Haskell JSON parsing with Aeson

Question

我有一个看起来像这样的JSON数据源：

{ "fields": [
{ "type": "datetime",
  "name": "Observation Valid",
  "description": "Observation Valid Time"},
{ "type": "datetime",
  "name": "Observation Valid UTC",
  "description": "Observation Valid Time UTC"},
{ "type": "number",
  "name": "Air Temperature[F]",
  "description": "Air Temperature at 2m AGL"},
{ "type": "number",
  "name": "Wind Speed[kt]",
  "description": "Wind Speed"},
{ "type": "number",
  "name": "Wind Gust[kt]",
  "description": "Wind Gust"},
{ "type": "number", "name":
  "Wind Direction[deg]",
  "description": "Wind Direction"}
  ],
"rows": [
["2018-04-22T00:10:00", "2018-04-22T05:10:00Z", 50.0, 9.0, null, 50.0],
["2018-04-22T00:15:00", "2018-04-22T05:15:00Z", 50.0, 9.0, null, 60.0],
["2018-04-22T00:20:00", "2018-04-22T05:20:00Z", 50.0, 8.0, null, 60.0],
["2018-04-22T00:30:00", "2018-04-22T05:30:00Z", 50.0, 9.0, null, 60.0]
]
}
            ( https://mesonet.agron.iastate.edu/json/obhistory.py?station=TVK&network=AWOS&date=2018-04-22 )

并尝试了几个数据描述，最后是：

data Entry =             -- Data entries
  Entry {  time      ::  Text     -- Observation Valid Time
         , timeUTC   ::  Text     -- Observation Valid Time UTC
         , airTemp   ::  Float    -- Air Temperature[F] at 2m AGL
         , wind      ::  Float    -- Wind Speed [kt]
         , gust      ::  Float    -- Wind Gust [kt]
         , direction ::  Int      -- Wind Direction[deg]
           } deriving (Show,Generic)
data Field =             -- Schema Definition
  Field {  ftype       :: String     -- 
         , name        :: String     -- 
         , description :: String    -- 
           } deriving (Show,Generic)
data Record =
  Record {  fields  :: [Field]     -- 
          , rows    :: [Entry]     -- data
           } deriving (Show,Generic)
-- Instances to convert our type to/from JSON.
instance FromJSON Entry
instance FromJSON Field
instance FromJSON Record
-- Get JSON data and decode it
dat <- (eitherDecode <$> getJSON) :: IO (Either String Record)

给出此错误： $ .fields [0]中的错误：键" ftype"不存在

(第一个(错误来自字段定义(我不使用(。在JSON中，条目是混合类型的数组，但是在Haskell中，它只是一个数据结构，而不是数组 - 不确定如何调和这些。

毫无疑问，初学者的错误 - 但我还没有找到任何似乎具有这种结构的例子。我需要为此写一个自定义解析器吗？

Answer 1

三件事阻止了此操作按预期工作：

• JSON数据包含一个名为"类型"的字段。Field记录类型的自定义FromJson实例可以处理此操作。
• Entry类型中的数据未命名，因此它可以更好地表示为没有字段名称的data记录或tuple。
• 代表风阵的Float有时为null，因此应该是Maybe Float

下面的代码包含所有这些修改并解析您的示例JSON数据：

{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE OverloadedStrings #-}
import Data.ByteString.Lazy as BSL
import Data.Text (Text)
import Data.Aeson 
import GHC.Generics
-- Either this tuple definition of Entry or the data definition without
-- names (commented out) will work.  
type Entry =             -- Data entries
  ( Text     -- Observation Valid Time
  , Text     -- Observation Valid Time UTC
  , Float    -- Air Temperature[F] at 2m AGL
  , Float    -- Wind Speed [kt]
  , Maybe Float    -- Wind Gust [kt]
  , Int      -- Wind Direction[deg]
  ) 
-- data Entry =             -- Data entries
--   Entry Text     -- Observation Valid Time
--         Text     -- Observation Valid Time UTC
--         Float    -- Air Temperature[F] at 2m AGL
--         Float    -- Wind Speed [kt]
--         (Maybe Float)    -- Wind Gust [kt]
--         Int      -- Wind Direction[deg]
--         deriving (Show,Generic)
-- instance FromJSON Entry
data Field =             -- Schema Definition
  Field {  ftype       :: String    -- 
        ,  name        :: String    -- 
        ,  description :: String    -- 
        } deriving (Show,Generic)
instance FromJSON Field where
  parseJSON = withObject "Field" $ v -> Field
    <$> v .: "type"
    <*> v .: "name"
    <*> v .: "description"
data Record =
  Record {  fields  :: [Field]     -- 
         ,  rows    :: [Entry]     -- data
         } deriving (Show,Generic)
instance FromJSON Record
getJSON :: IO ByteString
getJSON = BSL.readFile "json.txt" 
main :: IO()
main = do
  -- Get JSON data and decode it
  dat <- (eitherDecode <$> getJSON) :: IO (Either String Record)
  case dat of
    Right parsed -> print parsed
    Left err -> print err

Answer 2

字段具有ftype字段，因此Aeson试图在JSON中找到FTYPE，但无法(因为它包含FTYPE(。我了解您不能在Haskell中命名type字段，因此您需要找到一种方法，使Aeson使用其他名称。您需要相应地使用模板Haskell并设置FieldLabelModifier。另外，手动编写坚持可能会更简单。