Explanation of a core-lint warning (Bad getNth)

[Richard Eisenberg]

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.bracker at 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 <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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> 

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