properties and methods of cuts and goals

1. Cuts And Goal Model in
Prolog
Prof. Neeraj Bhargava
Kapil Chauhan
Department of Computer Science
School of Engineering & Systems Sciences
MDS University, Ajmer

2. Overview

3. The Cut
 Automatic backtracking is one of the most characteristic features of
Prolog.
There is an inbuilt Prolog predicate !, called cut, which offers a more
direct way of exercising control over the way Prolog looks for solutions.
Cut is simply a special atom that we can use when writing clauses
 Ex: p(X) :- b(X),c(X),!,d(X),e(X)

4. The Cut
consider the following piece of cut-free code
 p(X) :- a(X).
 p(X) :- b(X),c(X),d(X),e(X).
 p(X) :- f(X).
 a(1).
 b(1).
 c(1).
 b(2).
 c(2).
 d(2).
 e(2).
 f(3).
 If we pose the query p(X) we will get the following responses:
 X = 1 ;
 X = 2 ;
 X = 3 ;
 No

5. The Cut
But now suppose we insert a cut in the second clause:
 p(X) :- b(X),c(X),!,d(X),e(X).
 If we now pose the query p(X) we will get the following
responses:
 X = 1 ;
 No

6. Examples
 Suppose we have the following facts:
 teaches(dr_fred, history).
teaches(dr_fred, english).
teaches(dr_fred, drama).
teaches(dr_fiona, physics).
 studies(alice, english).
studies(angus, english).
studies(amelia, drama).
studies(alex, physics).

7. Example
 Output :-
 ?- teaches(dr_fred, Course), studies(Student, Course).
 Course = english
 Student = alice ;
 Course = english
 Student = angus ;
 Course = drama
 Student = amelia ;
 false.

8. IF-THEN-ELSE
In Prolog, if A then B else C is written as ( A -> B ; C).
 To Prolog this means: try A. If you can prove it, go on to
prove B and ignore C.
 If A fails, however, go on to prove C ignoring B.
 The max predicate using the if-then-else construct looks as
follows:
 max(X,Y,Z) :-
 ( X =< Y -> Z = Y; Z = X).

9. Negation as a Failure
 Predicate fail/0->As its name suggests, fail is a special symbol that will
immediately fail when Prolog encounters it as a goal.
 When Prolog fails, it tries to backtrack.
 Thus fail can be viewed as an instruction to force backtracking.
 And when used in combination with cut, which blocks backtracking,
fail enables us to write some interesting programs

10. Negation as a Failure
 suppose that we need to write code to capture the following condition:
 p holds if a and b hold, or if a does not hold and c holds too.
 This can be captured with the help of negation as failure very directly:
 p :- a,b.
 p :- + a, c.
 But suppose that a is a very complicated goal, it would be better to
 use the following program:
 p :- a,!,b.
 p :- c.

11. GOAL ORDER
 A query to the Prolog interpreter consists of one or more goals. For example,
?- lectures(john, Subject), studies(Student, Subject).
here are two goals, lectures(john, Subject) and studies(Student, Subject).
 A goal is something that Prolog tries to satisfy by finding values of the variables (in this
case Student and Subject) that make the goal succeed .
If all the goals in a query succeed, Prolog prints the bindings necessary to make the query succeed.
 Goals occur in rules as well as in queries. In
 happy(Dog) :-
 Is dog(Dog),
 go_for_walk(Dog).
 is_dog(Dog) and go_for_walk(Dog) are the two goals that form the body of the rule.

12. Assignment
 Explain Cuts And Goal Model in Prolog with suitable
example.