我做了两个类型家族：
PowerApps Custom Visual现在，我想结合两个类型族，以检查类型级别列表中的所有成员是否都是Mays。这是我的处理方式：
{-# Language KindSignatures, DataKinds, TypeOperators, TypeFamilies, FlexibleContexts #-}
import Data.Kind
type family All (c :: Type -> Constraint) (xs :: [Type]) :: Constraint where
All c '[] = ()
All c (x ': xs) = (c x, All c xs)
type family Is (f :: Type -> Type) (x :: Type) :: Constraint where
Is f (f _) = ()
现在，如果我尝试使用以下方法对此进行测试：
All (Is Maybe) tList
我收到编译错误：
f :: (All (Is Maybe) '[Maybe Bool]) => a -> a
f = id
基于此，我认为我会缺少[1 of 1] Compiling Main ( familyTest.hs, interpreted )
familyTest.hs:12:6: error:
• The type family ‘Is’ should have 2 arguments, but has been given 1
• In the type signature:
f :: (All (Is Maybe) '[Maybe Bool]) => a -> a
|
12 | f :: (All (Is Maybe) '[Maybe Bool]) => a -> a
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Failed, no modules loaded.
的论点。我的第一个想法是这与curring有关。也许类型家庭缺乏部分应用。但是，当我去弄清楚我错过了什么时，会发生一些奇怪的事情。首先，我检查所有基本组件的种类：
Is这里没有任何问题，所以我开始将它们放在一起：
[1 of 1] Compiling Main ( familyTest.hs, interpreted )
Ok, one module loaded.
*Main> :k All
All :: (* -> Constraint) -> [*] -> Constraint
*Main> :k Is
Is :: (* -> *) -> * -> Constraint
*Main> :k Maybe
Maybe :: * -> *
*Main> :k Bool
Bool :: *
这有点奇怪，*Main> :k Is Maybe
Is Maybe :: * -> Constraint
的类型为Is Maybe。我可以预料到这一点，但是先前的错误告诉我我在Type -> Constraint中输入了错误的参数数量。现在，我们尝试将其传递给Is：
All令人惊讶的是，即使我们希望这样做，它也不会出错。最后要尝试的是完成约束：
*Main> :k All (Is Maybe)
All (Is Maybe) :: [*] -> Constraint
并且它起作用了，尽管编译器早些时候抱怨这个确切的语句，但是现在该语句似乎是一个有效的约束。编译原始代码时，仍然出现相同的错误。
因此，在这一点上，我开始简化代码，以便可以隔离行为。
*Main> :k All (Is Maybe) '[Maybe Bool]
All (Is Maybe) '[Maybe Bool] :: Constraint
这仍然会导致类似的错误：
type family App (c :: Type -> Constraint) (x :: Type) :: Constraint where
App c x = c x
type family IsBool (x :: Type) :: Constraint where
IsBool Bool = ()
f :: (App IsBool Bool) => a -> a
f = id
当我尝试使用[1 of 1] Compiling Main ( familyTest.hs, interpreted )
familyTest.hs:19:6: error:
• The type family ‘IsBool’ should have 1 argument, but has been given none
• In the type signature: f :: (App IsBool Bool) => a -> a
|
19 | f :: (App IsBool Bool) => a -> a
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^
Failed, no modules loaded.
:k这是怎么回事？