I think you need a ‘Sub’. A cast (e `cast` g) requires a representational coercion. I think you have given it a (stronger) nominal one. Sub gets from one to t’other.
Simon From: Glasgow-haskell-users [mailto:glasgow-haskell-users-boun...@haskell.org] On Behalf Of Conal Elliott Sent: 14 April 2014 06:00 To: ghc-d...@haskell.org; glasgow-haskell-users@haskell.org Subject: Help with coercion & roles? I’m working on a GHC plugin (as part of my Haskell-to-hardware work) and running into trouble with coercions & roles. Error message from Core Lint: Warning: In the expression: LambdaCCC.Lambda.lamvP# @ (GHC.Types.Bool → GHC.Types.Bool → GHC.Types.Bool → GHC.Types.Bool) @ (Simple.HasIf GHC.Types.Bool) "tpl"# ((LambdaCCC.Lambda.varP# @ (GHC.Types.Bool → GHC.Types.Bool → GHC.Types.Bool → GHC.Types.Bool) "tpl"#) `cast` (<LambdaCCC.Lambda.EP>_N (Sym (Simple.NTCo:HasIf[0] <GHC.Types.Bool>_N)) ∷ LambdaCCC.Lambda.EP (GHC.Types.Bool → GHC.Types.Bool → GHC.Types.Bool → GHC.Types.Bool) ~# LambdaCCC.Lambda.EP (Simple.HasIf GHC.Types.Bool))) Role incompatibility: expected nominal, got representational in <LambdaCCC.Lambda.EP>_N (Sym (Simple.NTCo:HasIf[0] <GHC.Types.Bool>_N)) Do you see anything inconsistent/incompatible in the coercions or roles above? I constructed the nominal EP Refl coercion, and applied it (AppCo) an existing coercion of a simpler type. Thanks, -- Conal
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