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copied!<p>@mck, I see why you want to use <code>eval</code> now. But it's a very messy solution, and slow, as I mentioned in my answer to the previous question. The classic <em>On Lisp</em> says this about <code>eval</code>:</p> <p>"Generally it is not a good idea to call eval at runtime, for two reasons:</p> <ol> <li><p>It’s inefficient: eval is handed a raw list, and either has to compile it on the spot, or evaluate it in an interpreter. Either way is slower than compiling the code beforehand, and just calling it.</p></li> <li><p>It’s less powerful, because the expression is evaluated with no lexical context. Among other things, this means that you can’t refer to ordinary variables visible outside the expression being evaluated.</p></li> </ol> <p>Usually, calling eval explicitly is like buying something in an airport gift-shop. Having waited till the last moment, you have to pay high prices for a limited selection of second-rate goods."</p> <p>In this case the simplest thing is just to:</p> <pre><code>(defmacro testing-loop (var) (let ((g (gensym))) `(let ((,g ,var)) (if (consp ,g) (loop for x from 0 to 5 collect x) (loop for x from 0 to 5 and y in ,g collect y))))) </code></pre> <p>I know you want to factor out the common <code>loop for x from 0 to 5</code> (which isn't actually needed in the second branch anyways). But <code>loop</code> is itself a macro which is converted <em>at compile time</em> to efficient, low level code. So the call to <code>loop</code> has to be built <em>at compile time</em>, using values which are available at compile time. You can't just insert an <code>(if)</code> in there which is intended to be evaluated at run time.</p> <p>If you really don't want to repeat <code>loop for x from 0 to 5</code>, you could do something like:</p> <pre><code>(let ((a '(loop for x from 0 to 5))) `(if (consp ,var) (,@a collect x) (,@a and y in ,var collect y))) </code></pre> <p>That's just to give you the idea; if you really do this, make sure to <code>gensym</code>!</p> <p>A good lesson to learn from this is: when you are writing macros, you need to keep clearly in mind what is happening at compile time and what is happening at run time. The macro you wrote with <code>eval</code> compiles the <code>loop</code> macro dynamically, <em>every time it is run</em>, based on the return value of <code>consp</code>. You really want to compile the 2 different <code>loop</code> macros once, and just select the correct one at run-time.</p>

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