Between Haskell2010 and GHC2021

Phantoms.hs

{-# 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