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<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">I have created not one but three tickets arising from this thread:<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="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="font-family:Symbol;mso-fareast-language:EN-US"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><a href="https://ghc.haskell.org/trac/ghc/ticket/11449">https://ghc.haskell.org/trac/ghc/ticket/11449</a><o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="font-family:Symbol;mso-fareast-language:EN-US"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><a href="https://ghc.haskell.org/trac/ghc/ticket/11450">https://ghc.haskell.org/trac/ghc/ticket/11450</a><o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="font-family:Symbol;mso-fareast-language:EN-US"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><a href="https://ghc.haskell.org/trac/ghc/ticket/11451">https://ghc.haskell.org/trac/ghc/ticket/11451</a><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">I’d love comments on them: which of the three matter most to you folk? The first also involves a flag-naming question.<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"> Glasgow-haskell-users [mailto:glasgow-haskell-users-bounces@haskell.org]
<b>On Behalf Of </b>Edward Kmett<br>
<b>Sent:</b> 17 January 2016 14:44<br>
<b>To:</b> Andrew Farmer <xichekolas@gmail.com><br>
<b>Cc:</b> Henning Thielemann <lemming@henning-thielemann.de>; glasgow-haskell-users@haskell.org<br>
<b>Subject:</b> Re: suppress warning "Defined but not used: type variable ‘x’" in GHC-8.0<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" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
No, the type instance must match the class heading.<o:p></o:p></p>
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<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
I <b>can</b> use<o:p></o:p></p>
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<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span style="font-family:"Courier New"">instance Foo [_a] where</span><o:p></o:p></p>
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<span style="font-family:"Courier New""> type Bar [_a] = Int</span><o:p></o:p></p>
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<span style="font-family:"Courier New""> whatever = ... where</span><o:p></o:p></p>
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<span style="font-family:"Courier New""> bar :: _a -> Int</span><o:p></o:p></p>
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<span style="font-family:"Courier New""> bar = ...</span><o:p></o:p></p>
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<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
but that is a needlessly messy thing to request of every package everywhere.<o:p></o:p></p>
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<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
The arguments being pattern matched in a class associated type aren't really just bindings, they reference the surrounding context and so shouldn't be treated the same as the basic type family case.<o:p></o:p></p>
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<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
It isn't just the class associated type being mangled, it is every type variable that comes from the instance head in the entire body of every instance that happens to have a class associated type in it.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
Note that in the example above I added another ScopedTypeVariables reference to the same parameter, but it _also_ must be mangled despite having absolutely nothing to do with the class associated type.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
The existing convention has worked since 6.10 or so, when all of this stuff was invented in the first place, and the new state of affairs is clearly worse.<o:p></o:p></p>
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<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
-Edward<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
On Sun, Jan 17, 2016 at 3:16 AM, Andrew Farmer <<a href="mailto:xichekolas@gmail.com" target="_blank">xichekolas@gmail.com</a>> wrote:<o:p></o:p></p>
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<p>Can't you just:<o:p></o:p></p>
<p>instance Foo [a] where<br>
type Bar [_a] = Int<o:p></o:p></p>
<p>(At least I think I did that somewhere...)<o:p></o:p></p>
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<p class="MsoNormal">On Jan 16, 2016 9:24 PM, "Edward Kmett" <<a href="mailto:ekmett@gmail.com" target="_blank">ekmett@gmail.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">As a data point I now get thousands of occurrences of this warning across my packages.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">It is quite annoying.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">class Foo a where</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> type Bar a</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">instance Foo [a] where</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> type Bar [a] = Int</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">is enough to trigger it.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">And you can't turn it off by using _ as<o:p></o:p></p>
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<p><span style="font-family:"Courier New"">instance Foo [_] where<br>
type Bar [_] = Int</span><o:p></o:p></p>
<p>isn't legal.<o:p></o:p></p>
<p>I've been avoiding it for now by using<o:p></o:p></p>
<p><span style="font-family:"Courier New""> if impl(ghc >= 8)</span><o:p></o:p></p>
<p><span style="font-family:"Courier New""> ghc-options: -fno-warn-unused-matches</span><o:p></o:p></p>
<p><span style="font-family:"Arial",sans-serif">but this is a pretty awful addition to this warning as it stands.</span><o:p></o:p></p>
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<p class="MsoNormal">-Edward<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 Mon, Jan 11, 2016 at 2:12 PM, Henning Thielemann <<a href="mailto:lemming@henning-thielemann.de" target="_blank">lemming@henning-thielemann.de</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><br>
On Mon, 11 Jan 2016, Richard Eisenberg wrote:<o:p></o:p></p>
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<p class="MsoNormal">On Jan 9, 2016, at 6:44 PM, Henning Thielemann <<a href="mailto:lemming@henning-thielemann.de" target="_blank">lemming@henning-thielemann.de</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal"><br>
instance (Natural n) => Num.Integer (Un n) where<br>
type Repr (Un _n) = Unary<br>
<br>
<br>
GHC-7.6.3 and GHC-7.4.2 complain:<br>
Type indexes must match class instance head<br>
Found `Un _n' but expected `Un n'<br>
In the type synonym instance declaration for `Num.Repr'<br>
In the instance declaration for `Num.Integer (Un n)'<br>
<br>
<br>
GHC-7.8.4, GHC-7.10.3 and GHC-8.0 are happy with the difference.<o:p></o:p></p>
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<p class="MsoNormal"><br>
I'm surprised this is accepted at all. Looks like hogwash to me. I think you should post a bug report.<o:p></o:p></p>
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<p class="MsoNormal"><br>
Ok, but then GHC must not warn about the unused argument of Repr.<o:p></o:p></p>
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<p class="MsoNormal"><br>
_______________________________________________<br>
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<p class="MsoNormal"><o:p> </o:p></p>
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_______________________________________________<br>
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<p class="MsoNormal"><o:p> </o:p></p>
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