haskell - 类型系列 : top level vs. 相关联

Question

我刚刚开始学习类型家庭。 GHC 文档指出顶级和关联的类型系列具有相同的功能，但我编写的代码在顶级的行为与关联系列时的行为不同。这编译并运行良好:

{-# LANGUAGE TypeFamilies #-}
module Test where

-- type family R a
-- type instance R Maybe = Int

class C' a where
  type R a
  getInt' :: a Int
  getBool' :: R a -> a Bool

instance C' Maybe where
  type R Maybe = Int
  getInt' = Just 3
  getBool' i = Just $ i < 10

printer :: IO ()
printer = print $ (getBool' 5 :: Maybe Bool)

但这给了我一个类型错误:

{-# LANGUAGE TypeFamilies #-}
module Test where

type family R a
type instance R Maybe = Int

class C' a where
  -- type R a
  getInt' :: a Int
  getBool' :: R a -> a Bool

instance C' Maybe where
  -- type R Maybe = Int
  getInt' = Just 3
  getBool' i = Just $ i < 10

printer :: IO ()
printer = print $ (getBool' 5 :: Maybe Bool)

这些对我来说看起来一模一样；为什么一个编译而另一个不编译？

Answer 1

如果您注释种类，则第二个有效:

type family R (a :: * -> *)

我认为没有任何理由只为关联的类型系列推断正确的类型。