Question

我正在尝试解析RoseTree的JSON表示。这是我的快照：

module RoseTree2 where

import Data.Tree
import Data.Aeson
import qualified Data.Text as T
import Control.Applicative

data RoseTree2 = RoseNode Int [RoseTree2] deriving (Show)

instance ToJSON RoseTree2 where
toJSON (RoseNode n cs) =
    object [T.pack "value" .= show n
    , T.pack "children".= show cs]

instance FromJSON RoseTree2 where
    parseJSON (Object o) =
        RoseNode <$> o.: T.pack "value"
        <*> o.: T.pack "children"

但是我收到了关于文件加载的错误：

RoseTree2.hs:10:10:
    No instance for (GToJSON (GHC.Generics.Rep RoseTree2))
      arising from a use of `aeson-0.7.0.6:Data.Aeson.Types.Class.$gdmtoJSON'
    Possible fix:
      add an instance declaration for
      (GToJSON (GHC.Generics.Rep RoseTree2))
    In the expression:
      (aeson-0.7.0.6:Data.Aeson.Types.Class.$gdmtoJSON)
    In an equation for `toJSON':
        toJSON = (aeson-0.7.0.6:Data.Aeson.Types.Class.$gdmtoJSON)
    In the instance declaration for `ToJSON RoseTree2'
Failed, modules loaded: none.

请问有人告诉我我对JSON解析器的定义有什么问题，我该如何解决？谢谢！

Answer 1

您需要缩进toJSON

的定义

instance ToJSON RoseTree2 where
  toJSON (RoseNode n cs) =
    object [T.pack "value" .= show n
    , T.pack "children".= show cs]

Answer 2

您忘记在instance ToJSON RoseTree2之后缩进该行，因此实例块已关闭，默认为

default toJSON :: (Generic a, GToJSON (Rep a)) => a -> Value
toJSON = genericToJSON defaultOptions

在Haskell中解析RoseTree JSON

2 个答案: