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Dr. Muhammed Al-Mulhem ICS535-101 1 An Introduction to Logical Programming Slide 2 Dr. Muhammed Al-Mulhem ICS535-101 2 Predicate Calculus Logic programming is based on formal logic. Formal logic was developed to provide a method for describing propositions. Propositions are logical statements that may or may not be true. Symbolic logic can be used for the three basic needs of formal logic: To express propositions To express the relationship between propositions To describe how new propositions can be inferred from other propositions that are assumed to be true. Slide 3 Dr. Muhammed Al-Mulhem ICS535-101 3 Predicate Calculus One form of symbolic logic that is used for logic programming is called first-order predicate calculus (predicate calculus for short). A brief look at predicate calculus follows: Propositions Are logical statements that may or may not be true. There are two types of propositions; 1.Atomic Propositions: consists of two parts: a functor or relation and an ordered list of parameters. Examples: man (ahmed)1-tuple predicate likes (ahmed, steak)2-tuple predicate Slide 4 Dr. Muhammed Al-Mulhem ICS535-101 4 Predicate Calculus 2. Compound Proposition: have two or more atomic propositions, which are connected by Logical Connectors. Logical Connectors are operators that connects simple predicates to produce complex ones. A list of these connectors is: Slide 5 Dr. Muhammed Al-Mulhem ICS535-101 5 Examples The following are examples of Compound Propositions: The precedence of the operators is: The second example is equivalent to: Slide 6 Dr. Muhammed Al-Mulhem ICS535-101 6 Predicate Calculus Quantifiers We can introduce variables into propositions using Quantifiers. There are two quantifiers: 1. Universal quantifier (reads for all), and 2. Existential quantifier (reads there exists). For example: X.(man(X) human (X)) X.(mother (mary, X) male(X)) Slide 7 Dr. Muhammed Al-Mulhem ICS535-101 7 Clausal Form In predicate calculus we can represent the same proposition with more than one way. This will represent a problem if we want to use this technique with computers. Therefore, we must use a standard form to represent proposition called the clausal form. All propositions can be expressed in clausal form. Clausal Form Slide 8 Dr. Muhammed Al-Mulhem ICS535-101 8 A proposition in Clausal Form has the following general syntax: B 1 B 2 . B n A 1 A 2 . A m The meaning of this proposition is as follows: If all the A s are true, then at least one B is true. The characteristics of the clausal form proposition are: 1.Existential quantifiers are not required. 2.Universal quantifiers are implicit 3.No operators other than conjunction and disjunction are required 4.Disjunctions appear on the left side and conjunctions on the right side. 5.All predicate calculus propositions can be converted to clausal form. Clausal Form Slide 9 Dr. Muhammed Al-Mulhem ICS535-101 9 Clausal Form The right side of clausal form is called the antecedent or the head and the left side is the consequence or the tail. Examples of Clausal forms are: likes(ahmed, hamour) likes(ahmed, fish) fish(hamour) father( louis, al ) father( louis, violet ) father( al, bob ) mother( violet, bob ) grandfather( louis, bob ) Slide 10 Dr. Muhammed Al-Mulhem ICS535-101 10 Resolution is an inference rule that is used to infer propositions from given propositions. The concept of resolution is the following: From (P Q) and (R P) infer (R Q) Example From older(ahmed, ali) father(ahmed, ali) and wiser(ahmed, ali) older(ahmed, ali) Then infer wiser(ahmed, ali) father(ahmed, ali) PC and Theorem Proving Slide 11 Dr. Muhammed Al-Mulhem ICS535-101 11 Resolution The mechanics of this resolution construction are as follows: The left sides of the two propositions are ANDed together to make the left side of the new proposition. Same thing is done to get the right side. Any term that appears on both sides of the new propositions is removed from both sides. Slide 12 Dr. Muhammed Al-Mulhem ICS535-101 12 Unification The presence of variables in propositions requires resolution to find values for those variables that allow the matching process to succeed. This process of determining useful values for variables is called unification. The temporary assigning of values to variables to allow unification is called instantiation Example What if we have wiser(X,Y) older(X,Y) Then we need to do unification (instantiation) {X/ahmed and Y/ali } This will be discussed in the context of Prolog. Slide 13 Dr. Muhammed Al-Mulhem ICS535-101 13 When prepositions are used for resolution, only a restricted kind of clausal form called Horn clauses can be used. A Horn clause is a clause that has one of two forms: A single atomic proposition on the left side, or An empty left side. This mean that a Horn clause will contain one predicate head, in this case we call it headed Horn clause, or it has an empty head and we call it headless Horn clause. Horn Clauses Horn Clauses Slide 14 Dr. Muhammed Al-Mulhem ICS535-101 14 Horn Clauses Headed Horn clauses are used to state relationships, such as: likes(ahmed, hamour) likes(ahmed, fish) fish(hamour) Headedless Horn clauses are used to state facts, such as father(muhammed, khaled) father(muhammed, khaled) Most but not all, propositions can be stated in Horn clauses.