Implicit parameters

[Simon Peyton Jones]

Dear CLC

Can I ask for your advice?

I'm busy fixing GHC issue https://gitlab.haskell.org/ghc/ghc/-/issues/26737

The fix is easy: it simply means giving the built-in class IP a representational role, thus

type role IP nominal representational   -- New
class IP (x :: Symbol) a | x -> a where
  ip :: a

Now, this shouldn't be a CLC issue at all. IP is a purely internal matter.  It just happens to be the way that GHC implements the implicit-parameter feature of Haskell. The only user-visible change is that GHC will typecheck a few more programs that it previously rejected.

However IP is exported by base:GHC.Base.  That entire module should not be in `base` but today it is, so the change makes a user-visible change to `base`.

Would you like a proposal?  I ask because it's all busy-work if you plan to remove GHC.Base from `base` in any case.

I have only just realised this.  (It never occurred to me that IP would leak into `base`.)  The patch is ready to land.

You can see more background in #26737 and the MR https://gitlab.haskell.org/ghc/ghc/-/merge_requests/15337

Thanks

Simon

[Julian Ospald]

Please create a proposal. We'll fast-track it.

On 1/14/26 6:03 PM, Simon Peyton Jones wrote:
> Dear CLC
>
> Can I ask for your advice?
>
> I'm busy fixing GHC issue https://gitlab.haskell.org/ghc/ghc/-/

> issues/26737 <https://gitlab.haskell.org/ghc/ghc/-/issues/26737>

> gitlab.haskell.org/ghc/ghc/-/merge_requests/15337 <https://
> gitlab.haskell.org/ghc/ghc/-/merge_requests/15337>
>
> Thanks
>
> Simon
>
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