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<p class="MsoNormal">There is definitely no pure way to get from ‘a’ to its constraints.<o:p></o:p></p>
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<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1">It is far from clear what “its constraints” are. Is (C a b) such a constraint? C [a] b? What about superclasses?<o:p></o:p></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1">Constraints vary depending on where you are. GADT matches can bring into scope extra constraints on existing type variables.<o:p></o:p></li></ul>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">So as Ben says, to give a sensible response you’ll need to explain more about your goal<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Simon<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> ghc-devs <ghc-devs-bounces@haskell.org>
<b>On Behalf Of </b>Alejandro Serrano Mena<br>
<b>Sent:</b> 16 January 2020 16:19<br>
<b>To:</b> GHC developers <ghc-devs@haskell.org><br>
<b>Subject:</b> Find constraints over type<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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Dear GHC devs,<o:p></o:p></p>
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I am trying to figure out a way to obtain the constraints that hold over a type. Let me give you an example: suppose that I write the following function:<o:p></o:p></p>
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<o:p> </o:p></p>
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f :: Eq a => [a] -> Bool<o:p></o:p></p>
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f xs = xs == []<o:p></o:p></p>
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<o:p> </o:p></p>
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If I ask for the type of the Var ' xs', I get back '[a]'. This is correct, but I am missing the crucial information that '[a]' must be Eq.<o:p></o:p></p>
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<o:p> </o:p></p>
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Is there an easy way to get it? It seems that 'varType' doesn't give me enough information.<o:p></o:p></p>
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<o:p> </o:p></p>
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Regards,<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
Alejandro<o:p></o:p></p>
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