无法为关联数据族派生Typeable

Question

我正在尝试为关联数据系列派生Typeable，如下所示：

{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE TypeFamilies #-}
module Test where

import Data.Typeable

class Test a where
  data DTest a :: *

instance Test () where
  data DTest () = Foo
    deriving Typeable

但是我收到以下错误消息，这让我有点疑惑：

[1 of 1] Compiling Test             ( test.hs, test.o )

test.hs:12:14:
    Cannot eta-reduce to an instance of form
      instance (...) => Typeable DTest
    In the data instance declaration for ‘DTest’

有人可以详细说明这里出了什么问题吗？