Explanation of a core-lint warning (Bad getNth)

[Jan Bracker]

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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