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
?- snowy(columbo).
no
?- 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)

• 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).

• 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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