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<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">I agree too.<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">Is there a ticket? Does it emit a warning right now?<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>Richard Eisenberg<br>
<b>Sent:</b> 11 December 2015 16:02<br>
<b>To:</b> Michael Sloan <mgsloan@gmail.com><br>
<b>Cc:</b> ghc-devs@haskell.org<br>
<b>Subject:</b> Re: Unused type variables with non-linear patterns<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">I agree with Michael. This should not emit a warning.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">While you're in this area, make sure that<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">> type instance F _ _ = Int<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">indeed matches `F Char Bool`. It's easy to imagine both underscores being treated as the same variable, which is not what the user intends.<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,<o:p></o:p></p>
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<p class="MsoNormal">Richard<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Dec 10, 2015, at 4:32 PM, Michael Sloan <<a href="mailto:mgsloan@gmail.com">mgsloan@gmail.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">I definitely think this should be considered a usage. Ideally, you should always be able to resolve warnings, but there'd be no straightforward way to do so if LHS-only usages cause warnings. Also, It's quite useful to use type variables
only on the LHS. Usage example: <a href="https://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2fhackage.haskell.org%2fpackage%2fbase-4.8.1.0%2fdocs%2fsrc%2fData.Type.Equality.html%23EqStar&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7c65bff682e66a434f1f1708d302447d3f%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=gkgghrbPZ7shUtk4z2WesVVJaQD8wlKhlbXWmFHpjEs%3d">https://hackage.haskell.org/package/base-4.8.1.0/docs/src/Data.Type.Equality.html#EqStar</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">-Michael<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Thu, Dec 10, 2015 at 6:48 AM, Jan Stolarek <<a href="mailto:jan.stolarek@p.lodz.pl" target="_blank">jan.stolarek@p.lodz.pl</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">An intersting thing came up when implementing warnings for unused type variables in type family<br>
patterns (#10982 / D1576):<br>
<br>
type family X a b where<br>
X a a = Int<br>
<br>
Should we emit a warning that `a` is unused? It does not appear in the RHS but it is actually used<br>
because of non-linear pattern. We can fix this by prefixing both appearances of `a` with an<br>
underscore - this silences the warning and maintains semantics. Nevertheless, if a variable is<br>
unused it should be perfectly valid to replace it with an underscore - that is not the case in<br>
this example. Thoughts?<br>
<br>
Janek<br>
<br>
---<br>
Politechnika Łódzka<br>
Lodz University of Technology<br>
<br>
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