阅读Haskell中的YAML对象列表 [英] Reading YAML lists of objects in Haskell

问题描述

假设这个YAML（保存在一个名为 users.yml 的文件中）：

   -  id：1 
名称：未知用户
声望：0 
 
  -  id：2 
名称：Foo bar 
声望：4 
  

和这个Haskell data 类型：

  data MyUser = MyUser {id :: Int，
 name :: String，
 reputation :: Int} 
导出（显示）
  

我想使用 yaml 库将YAML读入 [MyUser] 。我该怎么做？

你需要创建一个 FromJSON 实例（请注意，这被称为 FromJSON ，因为 yaml 是从 Aeson 库），如 Data.Yaml 文档。

以前讨论过与Aeson类似的问题，此处，而讨论了Haskell YAML库的选择这里

这是一个最小的工作示例， YAML文件转换为 [MyUser] ：

  { - ＃LANGUAGE OverloadedStrings ＃ - } 
导入Data.Yaml 
导入Control.Applicative  - < $> ;, < * GT; 
 import Data.Maybe（fromJust）
 
将合格的Data.ByteString.Char8导入为BS 
 
 data MyUser = MyUser {id :: Int，
 name :: String，
 reputation :: Int} 
 derived（Show）
 
 instance FromJSON MyUser where 
 parseJSON（Object v）= MyUser< $> 
 v。：id< *> 
 v。：name< *> 
 v。：信誉
  - 非对象值的类型错误，因此失败。 
 parseJSON _ =错误无法从YAML / JSON解析MyUser
 
 main = do 
 ymlData<  -  BS.readFileusers.yml
 let users = Data.Yaml.decode ymlData :: Maybe [MyUser] 
  - 打印它，仅用于显示
打印$ fromJust用户
  

Assuming this YAML (saved in a file called users.yml):

- id: 1
  name: Unknown user
  reputation: 0

- id: 2
  name: Foo bar
  reputation: 4

and this Haskell data type:

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

I want to use the yaml library to read the YAML into a [MyUser]. How can I do that?

解决方案

You need to create a FromJSON instance (note that this is called FromJSON as yaml is derived from the Aeson library) as described in the Data.Yaml documentation.

A similar issue with Aeson was previously discussed here whereas the choice of a Haskell YAML library was discussed here

Here's a minimal working example that converts the YAML file into a [MyUser]:

{-# 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

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