara II)

Predicate Calculus Example

Q: Barbara is the grandmother of whom?

" x" y"z[mother(x, y) Ù parent(y, z) => grandmother(x, z)]

" x"y [father(x,y) => parent(x, y)]

" x"y [mother(x,y) => parent(x, y)]

Predicate Calculus Example
• mother(Barbara, George Jr.)
• father(George Jr., Jenna)
• father(George Jr., Barbara II)
• " x" y"z[mother(x, y) Ù parent(y, z) => grandmother(x, z)]
• " x"y [father(x,y) => parent(x, y)]
• " x"y [mother(x,y) => parent(x, y)]
Resolution in Predicate Calculus

" x" y"z[mother(x, y) Ùparent(y, z) => grandmother(x, z)]

mother(x, y) Ùparent(y, z) => grandmother(x, z)

Ø [mother(x, y) Ùparent(y, z)] Ú grandmother(x, z)

Ø mother(x, y) ÚØ parent(y, z) Ú grandmother(x, z)

Resolution in Predicate Calculus

" x"y [father(x,y) => parent(x, y)]

father(a, b) => parent(a, b)

Ø father(a, b) Úparent(a, b)

Resolution in Predicate Calculus

" x"y [mother(x,y) => parent(x, y)]

mother(c, d) => parent(c, d)

Ø mother(c, d) Úparent(c, d)

mother(Barbara, George Jr.)
• father(George Jr., Jenna)
• father(George Jr., Barbara II)
• Ø mother(x, y) ÚØ parent(y, z) Ú grandmother(x, z)
• Ø father(a, b) Úparent(a, b)
• Ø mother(c, d) Úparent(c, d)
• Ø grandmother(Barbara, Jenna)
• Ø mother(Barbara, e) ÚØ parent(e, Jenna) [7, 4]
• Ø mother(Barbara, f) ÚØ father(f, Jenna) [8, 5]
• Ø father(George Jr., Jenna) [9, 1]
• { }
Prolog Syntax
• Rules
• Facts
• Goals

They are all variants of an implication!

Rule

General

a => b

a Ù bÙ c => d

Prolog b :- a.

d :- a, b, c.

Fact

General

{ } => b

Prolog

b.

Goal

General

b => { }

Prolog

b.

Horn what?

Ø a Ú Ø b Ú Ø c Ú d

(Ø a Ú Ø b Ú Ø c) Ú d

Ø (a Ù bÙ c) Ú d

a Ù bÙ c => d

Horn what?

b

Ø { } Ú b

{ } => b

Horn what?

Ø a Ú Ø b Ú Ø c

Ø (a Ù bÙ c) Ú { }

a Ù bÙ c => { }

1

2

4

5

3

6

1

2

4

3

5

6

7

Uses of logic programming
• relational database management systems
• expert systems
• natural language processing
Weaknesses of Prolog
• Resolution order control
• Closed world assumption
Summary and Conclusions

Proponets say…

• Logical language => logically organized=> fewer errors, less maintenance
• Programs concise => less development time => good prototyping language

Opponents say…

• BUNK!

http://www.mozart-oz.org/features.html

Presentation Review
• Definition
• History and background
• Logical calculus
• Resolution theorem proving
• Prolog syntax and example program
• Strengths and weaknesses
• Summary and conclusions
References

Concepts of Programming Languages (Fourth Edition) by Robert Sebesta (An excellent overview of Prolog... very helpful!) Notes from CS 436 (Artificial Intelligence) Dr. John Paxton (My first introduction to predicate calculus and Prolog) Artificial Intelligence: A New Synthesis by Nils Nilsson (Excellent presentation of propositional and predicate calculus) Computer Science: An Overview by Glenn Brookshear (Nice short overview of logic programming and Prolog) Compiler Design by Reinhard Wilhelm and Dieter Maurer (Obtuse, highly mathemetized, and hard to read.... i.e. it's Greek to me) http://www.cs.waikato.ac.nz/Teaching/COMP313A/lecture_notes.html (6 excellent PowerPoint lectures on logic programming) Alain Colmerauer's web site (a cofounder of Prolog) History of PrologProlog compiler that ESUS uses