Re: [PATCH] Add new lisp function length= with bytecode support

[Richard Stallman]

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  > I would agree; having length= makes it seem strange not to have the others.

Why add length= as a primitive?  Why is it important enough to justify that?

Unless you're concerned about improper lists, 
length= can be implemented as follows:

(defun length= (list n)
  (if (= n 0)
      (null list)
    (let ((tail (nthcdr (1- n) list)))
      (and (consp tail)
           (not (cdr tail))))))

This will run almost as fast as a primitive, for long lists,
since nthcdr does most of the work.

If we want to make it safe for improper lists,
it would be better to add nthcdr-safe as a primitive.
That would make it easy to implement length= or length< or length>
or length>= if you want them.

-- 
Dr Richard Stallman
President, Free Software Foundation (gnu.org, fsf.org)
Internet Hall-of-Famer (internethalloffame.org)
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