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2013-10-21T11:56:36.507
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<h1>Use a function</h1> <p>If you want to use a function, you have to pass the symbol itself:</p> <pre><code>(defun mutate (symbol value) (setf (symbol-value symbol) value)) (mutate 'foo 42) foo ==> 42 </code></pre> <p>Note that the <code>symbol</code> argument to <code>mutate</code> is quoted.</p> <h1>Use a macro</h1> <pre><code>(defmacro mutate (symbol value) `(setf ,symbol ,value)) (mutate foo 42) foo ==> 42 </code></pre> <p>The <code>symbol</code> argument no longer has to be quoted.</p> <h1>Discussion</h1> <p>Arguments are <a href="http://www.lispworks.com/documentation/HyperSpec/Body/03_ababc.htm" rel="nofollow">passed by value</a> in Lisp, which means that a function sees the <em>value</em> of its argument, not the place where it is stored. Thus a function invoked as <code>(foo (! 5))</code>, <code>(foo 120)</code>, <code>(foo x)</code> sees only the number <code>120</code> and has absolutely no way of knowing whether this number is returned by a function (1st case), is a literal (2nd case) or stored in a variable (3rd case).</p> <p>Macros receive <a href="http://www.lispworks.com/documentation/HyperSpec/Body/03_ababb.htm" rel="nofollow">the <em>code</em> (the whole form)</a> which they <em>transform</em> and pass the new code to the compiler (or interpreter). Thus macros can determine how to modify their arguments (called <a href="http://www.lispworks.com/documentation/HyperSpec/Body/05_aa.htm" rel="nofollow"><em>places</em> or <em>generalized references</em></a> in that case).</p>
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1. POHow do I globally change a variable value within function in lisp
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1. COThis does not function unfortunately: If I set c as an empty list and try to change it within a function with: (mutate c (remove-duplicates (merge 'list a b #'<))), I still get NIL (even though (print (remove-duplicates (merge 'list a b #'<))) prints the correct output)
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2. CO@smihael: Which version are you talking about? Macro? Function?
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