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Using Cuts to Specify Defaults

Cuts are often used to give a range of behaviours of the program under special circumstances, with a ``catch--all'' default definition at the end. Consider, for example, the following program, which is discussed in Sterling & Shapiro ``The Arto of Prolog''. We want to define a predicate which will be able to determine a pension for some person (X), given certain characteristics of X. The default is nothing but, before that we put a series of tests to see if X qualifies for any pension. The code we use is:

pension(X, invalid_pension):-
    invalid(X), !.
pension(X, old_age_pension):-
    paid_up(X), !.
pension(X, supplementary_benefit):-
    over_65(X), !.
pension(_, nothing).    % This is the default if none of the above succeed.

Let's also define some characteristics for a sample of people.




This works fine for queries such as pension(fred,P), which would instantiate P to invalid and return no further results. However, there are ways of forcing the program to give incorrect recommendations. In particular, the query pension(fred,nothing) would succeed. How can this be fixed? One way is to make it explicit that the default of nothing only applies when there is no possible pension. In the code below, this is achieved by inventing a new predicate, entitlement/2 which makes this distinction.

entitlement(X, Y):-
    possible_pension(X, Y).
entitlement(X, nothing):-
    \+ possible_pension(X, _).

possible_pension(X, invalid_pension):-
possible_pension(X, old_age_pension):-
possible_pension(X, supplementary_benefit):-

Dave Stuart Robertson
Tue Jul 7 10:44:26 BST 1998