<div dir="ltr"><div><div>There is an example at <a href="http://lpaste.net/164532">http://lpaste.net/164532</a> (attached as well)<br><br></div>The derived Data instance fails with <br><br>/home/alanz/Example.hs:19:19:<br> Illegal type synonym family application in instance:<br> NameOrRdrName id<br> In the stand-alone deriving instance for ‘Data (NameOrRdrName id)’<br><br></div><div>And how would an instance for Outputable be defined in terms of the existing ones?<br><br></div>Alan<br><br></div><div class="gmail_extra"><br><div class="gmail_quote">On Wed, May 25, 2016 at 8:03 PM, Simon Peyton Jones <span dir="ltr"><<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">Can you give a small example? Certainly any instance like<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"> instance Data x => Data (F x)<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">is not allowed, of course, if F is a type function. It’s like not allowing<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">f (g x) = x<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">in the term language. Only constructors in patterns!<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"><br>
S<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"><u></u> <u></u></span></p>
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<p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif" lang="EN-US">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif" lang="EN-US"> ghc-devs [mailto:<a href="mailto:ghc-devs-bounces@haskell.org" target="_blank">ghc-devs-bounces@haskell.org</a>]
<b>On Behalf Of </b>Alan & Kim Zimmerman<br>
<b>Sent:</b> 25 May 2016 18:39<br>
<b>To:</b> <a href="mailto:ghc-devs@haskell.org" target="_blank">ghc-devs@haskell.org</a><br>
<b>Subject:</b> instances for closed type families<u></u><u></u></span></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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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>
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type family NameOrRdrName id where<br>
NameOrRdrName Id = Name<br>
NameOrRdrName Name = Name<br>
NameOrRdrName RdrName = RdrName<u></u><u></u></p>
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Is there any way to declare `Data` and `OutputableBndr` instances for this?<u></u><u></u></p>
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Without it I am having to do something like<br>
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instance (OutputableBndr name, OutputableBndr (NameOrRdrName name))<br>
=> Outputable (HsDecl name) where<u></u><u></u></p>
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which requires UndecidableInstances.<u></u><u></u></p>
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I get<br>
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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)’<u></u><u></u></p>
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for the parameterised version and<br>
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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)’<u></u><u></u></p>
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for the one specific to `RdrName`.<u></u><u></u></p>
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Regards<u></u><u></u></p>
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<p class="MsoNormal" style="margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
Alan<u></u><u></u></p>
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