如何构建自定义 reader monad 以及自定义类型类？

Question

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）中给出的方法与某种本土 reader monad 结合起来。

我试图解决的总体问题是避免将配置变量传递给几乎所有应用程序中的功能。我不能使用 ReaderT 的原因是因为我的很多功能都在 SqlPersistT 中，它本身在内部使用 ReaderT。另一个原因是为了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。也在这里复制它们：

什么是最合适的定义方式NwMonad？
因此，如何将NwMonad定义为HasNwConfig的实例？ askNwConfig的函数body怎么写?
我最终如何调用 runNwMonad？它的论点是什么？

代码如下：

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

Answer 1

下面是一些代码，展示了如何在同一个转换器堆栈中处理多个 Reader 环境。这里 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

如何构建自定义 reader monad 以及自定义类型类？

How to build a custom reader monad along with a custom typeclass?

haskell

monads

monad-transformers

reader-monad