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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Can you give a small example? Certainly any instance like<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"> instance Data x => Data (F x)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">is not allowed, of course, if F is a type function. It’s like not allowing<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">f (g x) = x<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">in the term language. Only constructors in patterns!<o:p></o:p></span></p>
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S<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><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> 25 May 2016 18:39<br>
<b>To:</b> ghc-devs@haskell.org<br>
<b>Subject:</b> instances for closed type families<o:p></o:p></span></p>
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<p class="MsoNormal"><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">
I am working on <a href="https://ghc.haskell.org/trac/ghc/ticket/12105" target="_blank">
https://ghc.haskell.org/trac/ghc/ticket/12105</a>, and have a type to ensure that the `HsMatchContext` has either a `RdrName` or a `Name`, not an `Id`.<br>
<br>
type family NameOrRdrName id where<br>
NameOrRdrName Id = Name<br>
NameOrRdrName Name = Name<br>
NameOrRdrName RdrName = RdrName<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">
Is there any way to declare `Data` and `OutputableBndr` instances for this?<o:p></o:p></p>
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Without it I am having to do something like<br>
<br>
instance (OutputableBndr name, OutputableBndr (NameOrRdrName name))<br>
=> Outputable (HsDecl name) where<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">
which requires UndecidableInstances.<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">
I get<br>
<br>
compiler/hsSyn/PlaceHolder.hs:114:19:<br>
Illegal type synonym family application in instance:<br>
NameOrRdrName id<br>
In the stand-alone deriving instance for ‘Data (NameOrRdrName id)’<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">
for the parameterised version and<br>
<br>
compiler/hsSyn/PlaceHolder.hs:115:19:<br>
Illegal type synonym family application in instance:<br>
NameOrRdrName RdrName<br>
In the stand-alone deriving instance for<br>
‘Data (NameOrRdrName RdrName)’<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">
for the one specific to `RdrName`.<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">
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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