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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">I think that’s ok. It’s the situation right now.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">Simon<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""> Richard Eisenberg [mailto:eir@cis.upenn.edu]
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<b>Sent:</b> 21 October 2013 18:08<br>
<b>To:</b> ghc-devs Devs; Simon Peyton-Jones; Edward Kmett<br>
<b>Subject:</b> small hiccup in new GND check<o:p></o:p></span></p>
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<p class="MsoNormal">Hi all,<o:p></o:p></p>
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<p class="MsoNormal">The implementation of the new GND check is done and was rather straightforward. But, it brings up a new design issue: With the old check, a writer of a class C could say `type role C nominal` to prevent GND use with that class. Now, all
classes have nominal parameters, and there is no (clean) way for a class writer to stop the use of GND. Perhaps this is OK -- classes can't in general be abstract -- but I thought we should pause a moment to reflect on this consequence of the design.<o:p></o:p></p>
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<p class="MsoNormal">For the curious, you can see the code in my github fork at <a href="https://raw.github.com/goldfirere/ghc/master/compiler/typecheck/TcDeriv.lhs">https://raw.github.com/goldfirere/ghc/master/compiler/typecheck/TcDeriv.lhs</a> (search for
`role_errs`). An example of the new error message is in <a href="https://github.com/goldfirere/testsuite/blob/master/tests/roles/should_fail/Roles10.stderr">https://github.com/goldfirere/testsuite/blob/master/tests/roles/should_fail/Roles10.stderr</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">I'm running further tests now, so wibbles to the above may happen.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Richard<o:p></o:p></p>
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