1:- module(canny_arity, [arities/2]).

 arities(?Arities0:compound, ?Arities:list) is semidet

Suppose that you want to accept arity arguments of the form {A, ...} where A is the first integer element of a comma-separated list of arity numbers. The Arities0 form is a compound term enclosed within braces, comprising integers delimited by commas. The arities/2 predicate extracts the arities as a list.

Empty lists fail. Also, lists containing non-integers fail to unify. The implementation works forwards and backwards: arity compound to arity list or vice versa, mode (+, -) or mode (-, +).

   15arities({Arity}, [Arity]) :- integer(Arity), !.
   16arities(Arities0, Arities) :-
   17    Arities0 =.. [{}, Arities_],
   18    arities_(Arities_, Arities).
   19
   20arities_((Arity0, Arity), [Arity0, Arity]) :-
   21    integer(Arity0),
   22    integer(Arity),
   23    !.
   24arities_(Arities0, [H|T]) :-
   25    Arities0 =.. [',', H, T0],
   26    integer(H),
   27    arities_(T0, T)