Generating Fibonacci Numbers

Prolog and foreign languages are generally intercallable in Quintus Prolog; in particular, arbitrarily nested calling is permitted. The following example uses recursive calling between Prolog and C to generate Fibonacci numbers:

                                     
fib.pl
     
     
     fib :-
         int(I),
         fib(I, F),
         write(fib(I,F)), nl,
         fail.
     
     int(I) :- int(0, I).
     
     int(I, I).
     int(I, K) :-
         J is I+1,
         int(J, K).
     
     fib(N, F) :-
         (   N =< 1 ->
             F = 1.0
         ;   N1 is N-1,
             N2 is N-2,
             c_fib(N1, F1),
             c_fib(N2, F2),
             F is F1+F2
         ).
     
     :- extern(fib(+integer, -float)).
     
     foreign(c_fib, c_fib(+integer, [-float])).
     foreign_file(fib, [c_fib]).
     
     :- load_foreign_files(fib, []).
     

                                      
fib.c
     
     
     #include <quintus/quintus.h>
     
     double c_fib(i)
         long int i;
         {
             float f1, f2;
             QP_pred_ref fib = QP_predicate("fib", 2, "user");
     
             if (i <= 1) {
                 return 1.0;
             } else {
                 QP_query(fib, i-1, &f1);
                 QP_query(fib, i-2, &f2);
                 return f1+f2;
             }
         }