Sophie: sbcl-1.0.31-1mdv2010.0 i586

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<h4 class="subsection">4.4.1 Type Errors at Compile Time</h4>

<p><a name="index-Compile-time-type-errors-51"></a><a name="index-Type-checking_002c-at-compile-time-52"></a>
If the compiler can prove at compile time that some portion of the
program cannot be executed without a type error, then it will give a
warning at compile time.

   <p>It is possible that the offending code would never actually be
executed at run-time due to some higher level consistency constraint
unknown to the compiler, so a type warning doesn't always indicate an
incorrect program.

   <p>For example, consider this code fragment:

<pre class="lisp">     (defun raz (foo)
       (let ((x (case foo
                   (:this 13)
                   (:that 9)
                   (:the-other 42))))
         (declare (fixnum x))
         (foo x)))
</pre>
   <p>Compilation produces this warning:

<pre class="example">     ; in: DEFUN RAZ
     ;     (CASE FOO (:THIS 13) (:THAT 9) (:THE-OTHER 42))
     ; --&gt; LET COND IF COND IF COND IF
     ; ==&gt;
     ;   (COND)
     ;
     ; caught WARNING:
     ;   This is not a FIXNUM:
     ;   NIL
</pre>
   <p>In this case, the warning means that if <code>foo</code> isn't any of
<code>:this</code>, <code>:that</code> or <code>:the-other</code>, then <code>x</code> will be
initialized to <code>nil</code>, which the <code>fixnum</code> declaration makes
illegal. The warning will go away if <code>ecase</code> is used instead of
<code>case</code>, or if <code>:the-other</code> is changed to <code>t</code>.

   <p>This sort of spurious type warning happens moderately often in the
expansion of complex macros and in inline functions. In such cases,
there may be dead code that is impossible to correctly execute. The
compiler can't always prove this code is dead (could never be
executed), so it compiles the erroneous code (which will always signal
an error if it is executed) and gives a warning.

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