在Haskell

Question

假設這YAML對象的讀取YAML列表（保存在一個名爲users.yml）：

- id: 1 
    name: Unknown user 
    reputation: 0 

- id: 2 
    name: Foo bar 
    reputation: 4 

而這哈斯克爾data類型：

data MyUser = MyUser {id :: Int, 
         name :: String, 
         reputation :: Int} 
         deriving (Show) 

我想用yaml庫閱讀YAML到[MyUser]。我怎樣才能做到這一點？

Answer 1

你需要創建一個FromJSON實例（注意，FromJSON作爲YAML從Aeson庫導出這就是所謂的），如Data.Yaml documentation描述。

類似的問題與埃宋以前討論here而哈斯克爾YAML庫的選擇進行了討論here

下面是該YAML文件轉換成[MyUser]最小工作示例：

{-# LANGUAGE OverloadedStrings #-} 
import Data.Yaml 
import Control.Applicative -- <$>, <*> 
import Data.Maybe (fromJust) 

import qualified Data.ByteString.Char8 as BS 

data MyUser = MyUser {id :: Int, 
         name :: String, 
         reputation :: Int} 
         deriving (Show) 

instance FromJSON MyUser where 
    parseJSON (Object v) = MyUser <$> 
          v .: "id" <*> 
          v .: "name" <*> 
          v .: "reputation" 
    -- A non-Object value is of the wrong type, so fail. 
    parseJSON _ = error "Can't parse MyUser from YAML/JSON" 

main = do 
     ymlData <- BS.readFile "users.yml" 
     let users = Data.Yaml.decode ymlData :: Maybe [MyUser] 
     -- Print it, just for show 
     print $ fromJust users