Closed World Assumption

1
Closed World Assumption

• Go through some last year exam questions

2
Closed World Assumption

• In logic program, we assume that the world is
  closed in the sense that everything that exists
  is in the program or can be derived from the
  program. Accordingly, if something is not in the
  program (or cannot be derived from it) then it is
  not true and consequently its negation is true.
  This deserves special care because we do not
  normally assume that the world is closed

3
An Example

• If we ask Prolog
• ?- not human('Mary').
• Prolog will probably answer yes. But this
  should not be understood as Prolog saying Mary is
  not human. What Prolog really means to say is
  there is not enough information in the program
  to prove that Mary is human.

4
Negation as Failure

• Q. How is not(Goal) implemented?
• A. Prolog does not try to prove this goal
  directly. Instead, it tries to prove the
  opposite, and if the opposite cannot be proved
  then Prolog assume that the not(Goal) succeeds.
  This is called negation as failure.

5
Negation as Failure (in code)

• not(G)-
• call(G), if G succeeds
• !, then we cut
• backtracking, then
• fail. make not(G) fail.
• not(G). otherwise, not(G)
• succeeds.

6
Two Control Facilities in Prolog call(G) and fail

• call(G) it treats the term G as a goal,
  executes G as if it appeared textually in its
  place.
• For example - if_then_else(P,Q,R) if P then Q
  else R
• if_then_else(P,Q,R)- call(P), !, call(Q).
• if_then_else(P,Q,R)- call(R).
• fail is a goal that always fails.
• For example a fail loop to print out all top
  boy names
• find_all- boy_names(X,Y,1), write((X,Y)), nl,
  fail.
• find_all.