An Overview of Prolog 1 What is Prolog

An Overview of Prolog 1

What is Prolog � Prolog is a powerful language for AI and nonnumerical 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. 2

Chapter One: Introduction to Prolog 1. 2. 3. 4. 5. 3 Defining relations by facts Defining relations by rules Recursive rules How Prolog answers questions Declarative and procedural meaning of programs

1. 1 Defining relations by facts Example The fact that tom is a parent of bob parent(ahemd, bob). the name of a relation argument 1 argument 2 Fact clause pam tom 4 bob liz ann pat jim

1. 1 Defining relations by facts � The family tree is defined by the following Prolog program: parent(pam, parent(tom, parent(bob, parent(pat, bob). liz). ann). pat). jim). pam tom bob liz ann pat This program consists of six clauses. Each clause declares one fact about parent relation (particular instance of the parent relation) * a relation is the set of all its instances 5 jim

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? pam ? - parent(bob, pat). Prolog answer: yes ? - parent(liz, pat). Prolog answer: no ? - parent(tom, pat). Prolog answer: no 6 tom bob liz ann pat jim

1. 1 Defining relations by facts Who is liz’s parent? ? - parent(X, liz). Prolog answer: X = tom pam Who are bob’s children? ? - parent(bob, X). tom bob liz ann pat Prolog answer: X = ann We can request anther solution by typing ; X = pat If we request more solution, Prolog will answer no 7 jim

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; bob tom liz X = tom Y = bob; ann pat X = tom Y = liz; … we can stop the stream of solutions by typing a return 8 jim

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 pam tom bob liz ann pat ? - parent(Y, jim), parent (X, Y) Prolog answer: X = bob Y = pat * Changing the order of the requirements will NOT affect the result. 9 jim

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? pam tom bob liz ? - parent (X, ann), parent(X, pat) Prolog answer: X = bob 10 ann pat jim

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). pam tom bob liz ann pat jim sex(pam, female). sex(tom, male). sex(bob, male). male and female are unary relations, whereas parent is a binary relation. 11

1. 2 Defining relations by rules Let us introduce the offspring relation (inverse of parent) Fact offspring(liz, tom). offspring(bob, tom). . caluses pam tom bob liz The logical statement is: For all X and Y, Y is an offspring of X is a parent of Y Offspring(Y, X) : - parent(X, Y). Rule clause 12 ann pat jim

1. 2 Defining relations by rules There is an important difference between facts and rules: � Facts is something that is always, unconditionally, true parent(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 13 body

1. 2 Defining relations by rules Questions � Is liz an offspring of tom? ? - offspring(liz, tom). How Prolog answer this question using the rule: Offspring(Y, X) : - parent(X, Y). Prolog answer: yes 14

1. 2 Defining relations by rules � How to express: � Mother relation � Grandparent relation � Sister relation in Prolog? 15

Important Points 16