haskell - 输入别名和 “instance … where”

Question

我有一个类型别名:

type MyId = UUID

和数据类型:

data MyData = MyData { myId :: MyId, .... }

然后我为 FromJSON和 ToJSON创建实例，为 MyData

instance FromJSON MyData where
  parseJSON (Object v) = ....

instance ToJSON MyData where
  toJSON (Object v) = ....

我想知道为什么会有错误:

1) No instance for (`FromJSON MyId`) arising from a use of ‘.:’
2) No instance for (ToJSON MyId) arising from a use of ‘.=’

instance FromJSON的 ToJSON和 UUID已经实现， MyId是否自动拥有它们？如果没有，我将如何实现它们？

instance FromJSON MyId where
  parseJSON (Object v) = --- ? It's already UUID for which FromJSON is implemented

instance ToJSON MyId where
  toJSON (Object v) = --- ? It's already UUID for which FromJSON is implemented

Answer 1

您要从Uuid-aeson包导入Data.UUID.Aeson吗？

这对我有用:

import Data.UUID
import Data.UUID.Aeson
import Data.Aeson

type MyUUID = UUID

data U2 = U2 MyUUID Int

instance ToJSON U2 where
  toJSON(U2 a b) = toJSON a