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    <p><code>defn</code> defines a function, <code>defmacro</code> defines a macro.</p> <p>The difference between functions and macros is that on a function call first the arguments of the function are evaluated then the body of the function is evaluated using the arguments.</p> <p>Macros on the other hand describe a transformation from one piece of code to another. Any evaluation takes place after the transformation.</p> <p>This means that arguments may be evaluated multiple times or not at all. As an example <code>or</code> is a macro. If the first argument of <code>or</code> is false, the second argument will never be evaluated. If <code>or</code> were a function, this would not be possible, because the arguments would always be evaluated before the function runs.</p> <p>Another consequence of this is that the arguments of a macro need not be a valid expression before the macro is expanded. For example you could define a macro <code>mymacro</code> such that <code>(mymacro (12 23 +))</code> expands to <code>(+ 23 12)</code>, so this will work even though <code>(12 23 +)</code> on its own would be nonsense. You couldn't do that with a function because <code>(12 23 +)</code> would be evaluated, and cause an error, before the function runs.</p> <p>A small example to illustrate the difference:</p> <pre><code>(defmacro twice [e] `(do ~e ~e)) (twice (println "foo")) </code></pre> <p>The macro <code>twice</code> gets the list <code>(println "foo")</code> as an argument. It then transforms it into the list <code>(do (println "foo") (println "foo"))</code>. This new code is what gets executed.</p> <pre><code>(defn twice [e] `(do ~e ~e)) (twice (println "foo")) </code></pre> <p>Here <code>println "foo"</code> is evaluted right away. Since <code>println</code> returns <code>nil</code>, twice is called with <code>nil</code> as its argument. <code>twice</code> now produces the list <code>(do nil nil)</code> and returns that as its result. Note that here <code>(do nil nil)</code> is not evaluated as code, it's just treated as a list.</p>
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    1. POWhat is the difference between defn and defmacro?
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    1. POWhat is the difference between defn and defmacro?
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    1. COwhat do you mean by "arguments may be evaluated multiple times" when referring to macros ? can you give an example or explain more ?
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      1. USBelun
    2. COI don't think anybody would do this, but imagine a forloop macro. You might invoke it as (forloop [x 0] (< x 10) (+ x 1) (print x)). The clauses (< x 10), (+ x 1), and (print x) would not get evaluated up front, but would get evaluated for every iteration of the loop. For that reason, this could only be implemented as a macro. A function would evaluate its arguments greedily. Macros are one way to achieve laziness in Clojure.
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      1. USDaniel Yankowsky
    3. CO@Belun: Most simple example: (defmacro twice [e] `(do ~e ~e)) (twice (println "foo")) will print foo twice.
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