我目前正在学习Haskell的类型系统，但是我在类型族和类示例方面遇到了一些麻烦。
下面是我的代码：

{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE StandaloneKindSignatures #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeFamilies #-}

module Test() where
import Data.Kind (Type)

data Color = ColorWhite | ColorBlack

type Board :: Color -> Type
data Board c where
  MkBoard :: (PrintableType c) => Board c

type ExBoard :: Type
data ExBoard where
  MkExBoard :: Board t -> ExBoard

type NextColor :: Color -> Color
type family NextColor c where
  NextColor 'ColorWhite = 'ColorBlack
  NextColor 'ColorBlack = 'ColorWhite

class PrintableType a where
  printT :: String

instance PrintableType 'ColorWhite where
  printT = "White"

instance PrintableType 'ColorBlack where
  printT = "Black"

withBoard :: ExBoard -> (forall c. Board c -> a) -> a
withBoard (MkExBoard b) f = f b

printInfo :: ExBoard -> String
printInfo  eb = withBoard eb $ \b@MkBoard -> printTurn b
printTurn :: forall c. PrintableType c => Board c -> String
printTurn _ = printT @c ++ " turn!"

compute :: Board c ->  Board (NextColor c)
compute _ = MkBoard

-- >>> printInfo $ MkExBoard $ MkBoard @'ColorWhite
-- >>> printInfo $ MkExBoard $ MkBoard @'ColorBlack
-- "White turn!"
-- "Black turn!"

现在，compute函数给我的错误是"No instance for (PrintableType (NextColor c)) arising from a use of ‘MkBoard’"，这让我很困惑，因为NextColor c的两个输出都已经定义为PrintableType的示例。
如果我从MkBoard中删除了(PrintableType c)，那么printTurn开始给我错误No instance for (PrintableType c) arising from a use of ‘printTurn’，而compute不再给予我错误。
考虑到这一点，我只能猜测编译器没有解析类型族结果的类示例。有没有什么方法可以让我在MkBoard中保留(PrintableType c)，因为print函数，并且还可以解决compute函数的问题？或者我在这里遗漏了什么？