I have condensed a self-contained plugin and an example application that reproduces the error. You can find it here: https://github.com/jbracker/polymonad-plugin/tree/master/examples/core-error
You just have to download the three files and run "cabal install" to reproduce the error. There is a high-level explanation of 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. 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. I hope this is comprehensible and you can help me with what is going on. Best, Jan [1] https://github.com/jbracker/polymonad-plugin/blob/master/examples/core-error/Main.hs#L83 [2] https://github.com/jbracker/polymonad-plugin/blob/master/examples/core-error/MinimalPlugin.hs [3] https://github.com/jbracker/polymonad-plugin/blob/master/examples/core-error/MinimalPlugin.hs#L198 [4] https://github.com/jbracker/polymonad-plugin/blob/master/examples/core-error/MinimalPlugin.hs#L130 [5] https://github.com/jbracker/polymonad-plugin/blob/master/examples/core-error/MinimalPlugin.hs#L556 2015-11-20 9:57 GMT+00:00 Simon Peyton Jones <simo...@microsoft.com>: > 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… > > > > So I feel a bit stalled on how to help. > > > > Simon > > > > *From:* ghc-devs [mailto:ghc-devs-boun...@haskell.org] *On Behalf Of *Richard > Eisenberg > *Sent:* 18 November 2015 17:14 > *To:* Jan Bracker > *Cc:* ghc-devs@haskell.org > *Subject:* Re: Explanation of a core-lint warning (Bad getNth) > > > > 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. > > > > Richard > > > > On Nov 18, 2015, at 12:11 PM, Jan Bracker <jan.brac...@googlemail.com> > wrote: > > > > Hi Richard, > > > > No "Split" is a class and is defined here: > http://hackage.haskell.org/package/effect-monad-0.6.1/docs/Control-Effect-State.html#t:Split > <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> > > "Union" is a type function (synonym that refers to a type function call): > http://hackage.haskell.org/package/effect-monad-0.6.1/docs/Control-Effect-Writer.html#t:Union > <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> > > > > thank you for your quick reply! > > > > Best, > > Jan > > > > 2015-11-18 17:05 GMT+00:00 Richard Eisenberg <e...@cis.upenn.edu>: > > 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. > > > > 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. > > > > I hope this helps! > > Richard > > > > On Nov 18, 2015, at 11:35 AM, Jan Bracker <jan.brac...@googlemail.com> > wrote: > > > > Hi, > > > > 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: > > > > *** Core Lint errors : in result of Simplifier *** > > <no location info>: Warning: > > [RHS of ds_a6bY :: (Set '["thres" :-> Int], Set (Unit Reader))] > > Bad getNth: > > Nth:0 > > (Nth:2 > > (Sub (Sym (TFCo:R:Inv[]Readerfg[0] <'["thres" :-> Int]>_N > <'[]>_N)) > > ; (Inv > > <Reader>_N <'["thres" :-> Int]>_N (Sym > TFCo:R:Unit[]Reader[0]))_R > > ; Sub > > (TFCo:R:Inv[]Readerfg[0] <'["thres" :-> Int]>_N <Unit > Reader>_N))) > > Split '["thres" :-> Int] '[] (Union '["thres" :-> Int] '[]) > > Split > > '["thres" :-> Int] > > (Unit Reader) > > (Union '["thres" :-> Int] (Unit Reader)) > > > > 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). > > > > 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? > > > > Thank you! > > Jan > > > > [1] > https://github.com/jbracker/polymonad-plugin/blob/144525886ec107af6f1283b26b19f8125c980aa4/src/Control/Polymonad/Plugin/Evidence.hs#L177 > > [2] > https://github.com/jbracker/polymonad-plugin/blob/144525886ec107af6f1283b26b19f8125c980aa4/src/Control/Polymonad/Plugin/Evaluate.hs#L29 > > [3] https://github.com/jbracker/polymonad-plugin > > _______________________________________________ > ghc-devs mailing list > ghc-devs@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs > <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> > > > > > > >
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