Question

I can easily write a predicate to get unique elements from a given list in Prolog e.g.

no_doubles( [], [] ).
no_doubles( [H|T], F ) :-
 member( H, T ),
 no_doubles( T, F ).
no_doubles( [H|T], [H|F] ) :-
 \+ member( H, T ),
 no_doubles( T, F ).

However, how can you do the same thing but for something other than a normal list i.e. not something like [a,b,c…]? So in my case, I want to extract unique atoms for a propositional formula e.g. unique_atoms(and(x,and(x,y),z),[x,y,z]). is satisfied. Do you use recursion just like in my no_doubles example but for a formula like this?

Any ideas are welcomed :). Thanks.

Answer 1

?- setof(X, member(X,[a,b,c,a,b,c]), L).
L = [a, b, c].

?- sort([a,b,c,a,b,c], L).
L = [a, b, c].

Propositional formulas:

get_atoms(X,[X]) :-
    atom(X).
get_atoms(and(P,Q),Atoms) :-
    get_atoms(P,Left),
    get_atoms(Q,Right),
    append(Left,Right,Atoms).

etc. Optimize using difference lists if necessary.

unique_atoms(P,UniqueAtoms) :- get_atoms(P,Atoms), sort(Atoms,UniqueAtoms).

A more direct way is to use sets.