An Overview of Prolog

1. An Overview of Prolog

2. What is Prolog • Prolog is a powerful language for AI and non-numerical programming • Stands for programming in logic • Centered around a small set of basic mechanisms, including: • Pattern matching • Tree-based data structuring • Automatic back tracking • Well suited for problems that involves structured objects and relations between them.

3. Chapter One: Introduction to Prolog • Defining relations by facts • Defining relations by rules • Recursive rules • How Prolog answers questions • Declarative and procedural meaning of programs

4. Chapter one: Defining relations by facts Defining relations by rules

5. 1.1 Defining relations by facts Example The fact that tom is a parent of bob parent(tom, bob). the name of a relation argument1 argument2 Fact clause

6. 1.1 Defining relations by facts • The family tree is defined by the following Prolog program:parent(pam, bob)parent(tom, bob)parent(tom, liz)parent(bob, ann)parent(bob, pat)parent(pat, jim)This program consists of six clauses. • Each clause declares onefact about parent relation (particular instance of the parentrelation) * a relation is the set of all its instances

7. 1.1 Defining relations by facts When the program communicated to Prolog system, Prolog can be posed some questions about the parent relation: Is bob a parent of pat? ?- parent(bob, pat) Prolog answer: True ?- parent(liz, pat) Prolog answer: No solutions ?- parent(tom, pat) Prolog answer:No solutions

8. 1.1 Defining relations by facts Who is liz’s parent? ?- parent(X, liz) Prolog answer: X = tom Who are bob’s children? ?- parent(bob, X) Prolog answer: X = ann. X = pat.

9. 1.1 Defining relations by facts Find X and Y such that X is a parent of Y. ?- parent (X, Y). Prolog answers: X = pam.Y = bob.X = tom.Y = bob.X = tom.Y = liz.X = bob.Y = ann.X = bob.Y = pat.X = pat.Y = jim.

10. 1.1 Defining relations by facts Who is a grandparent of jim? Who is the parent of jim? Assume that this is Y Who is the parent of Y? Assume that this is X ?- parent(Y, jim), parent (X, Y) Prolog answer: X = bob Y = pat • Changing the order of the requirements will affectthe result. ?- parent(X, jim), parent (Y, X) Prolog answer: X = pat. Y = bob. ?- parent(X, jim), parent (X, Y) Prolog answer: X = pat. Y = jim. √ x x

11. 1.1 Defining relations by facts Do ann and pat have a common parent? Who is a parent X of ann? Is X a parent of pat? ?- parent (X, ann), parent(X, pat) Prolog answer: X = bob

12. 1.2 Defining relations by rules • We can add the information on the sex of the people that occur in the parent relation: female(pam). male(tom). male(bob). female(liz). female(pat). female(ann). male(jim). male and female are unary relations, whereas parent is a binary relation. Gender(pam, female). Gender(tom, male). Gender(bob, male).

13. 1.2 Defining relations by rules Let us introduce the offspring relation (inverse of parent) offspring(liz, tom). offspring(bob, tom). ... The logical statement is: For all X and Y, Y is an offspring of X if X is a parent of Y offspring(Y, X) :- parent(X, Y). Rule clause Fact caluses

14. 1.2 Defining relations by rules There is an important difference between facts and rules: • Facts is something that is always, unconditionally, trueparent(tom, liz). • Rules specify things that are true if some condition is satisfied. Rules have: • Condition part (right-hand side) = body • Conclusion part (left-hand side) = head offspring(Y, X) :- parent(X, Y). head body

15. 1.2 Defining relations by rules Questions • Is liz an offspring of tom? ?- offspring(liz, tom). Prolog answer: True How Prolog answer this question using the rule: Offspring(Y, X) :- parent(X, Y). Prolog answer: True

16. 1.2 Defining relations by rules • How to express: • Mother relation • Grandparent relation • Sister relation in Prolog?

17. Important Points