Question

我有一个类型别名：

type MyId = UUID

数据类型：

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

然后我为FromJSON创建了ToJSON和MyData的实例

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

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

我想知道，为什么我有错误：

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

instance FromJSON和ToJSON UUID已经实施，MyId是否已自动拥有它们？如果没有，我将如何实施它们？

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

您是否导入Data.UUID.Aeson（来自uuid-aeson包）？

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

输入别名和“实例......在哪里”

1 个答案: