gistfile1.sls

;;; NQUEENS -- Compute number of solutions to 8-queens problem. 
(define (nqueens n)
  (define (dec-to n)
    (let loop ((i n) (l '()))
      (if (= i 0) l (loop (- i 1) (cons i l)))))
  (define (try x y z)
    (if (null? x)
        (if (null? y)
            1
            0)
        (+ (if (ok? (car x) 1 z)
               (try (append (cdr x) y) '() (cons (car x) z))
               0)
           (try (cdr x) (cons (car x) y) z))))
  (define (ok? row dist placed)
    (if (null? placed)
        #t
        (and (not (= (car placed) (+ row dist)))
             (not (= (car placed) (- row dist)))
             (ok? row (+ dist 1) (cdr placed)))))
  (try (dec-to n) '() '()))

(nqueens 12)