Compiler Error with Symbol

Test.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE InstanceSigs #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FlexibleContexts #-}

module Test where

import GHC.TypeLits (symbolVal, KnownSymbol)
import Data.Proxy (Proxy(..))

newtype s >> a = Named a

data HRec xs where
  HREmpty :: HRec '[]
  HRCons :: (s >> a) -> HRec xs -> HRec (s >> a ': xs)

instance Show (HRec '[]) where
  show :: HRec '[] -> String
  show _ = "HREmpty"

instance (KnownSymbol s, Show a, Show (HRec xs)) => Show (HRec ((s >> a) ': xs)) where
  show :: HRec ((s >> a) ': xs) -> String
  show (HRCons (Named a) as) = let val = show a
                                   key = (symbolVal @s) Proxy
                               in undefined