Haskell 实例包装 []

Question

我正在尝试创建一个包装 [] 的类型的实例；即，

instance Foo (NonNull []) where

(其中 NonNull 是 Michael Snoyman 的 Data.NonNull )这适用于，比如说，

instance Foo [] where

但是使用 NonNull []，我得到了

Expected kind ‘* -> *’, but ‘NonNull []’ has kind ‘*’

我已经尝试使用 RankNTypes 来提供一个明确的 forall:

instance ∀ α . Foo (NonNull [α]) where

结果类似:

Expected kind ‘* -> *’, but ‘NonNull [α]’ has kind ‘*’

还有一个类型同义词:

type NonNullList α = NonNull [α]
instance ToSeq NonNullList where

给出:

The type synonym ‘NonNullList’ should have 1 argument, but has been given none

我确信这一定是可能的，但我错过了咒语。非常感谢收到任何指示。

Answer 1

你最后一次尝试是最接近的一次，但你需要一个 newtype 而不是 type 同义词(data 也可以):

newtype NonNullList α = NonNullList (NonNull [α])

instance ToSeq NonNullList where ...

您首先尝试的

NonNull [] 没有意义，因为 NonNull 的参数必须是一个类型(如 [Int] ), 而不是像 [] 这样的类型构造函数。您应该会看到第二个错误，例如 in this similar example :

instance Functor (Maybe []) where

main.hs:5:19: error:
    • Expected kind ‘* -> *’, but ‘Maybe []’ has kind ‘*’
    • In the first argument of ‘Functor’, namely ‘(Maybe [])’
      In the instance declaration for ‘Functor (Maybe [])’
  |
5 | instance Functor (Maybe []) where
  |                   ^^^^^^^^
main.hs:5:25: error:
    • Expecting one more argument to ‘[]’
      Expected a type, but ‘[]’ has kind ‘* -> *’
    • In the first argument of ‘Maybe’, namely ‘[]’
      In the first argument of ‘Functor’, namely ‘(Maybe [])’
      In the instance declaration for ‘Functor (Maybe [])’
  |
5 | instance Functor (Maybe []) where
  |                         ^^