10.5 Practical Session 10

The purpose of Practical Session 10 is to help you get familiar with cuts and negation as failure.

First some keyboard exercises:

1. First of all, try out all three versions of the max/3 predicate defined in the text: the cut-free version, the green cut version, and the red cut version. As usual, ``try out'' means ``run traces on'', and you should make sure that you trace queries in which all three arguments are instantiated to integers, and queries where the third argument is given as a variable.

2. OK, time for a burger. Try out all the methods discussed in the text for coping with Vincent's preferences. That is, try out the program that uses a cut-fail combination, the program that uses negation as failure correctly, and also the program that gets it wrong by using negation in the wrong place.

Now for some programming:

1. Define a predicate nu/2 ("not unifiable") which takes two terms as arguments and succeeds if the two terms do not unify. For example:

       nu(foo,foo).
    no
 
    nu (foo,blob).
    yes
 
    nu(foo,X).
    no

   You should define this predicate in three different ways:

   1. First (and easiest) write it with the help of = and \+.

   2. Second write it with the help of =, but don't use \+.

   3. Third, write it using a cut-fail combination. Don't use = and don't use \+.

2. Define a predicate unifiable(List1,Term,List2) where List2 is the list of all members of List1 that match Term , but are not instantiated by the matching. For example,

       unifiable([X,b,t(Y)],t(a),List]

   should yield

       List = [X,t(Y)].

   Note that X and Y are still not instantiated. So the tricky part is: how do we check that they match with t(a) without instantiating them? (Hint: consider using the test \+ (term1 = term2). Why? Think about it. You might also like to think about the test \+(\+ (term1 = term2)).)