I agree with Simon, but just `Sub` the `<LambdaCCC.Lambda.EP>_N`, not the whole coercion. There are actually two problems here. The one Simon identified, and also the fact that Simple.NTCo:HasIf[0] produces a representational coercion. How do I know? Because of the `NT` -- it's a newtype axiom and must produce representational coercions. Furthermore, unless the newtype definition is inferred to require a nominal parameter, the newtype would expect a representational coercion, not the nominal one you are passing. Check the dump (using -ddump-tc) of the newtype axiom to be sure. Though putting a `Sub` on `<EP>` and changing the Refl constructor on `<Bool>` would work, you would then be violating an invariant of GHC's Coercion type: that we prefer `TyConAppCo tc ...` over `AppCo (Refl (TyConApp tc [])) ...`. In sum, here is the coercion I think you want: (LambdaCCC.Lambda.EP (Sym (Simple.NTCo:HasIf[0] <GHC.Types.Bool>_R)))_R This is one of the problems with roles -- they are *very* fiddly within GHC, and it's hard for a non-expert in roles to get them right. Have you seen docs/core-spec/core-spec.pdf? That is updated w.r.t. roles and may be of help. Let me know if I can help further! Richard On Apr 14, 2014, at 5:45 AM, Simon Peyton Jones <simonpj@microsoft.com> wrote:
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 _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users