输入别名和 "instance ... where"

Question

我有一个类型别名：

type MyId = UUID

还有一个数据类型：

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

然后我为 FromJSON 创建实例，为 MyData

创建实例 ToJSON

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 for UUID 已经实现了，难道 MyId 没有自动实现吗？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

您是否正在导入 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 "instance ... where"

Type alias and "instance ... where"

haskell

compiler-errors

typeclass