exercise-11-4.org

Exercise 11-4

It is somewhat confusing to see “no” printed after one or more valid answers have appeared. Modify the program to print “no” only when there are no answers at all, and “no more” in other cases.

(defvar valid-answers nil) 


(defun top-level-prove (goals)
  (setf valid-answers nil)
  (prove-all `(,@goals (show-prolog-vars ,@(variables-in goals)))
             no-bindings)
  (if valid-answers
      (format t "~&No-more.")
      (format t "~&No."))
  (values))


(defun show-prolog-vars (vars bindings other-goals)
  "Print each variable with its binding.
  Then ask the user if more solutions are desired."
  (if (null vars)
      (format t "~&Yes")
      (dolist (var vars)
        (format t "~&~a = ~a" var
                (subst-bindings bindings var))))
  (setf valid-answers t)e
  (if (continue-p)
      fail
      (prove-all other-goals bindings)))

Tests:

(?- (member 2 (1 2 3 2 1)))

(?- (member 2 (1 3 1)))