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224 lines
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<title>Throw and Catch (Guile Reference Manual)</title>
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<div class="subsubsection-level-extent" id="Throw-and-Catch">
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Next: <a href="Exceptions-and-C.html" accesskey="n" rel="next">Exceptions and C</a>, Previous: <a href="Raising-and-Handling-Exceptions.html" accesskey="p" rel="prev">Raising and Handling Exceptions</a>, Up: <a href="Exceptions.html" accesskey="u" rel="up">Exceptions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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<hr>
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<h4 class="subsubsection" id="Throw-and-Catch-1"><span>6.11.8.3 Throw and Catch<a class="copiable-link" href="#Throw-and-Catch-1"> ¶</a></span></h4>
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<p>Guile only adopted <code class="code">with-exception-handler</code> and
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<code class="code">raise-exception</code> as its primary exception-handling facility in
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2019. Before then, exception handling was fundamentally based on three
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other primitives with a somewhat more complex interface: <code class="code">catch</code>,
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<code class="code">with-throw-handler</code>, and <code class="code">throw</code>.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-catch"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">catch</strong> <var class="def-var-arguments">key thunk handler [pre-unwind-handler]</var><a class="copiable-link" href="#index-catch"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fcatch_005fwith_005fpre_005funwind_005fhandler"><span class="category-def">C Function: </span><span><strong class="def-name">scm_catch_with_pre_unwind_handler</strong> <var class="def-var-arguments">(key, thunk, handler, pre_unwind_handler)</var><a class="copiable-link" href="#index-scm_005fcatch_005fwith_005fpre_005funwind_005fhandler"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fcatch"><span class="category-def">C Function: </span><span><strong class="def-name">scm_catch</strong> <var class="def-var-arguments">(key, thunk, handler)</var><a class="copiable-link" href="#index-scm_005fcatch"> ¶</a></span></dt>
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<dd><p>Establish an exception handler during the dynamic extent of the call to
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<var class="var">thunk</var>. <var class="var">key</var> is either <code class="code">#t</code>, indicating that all
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exceptions should be handled, or a symbol, restricting the exceptions
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handled to those having the <var class="var">key</var> as their <code class="code">exception-kind</code>.
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</p>
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<p>If <var class="var">thunk</var> executes normally, meaning without throwing any
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exceptions, the handler procedures are not called at all and the result
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of the <code class="code">thunk</code> call is the result of the <code class="code">catch</code>. Otherwise
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if an exception is thrown that matches <var class="var">key</var>, <var class="var">handler</var> is
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called with the continuation of the <code class="code">catch</code> call.
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</p></dd></dl>
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<p>Given the discussion from the previous section, it is most precise and
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concise to specify what <code class="code">catch</code> does by expressing it in terms of
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<code class="code">with-exception-handler</code>. Calling <code class="code">catch</code> with the three
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arguments is the same as:
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</p>
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<div class="example">
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<pre class="example-preformatted">(define (catch key thunk handler)
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(with-exception-handler
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(lambda (exn)
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(apply handler (exception-kind exn) (exception-args exn)))
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thunk
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#:unwind? #t
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#:unwind-for-type key))
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</pre></div>
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<p>By invoking <code class="code">with-exception-handler</code> with <code class="code">#:unwind? #t</code>,
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<code class="code">catch</code> sets up an escape continuation that will be invoked in an
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exceptional situation before the handler is called.
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</p>
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<p>If <code class="code">catch</code> is called with four arguments, then the use of
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<var class="var">thunk</var> should be replaced with:
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</p>
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<div class="example">
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<pre class="example-preformatted"> (lambda ()
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(with-throw-handler key thunk pre-unwind-handler))
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</pre></div>
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<p>As can be seen above, if a pre-unwind-handler is passed to <code class="code">catch</code>,
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it’s like calling <code class="code">with-throw-handler</code> inside the body thunk.
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</p>
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<p><code class="code">with-throw-handler</code> is the second of the older primitives, and is
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used to be able to intercept an exception that is being thrown before
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the stack is unwound. This could be to clean up some related state, to
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print a backtrace, or to pass information about the exception to a
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debugger, for example.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-with_002dthrow_002dhandler"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">with-throw-handler</strong> <var class="def-var-arguments">key thunk handler</var><a class="copiable-link" href="#index-with_002dthrow_002dhandler"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fwith_005fthrow_005fhandler"><span class="category-def">C Function: </span><span><strong class="def-name">scm_with_throw_handler</strong> <var class="def-var-arguments">(key, thunk, handler)</var><a class="copiable-link" href="#index-scm_005fwith_005fthrow_005fhandler"> ¶</a></span></dt>
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<dd><p>Add <var class="var">handler</var> to the dynamic context as a throw handler
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for key <var class="var">key</var>, then invoke <var class="var">thunk</var>.
