Explanation of a core-lint warning (Bad getNth)

[Jan Bracker]

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 <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
> seriously.
>
> I hope this helps!
> Richard
>
> On Nov 18, 2015, at 11:35 AM, Jan Bracker <jan.bracker at 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
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