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<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Spot on Alexis.    Should not be hard to fix this.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">I think the best thing would be in mkCallUDs
<o:p></o:p></span></p>
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<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1"><span style="mso-fareast-language:EN-US">not to use SpecDict for implicit parameters; instead use UnspecArg<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1"><span style="mso-fareast-language:EN-US">don’t require length theta = length dicts.  Need to think about what else instead.  Isn’t it implied by the arity test?<o:p></o:p></span></li></ul>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Make a ticket!  Happy to help if you or Sandy need anything from me.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> ghc-devs <ghc-devs-bounces@haskell.org>
<b>On Behalf Of </b>Alexis King<br>
<b>Sent:</b> 15 March 2020 02:47<br>
<b>To:</b> Sandy Maguire <sandy@sandymaguire.me><br>
<b>Cc:</b> ghc-devs <ghc-devs@haskell.org><br>
<b>Subject:</b> Re: Specializing functions with implicit parameters<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">On Mar 14, 2020, at 20:03, Sandy Maguire <<a href="mailto:sandy@sandymaguire.me">sandy@sandymaguire.me</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">What GHC are you testing against? I suspect <a href="https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.haskell.org%2Fghc%2Fghc%2Fmerge_requests%2F668&data=02%7C01%7Csimonpj%40microsoft.com%7Cfc70cfc46c7146c59bd008d7c88b2c76%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637198372382352399&sdata=C%2BzhSCJ2EREtrQD%2FZedu03w0NxQKOnszWKXiKY5cSc4%3D&reserved=0">
https://gitlab.haskell.org/ghc/ghc/merge_requests/668</a> will fix this.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I’ve tested against HEAD. I think the change you link is helpful, but it doesn’t
<i>quite</i> get there: the usage gets dumped before specHeader even gets a chance to look at the call. The relevant bit of code is here:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><a href="https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.haskell.org%2Fghc%2Fghc%2Fblob%2F1de3ab4a147eeb0b34b24a3c0e91f174e6e5cb79%2Fcompiler%2Fspecialise%2FSpecialise.hs%23L2274-2302&data=02%7C01%7Csimonpj%40microsoft.com%7Cfc70cfc46c7146c59bd008d7c88b2c76%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637198372382352399&sdata=qfC7hSxoWNABKLpgJe6kqzTeDG1jmLwyane9W8UCOh4%3D&reserved=0">https://gitlab.haskell.org/ghc/ghc/blob/1de3ab4a147eeb0b34b24a3c0e91f174e6e5cb79/compiler/specialise/Specialise.hs#L2274-2302</a><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Specifically, this line seals the deal:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">    ClassPred cls _ -> not (isIPClass cls)  -- Superclasses can't be IPs<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">So maybe the right fix is just to change the role of type_determines_value so that it turns SpecDicts into UnspecArgs, and then with your change everything would just happily work out.<o:p></o:p></p>
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