Revert "add object helper functions"

This reverts commit 0d5f4545d0d3db7c9ec63cac005bb71b85fe6b23.

1 files changed, 61 insertions, 32 deletions

diff --git a/linked-list.scm b/linked-list.scm
index c9dff5b..356b2b2 100644
--- a/linked-list.scm
+++ b/linked-list.scm
@@ -12,42 +12,71 @@
 ;;; 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! 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.
+;;; (PUSH): Pushes a value to the head of the list.
+;;; (PUSH-TAIL): Pushes a value to the end of the list.
 ;;; (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 ()
-    (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)))
+    (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)
+