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</p></dd></dl>
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<p>It’s not possible to exactly express <code class="code">with-throw-handler</code> in terms
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of <code class="code">with-exception-handler</code>, but we can get close.
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</p>
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<div class="example">
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<pre class="example-preformatted">(define (with-throw-handler key thunk handler)
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(with-exception-handler
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(lambda (exn)
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(when (or (eq? key #t) (eq? key (exception-kind exn)))
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(apply handler (exception-kind exn) (exception-args exn)))
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(raise-exception exn))
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thunk))
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</pre></div>
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<p>As you can see, unlike in the case of <code class="code">catch</code>, the handler for
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<code class="code">with-throw-handler</code> is invoked within the continuation of
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<code class="code">raise-exception</code>, before unwinding the stack. If the throw
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handler returns normally, the exception will be re-raised, to be handled
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by the next exception handler.
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</p>
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<p>The special wrinkle of <code class="code">with-throw-handler</code> that can’t be shown
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above is that if invoking the handler causes a <code class="code">raise-exception</code>
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instead of completing normally, the exception is thrown in the
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<em class="emph">original</em> dynamic environment of the <code class="code">raise-exception</code>. Any
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inner exception handler will get another shot at handling the exception.
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Here is an example to illustrate this behavior:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(catch 'a
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(lambda ()
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(with-throw-handler 'b
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(lambda ()
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(catch 'a
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(lambda ()
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(throw 'b))
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inner-handler))
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(lambda (key . args)
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(throw 'a))))
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outer-handler)
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</pre></div>
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<p>This code will call <code class="code">inner-handler</code> and then continue with the
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continuation of the inner <code class="code">catch</code>.
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</p>
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<p>Finally, we get to <code class="code">throw</code>, which is the older equivalent to
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<code class="code">raise-exception</code>.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-throw"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">throw</strong> <var class="def-var-arguments">key arg …</var><a class="copiable-link" href="#index-throw"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fthrow"><span class="category-def">C Function: </span><span><strong class="def-name">scm_throw</strong> <var class="def-var-arguments">(key, args)</var><a class="copiable-link" href="#index-scm_005fthrow"> ¶</a></span></dt>
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<dd><p>Raise an exception with kind <var class="var">key</var> and arguments <var class="var">args</var>.
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<var class="var">key</var> is a symbol, denoting the “kind” of the exception.
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</p></dd></dl>
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<p>Again, we can specify what <code class="code">throw</code> does by expressing it in terms
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of <code class="code">raise-exception</code>.
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</p>
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<div class="example">
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<pre class="example-preformatted">(define (throw key . args)
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(raise-exception (make-exception-from-throw key args)))
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</pre></div>
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<p>At this point, we should mention the primitive that manage the
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relationship between structured exception objects <code class="code">throw</code>.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-make_002dexception_002dfrom_002dthrow"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">make-exception-from-throw</strong> <var class="def-var-arguments">key args</var><a class="copiable-link" href="#index-make_002dexception_002dfrom_002dthrow"> ¶</a></span></dt>
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<dd><p>Create an exception object for the given <var class="var">key</var> and <var class="var">args</var> passed
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to <code class="code">throw</code>. This may be a specific type of exception, for example
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<code class="code">&programming-error</code>; Guile maintains a set of custom transformers
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for the various <var class="var">key</var> values that have been used historically.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-exception_002dkind"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">exception-kind</strong> <var class="def-var-arguments">exn</var><a class="copiable-link" href="#index-exception_002dkind"> ¶</a></span></dt>
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<dd><p>If <var class="var">exn</var> is an exception created via
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<code class="code">make-exception-from-throw</code>, return the corresponding <var class="var">key</var> for
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the exception. Otherwise, unless <var class="var">exn</var> is an exception of a type
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with a known mapping to <code class="code">throw</code>, return the symbol
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<code class="code">%exception</code>.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-exception_002dargs"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">exception-args</strong> <var class="def-var-arguments">exn</var><a class="copiable-link" href="#index-exception_002dargs"> ¶</a></span></dt>
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<dd><p>If <var class="var">exn</var> is an exception created via
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<code class="code">make-exception-from-throw</code>, return the corresponding <var class="var">args</var>
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for the exception. Otherwise, unless <var class="var">exn</var> is an exception of a
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type with a known mapping to <code class="code">throw</code>, return <code class="code">(list <var class="var">exn</var>)</code>.
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</p></dd></dl>
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<hr>
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<p>
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Next: <a href="Exceptions-and-C.html">Exceptions and C</a>, Previous: <a href="Raising-and-Handling-Exceptions.html">Raising and Handling Exceptions</a>, Up: <a href="Exceptions.html">Exceptions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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