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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Greg<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">In GHC, big cases are done as tables (if dense) or trees (if sparse). If you have some examples where things go bad, do submit a bug report.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">For big dispatches on strings, I’m pretty sure we do something linear, top to bottom. I’d be strongly inclined to use a proper Map structure if you want good
performance on that. Is there some reason you don’t want to?<br>
<br>
Simon<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users-bounces@haskell.org]
<b>On Behalf Of </b>Greg Weber<br>
<b>Sent:</b> 09 October 2011 17:39<br>
<b>To:</b> GHC users<br>
<b>Subject:</b> log time instead of linear for case matching<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">We have a couple use cases in Yesod that can potentially match many different patterns. Routing connects the url of an http request to a Haskell function. The current code just uses a pattern match, which scales linearly. So if a site has
a hundred different routes (urls), it could take 100 comparisons to finally match the url to a function. Michael Snoyman is writing some code to make this issue obsolete. One of the things it does is use a Map lookup instead of a case match.<o:p></o:p></p>
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<p class="MsoNormal">More worrying is our system for internationalization of a website. A user is supposed to make a sum type with every custom message as a constructor in that sum type.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">data Msg = Model<o:p></o:p></p>
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<p class="MsoNormal"> | Year<o:p></o:p></p>
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<p class="MsoNormal"> | Please<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">-- Rendering function for English.<o:p></o:p></p>
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<p class="MsoNormal">renderEnglish Model = "Model"<o:p></o:p></p>
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<p class="MsoNormal">renderEnglish Year = "Year"<o:p></o:p></p>
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<p class="MsoNormal">renderEnglish Please = "Please fill in your car's details"<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">-- Rendering function for Swedish.<o:p></o:p></p>
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<p class="MsoNormal">renderSwedish Model = "Modell"<o:p></o:p></p>
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<p class="MsoNormal">renderSwedish Year = "Årgång"<o:p></o:p></p>
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<p class="MsoNormal">renderSwedish Please = "Vänligen fyll i uppgifterna för din bil"<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">So if there are 100 custom messages on a site, that will setup a case match with potentially 100 comparisons.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Note that we are using this technique for type safety- switching to a map here would be difficult. We want to know at compile time that a translation exists for every message.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">So first of all I am wondering if a sum type comparison does in fact scale linearly or if there are optimizations in place to make the lookup constant or logarithmic. Second, I as wondering (for the routing case) if Haskell can make a string
case comparison logarithmic so that users can use case comparisons instead of maps for string collections that are known at compile time and won't change.<o:p></o:p></p>
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