ProLog (Artificial Intelligence) Introduction

PROLOG PROgramming in LOGic A Brief Introduction

Compilers • PDC Visual Prolog • SWI Prolog • Bin-Prolog • K-Prolog • Prolog ++ http://swish.swi-prolog.org

PRO-LOG • Programming in Logic • 1975, Phillippe Roussell • Predicate Calculus • Declarative Language • Predicates and Rules • Inference Mechanism

PROLOG Paradigm • PROgramming in LOGic – Draw inferences from facts and rules • PROLOG is – declarative - specify facts and logical relationships • Non-procedural: "what", not "how". (we specify the problem but not the solution) • Database language with automated search and the ability to follow general rules. – symbolic - symbols are used to represent objects – high level - contains a built-in problem solving mechanism • PROLOG Programming (When we program in ProLog we need to provide following three things) – Declaring some facts about objects and their relationships – Defining some rules about objects and their relationships – Asking questions about objects and their relationships

PROLOG Paradigm • The PROLOG Programmer – Loads facts and rules into the database. – Makes queries to the database to see if a fact is: • in the database or • can be implied from the facts and rules therein Prolog Database Facts + Rules Query

System Interaction Problem solving in PROLOG – 1. insert facts and rules into the database – 2. ask questions (queries) based on the contents of the database • Facts – Used to represent unchanging information about objects and their relationships. – Only facts in the PROLOG database can be used for problem solving. – Insert facts into the database by, • typing the facts into a file and loading (consulting) the file into a running PROLOG system

System Interaction • Queries – Retrieve information from the database by entering QUERIES – A query, • is a pattern that PROLOG is asked to match against the database • has the syntax of a compound query • may contain variables – A query will cause PROLOG to • look at the database • try to find a match for the query pattern • execute the body of the matching head • return an answer

• Logic Programming • Logic programming is a form of declarative programming • A program is a collection of axioms – Each axiom is a Horn clause of the form: H :- B1, B2, ..., Bn. where H is the head term and Bi are the body terms – Meaning H is true if all Bi are true

Basic Proof technique - modus ponens A -> B A --------- B

• Prolog Uses backward chaining – More efficient than forward chaining for larger collections of axioms • Applications: expert systems, artificial intelligence, natural language understanding, logical puzzles and games

PROLOG Paradigm Examples (Facts) English PROLOG “A dog is a mammal” isa(dog, mammal). “A sparrow is a bird” isa(sparrow, bird).

PROLOG Paradigm Examples (Rules) English PROLOG “Something is an animal animal(X) :- isa(X,mammal). if it is a mammal or a bird” animal(X) :- isa(X, bird).

PROLOG Paradigm Examples (Queries) English PROLOG “is a sparrow an animal?” ?- animal(sparrow). answer: “yes” yes “is a table an animal?” ?- animal(table). answer: “no” no “what is a dog?” ?- isa(dog, X). answer: “a mammal” X = mammal

PROLOG syntax • Constants – Atoms • Alphanumeric atoms - alphabetic character sequence starting with a lower case letter Examples: apple a1 apple_cart • Quoted atoms “String” - sequence of characters surrounded by single quotes Examples: ‘Apple’ ‘hello world’ • Symbolic atoms - sequence of symbolic characters Examples: & < > * - + >> • Special atoms Examples: ! ; [ ] {} – Numbers • Integers and Floating Point numbers Examples: 0 1 9821 -10 1.3 -1.3E102

PROLOG syntax • Variable Names a sequence of alphanumeric characters beginning with an upper case letter or an underscore Examples: Anything _var X _ • Compound Terms (structures) – an atom followed by an argument list containing terms. The arguments are enclosed within brackets and separated by commas Example: isa(dog, mammal)

Prolog syntax • The names of all relationships and objects must begin with a lower case letter. For example studies, ali, programming • The relationship is written first and the objects are enclosed within parentheses and are written separated by commas. For example studies(ali, programming) • The full stop character ‘.’ must come at the end of a fact. • Order is arbitrary but it should be consistent

Example  Program with three facts and one rule: rainy(columbo). rainy(ayubia). cold(ayubia). Rule: snowy(X) :- rainy(X), cold(X).  Query and response: ?- snowy(ayubia). yes  Query and response: ?- snowy(columbo). no  Query and response: ?- snowy(lahore). No • ?- snowy(C). C = ayubia – because rainy(ayubia) and cold(ayubia) are sub-goals that are both true facts in the database – snowy(X) with X=columbo is a goal that fails, because cold(X) fails, triggering backtracking

snowy(C) snowy(X) AND rainy(X) cold(X) C = X rainy(columbo) rainy(ayubia) X = columbo X = ayubia cold(ayubia) C = X = ayubia C = ayubia rainy(columbo). rainy(ayubia). cold(ayubia). snowy(X) :- rainy(X), cold(X). ?- snowy(C). C = ayubia OR

