PROLOG KAIST Gunwoo Park 1

What’s Prolog? 2  General purpose logic programming language  Declarative, which means that the program logic is written in terms of relations (A → B)  The concept is developed by a group around Alan Colmerauer in Marseille, France, in the early 1970s  First implementation is launched at 1972  Initially aimed at natural language processing, but now its usage is stretched far into other areas  Even you can make Prolog program in GUI by using modern Prolog environments What than How

Schematic View of Prolog Programming 3  A program is written by many clauses (.pl )  A clause is a fact or rule  Fact Example: cindy is beautiful  Rule Example: If someone is beautiful, I love him/her  Then, how to use the program? It’s done by sending query to the virtual machine  Query Example: Do I love cindy?  Then the inference engine(virtual machine) analyzes the clauses to make an answer to the query  Program Answers: Yes

Schematic View of Prolog Programming (cont’d) 4 Program (Clauses) Inference Engine (Prolog Environment) Program User 1. User asks query “Do I love cindy?” 2. Inference engine gets query, and sees clauses. 3. Inference engine checks that cindy is beautiful, and I love any beautiful person. 4. So, inference engine concludes I love cindy, and answer “Yes”

Environment of Prolog 5  SWI-Prolog is a popular development environment for Prolog (  There are many versions of SWI to support various operating systems like Windows, Mac, and Linux

Data Type 6  Before seeing real examples, data type should be understood  Prolog’s single data type is the term. Terms are either atoms, numbers, variables or compound terms.  The first letter of data name is very important in Prolog because it determines the type  Type of cindy => Atom  Type of Cindy => Variable

Data Types (cont’d) 7  The types of the term  Atom : General-purpose name with no inherent meaning Starts with lower-case: x, blue, gunwoo With single quote: ‘Taco’, ‘some atom’  Number : Floats or integers (13, 14.23)  Variable : String consisting of letters and special characters, which is used as placeholder of arbitrary terms (X, Y, Z, Someone)  Compound terms: Composed of an atom called functor and a number of arguments, which are again terms trunk_year(‘Mazda’, 1986) Person_Friend(zelda, [tom, jim])

Rules and Facts  Program describes relations, defined by means of clauses.  A set of clauses is contained in.pl file  Two types of clauses – Facts and Rules  A rule is of the form of Head :- Body, which means that Head is true if Body is true  Clauses with empty bodies are called facts. An example of fact is cat(tom), means that tom is a cat. 8

Rules and Facts - Example  As mentioned, clauses can be either facts or rules. female(jessica). female(cindy). child(jessica, cindy). daughter(X, Y) :- female(X), child(X, Y). Semantics: jessica and cindy are female. jessica is cindy’s daughter. But there is a rule. If X is female and child of Y, then X is daughter of Y. Then let’s ask question to the set of clauses !!! Facts Rule The comma is AND operation The dot is end of a clause 9

Query  Let’s ask a query (question) to the virtual machine, then the machine give answer by backtracking declared facts and rules. ?- daughter(jessica, cindy). yes ?- professor(jessica, mack). no 10

Boolean Operation 11  Negation  failure(X) :- \+ success(X).  AND (Conjunction)  mom(X) :- female(X), parent(X)  OR (Disjunction)  good(X) :- money(X); power(X)

Knowledge Base Example 1 12 [kb1.pl] woman(mia). woman(jody). woman(yolanda). playsAirGuitar(jody). [Usage Scenario] ?- woman(mia). ?- playsAirGuitar(mia). ?- playsAirGuitar(yolanda). ?- tatooed(jody).

Knowledge Base Example 2 13 [kb2.pl] listenToMusic(mia). happy(yolanda). playsAirGuitar(mia) :- listensToMusic(mia). playsAirGuitar(yolanda) :- listensToMusic(yolanda). listensToMusic(yolanda) :- happy(mia). [Usage Scenario] ?- happy(yolanda). ?- playsAirGuitar(mia). ?- playsAirGuitar(yolanda). ?- listensToMusic(yolanda).

Knowledge Base Example 3 14 [kb3.pl] happy(vincent). listensToMusic(butch). playsAirGuitar(vincent) :- listensToMusic(vincent), happy(vincent). playsAirGuitar(butch) :- happy(butch). playsAirGuitar(butch) :- listensToMusic(butch). [Usage Scenario] ?- playsAirGuitar(vincent). ?- playsAirGuitar(butch).

Using Variables in Query  We can use variable in a query  After executing a query, value is bound to the variable as a result ?- daughter(X, cindy).=> X : jessica ?- female(X).=> X : jessica, cindy ?- daughter(jessica, _).=> yes ※ ’_’ means “there is anything which satisfies the question”. (Existential Quantification) 15

Knowledge Base Example 4 16 [kb4.pl] woman(mia). woman(jody). woman(yolanda). loves(vincent, mia). loves(marcellus, mia). loves(pumpkin, honey_bunny). loves(honey_bunny, pumpkin). [Usage Scenario] ?- woman(X). ?- loves(marcellus, X), woman(X).

Knowledge Base Example 5 17 [kb5.pl] loves(vincent, mia). loves(marcellus, mia). loves(pumpkin, honey_bunny). loves(honey_bunny, pumpkin). jealous(X, Y) :- loves(X, Z), loves(Y, Z). [Usage Scenario] ?- jealous(marcellus, W).

Other features  Built-in predicates  I/O, like printing a character in the screen, is almost impossible to be defined by user-defined clauses.  So, Prolog provides built-in predicates for systematic operation.  By using the built-in predicates we can do Using graphics Communication with operating system  Loop and recursion  Prolog provides the means of recursive predicates  Loop can be made by those recursive predicates 18

Example Program 1 19 [ example.pl ] son(jack, george). father(B, A) :- son(A, B). After loading example.pl to SWI, execute a simple query…

Example Program 2 Print “Hello World” in the screen using built-in predicate 20

Demonstration 21 readLine.pl

Applications of Prolog  Natural language processing (Relation)  Intelligent systems (Inference)  Complicated knowledge databases (Knowledge)  Logic data analysis (Inference) 22

Extensions of Prolog  Constraint logic programming  HiLog and λ Prolog  F-logic  OW Prolog  Logtalk  Prolog-MPI  Oblog  Although logic programming is lack of some aspects like efficiency or flexibility, its programming paradigm is used considerably  Prolog can be considered as the father of logic programming 23

Thank you 24