lisp,common-lisp,literals , What's the difference between (list nil) and '(nil) in Lisp? [duplicate]


What's the difference between (list nil) and '(nil) in Lisp? [duplicate]

Question:

Tag: lisp,common-lisp,literals

This question already has an answer here:

First of all, let me say I'm a beginner in Lisp. To be honest I have been a beginner for some time now, but there are still many things I don't know well.

While I was writing this question, I came up with a strange bug in my code.

Here is a function that will return the list (0 1 ... n) with the list e appended. It uses rplacd along the way to keep track of the last element, to avoid a final call to last.

For example, (foo 4 '(x)) returns (0 1 2 3 4 x).

The "head" is stored in a, which is not simply nil, because there is only one nil, and never a copy of it (if I understand correctly), hence I can't simply append to nil.

(defun foo (n e)
    (let* ((a (list nil)) (tail a))
        (loop for i to n
              do (rplacd tail (setf tail (list i)))
              finally (rplacd tail (setf tail e))
              (return (cdr a)))))

(defun bar (n e)
    (let* ((a '(nil)) (tail a))
        (loop for i to n
              do (rplacd tail (setf tail (list i)))
              finally (rplacd tail (setf tail e))
              (return (cdr a)))))

The only difference between these functions is the (list nil) replaced by '(nil) in bar. While foo works as expected, bar always returns nil.

My initial guess is this happens because the original cdr of a is indeed nil, and the quoted list may be considered constant. However, if I do (setf x '(nil)) (rplacd x 1) I get (nil . 1) as expected, so I must be at least partially wrong.


Answer:

When evaluated, '(nil) and (list nil) produce similar lists, but the former can be considered constant when present in source code. You should not perform any destructive operations on a constant quoted list in Common Lisp. See http://l1sp.org/cl/3.2.2.3 and http://l1sp.org/cl/quote. In particular, the latter says "The consequences are undefined if literal objects (including quoted objects) are destructively modified."


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