cl, scheme: Introduce `reduce' functions.

[fringe] / cl-fringe.lisp

diff --git a/cl-fringe.lisp b/cl-fringe.lisp
index a2ddf85..5af3cc7 100644 (file)
--- a/cl-fringe.lisp
+++ b/cl-fringe.lisp
@@ -10,14 +10,27 @@
 ;; new iterator function to produce the remaining items; if there are no more
 ;; items, then it returns NIL twice.
 
+(defun reduce-iterator (func init iter)
+  "Invoke FUNC on the elements of ITER.
+
+   We maintain a STATE whose value initially is INIT; for each ITEM, we
+   update the state with the value of (funcall FUNC ITEM STATE); the end
+   result is the final state."
+  (let ((state init) item)
+    (loop
+      (setf (values item iter) (funcall iter))
+      (unless iter (return state))
+      (setf state (funcall func item state)))))
+
 (defun list-iterator (iter)
   "Collect the items from ITER into a list and return it."
-  (labels ((recur (iter list)
-            (multiple-value-bind (data iter) (funcall iter)
-              (if iter
-                  (recur iter (cons data list))
-                  (nreverse list)))))
-    (recur iter nil)))
+  (nreverse (reduce-iterator #'cons nil iter)))
+
+(defun iterate-list (list)
+  "Return an iterator for LIST, according to our iteration protocol."
+  (if (endp list)
+      (lambda () (values nil nil))
+      (lambda () (values (car list) (iterate-list (cdr list))))))
 
 (defun same-iterators-p (iter-a iter-b &key (test #'eql))
   "Return whether ITER-A and ITER-B produce the same items."