Rewrite with continuation passing.

basic.lisp

@@ -9,21 +9,20 @@
 
 ;;; lazy results
 (defclass parse-result ()
-  ((top-results :initform nil :initarg :top-results :accessor top-results-of)
-   (promise-list :initform nil :initarg :promise-list :accessor promise-list-of)))
+  ((current-result :initform nil :initarg :top-results :accessor current-result-of)
+   (continuation :initform (constantly nil) :initarg :promise-list :accessor continuation-of)))
+
+(defun current-result (parser-result)
+  (with-accessors ((current-result current-result-of)
+		   (continuation continuation-of)) parse-result
+    (if current-result
+	current-result
+	(setf current-result (funcall continuation)))))
 
 (defun next-result (parse-result)
-  (with-accessors ((top-results top-results-of)
-		   (promise-list promise-list-of)) parse-result
-   (cond ((and (null top-results)
-	       (null promise-list))
-	  nil)
-	 ((and (null top-results)
-	       promise-list)
-	  (setf top-results (force (pop promise-list)))
-	  (next-result parse-result))
-	 (top-results
-	  (pop top-results)))))
+  (with-accessors ((current-result current-result-of)
+		   (continuation continuation-of)) parse-result
+    (setf current-result (funcall continuation))))
 
 (defun gather-results (parse-result)
   (iter (for result next (next-result parse-result))

primitives.lisp

@@ -54,8 +54,8 @@
   (delay
     #'(lambda (inp)
 	(make-instance 'parse-result
-		       :top-results (list (make-instance 'parser-possibility
-							 :tree v :suffix inp))))))
+		       :current-result (make-instance 'parser-possibility
+						      :tree v :suffix inp)))))
 
 (def-cached-parser zero
   "Primitive parser: parsing failure"
@@ -68,8 +68,8 @@
     #'(lambda (inp)
 	(if inp
 	    (make-instance 'parse-result
-			   :top-results (list (make-instance 'parser-possibility
-							     :tree (car inp) :suffix (cdr inp))))
+			   :current-result (make-instance 'parser-possibility
+							  :tree (car inp) :suffix (cdr inp)))
 	    (make-instance 'parse-result)))))
 
 (declaim (inline sat))
@@ -83,7 +83,10 @@
 (defun force? (parser)
   "Parser modifier: fully realize result from parser"
   (delay
-   #'(lambda (inp)
-       (let ((result (funcall parser inp)))
-	 (make-instance 'parse-result
-			:top-results (gather-results result))))))
+    #'(lambda (inp)
+	(let ((result (funcall parser inp)))
+	  (let ((all-results (gather-results result)))
+	   (make-instance 'parse-result
+			  :current-result (car all-results)
+			  :continuation #'(lambda ()
+					    (pop all-results))))))))