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<a href='https://github.com/akkartik/mu/blob/master/exception1.mu'>https://github.com/akkartik/mu/blob/master/exception1.mu</a>
<pre id='vimCodeElement'>
<span id="L1" class="LineNr"> 1 </span><span class="Comment"># Example program showing exceptions built out of delimited continuations.</span>
<span id="L2" class="LineNr"> 2 </span>
<span id="L3" class="LineNr"> 3 </span><span class="Comment"># Since Mu is statically typed, we can't build an all-purpose higher-order</span>
<span id="L4" class="LineNr"> 4 </span><span class="Comment"># function called 'try'; it wouldn't know how many arguments the function</span>
<span id="L5" class="LineNr"> 5 </span><span class="Comment"># passed to it needs to take, what their types are, etc. Instead, until Mu</span>
<span id="L6" class="LineNr"> 6 </span><span class="Comment"># gets macros we'll directly use the continuation primitives.</span>
<span id="L7" class="LineNr"> 7 </span>
<span id="L8" class="LineNr"> 8 </span><span class="muRecipe">def</span> <a href='exception1.mu.html#L8'>main</a> [
<span id="L9" class="LineNr"> 9 </span>  <span class="Constant">local-scope</span>
<span id="L10" class="LineNr">10 </span>  <a href='exception1.mu.html#L15'>foo</a><span class="Constant"> false/no-exception</span>
<span id="L11" class="LineNr">11 </span>  <a href='exception1.mu.html#L15'>foo</a><span class="Constant"> true/raise-exception</span>
<span id="L12" class="LineNr">12 </span>]
<span id="L13" class="LineNr">13 </span>
<span id="L14" class="LineNr">14 </span><span class="Comment"># example showing exception handling</span>
<span id="L15" class="LineNr">15 </span><span class="muRecipe">def</span> <a href='exception1.mu.html#L15'>foo</a> raise-exception?:bool [
<span id="L16" class="LineNr">16 </span>  <span class="Constant">local-scope</span>
<span id="L17" class="LineNr">17 </span>  <span class="Constant">load-inputs</span>
<span id="L18" class="LineNr">18 </span>  <span class="Comment"># To run an instruction of the form:</span>
<span id="L19" class="LineNr">19 </span>  <span class="Comment">#   try f ...</span>
<span id="L20" class="LineNr">20 </span>  <span class="Comment"># write this:</span>
<span id="L21" class="LineNr">21 </span>  <span class="Comment">#   call-with-continuation-mark 999/exception-tag, f, ...</span>
<span id="L22" class="LineNr">22 </span>  <span class="Comment"># By convention we reserve tag 999 for exceptions.</span>
<span id="L23" class="LineNr">23 </span>  <span class="Comment">#</span>
<span id="L24" class="LineNr">24 </span>  <span class="Comment"># 'f' above may terminate at either a regular 'return' or a 'return-with-continuation-mark'.</span>
<span id="L25" class="LineNr">25 </span>  <span class="Comment"># We never re-call the continuation returned in the latter case;</span>
<span id="L26" class="LineNr">26 </span>  <span class="Comment"># its existence merely signals that an exception was raised.</span>
<span id="L27" class="LineNr">27 </span>  <span class="Comment"># So just treat it as a boolean.</span>
<span id="L28" class="LineNr">28 </span>  <span class="Comment"># The other inputs and outputs to 'call-with-continuation-mark' depend on</span>
<span id="L29" class="LineNr">29 </span>  <span class="Comment"># the function it is called with.</span>
<span id="L30" class="LineNr">30 </span>  exception-raised?:bool, err:text, result:num <span class="Special">&lt;-</span> <span class="muControl">call-with-continuation-mark</span> <span class="Constant">999/exception-tag</span>, <a href='exception1.mu.html#L51'>f</a>, raise-exception?
<span id="L31" class="LineNr">31 </span>  <span class="Delimiter">{</span>
<span id="L32" class="LineNr">32 </span>    <span class="muControl">break-if</span> exception-raised?
<span id="L33" class="LineNr">33 </span>    $print <span class="Constant">[normal exit; result ]</span> result <span class="Constant">10/newline</span>
<span id="L34" class="LineNr">34 </span>  <span class="Delimiter">}</span>
<span id="L35" class="LineNr">35 </span>  <span class="Delimiter">{</span>
<span id="L36" class="LineNr">36 </span>    <span class="muControl">break-unless</span> exception-raised?
<span id="L37" class="LineNr">37 </span>    $print <span class="Constant">[error caught: ]</span> err <span class="Constant">10/newline</span>
<span id="L38" class="LineNr">38 </span>  <span class="Delimiter">}</span>
<span id="L39" class="LineNr">39 </span>]
<span id="L40" class="LineNr">40 </span>
<span id="L41" class="LineNr">41 </span><span class="Comment"># A callee function that can raise an exception has some weird constraints at</span>
<span id="L42" class="LineNr">42 </span><span class="Comment"># the moment.</span>
<span id="L43" class="LineNr">43 </span><span class="Comment">#</span>
<span id="L44" class="LineNr">44 </span><span class="Comment"># The caller's 'call-with-continuation-mark' instruction may return with</span>
<span id="L45" class="LineNr">45 </span><span class="Comment"># either a regular 'return' or a 'return-continuation-until-mark'.</span>
<span id="L46" class="LineNr">46 </span><span class="Comment"># To handle both cases, regular 'return' instructions in the callee must</span>
<span id="L47" class="LineNr">47 </span><span class="Comment"># prepend an extra 0 result, in place of the continuation that may have been</span>
<span id="L48" class="LineNr">48 </span><span class="Comment"># returned.</span>
<span id="L49" class="LineNr">49 </span><span class="Comment"># This change to number of outputs violates our type system, so the call has</span>
<span id="L50" class="LineNr">50 </span><span class="Comment"># to be dynamically typed. The callee can't have a header.</span>
<span id="L51" class="LineNr">51 </span><span class="muRecipe">def</span> <a href='exception1.mu.html#L51'>f</a> [
<span id="L52" class="LineNr">52 </span>  <span class="Constant">local-scope</span>
<span id="L53" class="LineNr">53 </span>  raise-exception?:bool <span class="Special">&lt;-</span> <span class="Constant">next-input</span>
<span id="L54" class="LineNr">54 </span>  <span class="Delimiter">{</span>
<span id="L55" class="LineNr">55 </span>    <span class="muControl">break-unless</span> raise-exception?
<span id="L56" class="LineNr">56 </span>    <span class="Comment"># throw/raise: 2 results + implicit continuation (ignoring the continuation tag)</span>
<span id="L57" class="LineNr">57 </span>    <span class="muControl">return-continuation-until-mark</span> <span class="Constant">999/exception-tag</span>, <span class="Constant">[error will robinson!]</span>, <span class="Constant">0/unused</span>
<span id="L58" class="LineNr">58 </span>  <span class="Delimiter">}</span>
<span id="L59" class="LineNr">59 </span>  <span class="Comment"># normal return: 3 results including 0 continuation placeholder at start</span>
<span id="L60" class="LineNr">60 </span> <span class="muControl"> return</span> <span class="Constant">0/continuation-placeholder</span>,<span class="Constant"> null/no-error,</span> <span class="Constant">34/regular-result</span>
<span id="L61" class="LineNr">61 </span>]
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