haskell - Aeson 解码结果为“无”

Question

我正在使用此代码:

{-# LANGUAGE OverloadedStrings #-}

import Data.Aeson
import Data.Maybe
import Data.ByteString.Lazy
import Control.Applicative
import Debug.Trace
import Control.Monad
import qualified Data.Aeson.Types as T

main = do
  res <- liftA show (liftA decodeOriginal (Data.ByteString.Lazy.readFile "./a.json"))
  Prelude.putStrLn res

interpretResult :: Maybe String -> String
interpretResult Nothing = "Error."
interpretResult x = fromJust x

data TotalLine1 = TotalLine1 {
  timestamp :: Integer,
  value :: Integer
} deriving (Eq, Show)

data Original = Original {
  totals :: [TotalLine1]
} deriving (Eq, Show)

instance FromJSON Original where
  parseJSON (Object v) = traceStack "Original" (Original <$> (parseJSON =<< (v .: "visitors.total")))
  parseJSON _ = mzero

instance FromJSON TotalLine1 where
  parseJSON (Object v) = TotalLine1 <$>
                         v .: "timestamp" <*>
                         v .: "value"
decodeOriginal :: ByteString -> Maybe Original
decodeOriginal b = traceStack "decoding" (do
  a <- decode b :: Maybe Original
  return a)

尝试像这样解析 JSON:

{
visitors.total: [
{
timestamp: 1365548400,
value: 1
},
{
timestamp: 1365548700,
value: 2
},
{
timestamp: 1365549000,
value: 5
},
]
}

但是 main 每次都返回 Nothing 。我做错了什么？看来 Original 甚至没有调用 parseJSON。

Answer 1

您的 JSON 文件无效。

一方面，字段的名称必须加引号，

"timestamp"

等等。另一方面，TotalLine1 列表中有一个尾随逗号，这会导致 ByteString 解码为 Value 失败。引用字段名称并删除结尾的逗号，这样就可以了。