Thanks for the summary and paper pointer! I'll study up. -- Conal On Thu, Apr 24, 2014 at 12:58 AM, Simon Peyton Jones wrote:

(a ~ b) is a *boxed*, *nominal* equality.

(a ~R# b) is an *unboxed, representational* equality

So it is rightly rejected.

For the boxed/unboxed thing, the paper “practical aspects…” gives more detail. http://research.microsoft.com/en-us/um/people/simonpj/papers/ext-f/

I think you already know about the nominal/representational distinction.

Simon

*From:* Glasgow-haskell-users [mailto: glasgow-haskell-users-bounces@haskell.org] *On Behalf Of *Conal Elliott *Sent:* 24 April 2014 01:29 *To:* glasgow-haskell-users@haskell.org; ghc-devs@haskell.org; Richard Eisenberg; Simon Peyton Jones *Subject:* Help with cast error

I'd appreciate help with a cast-related Core Lint error I'm getting with a GHC plugin I'm working on:

Argument value doesn't match argument type: Fun type: Enc (Vec ('S 'Z) Bool) ~ (Bool, ()) => EP (Enc (Vec ('S 'Z) Bool)) -> EP (Bool, ()) Arg type: ~R# (Enc (Vec ('S 'Z) Bool)) (Bool, ()) Arg: CO Sub (TFCo:R:EncVec[0] <'Z>_N <Bool>_N) ; (Sub TFCo:R:EncBool[0], Sub (TFCo:R:EncVec0[0] <Bool>_N))_R

(I omitted the module prefixes for brevity.) Do I have a role wrong here, or maybe something more fundamental?

I can easily supply more info if it'd help.

Thanks, -- Conal