Logic Representation : Propositional Calculus • Propositional Logic – Boolean statements – Logic connectives      Example – a family Atomic statements p: Ahmed is father of Belal q: Ahmed is brother of Aslam r : Aslam is uncle of Belal Complex statements  Statement Logic p Ahmed is not father of Belal p  q Ahmed is father of Belal or Ahmed is brother of Aslam p  q  r If Ahmed is father of Belal and brother of Aslam then Aslam is uncle of Belal

Prolog Programming Predicate Calculus • Predicates (Facts) – man(Ahmed) – father(Ahmed, Belal) – brother(Belal, Chand) – brother(Chand, Delawar) – owns(Belal, car) – tall(Belal) – hates(Ahmed, Chand) – family() • Formulae – X,Y,Z(man(X)  man(Y)  father(Z,Y)  father(Z,X)  brother(X,Y)) • Variables – X, Y and Z • Constants – Ahmed, Belal, Chand, Delawar and car Prolog code Predicates man(symbol) father(symbol, symbol) brother(symbol, symbol) owns(symbol, symbol) tall(symbol) hates(symbol, symbol) family() Clauses man(ahmed). father(ahmed, belal). brother(ahmed, chand). owns(belal, car). tall(belal). hates(ahmed, chand). family(). Rule: brother(X,Y):- man(X), man(Y), father(Z,Y), father(Z,X). Goal brother(ahmed,belal).

Sections of Prolog Program (1-3) • Predicates – Declarations of Relations or Rules – Just like function prototype (declaration). – Zero or more arguments – Example Predicates man(symbol) family() a_new_predicate (integer, char)

Sections of Prolog Program (2-3) • Clauses – Definition(s) of Predicate = Sentence OR Phrase – Types • Rules (function definitions) – 0 or more conditions (statements) • Facts – 0 conditions – All parameters constant – Examples • Fact brother(ahmed, chand). • Rule brother(X,Y):- man(X), man(Y), father(Z,Y), father(Z,X).

Sections of Prolog Program (3-3) • Goal – Goal of inference – Only and exactly one instance – The only tentative section of the program – The main() of prolog – Goal truth value = output of program – Syntactically and Semantically just another clause • Empty, simple (one goal), or compound (sub-goals) – Examples • Goal brother(X,chand). <or> brother(ahmed,chand). • Goal brother(X,chand), father().

Sample Program Update domains person = symbol predicates nondeterm father(person,person) nondeterm brother(person,person) nondeterm cousin(person,person) nondeterm grandfather(person,person) clauses father(a,b). father(a,c). father(a,d). father(b,e). father(b,f). father(b,g). father(c,h). father(c,i). father(d,j). father(d,k). brother(X,Y):- X<>Y, father(Z,X), father(Z,Y). cousin(X,Y):- father(A,X), father(B,Y), brother(A,B). grandfather(X,Y):- father(Z,Y), father(X,Z). goal cousin(X,Y).

Prolog Variables • Constant placeholders (NOT variables) – Bounded once • Loosely typed • Start with Capital letter or underscore • Examples – brother(ahmed, Ahmed) – brother(ahmed, _x) – Brother(ahmed, X) • Anonymous variable – The _ – Some value that isn’t required – Example brother(ahmed, _)

Expert system • A simple medical expert system • relieves(Drug, Symptom). – relieves(aspirin, headache). – relieves(aspirin, moderate_pain). – relieves(aspirin, moderate_arthritis). – relieves(aspirin_codine_combination, severe_pain). – relieves(cough_cure, cough). – relieves(pain_gone, severe_pain). – relieves(anti_diarrhea, diarrhea). – relieves(de_congest, cough). – relieves(de_congest, nasal_congestion). – relieves(penicilline, pneumonia). – relieves(bis_cure, diarrhea). – relieves(bis_cure, nausea). – relieves(new_med, headache). – relieves(new_med, moderate_pain). – relieves(cong_plus, nasal_congestion).

• aggravates(Drug, Condition). – aggravates(aspirin, asthma). – aggravates(aspirin, peptic_ulcer). – aggravates(anti-diarrhea, fever). – aggravates(de_congest, high_blood_pressure). – aggravates(de_congest, heart_disease). – aggravates(de_congest, diabetes). – aggravates(de_congest, glaucoma). – aggravates(penicilline, asthma). – aggravates(de_congest, high_blood_pressure). – aggravates(bis_cure, diabetes). – aggravates(bis_cure, fever).

should_take(Person, Drug) :- complains_of(Person, Symptom), relieves(Drug, Symptom), not(unsuitable_for(Person, Drug)). //conjunction of three classes. unsuitable_for(Person, Drug) :- aggravates(Drug, Condition), suffers_from(Person, Condition). complains_of(ali, headache). suffers_from(ali, peptic_ulcer). ?- should_take(ali, Drug). Drug = new_med;

Books • Prolog Programming for AI by Ivan Bratko • Visual Prolog; Language Tutorial by PDC

ProLog (Artificial Intelligence) Introduction

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