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 > "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 > > 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 > >
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