Rule Based Systems (Prolog)

1
Rule Based Systems(Prolog)

• M. Reza Zakerinasab
• mrzakeri_at_ucalgary.ca
• Please include CPSC433 in the subject line of
  any emails regarding this course.
• Slides originally created by Andrew M Kuipers.
• Some slides adopted from 600.325/425 Declarative
  Methods - J. Eisner

2
Prolog as constraint programming
Person Food
sam dal
sam curry
josie samosas
josie curry
rajiv dal
(Person, Food)

• An ordinary constraint between two variables
  Person and Food
• Prolog makes you name this constraint. Heres a
  program that defines it
• eats(sam, dal). eats(josie, samosas).
• eats(sam, curry). eats(josie, curry).
• eats(rajiv, dal).
• Now it acts like a subroutine! At the Prolog
  prompt you can type
• eats(Person1, Food1). constraint over two
  variables
• eats(Person2, Food2). constraint over two
  other variables

3
Using Prolog

• eats(sam, dal). eats(josie, samosas).
• eats(sam, curry). eats(josie, curry).
• eats(rajiv, burgers). eats(rajiv, dal).
• eats(Person1, Food), eats(Person2, Food).
• Person1sam, Person2josie, Foodcurry
• Person1josie, Person2sam, Foodcurry

Your program file (compiled) Sometimes called the
database
Query that you type interactively
Prologs answer
4
Adding more rules

• eats(sam, dal). eats(josie, samosas).
• eats(sam, curry). eats(josie, curry).
• eats(rajiv, burgers). eats(rajiv, dal).
• compatible(Person1, Person2) - eats(Person1,
  Food), eats(Person2, Food).
• compatible(Person1, Person2) - watches(Person1,
  Movie), watches(Person2, Movie).
• compatible(hal, Person2) - female(Person2),
  rich(Person2).

5
Basic Prolog Process

• Negate query, set as current goal.
• while current goal is non-empty
• choose the leftmost subgoal
• if a rule applies to the subgoal
• select the first applicable rule
  (top-to-bottom)
• perform resolution on subgoal and selected rule
• else
• backtrack if possible, otherwise fail
• success

6
Sir Bedeveres Infamous Deduction
witch(X) - burns(X),woman(X). woman(girl). burns(
X) - madeofwood(X). madeofwood(X) -
floats(X). floats(duck). floats(Y) -
floats(X),sameweight(X,Y). sameweight(duck,girl).
Logically sound?!!
?- witch(X). Goals
7
Sir Bedeveres Infamous Deduction
witch(X) - burns(X),woman(X). woman(girl). burns(
X) - madeofwood(X). madeofwood(X) -
floats(X). floats(duck). floats(Y) -
floats(X),sameweight(X,Y). sameweight(duck,girl).
?- witch(X). Goals ?witch(_G0)
8
Sir Bedeveres Infamous Deduction
witch(X) - burns(X),woman(X). woman(girl). burns(
X) - madeofwood(X). madeofwood(X) -
floats(X). floats(duck). floats(Y) -
floats(X),sameweight(X,Y). sameweight(duck,girl).
?- witch(X). Goals ?witch(_G0)
9
Sir Bedeveres Infamous Deduction
witch(X) - burns(X),woman(X). woman(girl). burns(
X) - madeofwood(X). madeofwood(X) -
floats(X). floats(duck). floats(Y) -
floats(X),sameweight(X,Y). sameweight(duck,girl).
?- witch(X). Goals ?witch(_G0) ?burns(_G0) ?
?woman(_G0)
10
Sir Bedeveres Infamous Deduction
witch(X) - burns(X),woman(X). woman(girl). burns(
X) - madeofwood(X). madeofwood(X) -
floats(X). floats(duck). floats(Y) -
floats(X),sameweight(X,Y). sameweight(duck,girl).
?- witch(X). Goals ?witch(_G0) ?burns(_G0) ?
?woman(_G0)
11
Sir Bedeveres Infamous Deduction
witch(X) - burns(X),woman(X). woman(girl). burns(
X) - madeofwood(X). madeofwood(X) -
floats(X). floats(duck). floats(Y) -
floats(X),sameweight(X,Y). sameweight(duck,girl).
?- witch(X). Goals ?witch(_G0) ?burns(_G0) ?
?woman(_G0) ? madeofwood(_G0)
12
Sir Bedeveres Infamous Deduction
witch(X) - burns(X),woman(X). woman(girl). burns(
X) - madeofwood(X). madeofwood(X) -
floats(X). floats(duck). floats(Y) -
floats(X),sameweight(X,Y). sameweight(duck,girl).
?- witch(X). Goals ?witch(_G0) ?burns(_G0) ?
?woman(_G0) ? madeofwood(_G0)
13
Sir Bedeveres Infamous Deduction
witch(X) - burns(X),woman(X). woman(girl). burns(
X) - madeofwood(X). madeofwood(X) -
floats(X). floats(duck). floats(Y) -
floats(X),sameweight(X,Y). sameweight(duck,girl).

