Re: [ghc-steering-committee] Mind the gap

This is PolyKinds, which generalizes the kind of D, meaning that satisfying the Typeable a superclass constraint of C also requires a Typeable k constraint, where (a :: k). Richard

On Jul 2, 2022, at 12:49 AM, Chris Dornan wrote:

Quick quiz: the below Haskell2010 ‘Phantoms' module (also in this Gist: https://gist.github.com/cdornan/f75cd8024434d998c87610cbb7fb6ab3) appears (for me on GHC 9.2.2 and 9.2.3) to not be a GHC2021 module, reporting this error under the latter configuration:

Phantoms.hs:22:10: error: • Could not deduce (Typeable k) arising from the superclasses of an instance declaration from the context: (C a, Typeable a) bound by the instance declaration at Phantoms.hs:22:10-36 • In the instance declaration for ‘S (D a)’ | 22 | instance (C a,Typeable a) => S (D a) where smethod = undefined | ^^^^^^^^^^^^^^^^^^^^^^^^^^^

Phantoms.hs:22:10: error: • Could not deduce (Typeable k) arising from the superclasses of an instance declaration from the context: (C a, Typeable a) bound by the instance declaration at Phantoms.hs:22:10-36 • In the instance declaration for ‘S (D a)’ | 22 | instance (C a,Typeable a) => S (D a) where smethod = undefined

The question is which language extension(s) are giving rise to these errors?

The ‘Phantoms' module is this:

{-# LANGUAGE DerivingVia #-}

module Phantoms where

import Data.Typeable

class C a where cmethod :: Proxy a -> ()

class (Show a, Typeable a) => S a where smethod :: a -> Int

newtype UsingD a = UsingD { getUsingD :: a }

newtype D a = D { getD :: Int } deriving (Show) via UsingD (D a)

instance (C a,Typeable a) => S (D a) where smethod = undefined

instance (C a,Typeable a) => Show (UsingD a) where showsPrec = undefined

instance (C a) => C (D a) where cmethod _ = undefined

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