08a0eed699
Couple more tweaks to html rendering: a) SubX definitions no longer link redundantly to themselves. This long-standing issue turns out to be tractable to solve for SubX files since the syntax is so simple. b) Fix links to SubX definitions in other directories. I forgot that I have to always generate tags from the directory of the file being linkified. c) Fix link colors. Before we lost all syntax highlighting for functions and Globals. Now they maintain their colors, just add an underline.
124 lines
8.6 KiB
HTML
124 lines
8.6 KiB
HTML
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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>
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<pre id='vimCodeElement'>
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<span id="L1" class="LineNr"> 1 </span><span class="Comment"># Example program showing exceptions built out of delimited continuations.</span>
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<span id="L2" class="LineNr"> 2 </span>
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<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>
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<span id="L4" class="LineNr"> 4 </span><span class="Comment"># function called 'try'; it wouldn't know how many arguments the function</span>
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<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>
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<span id="L6" class="LineNr"> 6 </span><span class="Comment"># gets macros we'll directly use the continuation primitives.</span>
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<span id="L7" class="LineNr"> 7 </span>
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<span id="L8" class="LineNr"> 8 </span><span class="muRecipe">def</span> <a href='exception1.mu.html#L8'>main</a> [
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<span id="L9" class="LineNr"> 9 </span> <span class="Constant">local-scope</span>
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<span id="L10" class="LineNr">10 </span> <a href='exception1.mu.html#L15'>foo</a><span class="Constant"> false/no-exception</span>
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<span id="L11" class="LineNr">11 </span> <a href='exception1.mu.html#L15'>foo</a><span class="Constant"> true/raise-exception</span>
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<span id="L12" class="LineNr">12 </span>]
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<span id="L13" class="LineNr">13 </span>
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<span id="L14" class="LineNr">14 </span><span class="Comment"># example showing exception handling</span>
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<span id="L15" class="LineNr">15 </span><span class="muRecipe">def</span> <a href='exception1.mu.html#L15'>foo</a> raise-exception?:bool [
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<span id="L16" class="LineNr">16 </span> <span class="Constant">local-scope</span>
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<span id="L17" class="LineNr">17 </span> <span class="Constant">load-inputs</span>
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<span id="L18" class="LineNr">18 </span> <span class="Comment"># To run an instruction of the form:</span>
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<span id="L19" class="LineNr">19 </span> <span class="Comment"># try f ...</span>
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<span id="L20" class="LineNr">20 </span> <span class="Comment"># write this:</span>
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<span id="L21" class="LineNr">21 </span> <span class="Comment"># call-with-continuation-mark 999/exception-tag, f, ...</span>
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<span id="L22" class="LineNr">22 </span> <span class="Comment"># By convention we reserve tag 999 for exceptions.</span>
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<span id="L23" class="LineNr">23 </span> <span class="Comment">#</span>
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<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>
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<span id="L25" class="LineNr">25 </span> <span class="Comment"># We never re-call the continuation returned in the latter case;</span>
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<span id="L26" class="LineNr">26 </span> <span class="Comment"># its existence merely signals that an exception was raised.</span>
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<span id="L27" class="LineNr">27 </span> <span class="Comment"># So just treat it as a boolean.</span>
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<span id="L28" class="LineNr">28 </span> <span class="Comment"># The other inputs and outputs to 'call-with-continuation-mark' depend on</span>
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<span id="L29" class="LineNr">29 </span> <span class="Comment"># the function it is called with.</span>
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<span id="L30" class="LineNr">30 </span> exception-raised?:bool, err:text, result:num <span class="Special"><-</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?
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<span id="L31" class="LineNr">31 </span> <span class="Delimiter">{</span>
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<span id="L32" class="LineNr">32 </span> <span class="muControl">break-if</span> exception-raised?
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<span id="L33" class="LineNr">33 </span> $print <span class="Constant">[normal exit; result ]</span> result <span class="Constant">10/newline</span>
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<span id="L34" class="LineNr">34 </span> <span class="Delimiter">}</span>
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<span id="L35" class="LineNr">35 </span> <span class="Delimiter">{</span>
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<span id="L36" class="LineNr">36 </span> <span class="muControl">break-unless</span> exception-raised?
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<span id="L37" class="LineNr">37 </span> $print <span class="Constant">[error caught: ]</span> err <span class="Constant">10/newline</span>
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<span id="L38" class="LineNr">38 </span> <span class="Delimiter">}</span>
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<span id="L39" class="LineNr">39 </span>]
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<span id="L40" class="LineNr">40 </span>
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<span id="L41" class="LineNr">41 </span><span class="Comment"># A callee function that can raise an exception has some weird constraints at</span>
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<span id="L42" class="LineNr">42 </span><span class="Comment"># the moment.</span>
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<span id="L43" class="LineNr">43 </span><span class="Comment">#</span>
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<span id="L44" class="LineNr">44 </span><span class="Comment"># The caller's 'call-with-continuation-mark' instruction may return with</span>
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<span id="L45" class="LineNr">45 </span><span class="Comment"># either a regular 'return' or a 'return-continuation-until-mark'.</span>
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<span id="L46" class="LineNr">46 </span><span class="Comment"># To handle both cases, regular 'return' instructions in the callee must</span>
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<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>
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<span id="L48" class="LineNr">48 </span><span class="Comment"># returned.</span>
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<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>
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<span id="L50" class="LineNr">50 </span><span class="Comment"># to be dynamically typed. The callee can't have a header.</span>
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<span id="L51" class="LineNr">51 </span><span class="muRecipe">def</span> <a href='exception1.mu.html#L51'>f</a> [
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<span id="L52" class="LineNr">52 </span> <span class="Constant">local-scope</span>
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<span id="L53" class="LineNr">53 </span> raise-exception?:bool <span class="Special"><-</span> <span class="Constant">next-input</span>
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<span id="L54" class="LineNr">54 </span> <span class="Delimiter">{</span>
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<span id="L55" class="LineNr">55 </span> <span class="muControl">break-unless</span> raise-exception?
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<span id="L56" class="LineNr">56 </span> <span class="Comment"># throw/raise: 2 results + implicit continuation (ignoring the continuation tag)</span>
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<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>
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<span id="L58" class="LineNr">58 </span> <span class="Delimiter">}</span>
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<span id="L59" class="LineNr">59 </span> <span class="Comment"># normal return: 3 results including 0 continuation placeholder at start</span>
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<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>
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<span id="L61" class="LineNr">61 </span>]
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</pre>
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