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/144525886ec107af6f1283b26b... [2] https://github.com/jbracker/polymonad-plugin/blob/144525886ec107af6f1283b26b... [3] https://github.com/jbracker/polymonad-plugin