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<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Maybe put this all on a ticket?<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">Everyone: would someone like to dig into this? I’m so under water that I just can’t<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"> Jan Bracker [mailto:jan.bracker@googlemail.com]
<br>
<b>Sent:</b> 15 January 2016 14:03<br>
<b>To:</b> Simon Peyton Jones <simonpj@microsoft.com><br>
<b>Cc:</b> Richard Eisenberg <eir@cis.upenn.edu>; ghc-devs@haskell.org<br>
<b>Subject:</b> Re: Explanation of a core-lint warning (Bad getNth)<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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I have condensed a self-contained plugin and an example application that reproduces the error. You can find it here:<o:p></o:p></p>
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<a href="https://github.com/jbracker/polymonad-plugin/tree/master/examples/core-error">https://github.com/jbracker/polymonad-plugin/tree/master/examples/core-error</a><o:p></o:p></p>
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<o:p> </o:p></p>
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You just have to download the three files and run "cabal install" to reproduce the error. There is a high-level explanation of <o:p></o:p></p>
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what is going on in [1]. The plugin [2] is still around 600 lines long, but I have added a lot of comments to make it comprehensible.<o:p></o:p></p>
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I suppose the most interesting part is the production of evidence [3]. I have added checks to see if the evidence I produce contains the 'Nth' constructors the core-linter refers to [4,5], but the evidence produced does not contain them. So somehow the evidence
triggers GHC to produce evidence that the core-linter warns about.<o:p></o:p></p>
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<o:p> </o:p></p>
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I hope this is comprehensible and you can help me with what is going on.<o:p></o:p></p>
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<o:p> </o:p></p>
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Best,<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">
Jan<o:p></o:p></p>
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<o:p> </o:p></p>
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[1] <a href="https://github.com/jbracker/polymonad-plugin/blob/master/examples/core-error/Main.hs#L83">
https://github.com/jbracker/polymonad-plugin/blob/master/examples/core-error/Main.hs#L83</a><o:p></o:p></p>
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[2] <a href="https://github.com/jbracker/polymonad-plugin/blob/master/examples/core-error/MinimalPlugin.hs">
https://github.com/jbracker/polymonad-plugin/blob/master/examples/core-error/MinimalPlugin.hs</a><o:p></o:p></p>
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[3] <a href="https://github.com/jbracker/polymonad-plugin/blob/master/examples/core-error/MinimalPlugin.hs#L198">https://github.com/jbracker/polymonad-plugin/blob/master/examples/core-error/MinimalPlugin.hs#L198</a><o:p></o:p></p>
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[4] <a href="https://github.com/jbracker/polymonad-plugin/blob/master/examples/core-error/MinimalPlugin.hs#L130">https://github.com/jbracker/polymonad-plugin/blob/master/examples/core-error/MinimalPlugin.hs#L130</a><o:p></o:p></p>
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[5] <a href="https://github.com/jbracker/polymonad-plugin/blob/master/examples/core-error/MinimalPlugin.hs#L556">
https://github.com/jbracker/polymonad-plugin/blob/master/examples/core-error/MinimalPlugin.hs#L556</a><o:p></o:p></p>
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<o:p> </o:p></p>
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2015-11-20 9:57 GMT+00:00 Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>>:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Calibri",sans-serif">I don’t know how to help either, because there’s no way to reproduce it. Can you find a Haskell program that, when GHC compiles it,
produces this Lint error? Or does it require your plugin? If the latter, it’s hard to know what your plugin might be doing…</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Calibri",sans-serif"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Calibri",sans-serif">So I feel a bit stalled on how to help.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Calibri",sans-serif"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Calibri",sans-serif">Simon</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Calibri",sans-serif"> </span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><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:<a href="mailto:ghc-devs-bounces@haskell.org" target="_blank">ghc-devs-bounces@haskell.org</a>]
<b>On Behalf Of </b>Richard Eisenberg<br>
<b>Sent:</b> 18 November 2015 17:14<br>
<b>To:</b> Jan Bracker<br>
<b>Cc:</b> <a href="mailto:ghc-devs@haskell.org" target="_blank">ghc-devs@haskell.org</a><br>
<b>Subject:</b> Re: Explanation of a core-lint warning (Bad getNth)</span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Ah yes. I looked too quickly. Note that there are two NthCo's listed. Its the outermost that's the problem, which is deconstructing the Union. But it's doing so to prove that '["thres"
:-> Int] ~ '["thres" :-> Int] which is rather easy to prove without NthCo. I'm not sure why GHC is doing this.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Richard<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">On Nov 18, 2015, at 12:11 PM, Jan Bracker <<a href="mailto:jan.bracker@googlemail.com" target="_blank">jan.bracker@googlemail.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt"><o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Hi Richard,<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">No "Split" is a class and is defined here: <a href="https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fhackage.haskell.org%2fpackage%2feffect-monad-0.6.1%2fdocs%2fControl-Effect-State.html%23t%3aSplit&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7c7aa4db864c994205421c08d2f03ba9ca%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=FKQH9CvKCu6KszP1JbOnXOiu4DhhdgpRPWol6zQG41o%3d" target="_blank">http://hackage.haskell.org/package/effect-monad-0.6.1/docs/Control-Effect-State.html#t:Split</a><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">"Union" is a type function (synonym that refers to a type function call): <a href="https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fhackage.haskell.org%2fpackage%2feffect-monad-0.6.1%2fdocs%2fControl-Effect-Writer.html%23t%3aUnion&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7c7aa4db864c994205421c08d2f03ba9ca%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=%2f174TvSeTQ8J%2bAw3VXtSzNKsy8JpFsfDffQvkD7WtkE%3d" target="_blank">http://hackage.haskell.org/package/effect-monad-0.6.1/docs/Control-Effect-Writer.html#t:Union</a><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">thank you for your quick reply!<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Best,<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Jan<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">2015-11-18 17:05 GMT+00:00 Richard Eisenberg <<a href="mailto:eir@cis.upenn.edu" target="_blank">eir@cis.upenn.edu</a>>:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I took just a quick look at this. Is Split a type family? The NthCo coercion form takes apart a composite equality into its pieces. For example, if we know (Maybe a ~ Maybe b),
then NthCo:0 will tell us that (a ~ b). In your case, it looks like GHC is trying to deduce (Union '["thres" :-> Int] []) ~ (Union '["thres" :-> Int] (Unit Reader)) from an equality of two (Split ...) types. If Split is a type family, this deduction is unsound.
