哈斯克爾埃宋：如何獲取價值解析器的IO單子

Question

我試圖做的JSON解析在IO：

{-# LANGUAGE OverloadedStrings #-} 
import Network.HTTP.Simple 
import Data.Aeson 
import Data.Maybe (fromJust) 

main :: IO() 
main = do 
    response <- getResponseBody <$> httpJSON "http://localhost:9200" :: IO Object 
    name <- fromJust <$> response .: "name" :: Parser String 
    print "hi" 

我得到的錯誤：

/home/nut/dev/haskell/elastic/app/Main.hs:39:11: error: 
    • Couldn't match type ‘Parser’ with ‘IO’ 
     Expected type: IO String 
     Actual type: Parser String 
    • In a stmt of a 'do' block: 

所以，我該怎麼辦從json結果中獲得name？

Answer 1

埃宋有一堆的功能，從Parser a到a去：

parse  :: (a -> Parser b) -> a -> Result b 
parseEither :: (a -> Parser b) -> a -> Either String b 
parseMaybe :: (a -> Parser b) -> a -> Maybe b 

所以如果你有例如

(.: "name") :: Object -> Parser String 

，那麼你必須

parseMaybe (.: "name") :: Object -> Maybe String 

所以你可以做

{-# LANGUAGE OverloadedStrings #-} 
import Network.HTTP.Simple 
import Data.Aeson 
import Data.Maybe (fromJust) 
import Data.Aeson.Types -- new import for parseMaybe 

main :: IO() 
main = do 
    response <- getResponseBody <$> httpJSON "http://localhost:9200" 
    let name = fromJust $ parseMaybe (.: "name") response :: String 
    print "hi"