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Logic Programming

Logic Programming. Tarik Booker. What will we cover?. Introduction Definitions Predicate Calculus Prolog Applications. What is Logic Programming?. Simply programming that uses a form of symbolic logic (predicate calculus) as a programming language

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Logic Programming

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  1. Logic Programming Tarik Booker

  2. What will we cover? • Introduction • Definitions • Predicate Calculus • Prolog • Applications

  3. What is Logic Programming? • Simply programming that uses a form of symbolic logic (predicate calculus) as a programming language • Languages based on this type of programming are called Logic Programming Languages • (Also known as declarative languages)

  4. Definitions (1) Predicate Calculus: • Proposition – a logical statement that may or may not be true • Atomic – consist of compound terms • Compound Terms – element of a mathematical relation • Functor – function symbol that names the relation • Ordered list – parameters man(bob) like(bob, steak)

  5. Definitions (2) Clausal Form: B1 B2  B3  A1  A2  A3 • Antecedent – right side of a clausal form • Consequent – left side of a clausal form • Declarative Semantics – there is a simple way to determine the meaning of each statement

  6. Prolog • Logic Programming Language • Definitions: • Term – a constant, variable or structure • Constant – only an atom (synbolic value) or an integer • Variable – any string of letters, digits, or underscores that begins with an uppercase letter • Structure – atomic propositions of predicate calculus • Ex: mystruct(parameter list) • Instantiation – a binding of a value to a variable

  7. Sample Prolog (Fact Statements) Fact Statements: • male(bill). • male(jack). • father(bill, jack). Which is Correct? • Bill is Jack’s father? • Jack is Bill’s father?

  8. (Answer) • Either way! • There are no intrinsic semantics in Prolog (just like Predicate Calculus) • Can be interpreted by the programmer in any way he/she likes.

  9. Sample Prolog (Rule Statements) • Consequence_1 :- Antecedent_expression • Examples: • female(shelley), child(shelley). • ancestor(mary, shelley) :- mother(mary, shelley). (If Mary is the mother of Shelley, then Mary is the ancestor of Shelley)

  10. Goal Statements • Queries are known as goals in Prolog. • Example: father(jason, freddy). father(X, freddy).

  11. Inferencing • You want a goal. • When the goal is a compound proposition, each of the facts (structures) is called a subgoal • To prove a goal is true, the inferencing process muse connect the goal to one or more facts in the database • Proving a subgoal is known as satisfying the subgoal

  12. Inferencing Example • Database contains: • father(bob). • man(X) :- father(X). • Your goal (query): • man(bob). • Forward Chaining – bottom-up resolution (start with facts, find goal) • Backward Chaining – top-down resolution(start with goal, find facts)

  13. Inferencing (2) • Forward Chaining: better when the number of possible correct answers is large • Backward Chaining: better when there is a reasonable small set of candidate answers • When goal has more than one structure, we must search: • Depth-first – finds a proof for the first subgoal beforw working on the others • Breadth-first – works on all subgoals in parallel • Prolog uses depth-first, because it utilizes fewer resources • Backing up in a goal to a previously proven subgoal is known as backtracking

  14. Applications of Logic Programming • Relational Database Management Systems • Expert Systems – emulate human expertise in some particular domain • Natural Language Processing

  15. Resources Used Sebesta, Robert W. Concepts of Programming Languages (4th Edition)

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