[Haskell-cafe] Combining RequiredTypeArguments and type class methods?

[Tom Ellis]

On Sun, Jul 13, 2025 at 04:03:08PM +1000, Viktor Dukhovni wrote:
> The user guide documentation of RequiredTypeArguments highlights a
> hypothetical alternative to the interface of the `sizeOf` method of
> the Storable class:
> 
>     sizeOf :: forall a -> Storable a => Int
> 
> This is fine in isolation, but how would this actually work in a type
> class method definition?
>
> Is the hypothetical `sizeOf` actually realisable as a type class method?
> Or can it only be a module-level wrapper?  Something like the below,
> which does work?

Yeah, there's definitely something that doesn't quite work with
RequiredTypeArguments and type class methods. After all, the type of a
type class `C` is implicitly prefixed with `forall a. C a =>`, so you
can't add your own `forall a ->`. It would shadow.

There is, however, a known trick. This works:

    {-# LANGUAGE AllowAmbiguousTypes #-}
        {-# LANGUAGE RequiredTypeArguments #-}

    class MyStorable a where
          sizeOf :: forall b -> b ~ a => (MyStorable a) => Int

    instance MyStorable Bool where
          sizeOf (type Bool) = 1

(But I prefer your solution: the wrapper.)

Tom