从特定类型泛化为 GADT 中的 class

Question

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，我定义了一个在类型签名中使用 ExampleCharacters 的 Symbol class。示例用法是 let sym = Terminal "xyy" :: Symbol Variable 创建名称为 "xyy".

的变量符号

现在，显而易见的下一步是将 ExampleCharacter 推广到 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’

我不太确定这个错误想告诉我什么。为什么 forall (c :: * -> *) a. Character c 是非法约束，怎么会这样？有什么解决办法吗？

Answer 1

来自docs：

As kinds and types are the same, kinds can now (with -XTypeInType) contain type constraints. Only equality constraints are currently supported, however. We expect this to extend to other constraints in the future.

所以原则上你写的是合理的；它根本不受支持（截至撰写本文时，最新的 GHC 是 8.2.2）。

从特定类型泛化为 GADT 中的 class

Generalizing from a specific type to a class in a GADT

haskell

gadt

rank-n-types