<div dir="ltr">I have condensed a self-contained plugin and an example application that reproduces the error. You can find it here:<div><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></div><div><br></div><div>You just have to download the three files and run "cabal install" to reproduce the error. There is a high-level explanation of </div><div>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.</div><div>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.</div><div><br></div><div>I hope this is comprehensible and you can help me with what is going on.</div><div><br></div><div>Best,</div><div>Jan</div><div><br></div><div>[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></div><div>[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></div><div>[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></div><div>[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></div><div>[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></div></div><div class="gmail_extra"><br><div class="gmail_quote">2015-11-20 9:57 GMT+00:00 Simon Peyton Jones <span dir="ltr"><<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>></span>:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<p class="MsoNormal"><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…<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">So I feel a bit stalled on how to help.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Simon<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><u></u> <u></u></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:<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)<u></u><u></u></span></p>
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<p class="MsoNormal">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.<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">Richard<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">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:<u></u><u></u></p>
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<p class="MsoNormal">Hi Richard,<u></u><u></u></p>
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<p class="MsoNormal">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><u></u><u></u></p>
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<p class="MsoNormal">"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><u></u><u></u></p>
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<p class="MsoNormal">thank you for your quick reply!<u></u><u></u></p>
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<p class="MsoNormal">Best,<u></u><u></u></p>
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<p class="MsoNormal">Jan<u></u><u></u></p>
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<p class="MsoNormal">2015-11-18 17:05 GMT+00:00 Richard Eisenberg <<a href="mailto:eir@cis.upenn.edu" target="_blank">eir@cis.upenn.edu</a>>:<u></u><u></u></p>
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<p class="MsoNormal">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.<u></u><u></u></p>
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<p class="MsoNormal">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.<u></u><u></u></p>
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<p class="MsoNormal">I hope this helps!<u></u><u></u></p>
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<p class="MsoNormal">Richard<u></u><u></u></p>
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<p class="MsoNormal">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:<u></u><u></u></p>
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<p class="MsoNormal">Hi,<u></u><u></u></p>
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<p class="MsoNormal">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:<u></u><u></u></p>
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<p class="MsoNormal">*** Core Lint errors : in result of Simplifier ***<u></u><u></u></p>
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<p class="MsoNormal"><no location info>: Warning:<u></u><u></u></p>
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<p class="MsoNormal"> [RHS of ds_a6bY :: (Set '["thres" :-> Int], Set (Unit Reader))]<u></u><u></u></p>
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<p class="MsoNormal"> Bad getNth:<u></u><u></u></p>
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<p class="MsoNormal"> Nth:0<u></u><u></u></p>
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<p class="MsoNormal"> (Nth:2<u></u><u></u></p>
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<p class="MsoNormal"> (Sub (Sym (TFCo:R:Inv[]Readerfg[0] <'["thres" :-> Int]>_N <'[]>_N))<u></u><u></u></p>
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<p class="MsoNormal"> ; (Inv<u></u><u></u></p>
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<p class="MsoNormal"> <Reader>_N <'["thres" :-> Int]>_N (Sym TFCo:R:Unit[]Reader[0]))_R<u></u><u></u></p>
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<p class="MsoNormal"> ; Sub<u></u><u></u></p>
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<p class="MsoNormal"> (TFCo:R:Inv[]Readerfg[0] <'["thres" :-> Int]>_N <Unit Reader>_N)))<u></u><u></u></p>
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<p class="MsoNormal"> Split '["thres" :-> Int] '[] (Union '["thres" :-> Int] '[])<u></u><u></u></p>
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<p class="MsoNormal"> Split<u></u><u></u></p>
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<p class="MsoNormal"> '["thres" :-> Int]<u></u><u></u></p>
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<p class="MsoNormal"> (Unit Reader)<u></u><u></u></p>
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<p class="MsoNormal"> (Union '["thres" :-> Int] (Unit Reader))<u></u><u></u></p>
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<p class="MsoNormal">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).<u></u><u></u></p>
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<p class="MsoNormal">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?<u></u><u></u></p>
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<p class="MsoNormal">Thank you!<u></u><u></u></p>
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<p class="MsoNormal">Jan<u></u><u></u></p>
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<p class="MsoNormal">[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><u></u><u></u></p>
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<p class="MsoNormal">[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><u></u><u></u></p>
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<p class="MsoNormal">[3] <a href="https://github.com/jbracker/polymonad-plugin" target="_blank">https://github.com/jbracker/polymonad-plugin</a><u></u><u></u></p>
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