Unification with STO detection

Question

In ISO Prolog unification is defined only for those cases that are NSTO (not subject to occurs-check). The idea behind is to cover those cases of unifications that are mostly u

Answer 1

Here goes my attempt:

unify_sto(X,Y):-
  unify_with_occurs_check(X,Y) -> true ;
  (
    term_general(X, XG),
    term_general(Y, YG),
    \+(unify_sto1(XG,YG)),
    throw(error(type_error(acyclic,unify(X,Y)),_))
  ).

unify_sto1(X, Y):-
   unify_with_occurs_check(X,Y).
unify_sto1(X, Y):-
   X\=Y.

term_general(X, Y):-
  (var(X) ->  Y=X ;
  (atomic(X) -> Y=_ ;
  (
    X=..[Functor|L],
    term_general1(L, NL),
    Y=..[Functor|NL]
  ))).

term_general1([X|XTail], [Y|YTail]):-
  term_general(X, Y),
  term_general1(XTail, YTail).
term_general1([], []).

It first tries to unify_with_occurs_check, and if it does not succeed then it proceed to build a more general term for each argument, then it tries to unify such a term and test to see if it is cyclic. If it is cyclic a type_error of the kind acyclic is thrown.

E.g:

?- unify_sto(1+A,2+s(A)).
ERROR: Unhandled exception: error(type_error(acyclic,unify(1+_G3620,2+s(_G3620))))