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)