/***********************************************************************/
/* Code for a new Prolog prompt                                        */
/*                                                                     */
/* Based on Sterling & Shapiro, p.186                                  */
/***********************************************************************/

prompt_me :- 
   special_prompt, read(Goal), prompt_me(Goal).

special_prompt :- write('Yes, spod? ').

prompt_me(halt) :- !.

prompt_me(Goal) :-
   isground(Goal), !, ground_solve(Goal), prompt_me.

prompt_me(Goal) :-
   nonground_solve(Goal), prompt_me.

ground_solve(Goal) :-
   solve(Goal), !, write('Oh, yes'), nl.

ground_solve(Goal) :-
   write('Oh, no'), nl. 

nonground_solve(Goal) :-
   solve(Goal), write(Goal), nl, fail.

nonground_solve(Goal) :-
   write('That''s your lot'), nl.

solve(Goal) :- call(Goal).

isground(Term) :-
   nonvar(Term), constant(Term).

isground(Term) :-
   nonvar(Term), compound(Term), functor(Term, F, N), isground(N, Term).

isground(N, Term) :-
   N > 0, arg(N, Term, Arg), ground(Arg), N1 is N - 1, isground(N1, Term).

isground(0, Term).

constant(X) :- integer(X).

constant(X) :- atom(X).

compound(X) :- not atomic(X).