Haskell 实例包装 []

Question

>我正在尝试创建一个包装 [] 的类型实例;即

instance Foo (NonNull []) where

(其中NonNull是Michael Snoyman的数据.NonNull( 这适用于，比如说，

instance Foo [] where

但是有了NonNull []，我得到了

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

我尝试使用RankNTypes来提供明确的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

这给了：

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

我相信这一定是可能的，但我错过了咒语。 任何指针都感激地收到。

Answer 1

您的最后一次尝试是最接近的一次，但您需要一个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NonNull []没有意义，因为NonNull的参数必须是类型(如[Int](，而不是像[]那样的类型构造函数。您应该会看到第二个错误，如以下类似示例所示：

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
|                         ^^