RE: Help with coercion & roles?

14 Apr 2014

      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` (_N (Sym (Simple.NTCo:HasIf[0] _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 _N (Sym (Simple.NTCo:HasIf[0] _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

RE: Help with coercion & roles?

Simon Peyton Jones