如何在具有自定义类型 class 约束的 GADT 上实现 fromJSON？

Question

我有以下 GADT：

{-# LANGUAGE GADTs #-}

data LogProtocol a where
  Message :: String -> LogProtocol String
  StartRun :: forall rc. (Show rc, Eq rc, Titled rc, ToJSON rc, FromJSON rc) 
   => rc -> LogProtocol rc
  ... and many more...

toJSON 很简单，没有显示。 fromJSON 实现基于：

和 This Blog Post - pattern 2

又如下：

{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE TemplateHaskell #-}

-- tag type is used in to/ from JSON to reduce the use of magic strings
data LPTag = MessageT |
             StartRunT |
             ... and many more...
             deriving (Show, Eq, Enum)

tagList :: Enum a => [a]
tagList = enumFrom $ toEnum 0

$(deriveJSON defaultOptions ''LPTag) 

-- a wrapper to hide the a type param in the GADT
data Some (t :: k -> *) where
  Some :: t x -> Some t

instance FromJSON (Some LogProtocol) where

parseJSON :: Value -> Parser (Some LogProtocol)
parseJSON v@(Object o) =
  let 
    tag :: Maybe LPTag 
    tag = do 
      t <- (HML.lookup "type" o) 
      parseMaybe parseJSON t 

    failMessage :: [Char]
    failMessage = toS $ "Could not parse LogProtocol no type field or type field value is not a member of specified in: " 
                    <> (show(tagList :: [LPTag])) 
                    <> show v

  in 
    maybe  
      (fail failMessage )
      (
        \case 
          MessageT -> Some <$> (Message <$> o .: "txt")    
          StartRunT -> Some <$> (StartRun <$> o .: "runConfig")
      )
      tag        

parseJSON wrng = typeMismatch "LogProtocol" wrng

“'Message'”的大小写没问题。我遇到的问题是错误，例如：

* No instance for (Titled x2) arising from a use of `StartRun'
* In the first argument of `(<$>)', namely `StartRun'
  In the second argument of `(<$>)', namely
    `(StartRun <$> o .: "runConfig")'
  In the expression: Some <$> (StartRun <$> o .: "runConfig")

任何地方我都有自己的类型 class 约束（例如 Titled）在数据构造函数中，编译器说 "No"。有办法解决这个问题吗？

Answer 1

存在类型是一种反模式，尤其是当您需要进行反序列化时。 StartRun 应该包含具体类型。无论如何，反序列化都需要一个具体的类型，因此您不妨将 StartRun 专门化为它。

如何在具有自定义类型 class 约束的 GADT 上实现 fromJSON？

How can I implement fromJSON on a GADT with custom type class constraints?

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aeson