That may be what Core Lint is worried about.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I'm not surprised that the executable would run with an error. But it might not in the future. If -dcore-lint fails, it means that there is a potential type safety issue in the
Core code, and this should be taken seriously.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I hope this helps!<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Richard<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">On Nov 18, 2015, at 11:35 AM, Jan Bracker <<a href="mailto:jan.bracker@googlemail.com" target="_blank">jan.bracker@googlemail.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Hi,<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I am using the type checker plugin interface and I am trying to produce some evidence for type class instances. During compilation of one of my examples I get this core-lint error:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">*** Core Lint errors : in result of Simplifier ***<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><no location info>: Warning:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> [RHS of ds_a6bY :: (Set '["thres" :-> Int], Set (Unit Reader))]<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> Bad getNth:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> Nth:0<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> (Nth:2<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> (Sub (Sym (TFCo:R:Inv[]Readerfg[0] <'["thres" :-> Int]>_N <'[]>_N))<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> ; (Inv<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <Reader>_N <'["thres" :-> Int]>_N (Sym TFCo:R:Unit[]Reader[0]))_R<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> ; Sub<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> (TFCo:R:Inv[]Readerfg[0] <'["thres" :-> Int]>_N <Unit Reader>_N)))<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> Split '["thres" :-> Int] '[] (Union '["thres" :-> Int] '[])<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> Split<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> '["thres" :-> Int]<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> (Unit Reader)<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> (Union '["thres" :-> Int] (Unit Reader))<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I suppose "getNth" refers to the constructor "EvTupleSel" from "EvTerm", "TcNthCo" from "TcCoercion" or to "NthCo" from "Coercion". But I never produce evidence of the shape "getNth". My
evidence production code can be found at [1] and the only place where evidence of this shape can come from is my "evaluateType" function [2] that calls "normaliseType" from the GHC module "FamInstEnv". You can reproduce the error by checking out commit 144525886ec107af6f1283b26b19f8125c980aa4
from [3] and running "make effect-example" in the top directory of the repository (GHC 7.10 or better is required and a sandbox is automatically created).<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">The core-lint error does not seem to have any negative consequences when ignored. The produced executable works fine. Can somebody explain why it appears and maybe how I can fix
it?<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Thank you!<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Jan<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">[1] <a href="https://github.com/jbracker/polymonad-plugin/blob/144525886ec107af6f1283b26b19f8125c980aa4/src/Control/Polymonad/Plugin/Evidence.hs#L177" target="_blank">https://github.com/jbracker/polymonad-plugin/blob/144525886ec107af6f1283b26b19f8125c980aa4/src/Control/Polymonad/Plugin/Evidence.hs#L177</a><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">[2] <a href="https://github.com/jbracker/polymonad-plugin/blob/144525886ec107af6f1283b26b19f8125c980aa4/src/Control/Polymonad/Plugin/Evaluate.hs#L29" target="_blank">https://github.com/jbracker/polymonad-plugin/blob/144525886ec107af6f1283b26b19f8125c980aa4/src/Control/Polymonad/Plugin/Evaluate.hs#L29</a><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">[3] <a href="https://github.com/jbracker/polymonad-plugin" target="_blank">https://github.com/jbracker/polymonad-plugin</a><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">_______________________________________________<br>
ghc-devs mailing list<br>
<a href="mailto:ghc-devs@haskell.org" target="_blank">ghc-devs@haskell.org</a><br>
<a href="https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fmail.haskell.org%2fcgi-bin%2fmailman%2flistinfo%2fghc-devs&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7c7aa4db864c994205421c08d2f03ba9ca%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=tediRvsqUcgRC%2bw5GVLOmxMMHWvYS%2bYai8rQbOtj7yM%3d" target="_blank">http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs</a><o:p></o:p></p>
</blockquote>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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</blockquote>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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</blockquote>
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<p class="MsoNormal"><o:p> </o:p></p>
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