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<p><strong>handler-case</strong> <span><i>Macro</i></span></p>
<p><strong>Syntax:</strong></p>
<p><span><b>handler-case</b></span> <span><i>expression</i></span> [[ <em>{↓error-clause}</em>* <em>| ↓no-error-clause</em> ]] → {result}*</p>
<p><em>clause::</em>=<em>↓error-clause | ↓no-error-clause</em></p>
<p><em>error-clause::</em>=(<em>typespec</em> ([<em>var</em>]) <span><i>{declaration}</i></span>* <span><i>{form}</i></span>*)</p>
<p><em>no-error-clause::</em>=(<!-- -->:no-error<!-- --> <em>lambda-list {declaration}</em>* <span><i>{form}</i></span>*)</p>
<p><strong>Arguments and Values:</strong></p>
<p><span><i>expression</i></span>—a <span><i>form</i></span>.</p>
<p><em>typespec</em>—a <span><i>type specifier</i></span> .</p>
<span><b>handler-case</b></span>
<p><em>var</em>—a <em>variable name</em>.</p>
<p><em>lambda-list</em>—an <span><i>ordinary lambda list</i></span>.</p>
<p><span><i>declaration</i></span>—a <span><b>declare</b></span> <span><i>expression</i></span>; not evaluated.</p>
<p><span><i>form</i></span>—a <span><i>form</i></span>.</p>
<p><em>results</em>—In the normal situation, the values returned are those that result from the evaluation of <span><i>expression</i></span>; in the exceptional situation when control is transferred to a <em>clause</em>, the value of the last <span><i>form</i></span> in that <em>clause</em> is returned.</p>
<p><strong>Description:</strong></p>
<p><span><b>handler-case</b></span> executes <span><i>expression</i></span> in a <span><i>dynamic environment</i></span> where various handlers are active. Each <em>error-clause</em> specifies how to handle a <span><i>condition</i></span> matching the indicated <em>typespec</em>. A <em>no-error-clause</em> allows the specification of a particular action if control returns normally.</p>
<p>If a <span><i>condition</i></span> is signaled for which there is an appropriate <em>error-clause</em> during the execution of <span><i>expression</i></span> (<em>i.e.</em>, one for which (typep <span><i>condition</i></span> ’<em>typespec</em>) returns <span><i>true</i></span>) and if there is no intervening handler for a <span><i>condition</i></span> of that <span><i>type</i></span>, then control is transferred to the body of the relevant <em>error-clause</em>. In this case, the dynamic state is unwound appropriately (so that the handlers established around the <span><i>expression</i></span> are no longer active), and <em>var</em> is bound to the <span><i>condition</i></span> that had been signaled. If more than one case is provided, those cases are made accessible in parallel. That is, in</p>
<p>(handler-case <span><i>form</i></span></p>
<p>(<em>typespec1</em> (<em>var1</em>) <em>form1</em>)</p>
<p>(<em>typespec2</em> (<em>var2</em>) <em>form2</em>))</p>
<p>if the first <em>clause</em> (containing <em>form1</em>) has been selected, the handler for the second is no longer visible (or vice versa).</p>
<p>The <em>clauses</em> are searched sequentially from top to bottom. If there is <span><i>type</i></span> overlap between <em>typespecs</em>, the earlier of the <em>clauses</em> is selected.</p>
<p>If <em>var</em> is not needed, it can be omitted. That is, a <em>clause</em> such as:</p>
<p>(<em>typespec</em> (<em>var</em>) (declare (ignore <em>var</em>)) <span><i>form</i></span>)</p>
<p>can be written (<em>typespec</em> () <span><i>form</i></span>).</p>
<p>If there are no <span><i>forms</i></span> in a selected <em>clause</em>, the case, and therefore <span><b>handler-case</b></span>, returns <span><b>nil</b></span>. If execution of <span><i>expression</i></span> returns normally and no <em>no-error-clause</em> exists, the values returned by <span><i>expression</i></span> are returned by <span><b>handler-case</b></span>. If execution of <span><i>expression</i></span> returns normally and a <em>no-error-clause</em> does exist, the values returned are used as arguments to the function described by constructing (lambda <em>lambda-list {form}</em>*) from the <em>no-error-clause</em>, and the <span><i>values</i></span> of that function call are returned by <span><b>handler-case</b></span>. The handlers which were established around the <span><i>expression</i></span> are no longer active at the time of this call.</p>
