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    <p>Taking a look at how the existing one (in SBCL, at least) does things, I see:</p> <pre><code>* (macroexpand-1 '(push 1 *foo*)) (LET* ((#:G823 1) (#:NEW822 (CONS #:G823 *FOO*))) (SETQ *FOO* #:NEW822)) T </code></pre> <p>So, I imagine, mixing in a combination of your version and what this generates, one might do:</p> <pre><code>(defmacro my-push (element place) (let ((el-sym (gensym)) (new-sym (gensym "NEW"))) `(let* ((,el-sym ,element) (,new-sym (cons ,el-sym ,place))) (setq ,place ,new-sym))))) </code></pre> <p>A few observations:</p> <ol> <li><p>This seems to work with either <code>setq</code> or <code>setf</code>. Depending on what problem you're actually trying to solve (I presume re-writing <code>push</code> isn't the actual end goal), you may favor one or the other.</p></li> <li><p>Note that <code>place</code> does still get evaluated twice... though it does at least do so only after evaluating <code>element</code>. Is the double evaluation something you actually need to avoid? (Given that the built-in <code>push</code> doesn't, I'm left wondering if/how you'd be able to... though I'm writing this up before spending terribly much time thinking about it.) Given that it's something that needs to evaluate as a "<a href="http://clhs.lisp.se/Body/26_glo_p.htm#place" rel="nofollow">place</a>", perhaps this is normal?</p></li> <li><p>Using <code>let*</code> instead of <code>let</code> allows us to use <code>,el-sym</code> in the setting of <code>,new-sym</code>. This moves where the <code>cons</code> happens, such that it's evaluated in the first evaluation of <code>,place</code>, and after the evaluation of <code>,element</code>. Perhaps this gets you what you need, with respect to evaluation ordering?</p></li> <li><p>I think the biggest problem with your second version is that your <code>setf</code> really does need to operate on the symbol passed in, not on a <code>gensym</code> symbol.</p></li> </ol> <p>Hopefully this helps... (I'm still somewhat new to all this myself, so I'm making some guesses here.)</p>
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    1. POHow could I implement the push macro?
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    1. COInteresting. The HyperSpec says the place form is only evaluated once, but it looks like SBCL knows that getting the value from a symbol twice is harmless. For a place form with a possible side effect it does: CL-USER> (macroexpand-1 '(push 4 (my-list-func))) (LET* ((#:G31 4) (#:G30 (CONS #:G31 (MY-LIST-FUNC)))) #:G30 (FUNCALL #'(SETF MY-LIST-FUNC) #:G30)). Does this form of SETF avoid a second evaluation somehow?
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    2. COWhen I actually run the above I get "Undefined function (SETF MY-LIST-FUNC)". Looks like the double evaluation problem is avoided by disallowing place forms which are not SETF-able on their own. Makes sense.
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