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<title>Control/Parallel.hs</title>
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<pre><a name="line-1"></a><span class='hs-comment'>{-# LANGUAGE CPP #-}</span>
<a name="line-2"></a>
<a name="line-3"></a><span class='hs-comment'>-----------------------------------------------------------------------------</span>
<a name="line-4"></a><span class='hs-comment'>-- |</span>
<a name="line-5"></a><span class='hs-comment'>-- Module : Control.Parallel</span>
<a name="line-6"></a><span class='hs-comment'>-- Copyright : (c) The University of Glasgow 2001</span>
<a name="line-7"></a><span class='hs-comment'>-- License : BSD-style (see the file libraries/base/LICENSE)</span>
<a name="line-8"></a><span class='hs-comment'>--</span>
<a name="line-9"></a><span class='hs-comment'>-- Maintainer : libraries@haskell.org</span>
<a name="line-10"></a><span class='hs-comment'>-- Stability : stable</span>
<a name="line-11"></a><span class='hs-comment'>-- Portability : portable</span>
<a name="line-12"></a><span class='hs-comment'>--</span>
<a name="line-13"></a><span class='hs-comment'>-- Parallel Constructs</span>
<a name="line-14"></a><span class='hs-comment'>--</span>
<a name="line-15"></a><span class='hs-comment'>-----------------------------------------------------------------------------</span>
<a name="line-16"></a>
<a name="line-17"></a><span class='hs-keyword'>module</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Parallel</span> <span class='hs-layout'>(</span>
<a name="line-18"></a> <span class='hs-varid'>par</span><span class='hs-layout'>,</span> <span class='hs-varid'>pseq</span>
<a name="line-19"></a> <span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-20"></a>
<a name="line-21"></a><span class='hs-cpp'>#ifdef __GLASGOW_HASKELL__</span>
<a name="line-22"></a><span class='hs-keyword'>import</span> <span class='hs-keyword'>qualified</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'>par</span><span class='hs-layout'>,</span> <span class='hs-varid'>pseq</span> <span class='hs-layout'>)</span>
<a name="line-23"></a>
<a name="line-24"></a><span class='hs-keyword'>infixr</span> <span class='hs-num'>0</span> <span class='hs-varop'>`par`</span><span class='hs-layout'>,</span> <span class='hs-varop'>`pseq`</span>
<a name="line-25"></a><span class='hs-cpp'>#endif</span>
<a name="line-26"></a>
<a name="line-27"></a><span class='hs-comment'>-- Maybe parIO and the like could be added here later.</span>
<a name="line-28"></a>
<a name="line-29"></a><a name="par"></a><span class='hs-comment'>-- | Indicates that it may be beneficial to evaluate the first</span>
<a name="line-30"></a><span class='hs-comment'>-- argument in parallel with the second. Returns the value of the</span>
<a name="line-31"></a><span class='hs-comment'>-- second argument.</span>
<a name="line-32"></a><span class='hs-comment'>--</span>
<a name="line-33"></a><span class='hs-comment'>-- @a ``par`` b@ is exactly equivalent semantically to @b@.</span>
<a name="line-34"></a><span class='hs-comment'>--</span>
<a name="line-35"></a><span class='hs-comment'>-- @par@ is generally used when the value of @a@ is likely to be</span>
<a name="line-36"></a><span class='hs-comment'>-- required later, but not immediately. Also it is a good idea to</span>
<a name="line-37"></a><span class='hs-comment'>-- ensure that @a@ is not a trivial computation, otherwise the cost of</span>
<a name="line-38"></a><span class='hs-comment'>-- spawning it in parallel overshadows the benefits obtained by</span>
<a name="line-39"></a><span class='hs-comment'>-- running it in parallel.</span>
<a name="line-40"></a><span class='hs-comment'>--</span>
<a name="line-41"></a><span class='hs-comment'>-- Note that actual parallelism is only supported by certain</span>
<a name="line-42"></a><span class='hs-comment'>-- implementations (GHC with the @-threaded@ option, and GPH, for</span>
<a name="line-43"></a><span class='hs-comment'>-- now). On other implementations, @par a b = b@.</span>
<a name="line-44"></a><span class='hs-comment'>--</span>
<a name="line-45"></a><span class='hs-definition'>par</span> <span class='hs-keyglyph'>::</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>b</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>b</span>
<a name="line-46"></a><span class='hs-cpp'>#ifdef __GLASGOW_HASKELL__</span>
<a name="line-47"></a><span class='hs-definition'>par</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>GHC</span><span class='hs-varop'>.</span><span class='hs-conid'>Conc</span><span class='hs-varop'>.</span><span class='hs-varid'>par</span>
<a name="line-48"></a><span class='hs-cpp'>#else</span>
<a name="line-49"></a><span class='hs-comment'>-- For now, Hugs does not support par properly.</span>
<a name="line-50"></a><span class='hs-definition'>par</span> <span class='hs-varid'>a</span> <span class='hs-varid'>b</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>b</span>
<a name="line-51"></a><span class='hs-cpp'>#endif</span>
<a name="line-52"></a>
<a name="line-53"></a><a name="pseq"></a><span class='hs-comment'>-- | Semantically identical to 'seq', but with a subtle operational</span>
<a name="line-54"></a><span class='hs-comment'>-- difference: 'seq' is strict in both its arguments, so the compiler</span>
<a name="line-55"></a><span class='hs-comment'>-- may, for example, rearrange @a ``seq`` b@ into @b ``seq`` a ``seq`` b@.</span>
<a name="line-56"></a><span class='hs-comment'>-- This is normally no problem when using 'seq' to express strictness,</span>
<a name="line-57"></a><span class='hs-comment'>-- but it can be a problem when annotating code for parallelism,</span>
<a name="line-58"></a><span class='hs-comment'>-- because we need more control over the order of evaluation; we may</span>
<a name="line-59"></a><span class='hs-comment'>-- want to evaluate @a@ before @b@, because we know that @b@ has</span>
<a name="line-60"></a><span class='hs-comment'>-- already been sparked in parallel with 'par'.</span>
<a name="line-61"></a><span class='hs-comment'>--</span>
<a name="line-62"></a><span class='hs-comment'>-- This is why we have 'pseq'. In contrast to 'seq', 'pseq' is only</span>
<a name="line-63"></a><span class='hs-comment'>-- strict in its first argument (as far as the compiler is concerned),</span>
<a name="line-64"></a><span class='hs-comment'>-- which restricts the transformations that the compiler can do, and</span>
<a name="line-65"></a><span class='hs-comment'>-- ensures that the user can retain control of the evaluation order.</span>
<a name="line-66"></a><span class='hs-comment'>--</span>
<a name="line-67"></a><span class='hs-definition'>pseq</span> <span class='hs-keyglyph'>::</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>b</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>b</span>
<a name="line-68"></a><span class='hs-cpp'>#ifdef __GLASGOW_HASKELL__</span>
<a name="line-69"></a><span class='hs-definition'>pseq</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>GHC</span><span class='hs-varop'>.</span><span class='hs-conid'>Conc</span><span class='hs-varop'>.</span><span class='hs-varid'>pseq</span>
<a name="line-70"></a><span class='hs-cpp'>#else</span>
<a name="line-71"></a><span class='hs-definition'>pseq</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>seq</span>
<a name="line-72"></a><span class='hs-cpp'>#endif</span>
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