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copied!<p>This form:</p> <pre><code>(defmacro test (lst) (dolist (x lst) (print x))) </code></pre> <p>defines a <em>macro</em>, which is a "code transformation function" which gets applied to a form using this macro at macro expansion time. So, after you have defined this macro, when you evaluate this expression:</p> <pre><code>(test (1 2 3)) </code></pre> <p>it first gets read to this list:</p> <pre><code>(test (1 2 3)) </code></pre> <p>Then, since Lisp reads <code>test</code> at the operator position, it gets macro-expanded by passing the argument, which is the literal list <code>(1 2 3)</code>, to the macro expansion function defined above. This means that the following gets evaluated at macro-expansion time:</p> <pre><code>(dolist (x '(1 2 3)) (print x)) </code></pre> <p>So, at macro-expansion time, the three values get printed. Finally, the return value of that form is returned as the code to be compiled and executed. <code>Dolist</code> returns <code>nil</code> here, so this is the code returned:</p> <pre><code>nil </code></pre> <p><code>Nil</code> evaluates to <code>nil</code>, which is returned.</p> <p>Generally, such a macro is not very useful. See "Practical Common Lisp" by Peter Seibel or "On Lisp" by Paul Graham for an introduction to useful macros.</p> <hr> <p><em>Update:</em> Perhaps it is useful to recapitulate the order of reading, expanding, and evaluating Lisp code.</p> <p>First, the REPL takes in a stream of characters: <code>(</code> <code>t</code> <code>e</code> <code>s</code> <code>t</code> <code> </code> <code>(</code> <code>1</code> <code></code> <code>2</code> <code></code> <code>3</code> <code>)</code> <code>)</code>, which it assembles into tokens: <code>(</code> <code>test</code> <code>(</code> <code>1</code> <code>2</code> <code>3</code> <code>)</code> <code>)</code>.</p> <p>Then, this is translated into a tree of symbols: (<code>test</code> (<code>1</code> <code>2</code> <code>3</code>)). There may be so-called <em>reader macros</em> involved in this step. For example, <code>'x</code> is translated to (<code>quote</code> <code>x</code>).</p> <p>Then, from the outside in, each symbol in operator position (i.e., the first position in a form) is examined. If it names a macro, then the corresponding macro function is invoked with the <em>code</em> (i.e., the subtrees of symbols) that is the rest of the form as arguments. The macro function is supposed to return a new form, i.e. <em>code</em>, which replaces the macro form. In your case, the macro <code>test</code> gets the code (<code>1</code> <code>2</code> <code>3</code>) as argument, prints each of the symbols contained within (note that this is even before compile time), and returns <code>nil</code>, throwing its arguments away (the compiler never even sees your little list). The returned code is then examined again for possible macroexpansions.</p> <p>Finally, the expanded code that does not contain any macro invocations anymore is <em>evaluated</em>, i.e. compiled and executed. <code>Nil</code> happens to be a self-evaluating symbol; it evaluates to <code>nil</code>.</p> <p>This is just a rough sketch, but I hope that it clears some things up.</p>

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