haskell处理JSON(aeson)

aeson是haskell的一个库，其实我也不太懂，不过大概是这样的：

 

定义一个类型

如

 

data Person =
  Person { firstName  :: String
         , lastName   :: String
         , age        :: Int
         , likesPizza :: Bool
           } deriving Show
 

然后我们来实现FromJSON , ToJSON这两个class

 

instance FromJSON Person where
parseJSON (Object v) =
    Person <$> v .: "firstName"
           <*> v .: "lastName"
           <*> v .: "age"
           <*> v .: "likesPizza"
parseJSON _ = mzero

instance ToJSON Person where
toJSON (Person firstName lastName age likesPizza) =
    object [ "firstName"  .= firstName
           , "lastName"   .= lastName
           , "age"        .= age
           , "likesPizza" .= likesPizza
             ]
 

不过呢，也不用都这么写，打开ghc的DeriveGeneric扩展，就可以让编译器帮我们自动生成了

 

data Person =
  Person { firstName  :: String
         , lastName   :: String
         , age        :: Int
         , likesPizza :: Bool
           } deriving (Show , Generic)
 
instance FromJSON Person
instance ToJSON Person

 

就这么简单！

 

处理JSON为两个，encode和decode

decode :: FromJSON a => ByteString -> Maybe a
encode :: ToJSON a => a -> ByteString

 

下面是一个完整的读取acmicpc.info提供的比赛的JSON的代码

{-# LANGUAGE OverloadedStrings, DeriveGeneric #-}

import Data.Aeson
import Control.Applicative
import Control.Monad
import qualified Data.ByteString.Lazy as B
import Network.HTTP.Conduit (simpleHttp)
import Network.Socket (withSocketsDo)

import Data.Map
import GHC.Generics
data Contest = 
  Contest{
    id :: String , 
    oj :: String ,
    link :: String ,
    name :: String ,
    start_time :: String ,
    week :: String , 
    access :: String
  } deriving (Show , Generic)

instance FromJSON Contest
instance ToJSON Contest


jsonURL :: String
jsonURL = "http://contests.acmicpc.info/contests.json"

getJSON :: IO B.ByteString
getJSON = withSocketsDo $ simpleHttp jsonURL

main :: IO ()
main = do
  d <- (eitherDecode <$> getJSON) :: IO (Either String [Contest])
  case d of
    Left err -> putStrLn err
    Right ps -> print ps

　　

 

- -基本都是抄的例子，不过要主意的是simpleHttp在windows下面要在前面加个withSocketsDo...不然会有一个10093的错误，说file not exists

getAddrInfo: does not exist (error 10093)