Clause Compilation swallows Cyclic Equations

General
1

I was testing top-level printing, and found a clause
consult and execution bug. Try entering these clauses:

?- [user].
p(X,Y) :- X = f(f(f(X))), Y = f(f(Y)).
p(X,Y) :- X = a(f(X,Y)), Y = b(g(X,Y)).
p(X,Y) :- X = s(s(X,Y),_), Y = s(Y,X).

Querying and Listing in SWI-Prolog gives, it is buggy:

/* SWI-Prolog 9.3.25 */
?- p(X,Y).
X = Y, Y = f(f(Y)) ;
X = a(f(X, Y)) ;           /* bug, Y = b(g(X, Y)) missing */
X = s(s(X, Y), _).         /* bug, Y = s(Y, X) missing */

?- listing(p/2).
p(A, B) :-
    A=f(f(f(A))),
    B=f(f(B)).
p(A, B) :-
    A=a(f(A, B)),
    B=B.                   /* bug, B = b(g(A, B)) expected */
p(A, B) :-
    A=s(s(A, B), _),
    B=B.                   /* bug, B = s(B, A) expected */

Querying and Listing in Trealla Prolog gives, it is buggy:

/* Trealla Prolog 2.78.24 */
?- p(X,Y).
   X = f(f(f(_A))), Y = f(f(_B))      /* bug what are _A and _B */
;  X = a(f(_A,_B)), Y = b(g(_A,_B))   /* bug what are _A and _B */
;  X = s(s(_A,_B),X), Y = s(_B,_A).   /* bug what are _A and _B */

?- listing(p/2).
p(A,B) :-
   A=f(f(f(A))),B=f(f(B)).
p(A,B) :-
   A=a(f(A,B)),B=b(g(A,B)).
p(A,B) :-
   A=s(s(A,B),_),B=s(B,A).

Edit 19.07.2025:
Querying and Listing in Dogelog Player gives, it is ok:

/* Dogelog Player 1.3.5 */
?- p(X,Y).
X = f(X), Y = X;
X = a(f(X, Y)), Y = b(g(X, Y));
X = s(s(X, Y), _), Y = s(Y, X).

?- listing(p/2).
p(A, B) :-
   A = f(f(f(A))),
   B = f(f(B)).
p(A, B) :-
   A = a(f(A, B)),
   B = b(g(A, B)).
p(A, B) :-
   A = s(s(A, B), _),
   B = s(B, A).
2

Thanks. The simplest case seems to be

p(X,Y) :- X = a(Y), Y = b.

Looks like it is related to migrating unification into the head.

3

(Nothing of importance)

6

Fixed compilation with 62a8539fdeb4fd9c139152a3a667aba843e84645

The toplevel could be better, but that seems way less important.

7

(Nothing of importance)