<div dir="ltr"><div>Hi Simon. Thanks for the reply. My plugin appears to produce the HoleProv indirectly (and to my surprise) while building a dictionary to satisfy a given constraint, using mkNonCanonical and solveWanteds. I don't know why solveWanteds produces a HoleProv in this case and not in the many others I've tried. The constraint being solved in the example I included was 'HasRep (Vec ('S n_aCj7) (Key h_aCj6))', where HasRep is similar to Generic (from GHC.Generics), and there is a HasRep instance for Vec ('S n x). Come to think of it, the free type variable s_aD1S troubles me as well.<br><br>I'm not terribly confident in the code I use for constructing these dictionaries (setting up and calling solveWanteds) during Core-to-Core transformation. Do you know of any relevant example code, docs, etc?<br><br>Yes, I'm using -dcore-lint, as well explicitly linting each small transformation step (while debugging). Doing so has been very helpful in finding bugs quickly.<br><br></div>Cheers, - Conal<br></div><div class="gmail_extra"><br><div class="gmail_quote">On Mon, Apr 4, 2016 at 2:48 AM, 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-family:"Calibri",sans-serif">Definitely a bug. All HoleProvs should be eliminated by the type checker.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Did your plugin produce a HoleProv? It definitely should not do so; see the notes with that constructor.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Lint checks for this – did you run with –dcore-lint?<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Simon<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><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>Conal Elliott<br>
<b>Sent:</b> 02 April 2016 20:24<br>
<b>To:</b> <a href="mailto:ghc-devs@haskell.org" target="_blank">ghc-devs@haskell.org</a><br>
<b>Subject:</b> "opt_univ fell into a hole"<u></u><u></u></span></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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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<u></u><u></u></p>
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