haskell - 有没有办法使用语法糖来构建存在类型列表？

Question

考虑一个类似列表的类型：

{-# LANGUAGE ExistentialQuantification #-}

data ShowList = Nil | forall a. Show a => a :* ShowList
infixr 5 :*

myShowList :: ShowList
myShowList = 'x' :* () :* Nil

我希望能够使用语法糖来构建这些列表，如下所示：

myShowList :: ShowList
myShowList = ['x', ()]

但即使使用RebindableSyntaxand OverloadedLists，我似乎仍然无法通过内置列表类型，试图统一所有元素的类型，这让我很困惑：

$ ghci -XRebindableSyntax -XOverloadedLists
GHCi, version 9.2.1: https://www.haskell.org/ghc/  :? for help
ghci> fromListN = fromListN
ghci> myShowList = ['x', ()]

<interactive>:2:20: error:
    • Couldn't match expected type ‘GHC.Types.Char’
                  with actual type ‘()’
    • In the expression: ()
      In the expression: ['x', ()]
      In an equation for ‘myShowList’: myShowList = ['x', ()]
ghci> :info []
type [] :: * -> *
data [] a = [] | a : [a]
        -- Defined in ‘GHC.Types’
ghci> 

有什么办法可以解决这个问题，还是我坚持写这些不加糖？