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<p class="MsoNormal"><span style="font-size:12.0pt">Richard is right. <o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Let’s attach this useful info to the
<i>ticket</i>, rather than ghc-devs. I’ve done that for this exchange.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><a href="https://ghc.haskell.org/trac/ghc/ticket/10789#comment:18">https://ghc.haskell.org/trac/ghc/ticket/10789#comment:18</a><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><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>Richard Eisenberg<br>
<b>Sent:</b> 18 June 2018 04:21<br>
<b>To:</b> sasa bogicevic <brutallesale@gmail.com><br>
<b>Cc:</b> ghc-devs@haskell.org<br>
<b>Subject:</b> Re: #10789<o:p></o:p></span></p>
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<p class="MsoNormal">On Jun 17, 2018, at 10:01 AM, sasa bogicevic <<a href="mailto:brutallesale@gmail.com">brutallesale@gmail.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif">So I guess that we need to check if one of the kinds of two types we are comparing defaults to * (or Type if you will) and then<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif">add new warning that will be more descriptive as to why the failure happened. Maybe there is a way to check if what we are<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif">comparing are actually type families so that would make the job easier I guess.<o:p></o:p></span></p>
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<p class="MsoNormal">I don't think the problem is particular to `Type` or defaulting. Instead, the problem is when one of the two mismatched types is a type family application where the type family has equations that pattern-match on an invisible parameter,
and it's that invisible-parameter matching that's gone awry. Now that I think about it, detecting these particular conditions might be tricky: you might need to edit code in FamInstEnv that does type family equation lookup to return diagnostic information
if a match fails. (I would look at reduceTyFamApp_maybe, and perhaps it can return something more interesting than Nothing in the failure case.) <o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif">Richard Eisenberg offered some help on this but I am not sure how to grab hold of him so I'd appreciate any help I could get.<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">Just email! :)<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Thanks for looking into this!<o:p></o:p></p>
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<p class="MsoNormal">Richard<o:p></o:p></p>
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