Pravin Mittal
CS 538, Lecture 38
Date: 4/26/2000 


TERMINATION ISSUES AND EXTRALOGICAL ISSUES CONTINUED:


fact(N, F) :- N =< 1, F = 1.
fact(N, F) :- N > 1, M is N - 1, fact(M, G), F is N*G. 

This program can be applied in four different ways: 

1. ?- (3, 6).
N and F are fixed/grounded. The program works. 

2. ?- (3, Y).
N is fixed, F is free. The program works. 

3. ?- (X, 6).
F is fixed, N is free. The program DOES NOT WORK. 

4. ?- (X, Y).
Both N and F are free. The program DOES NOT WORK. 

If N is free, fact can't work because fact can't guess whether N =< 1 or N > 1. You can use nonvar and ! to solve this problem in a better version of
the factorial function: 

fact(1, 1).
fact(N, 1) :- nonvar(N), N <= 1, !.
fact(N, F) :- nonvar(N), N > 1, !, M is N-1, fact(M, G), F is N*G, !.
fact(N, F) :- nonvar(F), !, fact(M, G), N is M+1, F2 is N*G, F is =< F2, !, F = F2.
fact(N, F) :- fact(M, G), N is M+1, F is N*G. 


!, are very important as it locks the program to the specific rule,

Consider rules of the form B:- !,C, in which a cut appears as the first condition. If the goal C fails, then control backtracks past B without considering any remaining rules for B. Thus, the cut has the effect of making B fail if C fails. 

There are two modes: 
1) Query mode
2) Input mode
 
Handy pieces of information and syntax in prolog:

^D = end of file

It is also possible to read the file with the help of command consult ('filename.pro')

, = AND
; = OR
\+ = NOT

read (X) just takes one argument from input board

N is M+1 is mathematical comparison whereas N= M+1 is a setting. 

var (X), X is a variable and do not have binding
norvar (X), X is bounded

integer (X) , X is integer
atomic (X), X is not a complex predicate

Debugging

trace to turn on the debugger
notrace to turn off the debugger