Chapter 16

Mutable pairs

The procedures provided by the (r6rs mutable-pairs)library allow new values to be assigned to the car and cdr fields of previously allocated pairs.

16.1  Procedures

Stores obj in the car field of pair. Returns the unspecified value.

(define (f) (list ’not-a-constant-list))
(define (g) ’(constant-list))
(set-car! (f) 3)                     ===⇒  
(set-car! (g) 3)                     ===⇒  unspecified
; should raise  &assertion exception

If an immutable pair is passed to set-car!, an exception with condition type &assertion should be raised.

Stores obj in the cdr field of pair. Returns the unspecified value.

If an immutable pair is passed to set-cdr!, an exception with condition type &assertion should be raised.

(let ((x (list ’a ’b ’c ’a))
      (y (list ’a ’b ’c ’a ’b ’c ’a)))
  (set-cdr! (list-tail x 2) x)
  (set-cdr! (list-tail y 5) y)
  (list
   (equal? x x)
   (equal? x y)
   (equal? (list x y ’a) (list y x ’b)))) 
                ===⇒  (#t #t #f)