• ?- witch(X).
• Goals
• ?witch(_G0)
• ?burns(_G0) ? ?woman(_G0)
• madeofwood(_G0)
• floats(_G0)

14
Sir Bedeveres Infamous Deduction
witch(X) - burns(X),woman(X). woman(girl). burns(
X) - madeofwood(X). madeofwood(X) -
floats(X). floats(duck). floats(Y) -
floats(X),sameweight(X,Y). sameweight(duck,girl).

• ?- witch(X).
• Goals
• ?witch(_G0)
• ?burns(_G0) ? ?woman(_G0)
• madeofwood(_G0)
• floats(_G0)

15
Sir Bedeveres Infamous Deduction
witch(X) - burns(X),woman(X). woman(girl). burns(
X) - madeofwood(X). madeofwood(X) -
floats(X). floats(duck). floats(Y) -
floats(X),sameweight(X,Y). sameweight(duck,girl).

• ?- witch(X).
• Goals
• ?witch(_G0)
• ?burns(_G0) ? ?woman(_G0)
• madeofwood(_G0)
• floats(duck)

16
Sir Bedeveres Infamous Deduction
witch(X) - burns(X),woman(X). woman(girl). burns(
X) - madeofwood(X). madeofwood(X) -
floats(X). floats(duck). floats(Y) -
floats(X),sameweight(X,Y). sameweight(duck,girl).

• ?- witch(X).
• Goals
• ?witch(_G0)
• ?burns(_G0) ? ?woman(_G0)
• madeofwood(duck)

17
Sir Bedeveres Infamous Deduction
witch(X) - burns(X),woman(X). woman(girl). burns(
X) - madeofwood(X). madeofwood(X) -
floats(X). floats(duck). floats(Y) -
floats(X),sameweight(X,Y). sameweight(duck,girl).

• ?- witch(X).
• Goals
• ?witch(_G0)
• ?burns(duck) ? ?woman(duck)

18
Sir Bedeveres Infamous Deduction
witch(X) - burns(X),woman(X). woman(girl). burns(
X) - madeofwood(X). madeofwood(X) -
floats(X). floats(duck). floats(Y) -
floats(X),sameweight(X,Y). sameweight(duck,girl).

• ?- witch(X).
• Goals
• ?witch(_G0)
• ?burns(duck) ? ?woman(duck)

fail! backtrack...
19
Sir Bedeveres Infamous Deduction
witch(X) - burns(X),woman(X). woman(girl). burns(
X) - madeofwood(X). madeofwood(X) -
floats(X). floats(duck). floats(Y) -
floats(X),sameweight(X,Y). sameweight(duck,girl).

• ?- witch(X).
• Goals
• ?witch(_G0)
• ?burns(_G0) ? ?woman(_G0)
• madeofwood(_G0)
• floats(_G0)

20
Sir Bedeveres Infamous Deduction
witch(X) - burns(X),woman(X). woman(girl). burns(
X) - madeofwood(X). madeofwood(X) -
floats(X). floats(duck). floats(Y) -
floats(X),sameweight(X,Y). sameweight(duck,girl).

• ?- witch(X).
• Goals
• ?witch(_G0)
• ?burns(_G0) ? ?woman(_G0)
• madeofwood(_G0)
• floats(_G0)
• floats(_G1) ? ?sameweight(_G1, _G0)

21
Sir Bedeveres Infamous Deduction
witch(X) - burns(X),woman(X). woman(girl). burns(
X) - madeofwood(X). madeofwood(X) -
floats(X). floats(duck). floats(Y) -
floats(X),sameweight(X,Y). sameweight(duck,girl).

