haskell - 尝试摆脱嵌套表达式

Question

我有一个 yaml 文件:

base123:
  key1: "key1"
  key2: "key2"
  key3: "key3"

以及应该从中读取所有 3 个值的代码:

read123 :: IO (String, String, String)
read123 = do
  rawConfig <- Y.decodeFile "config.yml" :: IO (Maybe Y.Value)
  case rawConfig of
    Just res1 ->
      case res1 of
        Object res2 ->
          case (LHashMap.lookup "base123" res2) of
            Just (Object res3) -> 
              case (LHashMap.lookup "key1" res3) of
                Just (String key1) -> 
                  case (LHashMap.lookup "key2" res3) of
                    Just (String key2) -> 
                      case (LHashMap.lookup "key3" res3) of
                        Just (String key3) -> return (key1, key2, key3)
        _ -> error "some error"        

    Nothing -> error "error123"

看起来效果不错。但我相信，一定有一种方法可以在不使用 Lens 的情况下摆脱嵌套表达式。有没有？或者有什么方法可以更简单地完成同样的事情？

更新:

  rawConfig <- Y.decodeFile "config.yml" :: IO (Maybe Y.Value)
  case rawConfig of
    Just (Object res1) -> LHashMap.lookup "base123" res1
    Nothing -> error "error"

  return ("", "", "") -- stub 

错误:

Couldn't match type `Maybe' with `IO'
    Expected type: IO Value
      Actual type: Maybe Value
    In the return type of a call of `LHashMap.lookup'

Answer 1

我建议您使用正确的数据类型来存储 YAML 数据。迈克尔·斯诺曼的yaml库重用了 aeson 的大部分 API。所以它与使用 aeson 包的方式非常相似。这是一个有效的示例代码:

{-# LANGUAGE OverloadedStrings #-}

import Control.Applicative
import Control.Monad (mzero)
import Data.Text
import Data.Yaml

data Base = Base {
  key1 :: Text,
  key2 :: Text,
  key3 :: Text
  } deriving (Show)

instance FromJSON Base where
  parseJSON (Object v) = Base <$>
                         ((v .: "base123") >>= (.: "key1")) <*>
                         ((v .: "base123") >>= (.: "key2")) <*>
                         ((v .: "base123") >>= (.: "key3"))

  parseJSON _ = mzero

main = do
  b <- decodeFile "/home/sibi/yaml.yml" :: IO (Maybe Base)
  print b

在 ghci 中:

λ> main
Just (Base {key1 = "key1", key2 = "key2", key3 = "key3"})