Lisp macro set-or-nconc not able to get working

I were trying to make a macro for binding variable if it is unbind, else if it is binded than appending to its value

(defmacro set-or-nconc (var &rest args)
         `(,(if (and (boundp var) (not (null ,var)))
                'nconc 'setq)
            ,var ,@args))

The intended output wanted is

(set-or-nconc i '(a b)) => '(a b)
i                       => '(a b)
(set-开发者_如何转开发or-nconc i '(a b)) => '(a b a b)
i                       => '(a b a b)

But my defined macro is not working especially if i is binded to nil Do somebody have idea what is the problem with this macro ?

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The determination as to use setq or nconc is done at macro-expansion time, not at run-time. This is a bit of a problem. There's also some issues with your backquote expression, as there's either a "," too many (in (null ,var)) or one too few (in (boundp var), with the need for another backquote).

Below is at least something approaching a working solution, delaying the choice of using setq or nconc until run-time:

(defmacro set-or-nconc (var &rest args)
  `(if (and (boundp ',var) (not (null ,var)))
       (nconc ,var ,@args)
     (setq ,var ,@args)))

Also be aware that using this to gradually build up a list will have more-or-less O(n^2) behaviour, it's usually faster to accumulate by consing, then (if necessary) do a reverse at the end.