add object helper functions

1 files changed, 32 insertions, 61 deletions

diff --git a/linked-list.scm b/linked-list.scm
index 356b2b2..c9dff5b 100644
--- a/linked-list.scm
+++ b/linked-list.scm
@@ -12,71 +12,42 @@
 ;;; along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
 ;;; Singly linked list with tail pointer, message passing style.
-
-;;; LINKED-LIST-ELEM:
 ;;; 
-;;; (NEXT): Goes to the next element, if it exists. Does nothing if the
-;;; iterator is at the end of the list.
-;;; (GET): Gets the element at this list, signals error if at the end
-;;; of the list.
-;;; (EMPTY?): True if there are no more values to go to.
-
-(define linked-list-elem:new
-  (lambda (ptr)
-    (lambda args
-      (let ((op (car args)))
-        (cond
-          ((eq? op 'next)
-           (if (not (null? ptr))
-               (set! ptr (cdr ptr))
-               #f)
-           (null? ptr))
-          ((eq? op 'get) (car ptr))
-          ((eq? op 'empty?) (null? ptr)))))))
-
 ;;; LINKED-LIST:
 ;;; 
-;;; (PUSH): Pushes a value to the head of the list.
-;;; (PUSH-TAIL): Pushes a value to the end of the list.
+;;; (PUSH! VAL): Pushes a value to the head of the list.
+;;; (PUSH-TAIL! VAL): Pushes a value to the end of the list.
+;;; (SET-CDR! VAL): Set the cdr of TAIL to VAL. This will be overrwitten
+;;;    if PUSH-TAIL! is called again.
 ;;; (TO-LIST): Returns a list structure.
-;;; (TRAVERSE-FROM-HEAD): Returns an instance of LINKED-LIST-ELEM pointing
-;;; to the head of the list.
 
 (define linked-list:new
   (lambda ()
-    (let ((head '())
-          (tail '()))
-      (letrec ((push
-                (lambda (val)
-                  (set! head (cons val head))
-                  (if (null? tail)
-                      (set! tail head)
-                      #f)))
-               (push-tail
-                (lambda (val)
-                  (if (null? tail)
-                      (push val)
-                      (begin
-                        (set-cdr! tail (cons val '()))
-                        (set! tail (cdr tail))))))
-               (%set-cdr!
-                (lambda (val)
-                  (if (null? tail)
-                      (error "cannot set cdr of empty list")
-                      (set-cdr! tail val)))))
-        (lambda args
-          (let ((op (car args)))
-            (cond
-              ((eq? op 'push) (apply push (cdr args)))
-              ((eq? op 'push-tail) (apply push-tail (cdr args)))
-              ((eq? op 'set-cdr!) (apply %set-cdr! (cdr args)))
-              ((eq? op 'to-list) head)
-              ((eq? op 'traverse-from-head) (linked-list-elem:new head))
-              (else (error (cons "invalid operation" args))))))))))
-
-(define x (linked-list:new))
-(x 'push-tail 1)
-(x 'push-tail 2)
-(x 'push-tail 3)
-(x 'to-list)
-
+    (letrec
+        ((head '())
+         (tail '())
+         (this
+          (object/procedures
+           'push!
+           (lambda (val)
+             (set! head (cons val head))
+             (if (null? tail)
+                 (set! tail head))
+             this)
+           'push-tail!
+           (lambda (val)
+             (if (null? tail)
+                 (this 'push! val)
+                 (begin
+                   (set-cdr! tail (cons val '()))
+                   (set! tail (cdr tail))))
+             this)
+           'set-cdr!
+           (lambda (val)
+             (if (null? tail)
+                 (error "cannot set cdr of empty list")
+                 (set-cdr! tail val))
+             this)
+           'to-list
+           (lambda () head))))
+      this)))