# PLC Session 13

This marks the last session that we had in semester one. This time our friend’s group will brought the presentation for us. This session talked about Logic Programming Languages As usual This session is divided into some topics.

• Introduction
• A Brief Introduction to Predicate Calculus
• An Overview of Logic Programming
• The Origins of Prolog
• The Basic Elements of Prolog
• Deficiencies of Prolog
• Applications of Logic Programming

First, we will talk about Introduction,Programming that uses a form of symbolic logic as a programming language is often called logic programming, and languages based on symbolic logic are called logic programming languages, or declarative languages. The syntax of logic programming languages is remarkably different from that of the imperative and functional languages. Next topic talks about A Brief Introduction to Predicate Calculus. A proposition can be thought of as a logical statement that may or may not be true. It consists of objects and the relationships among objects. Formal logic was developed to provide a method for describing propositions, with the goal of allowing those formally stated propositions to be checked for validity. Symbolic logic can be used for the three basic needs of formal logic: to express propositions, to express the relationships between propositions, and to describe how new propositions can be inferred from other propositions that are assumed to be true. The particular form of symbolic logic that is used for logic programming is called first-order predicate calculus.The simplest propositions, which are called atomic propositions, consist of compound terms. A compound term is one element of a mathematical relation, written in a form that has the appearance of mathematical function notation. A compound term is composed of two parts: a functor, which is the function symbol that names the relation, and an ordered list of parameters, which together represent an element of the relation. A compound term with a single parameter is a 1-tuple; one with two parameters is a 2-tuple, and so forth. For example, we might have the two propositions
man(jake)
like(bob, steak)
which state that {jake} is a 1-tuple in the relation named man, and that {bob, steak} is a 2-tuple in the relation named like. Propositions can be stated in two modes: one in which the proposition is defined to be true, and one in which the truth of the proposition is something that is to be determined. In other words, propositions can be stated to be facts or queries. The example propositions could be either. One problem with predicate calculus as we have described it thus far is that there are too many different ways of stating propositions that have the same meaning; that is, there is a great deal of redundancy. Next we are going to discuss about An Overview of Logic Programming. In this topic we learned that Languages used for logic programming are called declarative languages, because programs written in them consist of declarations rather than assignments and control flow statements. These declarations are actually statements, or propositions, in symbolic logic. One of the essential characteristics of logic programming languages is their semantics, which is called declarative semantics. We also learned that declarative semantics is far simpler than the semantics of imperative languages. Programming in a logic programming language is nonprocedural An example commonly used to illustrate the difference between procedural and nonprocedural systems is sorting. In a language like Java, sorting is done by explaining in a Java program all of the details of some sorting algorithm to a computer that has a Java compiler. The computer, after translating the Java program into machine code or some interpretive intermediate code, follows the instructions and produces the sorted list. In a nonprocedural language, it is necessary only to describe the characteristics of the sorted list: It is some permutation of the given list such that for each pair of adjacent elements, a given relationship holds between the two elements. To state this formally, suppose the list to be sorted is in an array named list that has a subscript range 1 . . . n. Let’s move on to the next topic about The Origins of Prolog. The development of Prolog and other research efforts in logic programming received limited attention outside of Edinburgh and Marseille until the announcement in 1981 that the Japanese government was launching a large research project called the Fifth Generation Computing Systems.  One of the primary objectives of the project was to develop intelligent machines, and Prolog was chosen as the basis for this effort. The announcement of FGCS aroused in researchers and the governments of the United States and several European countries a sudden strong interest in artificial intelligence and logic programming. Next topic is about The Basic Elements of Prolog. Variables are not bound to types by declarations. The binding of a value, and thus a type, to a variable is called an instantiation. Instantiation occurs only in the resolution process. A variable that has not been assigned a value is called uninstantiated The last kind of term is called a structure. Structures represent the atomic propositions of predicate calculus, and their general form is the same: functor(parameter list) .

The functor is any atom and is used to identify the structure.The parameter list can be any list of atoms, variables, or other structures. The other basic form of Prolog statement for constructing the database corresponds to a headed Horn clause. This form can be related to a known theorem in mathematics from which a conclusion can be drawn if the set of given conditions is satisfied. Next we are going to talk about deficiencies of prolog. Prolog, for reasons of efficiency, allows the user to control the ordering of pattern matching during resolution. In a pure logic programming environment, the order of attempted matches that take place during resolution is nondeterministic, and all matches could be attempted concurrently. However, because Prolog always matches in the same order, starting at the beginning of the database and at the left end of a given goal, the user can profoundly affect efficiency by ordering the database statements to optimize a particular application. The last topic is about applications of Logic Programming .Relational database management systems (RDBMSs) store data in the form of tables. Queries on such databases are often stated in Structured Query Language (SQL). SQL is nonprocedural in the same sense that logic programming is nonprocedural. The user does not describe how to retrieve the answer; rather, he or she describes only the characteristics of the answer. One of the advantages of using logic programming to implement an RDBMS is that only a single language is required. Another advantage of using logic programming to implement an RDBMS is that deductive capability is built in. We also learned about Expert system, Expert systems are computer systems designed to emulate human expertise in some particular domain. They consist of a database of facts, an inferencing process, some heuristics about the domain, and some friendly human interface that makes the system appear much like an expert human consultant. Prolog can and has been used to construct expert systems. It can easily fulfill the basic needs of expert systems, using resolution as the basis for query processing, using its ability to add facts and rules to provide the learning capability, and using its trace facility to inform the user of the “reasoning” behind a given result.

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