To be super-clear about at least one aspect: I don't want Tidy Core to ever contain something that looks like this:

GADTTest.potato
:: GHC.Types.Int -> GADTTest.Silly GHC.Types.Int -> GHC.Types.Int
GADTTest.potato =
\ (x_asZ :: GHC.Types.Int)
    (ds_dPR :: GADTTest.Silly GHC.Types.Int) ->
    case ds_dPR of _ { GADTTest.Silly $dNum_aLV ds1_dPS ->
    GHC.Num.+ @ GHC.Types.Int $dNum_aLV x_asZ x_asZ
    }

Here we see GHC.Num.+ applied to GHC.Types.Int and $dNum_aLV. We therefore know that $dNum_aLV must be GHC.Num.$fNumInt, so GHC.Num.+ can eat these arguments and produce GHC.Num.$fNumInt_$c+. But for some reason, GHC fails to recognize and exploit this fact! I would like help understanding why that is, and what I can do to fix it.

On Mon, Oct 20, 2014 at 7:53 AM, David Feuer <david.feuer@gmail.com> wrote:

On Oct 20, 2014 5:05 AM, "Simon Peyton Jones" <simonpj@microsoft.com> wrote:
> I’m unclear what you are trying to achieve with #9701. I urge you to write a clear specification that we all agree about before burning cycles hacking code.

What I'm trying to achieve is to make specialization work in a situation where it currently does not. It appears that when the type checker determines that a GADT carries a certain dictionary, the specializer happily uses it *even once the concrete type is completely known*. What we would want to do in that case is to replace the use of the GADT-carried dictionary with a use of the known dictionary for that type.

> There are a lot of comments at the top of Specialise.lhs. But it is, I’m afraid, a tricky pass. I could skype.

I would appreciate that. What day/time are you available?