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<pre><a name="line-1"></a><span class='hs-comment'>{-
<a name="line-2"></a>The parallel function (specialised to lists) is equivalent to:
<a name="line-3"></a>
<a name="line-4"></a>import Control.Parallel.Strategies
<a name="line-5"></a>parallel :: [IO [a]] -> IO [[a]]
<a name="line-6"></a>parallel = return . withStrategy (parList $ seqList r0) . map unsafePerformIO
<a name="line-7"></a>
<a name="line-8"></a>However, this version performs about 10% slower with 2 processors in GHC 6.12.1
<a name="line-9"></a>-}</span>
<a name="line-10"></a>
<a name="line-11"></a><span class='hs-keyword'>module</span> <span class='hs-conid'>Parallel</span><span class='hs-layout'>(</span><span class='hs-varid'>parallel</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-12"></a>
<a name="line-13"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>System</span><span class='hs-varop'>.</span><span class='hs-conid'>IO</span><span class='hs-varop'>.</span><span class='hs-conid'>Unsafe</span>
<a name="line-14"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>GHC</span><span class='hs-varop'>.</span><span class='hs-conid'>Conc</span><span class='hs-layout'>(</span><span class='hs-varid'>numCapabilities</span><span class='hs-layout'>)</span>
<a name="line-15"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Concurrent</span>
<a name="line-16"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span>
<a name="line-17"></a>
<a name="line-18"></a>
<a name="line-19"></a><a name="parallel"></a><span class='hs-definition'>parallel</span> <span class='hs-keyglyph'>::</span> <span class='hs-keyglyph'>[</span><span class='hs-conid'>IO</span> <span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>IO</span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span>
<a name="line-20"></a><span class='hs-definition'>parallel</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyword'>if</span> <span class='hs-varid'>numCapabilities</span> <span class='hs-varop'><=</span> <span class='hs-num'>1</span> <span class='hs-keyword'>then</span> <span class='hs-varid'>parallel1</span> <span class='hs-keyword'>else</span> <span class='hs-varid'>parallelN</span>
<a name="line-21"></a>
<a name="line-22"></a>
<a name="line-23"></a><a name="parallel1"></a><span class='hs-definition'>parallel1</span> <span class='hs-keyglyph'>::</span> <span class='hs-keyglyph'>[</span><span class='hs-conid'>IO</span> <span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>IO</span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span>
<a name="line-24"></a><span class='hs-definition'>parallel1</span> <span class='hs-conid'>[]</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>return</span> <span class='hs-conid'>[]</span>
<a name="line-25"></a><span class='hs-definition'>parallel1</span> <span class='hs-layout'>(</span><span class='hs-varid'>x</span><span class='hs-conop'>:</span><span class='hs-varid'>xs</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyword'>do</span>
<a name="line-26"></a> <span class='hs-varid'>x2</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>x</span>
<a name="line-27"></a> <span class='hs-varid'>xs2</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>unsafeInterleaveIO</span> <span class='hs-varop'>$</span> <span class='hs-varid'>parallel1</span> <span class='hs-varid'>xs</span>
<a name="line-28"></a> <span class='hs-varid'>return</span> <span class='hs-varop'>$</span> <span class='hs-varid'>x2</span><span class='hs-conop'>:</span><span class='hs-varid'>xs2</span>
<a name="line-29"></a>
<a name="line-30"></a>
<a name="line-31"></a><a name="parallelN"></a><span class='hs-definition'>parallelN</span> <span class='hs-keyglyph'>::</span> <span class='hs-keyglyph'>[</span><span class='hs-conid'>IO</span> <span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>IO</span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span>
<a name="line-32"></a><span class='hs-definition'>parallelN</span> <span class='hs-varid'>xs</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyword'>do</span>
<a name="line-33"></a> <span class='hs-varid'>ms</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>mapM</span> <span class='hs-layout'>(</span><span class='hs-varid'>const</span> <span class='hs-varid'>newEmptyMVar</span><span class='hs-layout'>)</span> <span class='hs-varid'>xs</span>
<a name="line-34"></a> <span class='hs-varid'>chan</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>newChan</span>
<a name="line-35"></a> <span class='hs-varid'>mapM_</span> <span class='hs-layout'>(</span><span class='hs-varid'>writeChan</span> <span class='hs-varid'>chan</span> <span class='hs-varop'>.</span> <span class='hs-conid'>Just</span><span class='hs-layout'>)</span> <span class='hs-varop'>$</span> <span class='hs-varid'>zip</span> <span class='hs-varid'>ms</span> <span class='hs-varid'>xs</span>
<a name="line-36"></a> <span class='hs-varid'>replicateM_</span> <span class='hs-varid'>numCapabilities</span> <span class='hs-layout'>(</span><span class='hs-varid'>writeChan</span> <span class='hs-varid'>chan</span> <span class='hs-conid'>Nothing</span> <span class='hs-varop'>>></span> <span class='hs-varid'>forkIO</span> <span class='hs-layout'>(</span><span class='hs-varid'>f</span> <span class='hs-varid'>chan</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span>
<a name="line-37"></a> <span class='hs-varid'>parallel1</span> <span class='hs-varop'>$</span> <span class='hs-varid'>map</span> <span class='hs-varid'>takeMVar</span> <span class='hs-varid'>ms</span>
<a name="line-38"></a> <span class='hs-keyword'>where</span>
<a name="line-39"></a> <span class='hs-varid'>f</span> <span class='hs-varid'>chan</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyword'>do</span>
<a name="line-40"></a> <span class='hs-varid'>v</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>readChan</span> <span class='hs-varid'>chan</span>
<a name="line-41"></a> <span class='hs-keyword'>case</span> <span class='hs-varid'>v</span> <span class='hs-keyword'>of</span>
<a name="line-42"></a> <span class='hs-conid'>Nothing</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>return</span> <span class='hs-conid'>()</span>
<a name="line-43"></a> <span class='hs-conid'>Just</span> <span class='hs-layout'>(</span><span class='hs-varid'>m</span><span class='hs-layout'>,</span><span class='hs-varid'>x</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-keyword'>do</span>
<a name="line-44"></a> <span class='hs-varid'>x'</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>x</span>
<a name="line-45"></a> <span class='hs-varid'>putMVar</span> <span class='hs-varid'>m</span> <span class='hs-varid'>x'</span>
<a name="line-46"></a> <span class='hs-varid'>f</span> <span class='hs-varid'>chan</span>
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