• ?- witch(X).
• Goals
• ?witch(_G0)
• ?burns(_G0) ? ?woman(_G0)
• madeofwood(_G0)
• floats(_G0)
• floats(_G1) ? ?sameweight(_G1, _G0)

22
Sir Bedeveres Infamous Deduction
witch(X) - burns(X),woman(X). woman(girl). burns(
X) - madeofwood(X). madeofwood(X) -
floats(X). floats(duck). floats(Y) -
floats(X),sameweight(X,Y). sameweight(duck,girl).

• ?- witch(X).
• Goals
• ?witch(_G0)
• ?burns(_G0) ? ?woman(_G0)
• madeofwood(_G0)
• floats(_G0)
• floats(duck) ? ?sameweight(duck, _G0)

23
Sir Bedeveres Infamous Deduction
witch(X) - burns(X),woman(X). woman(girl). burns(
X) - madeofwood(X). madeofwood(X) -
floats(X). floats(duck). floats(Y) -
floats(X),sameweight(X,Y). sameweight(duck,girl).

• ?- witch(X).
• Goals
• ?witch(_G0)
• ?burns(_G0) ? ?woman(_G0)
• madeofwood(_G0)
• floats(_G0)
• floats(duck) ? ?sameweight(duck, _G0)

24
Sir Bedeveres Infamous Deduction
witch(X) - burns(X),woman(X). woman(girl). burns(
X) - madeofwood(X). madeofwood(X) -
floats(X). floats(duck). floats(Y) -
floats(X),sameweight(X,Y). sameweight(duck,girl).

• ?- witch(X).
• Goals
• ?witch(_G0)
• ?burns(_G0) ? ?woman(_G0)
• madeofwood(_G0)
• floats(_G0)
• floats(duck) ? ?sameweight(duck, girl)

25
Sir Bedeveres Infamous Deduction
witch(X) - burns(X),woman(X). woman(girl). burns(
X) - madeofwood(X). madeofwood(X) -
floats(X). floats(duck). floats(Y) -
floats(X),sameweight(X,Y). sameweight(duck,girl).

• ?- witch(X).
• Goals
• ?witch(_G0)
• ?burns(_G0) ? ?woman(_G0)
• madeofwood(_G0)
• floats(girl)

26
Sir Bedeveres Infamous Deduction
witch(X) - burns(X),woman(X). woman(girl). burns(
X) - madeofwood(X). madeofwood(X) -
floats(X). floats(duck). floats(Y) -
floats(X),sameweight(X,Y). sameweight(duck,girl).

• ?- witch(X).
• Goals
• ?witch(_G0)
• ?burns(_G0) ? ?woman(_G0)
• madeofwood(girl)

27
Sir Bedeveres Infamous Deduction
witch(X) - burns(X),woman(X). woman(girl). burns(
X) - madeofwood(X). madeofwood(X) -
floats(X). floats(duck). floats(Y) -
floats(X),sameweight(X,Y). sameweight(duck,girl).

• ?- witch(X).
• Goals
• ?witch(_G0)
• ?burns(girl) ? ?woman(girl)

28
Sir Bedeveres Infamous Deduction
witch(X) - burns(X),woman(X). woman(girl). burns(
X) - madeofwood(X). madeofwood(X) -
floats(X). floats(duck). floats(Y) -
floats(X),sameweight(X,Y). sameweight(duck,girl).

• ?- witch(X).
• Goals
• ?witch(_G0)
• ?burns(girl) ? ?woman(girl)

works this time
29
Sir Bedeveres Infamous Deduction
witch(X) - burns(X),woman(X). woman(girl). burns(
X) - madeofwood(X). madeofwood(X) -
floats(X). floats(duck). floats(Y) -
floats(X),sameweight(X,Y). sameweight(duck,girl).

• ?- witch(X).
• Goals
• ?witch(girl)

30
Sir Bedeveres Infamous Deduction
witch(X) - burns(X),woman(X). woman(girl). burns(
X) - madeofwood(X). madeofwood(X) -
floats(X). floats(duck). floats(Y) -
floats(X),sameweight(X,Y). sameweight(duck,girl).

• ?- witch(X).
• X girl