<span><b>handler-case</b></span>
<p><strong>Examples:</strong></p>
<div class="language-lisp codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-lisp codeBlock_bY9V thin-scrollbar" style="color:#393A34;background-color:#f6f8fa"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token punctuation" style="color:#393A34">(</span><span class="token defun keyword" style="color:#00009f">defun</span><span class="token defun"> </span><span class="token defun function" style="color:#d73a49">assess-condition</span><span class="token defun"> </span><span class="token defun punctuation" style="color:#393A34">(</span><span class="token defun arguments argument variable" style="color:#36acaa">condition</span><span class="token defun punctuation" style="color:#393A34">)</span><span class="token plain"> </span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">handler-case</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">signal</span><span class="token plain"> condition</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">warning</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">&quot;Lots of smoke, but no fire.&quot;</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">(</span><span class="token keyword" style="color:#00009f">or</span><span class="token plain"> arithmetic-error control-error cell-error stream-error</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">condition</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">format</span><span class="token plain"> </span><span class="token boolean" style="color:#36acaa">nil</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">&quot;~</span><span class="token string argument" style="color:#e3116c">S</span><span class="token string" style="color:#e3116c"> looks especially bad.&quot;</span><span class="token plain"> condition</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">serious-condition</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">condition</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">format</span><span cla
<p><strong>See Also:</strong></p>
<p><span><b>handler-bind</b></span>, <span><b>ignore-errors</b></span>, Section 9.1 (Condition System Concepts) <strong>Notes:</strong></p>
<p>(handler-case form</p>
<p>(<em>type1</em> (<em>var1</em>) . <em>body1</em>)</p>
<p>(<em>type2</em> (<em>var2</em>) . <em>body2</em>) ...)</p>
<p>is approximately equivalent to:</p>
<p>(block #1=#<!-- -->:g0001</p>
<p>(let ((#2=#<!-- -->:g0002<!-- --> nil))</p>
<p>(tagbody</p>
<p>(handler-bind ((<em>type1</em> #’(lambda (temp)</p>
<p>(setq #1# temp)</p>
<p>(go #3=#<!-- -->:g0003<!-- -->)))</p>
<p>(<em>type2</em> #’(lambda (temp)</p>
<p>(setq #2# temp)</p>
<p>(go #4=#<!-- -->:g0004<!-- -->))) ...)</p>
<p>(return-from #1# form))</p>
<p>#3# (return-from #1# (let ((<em>var1</em> #2#)) . <em>body1</em>))</p>
<p>#4# (return-from #1# (let ((<em>var2</em> #2#)) . <em>body2</em>)) ...)))</p>
<p>(handler-case form</p>
<p>(<em>type1 (var1)</em> . <em>body1</em>)</p>
<p>...</p>
<p>(<!-- -->:no-error<!-- --> (<em>varN-1 varN-2</em> ...) . <em>bodyN</em>))</p>
<p>is approximately equivalent to:</p>
<p>(block #1=#<!-- -->:error-return</p>
<p>(multiple-value-call #’(lambda (<em>varN-1 varN-2</em> ...) . <em>bodyN</em>)</p>
<p>(block #2=#<!-- -->:normal-return</p>
<p>(return-from #1#</p>
<p>(handler-case (return-from #2# form)</p>
<p>(<em>type1</em> (<em>var1</em>) . <em>body1</em>) ...)))))</p>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="expanded-reference-handler-case">Expanded Reference: handler-case<a href="handler-case_macro.html#expanded-reference-handler-case" class="hash-link" aria-label="Direct link to Expanded Reference: handler-case" title="Direct link to Expanded Reference: handler-case">​</a></h2>
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