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我正在尝试将http://lexi-lambda.github.io/blog/2016/06/12/four-months-with-haskell/（标题为“Typeclasses can emulate effects”的部分）与一些方法结合起来有点像本土读者单子。

我要解决的总体问题是避免将配置变量传递给我的应用程序中几乎所有功能。我不能使用 a 的原因ReaderT是因为我的很多函数都在 in SqlPersistT，它本身在ReaderT内部使用 a 。另一个原因是更好地学习所有这些心理体操。

我的两个问题在下面的代码中作为注释给出。在这里也复制它们：

  • 最合适的定义方式是NwMonad什么？
  • 因此，如何定义NwMonad为HasNwConfig? 函数体怎么写askNwConfig？
  • 我最后怎么打电话runNwMonad？它的论点是什么？

这是代码：

data NwConfig = NwConfig {
  _googleClientId :: T.Text,
  _googleClientSecret :: T.Text,
  _tgramBotToken :: String,
  _aria2Command :: String,
  _aria2DownloadDir :: String
  }
$(makeLenses ''NwConfig)

instance Default NwConfig where
  def = NwConfig{}

class MonadIO m => HasNwConfig m where
  askNwConfig :: m NwConfig

startAria2 :: (HasNwConfig m) => m Sytem.Process.ProcessHandle
  cfg <- askNwConfig
  (_, _, _, processHandle) <- createProcess $ proc (cfg ^. aria2Command) []
return processHandle


-- QUESTION: Is this correct?
data NwMonad a = NwMonad{runNwMonad :: (NwConfig -> IO a)}
               deriving (Functor, Applicative, Monad, MonadIO)

-- Or is this the way to do it?
data NwMonad a = NwMonad{runNwMonad :: IO a, nwConfig :: NwConfig}
               deriving (Functor, Applicative, Monad, MonadIO)

instance HasNwConfig NwMonad where
  askNwConfig = return . nwConfig -- QUESTION: How to write this?

main :: IO ()
main = do
  [cId, cSecret, botToken] <- sequence [getEnv "GOOGLE_CLIENT_ID", getEnv "GOOGLE_CLIENT_SECRET", getEnv "TELEGRAM_TOKEN"]
  let cfg = (def :: NwConfig)
        & googleClientId .~ (T.pack cId)
        & googleClientSecret .~ (T.pack cSecret)
        & tgramBotToken .~ botToken
        & aria2Command .~ "/Users/saurabhnanda/projects/nightwatch/aria2-1.19.3/bin/aria2c"
  -- QUESTION: How do I use this now?
  runNwMonad $ (?????) $ startAria2
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1 回答 1

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这是一些代码，展示了如何在同一个变压器堆栈中使用多个阅读器环境。这BaseMonad就像你的SqlPersistT：

import Control.Monad.Reader
import Control.Monad.State
import Control.Monad.IO.Class

type BaseMonad = ReaderT String IO

type NwMonad = ReaderT Int BaseMonad

askString :: NwMonad String
askString =  lift ask

askInt :: NwMonad Int
askInt = ask

startAria :: NwMonad ()
startAria = do
  i <- askInt
  s <- askString
  liftIO $ putStrLn $ "i: " ++ show i ++ " s: " ++ s

main = do
  let cfg = 10       -- i.e. your google client data
      s = "asd"      -- whatever is needed for SqlPersistT
  runReaderT (runReaderT startAria cfg) s

下面是一些使用 SqlPersisT 类型和 runSqlConn 的代码：

import Control.Monad.Reader
import Control.Monad.State
import Control.Monad.IO.Class
import Database.Persist.Sql

data Config = Config { _clientId :: String }

type BaseMonad = SqlPersistT IO

type NwMonad = ReaderT Config BaseMonad

askBackend:: NwMonad SqlBackend
askBackend =  lift ask

askConfig :: NwMonad Config
askConfig = ask

startAria :: NwMonad ()
startAria = do
  cfg <- askConfig
  liftIO $ putStrLn $ "client id: " ++ (_clientId cfg)

main = do
  let cfg = Config "foobar"
      backend = undefined :: SqlBackend -- however you get this
      sqlComputation = runReaderT startAria cfg :: SqlPersistT IO ()
  runSqlConn sqlComputation backend :: IO ()

更新

环境类型无关紧要。

import Control.Monad.Reader
import Control.Monad.IO.Class

type Level1  =  ReaderT Int IO
type Level2  =  ReaderT Int Level1
type Level3  =  ReaderT Int Level2

ask3 :: Level3 Int
ask3 = ask

ask2 :: Level3 Int
ask2 =  lift ask

ask1 :: Level3 Int
ask1 =  lift $ lift $ ask

doit :: Level3 ()
doit = do
  r1 <- ask1
  r2 <- ask2
  r3 <- ask3
  liftIO $ print (r1, r2, r3)

main = do
  runReaderT (runReaderT (runReaderT doit 333) 222) 111
于 2016-06-26T07:04:58.307 回答