tree->generator with explicit CPS

2008-09-03

This is just a little follow-up to my last post. As another exercise in continuations, I transformed tree->generator to explicit continuation-passing style. I guess I could also have looked at the expansions of the cl-cont macros, but this is the result of a manual transformation:

(defun tree->generator (tree)
  (let (generate-leaves)
    (setf generate-leaves
	  (lambda ()
	    (labels ((recur (tree cont)
		       (cond ((null tree) ; empty leaf: continue.
			      (funcall cont))
			     ((consp tree) ; recurse into branches:
			      (recur (car tree)
				     (lambda ()
				       (recur (cdr tree) cont))))
			     ('otherwise ; leaf with a value:
			      (setf generate-leaves ; update generator
				    cont)
			      tree))))	      ; and return the value.
	      (recur tree
		     (lambda () nil))))) ; return nil after exhaustion.
    (lambda ()
      (funcall generate-leaves))))

[EDIT: I realized the potential for further simplification right after publishing the post. So now the call to and return from generate-leaves in the above code aren’t in CPS anymore, only recur has to be in CPS.]

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