Sophie: sbcl-1.0.31-1mdv2010.0 i586

sbcl-1.0.31-1mdv2010.0.i586.rpm

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<h5 class="subsubsection">4.1.3.3 The Processing Path</h5>

<p><a name="index-Processing-Path-41"></a><a name="index-Macroexpansion-42"></a><a name="index-Source_002dto_002dsource-transformation-43"></a>
The processing path is mainly useful for debugging macros, so if you
don't write macros, you can probably ignore it. Consider this example:

<pre class="lisp">     (defun foo (n)
       (dotimes (i n *undefined*)))
</pre>
   <p>Compiling results in this error message:

<pre class="example">     ; in: DEFUN FOO
     ;     (DOTIMES (I N *UNDEFINED*))
     ; --&gt; DO BLOCK LET TAGBODY RETURN-FROM
     ; ==&gt;
     ;   (PROGN *UNDEFINED*)
     ;
     ; caught WARNING:
     ;   undefined variable: *UNDEFINED*
</pre>
   <p>Note that <code>do</code> appears in the processing path. This is because
<code>dotimes</code> expands into:

<pre class="lisp">     (do ((i 0 (1+ i)) (#:g1 n))
         ((&gt;= i #:g1) *undefined*)
       (declare (type unsigned-byte i)))
</pre>
   <p>The rest of the processing path results from the expansion of
<code>do</code>:

<pre class="lisp">     (block nil
       (let ((i 0) (#:g1 n))
         (declare (type unsigned-byte i))
         (tagbody (go #:g3)
           #:g2    (psetq i (1+ i))
           #:g3    (unless (&gt;= i #:g1) (go #:g2))
           (return-from nil (progn *undefined*)))))
</pre>
   <p>In this example, the compiler descended into the <code>block</code>,
<code>let</code>, <code>tagbody</code> and <code>return-from</code> to reach the
<code>progn</code> printed as the actual source. This is a place where the
&ldquo;actual source appears in explanation&rdquo; rule was applied. The
innermost actual source form was the symbol <code>*undefined*</code> itself,
but that also appeared in the explanation, so the compiler backed out
one level.

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