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    <p>Your forms appear to be nested improperly.</p> <pre><code>(if test true-branch &amp;optional false-branch) </code></pre> <p>in this case is binding</p> <pre><code> test = ((and (car p1) (car p2)) nil) true-branch = (princ (car p1)) false-branch = nil </code></pre> <p>you probably wanted</p> <pre><code> (if (and (car p1) (car p2)) nil (princ (car p1))) </code></pre> <p>which is more usually written as</p> <pre><code> (unless (and (car p1) (car p2)) (princ (car p1))) </code></pre> <p>which is somewhat strange because you know that <code>(car p1)</code> <em>may be</em> <code>nil</code> at that point, (except <code>(car p2)</code> could be <code>nil</code>), so it may then be the same as</p> <pre><code> (unless (and (car p1) (car p2)) (princ nil)) </code></pre> <p>half the time, so perhaps you meant rather like</p> <pre><code> (when (and (car p1) (car p2)) (princ (car p1)) </code></pre> <p>which is the same as</p> <pre><code> (if (and (car p1) (car p2)) (princ (car p1))) </code></pre> <p>(but the use of <code>when</code> or <code>unless</code> helps others reading your code, so we won't have to puzzle over the forms to make sure that there isn't an “<code>else</code>-clause” hiding in there that we haven't noticed)</p> <p>So, I'm not really sure what you're trying to do there.</p> <p>⁂ Note on how your compiler will interpret what you've written a bit further:</p> <pre><code> ((and (car p1) (car p2)) nil) </code></pre> <p>is a particularly problematic-looking form, because this is what will happen:</p> <pre><code> (eval (car p1)) (eval (car p2)) </code></pre> <p>let's call these results for now <code>cp1</code> and <code>cp2</code></p> <pre><code> (eval (and cp1 cp2)) </code></pre> <p>OK, so if <code>cp1</code> (i.e. <code>(car p1)</code>) is <code>nil</code>, it has just become the value of the form; if it <em>is not</em> <code>nil</code>, then <code>cp2</code> will become the value of the form.</p> <p>So, the results of this form are one of two things: <code>nil</code>, or <code>cp2</code> (which could be <code>nil</code>) Let's call this result <code>f</code></p> <p>The next form outward, though, is the problem:</p> <pre><code> (eval ((function f) nil)) </code></pre> <p>You see, the <code>(and…)</code> form is in the <code>car</code> of the surrounding form, so the compiler is going to have to try to coërce the results of that form into a <code>funcall</code>able object, as though you'd called the function <code>function</code> on it.</p> <p>Now, if <code>f</code> (i.e. <code>(and (car p1) (car p2))</code>) happens to be a reference to a function, <em>and</em> that function can be called with a single parameter, then you might be OK.</p> <p>But the fact that you're trying to <code>princ</code> the results of <code>(car p1)</code> suggests that the contents are likely a <code>string</code>, <code>character</code>, or <code>number</code>, and none of those are going to return a function pointer when passed to <code>function</code>. </p> <p><strong>Lesson to take home</strong>: Watch your parentheses :-)</p> <p>Also… the output from your compiler or debugger should probably have given you some strong hints about what was going wrong here. In future, it'd be quite helpful to show</p> <ul> <li>what parameters you're passing in to the function, and</li> <li>what your output from the compiler or debugger is that let you know you have a problem</li> </ul>
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    1. COWow thanks for that, I'll give it ago!
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    2. COSo how would I change it so that it is only true if (car p1) AND (car p1) is nil?
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    3. COUm, that is ambiguous in English; if you mean `(and (car p1) (not (car p2)))` (like: `(car p1) & ¬(car p2)` infix-style), then you might use `(if (and (car p1) (not (car p2))) (do-it))` (or `when`), but if you mean `(and (not (car p1)) (not (car p2)))` (like `(¬(car p1) & ¬(car p2))` – the latter is more clearly written as `(not (or (car p1) (car p2)))`… which would be `(unless (or (car p1) (car p2)) (do-something))`
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