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-bounces@haskell.org] On Behalf Of Conal Elliott
Sent: 14 April 2014 06:00
To: ghc-devs@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