Explanation of a core-lint warning (Bad getNth)
jan.bracker at googlemail.com
Wed Nov 18 17:11:43 UTC 2015
No "Split" is a class and is defined here:
"Union" is a type function (synonym that refers to a type function call):
thank you for your quick reply!
2015-11-18 17:05 GMT+00:00 Richard Eisenberg <eir at 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
> I hope this helps!
> On Nov 18, 2015, at 11:35 AM, Jan Bracker <jan.bracker at googlemail.com>
> 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:
> (Sub (Sym (TFCo:R:InvReaderfg <'["thres" :-> Int]>_N
> ; (Inv
> <Reader>_N <'["thres" :-> Int]>_N (Sym
> ; Sub
> (TFCo:R:InvReaderfg <'["thres" :-> Int]>_N <Unit
> Split '["thres" :-> Int] ' (Union '["thres" :-> Int] ')
> '["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  and the only place where evidence of this shape can come
> from is my "evaluateType" function  that calls "normaliseType" from the
> GHC module "FamInstEnv". You can reproduce the error by checking out
> commit 144525886ec107af6f1283b26b19f8125c980aa4 from  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!
>  https://github.com/jbracker/polymonad-plugin
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> ghc-devs at haskell.org
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