prolog

# Homework, want to solve with Prolog

Can I solve the homework for my logic class with Prolog?

``````5 people are interrogated. The statements from A and B are lost.
C says: If A is truthful then B is truthful.
D says: If C is truthful then A is truthful.
E says: If D is truthful then A is truthful.

1) Is it possible that C lies?
2) Is it possible that D lies?
3) Is it possible that E lies?
``````

My latest attempt was

``````f(A, B, C, D, E):-
(C->(A->B)),
(D->(C->A)),
(E->(A->D)).
``````

This compiles, but when I query Prolog with

``````f(A, B, C, D, E).
``````

I get the error "Arguments are not sufficiently instantiated".

Solution

• Can model with:

``````stat(truth).
stat(lies).

go(A, B, C, D, E) :-
maplist(stat, [A, B, C, D, E]),
stat_if(C, A, B),
stat_if(D, C, A),
stat_if(E, D, A).

stat_if(truth, A, B) :-
implies(A, B).
stat_if(lies, A, B) :-
\+ implies(A, B).

implies(truth, truth).
% If lying, cannot determine
implies(lies, _).
``````

Then run with e.g.:

``````?- go(A, B, C, D, truth).
A = B, B = C, C = D, D = truth ;
A = D, D = truth,
B = C, C = lies ;
A = D, D = lies,
B = C, C = truth ;
A = B, B = D, D = lies,
C = truth ;
false.

?- go(A, B, C, D, lies).
false.
``````