我有以下定义
{-# LANGUAGE GADTs, TypeInType, RankNTypes #-}
import Data.Kind
class Character (a :: * -> *) where
showVal :: a b -> b -> String
data ExampleCharacter a where
Variable :: ExampleCharacter String
EqualSign :: ExampleCharacter ()
Deref :: ExampleCharacter ()
instance Character ExampleCharacter where
showVal Variable = id
showVal EqualSign = const "="
showVal Deref = const "*"
data Symbol :: forall a. ExampleCharacter a -> * where
Terminal :: a -> Symbol (b :: ExampleCharacter a)
如您所见,我已经定义了一个 Symbol 类,它在类型签名中使用 ExampleCharacters。示例用法是使用let sym = Terminal "xyy" :: Symbol Variable
名称创建变量符号"xyy"
。
现在,显而易见的下一步是推广ExampleCharacter
到Character c
. 我尝试了以下方法:
data Symbol :: (forall c (a :: *). Character c) => c a -> * where
Terminal :: a -> Symbol (b :: c a)
但我收到以下错误:
main.hs:22:20: error:
• Illegal constraint in a type: forall (c :: * -> *) a. Character c
• In the type ‘Symbol (b :: c a)’
In the definition of data constructor ‘Terminal’
In the data declaration for ‘Symbol’
我不太确定错误想告诉我什么。为什么是forall (c :: * -> *) a. Character c
非法约束以及如何呢?有什么办法吗?