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<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Definitely a bug. All HoleProvs should be eliminated by the type checker.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Did your plugin produce a HoleProv? It definitely should not do so; see the notes with that constructor.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Lint checks for this – did you run with –dcore-lint?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;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>Conal Elliott<br>
<b>Sent:</b> 02 April 2016 20:24<br>
<b>To:</b> ghc-devs@haskell.org<br>
<b>Subject:</b> "opt_univ fell into a hole"<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:6.0pt;margin-left:0cm">
I'm getting the following error message from a GHC plugin I'm developing (note GHC version):<br>
<br>
ghc-stage2: panic! (the 'impossible' happened)<br>
(GHC version 8.1.20160307 for x86_64-apple-darwin):<br>
opt_univ fell into a hole {aD1S}<br>
<br>
I don't get this error when compiling without my plugin, so I may well be violating a compiler invariant.<br>
<br>
Shortly before the error, the plugin produced the following dictionary expression, which does indeed contain a `UnivCo` with a `HoleProv`. The plugin does sometimes generate `UnivCo`s but not with `HoleProv`.<br>
<br>
let {<br>
$dHasRep_aD1T :: HasRep (Vec 'Z s_aD1R[fuv:0])<br>
$dHasRep_aD1T = $fHasRepVec0 @ s_aD1R[fuv:0] } in<br>
let {<br>
$dHasRepZLVeczqZZZLKeyhZRZR_Ii5CR :: HasRep (Vec 'Z (Key h_aCnh))<br>
$dHasRepZLVeczqZZZLKeyhZRZR_Ii5CR =<br>
$dHasRep_aD1T<br>
`cast` ((HasRep<br>
(Vec<br>
<'Z>_N<br>
(Sym U(hole:{aD1S}, Key h_aCnh, s_aD1R[fuv:0])_N))_N)_R<br>
:: HasRep (Vec 'Z s_aD1R[fuv:0])<br>
~R# HasRep (Vec 'Z (Key h_aCnh))) } in<br>
$dHasRepZLVeczqZZZLKeyhZRZR_Ii5CR<br>
<br>
Here, `Key` is a type family from `Data.Key` in the keys package, and `Vec` is a GADT of statically length-indexed lists.<br>
<br>
Suggestions?<br>
<br>
Thanks, - Conal<o:p></o:p></p>
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