ACSC300: Logic Programming

Transcription

1 ACSC300: Logic Programming Lecture 1: Introduction to Logic Programming and Prolog Harris Papadopoulos Course Aims This course aims to help you to: Understand a radically different programming paradigm: Declarative programming See how logic programming contributes to the quest for Artificial Intelligence Learn a new programming language called Prolog Appreciate the strengths and weaknesses of Prolog Useful Books Course Details Textbook: Ivan Bratko, Prolog-Programming for Artificial Intelligence, 3rd Edition, Addison Wesley, Optional books for further reading: Leon Sterling and Ehud Shapiro, The Art of Prolog, 2nd Edition, MIT Press, W. F. Clocksin, C. S. Mellish, Programming in Prolog: Using the Iso Standard, 5th edition, Springer-Verlag, Books: You are expected to have access to the textbook. The optional books on slide 3 have contributed material to the course, and are appropriate for further reading. Lectures: You will be able to find copies of all slides on the course homepage: However, further examinable material will be presented in lectures, and you will be expected to take notes. Course Details Workload 36 hours of classes 70 hours coursework and private study Coursework (40% of final mark) Test 50% Assignments 40% Practical participation 10% How to Succeed in this Course Attend all classes and make sure you understand everything covered in each class before coming to the next. If you need help come to see me during my office hours. In case you miss a class, make sure you cover the material of that class at home from the notes and the textbook. 1

2 How to Succeed in this Course Do all homework and assignments these are designed to help you understand and use the material taught. Read the relevant sections of the textbook Practice with Prolog at home Logic Programming Problem Solving = Problem Description + Logical Deductions How do we build in the ability to do the deductions? Ideally we would like to be able to tell the computer what we want it to do and not how we want it to do it. Imperative vs Declarative Programming In imperative programming (e.g. Java, Pascal, C++), you tell the computer HOW to solve the problem, i.e. do this, then do that. In declarative programming (e.g. Prolog), you declare WHAT the problem is, and leave it to the computer to use built-in reasoning to solve the problem. Introducing Prolog Prolog is the first attempt to design a purely declarative programming language. It is short for Programmation en Logique. It was developed in the 1970 s by Robert Kowalski and Maarten Van Emden (Edinburgh) Alain Colmerauer (Marseille) It was efficiently implemented by David Warren. Introducing Prolog When you write a Prolog program, you are writing down your knowledge of the problem you are modelling the problem. Prolog is widely used for specifying and modelling systems (e.g. software prototyping, design of circuits), and for many artificial intelligence applications such as expert systems and natural language processing. Prolog Concepts For a powerful, all-purpose programming language prolog has remarkably few concepts. A Prolog program consists of a set of clauses Each clause is either a fact or a rule 2

3 Facts: Rules: Examples of Clauses female(anne). parent(philip,anne). father(x,y) :- parent(x,y), male(x). ancestor(x,y) :- parent(x,y). ancestor(x,y) :- ancestor(x,z), parent(z,y). Terms A term represents some object or class of objects we want to talk about. It may be: A constant, which can be A number 37, 12.4, -5 A string hello world, Pete An atom philip, monkey Terms A variable X, Person, _3 (variable names begin with an upper-case letter or underscore) A compound term date (may, 1, 2004) In general a compound term is an expression of the form ƒ(t 1, T 2,, T n ), where ƒ is called a functor and T 1, T 2,, T n are terms. Facts A fact represents a unit of information that is assumed to be true. its_raining. Often, a fact asserts some property of a term, or a list of terms. parent(philip,anne). Facts In general a fact is an expression of the form p(t 1, T 2,, T n ), where p is called a predicate symbol and T 1, T 2,, T n are terms. (Predicate symbols start with a lower-case letter) Rules A rule represents a conditional assertion ( this is true if this is true ). need_umbrella :- its_raining. father(x,y) :- parent(x,y), male(x). IF AND 3

4 Rules In general a rule is an expression of the form A :- B 1, B 2,, B n, where A and B 1, B 2,, B n are atomic formulas. Rules An atomic formula is an expression of the form p(t 1, T 2,, T n ), where p is a predicate symbol and T 1, T 2,, T n are terms. A is called the head of the rule. B 1, B 2,, B n is called the body of the rule. Example cat(349, Dickens, Oliver Twist ). cat(487, Bronte, Jane Eyre ). cat(187, Boole, Laws of Thought ).. fiction(author, Title) :- cat(num, Author, Title), Num > 200, Num < 600. Variables Always begin with upper case letters or underscores (e.g. X, TimeTable, _24). Anything that begins with an upper case letter in Prolog is a variable (unless in quotes). A variable such as X in one clause is completely independent of variables with the same name in another clause each clause stands alone. Can take any value There are no types in Prolog Note Remember: A clause is a Prolog statement (i.e. a fact or a rule). All clauses in Prolog must end with a dot. Many Prolog systems require all the clauses for a particular predicate to appear together. The group of clauses for a predicate pred is called the procedure of pred. Queries Once we have a Prolog program we can use it to answer a query.?- parent(philip, anne).?- border(wales, scotland). The Prolog interpreter responds yes or no. Note that all queries must end with a dot. 4

5 Queries A query may contain variables:?- parent(philip, Who). The Prolog interpreter will respond with the values for the variables which make the query true (if any).?- parent(philip, Who). Who = anne; Who = charles; Who = andrew; no Example Otherwise it will respond with a no. Queries In general a query is an expression of the form?- A 1, A 2,, A n, where A 1, A 2,, A n are atomic formulas, known as goals. Queries The Prolog system will try to find values for the variables which make all the goals true. The same variable may occur in more than one goal. After each answer, typing a semi-colon (;) asks for any further answers. Example If we have the Prolog program: male(charles). male(edward). parent(philip, anne). parent(philip, edward). parent(philip, charles). Example (cont.) And we enter the query:?- parent(x, charles), parent(x,y), male(y). Then we get the response X = philip, Y = edward; What happens next? 5

6 Side-effects Some queries may cause the system to carry out certain actions.?- halt. this causes the Prolog system to exit.?- consult( myfile ). this causes the Prolog system to read the contents of myfile and add the clauses to the current program. Side-effects?- consult(user). this causes the Prolog system to read the user input as a Prolog program until terminated by a CTRL-D.?- listing(predname). this causes the Prolog system to output a listing of the clauses defining the predicate predname in the current program. Exercise Suppose we have the following Prolog program: male(charles). : parent(philip, anne). parent(elizabeth, edward). : wife(elizabeth, philip). wife(mary, george). Write queries to find: Exercise (cont.) 1. The parents of charles. 2. The father of charles. 3. The grandparents of charles. 4. All the grandchildren of the grandparents of charles. 6