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<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Yes that looks right. A class instance can’t dispatch on a type family application. In haskell we don’t allow<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">reverse (a ++ b) = reverse a ++ reverse b<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">and it’s the same for type families and class instances. You should do the type-family reduction yourself, as you do below.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Simon<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif"> ghc-devs [mailto:ghc-devs-bounces@haskell.org]
<b>On Behalf Of </b>Alan & Kim Zimmerman<br>
<b>Sent:</b> 18 May 2017 08:15<br>
<b>To:</b> ghc-devs@haskell.org<br>
<b>Subject:</b> Re: Type families and classes<o:p></o:p></span></p>
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And to answer my own question, it seems<br>
<br>
<span style="font-size:10.0pt;font-family:"Courier New"">instance HasSourceText SourceText where<br>
noSourceText = NoSourceText<br>
sourceText s = SourceText s<br>
</span><span style="font-size:10.0pt"> getSourceText a = a<br>
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</span><o:p></o:p></p>
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And then applying the appropriate constraint at the use-site does the job.<o:p></o:p></p>
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So<br>
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<span style="font-family:"Courier New"">noSyntaxExpr :: (HasSourceText (XHsString x)) => SyntaxExpr x id</span><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">
Alan<o:p></o:p></p>
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<o:p> </o:p></p>
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<o:p> </o:p></p>
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<o:p> </o:p></p>
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On 18 May 2017 at 09:04, Alan & Kim Zimmerman <<a href="mailto:alan.zimm@gmail.com" target="_blank">alan.zimm@gmail.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:12.0pt;margin-left:0cm">
Hi all<o:p></o:p></p>
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I am experimenting with Trees that Grow [1] in the context of the GHC HsSyn AST, and wanting to express that a given extension point needs to have certain properties.<o:p></o:p></p>
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The specific case is to be able to contain a SourceText, in the context of HsLit<o:p></o:p></p>
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So I have (stripped down)<br>
<br>
<span style="font-size:10.0pt;font-family:"Courier New"">data GHCX<br>
<br>
type family XHsString x<br>
<br>
type instance XHsString GHCX = SourceText<br>
<br>
class HasSourceText a where<br>
-- Provide setters to mimic existing constructors<br>
noSourceText :: a<br>
sourceText :: String -> a<br>
<br>
getSourceText :: a -> SourceText<br>
<br>
instance HasSourceText (XHsString GHCX) where<br>
noSourceText = NoSourceText<br>
sourceText s = SourceText s<br>
</span><span style="font-size:10.0pt"> getSourceText a = a<br>
<br>
</span><o:p></o:p></p>
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But this gives an error<br>
<br>
compiler/hsSyn/HsExtension.hs:80:10: error:<br>
• Illegal type synonym family application in instance:<br>
XHsString GHCX<br>
• In the instance declaration for ‘HasSourceText (XHsString GHCX)’<o:p></o:p></p>
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Is there some way to achieve this, or is it simply impossible?<o:p></o:p></p>
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The full work-in-progress source is here[2]<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">
Alan<o:p></o:p></p>
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<o:p> </o:p></p>
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<br>
[1] <a href="https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Farxiv.org%2Fabs%2F1610.04799&data=02%7C01%7Csimonpj%40microsoft.com%7Cec7c24b0b3aa4d0b0af608d49dbdc4c5%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636306885764359008&sdata=ojtHcyFU%2BooW%2FdxDMaPpsDSUH8kfEF6y0NF4Encv1WE%3D&reserved=0" target="_blank">
https://arxiv.org/abs/1610.04799</a><br>
[2] <a href="https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Falanz%2Fghc%2Fblob%2Fdf1c3b3d42284dd121086e6c571793f19e758977%2Fcompiler%2FhsSyn%2FHsExtension.hs%23L73&data=02%7C01%7Csimonpj%40microsoft.com%7Cec7c24b0b3aa4d0b0af608d49dbdc4c5%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636306885764359008&sdata=SSvg%2FOyBUAJoZ3HMAEpaUYuYdXROVNGeyo7bXTnUe2k%3D&reserved=0" target="_blank">
https://github.com/alanz/ghc/blob/df1c3b3d42284dd121086e6c571793f19e758977/compiler/hsSyn/HsExtension.hs#L73</a><o:p></o:p></p>
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