在 ScottyT 中使用 ReaderT 转换器（对比 ActionT）

Question

我正在尝试使用 ReaderT monad 转换器方法通过基于 Scotty 的应用程序进行线程配置，但在这样做时遇到了问题。在定义路由（因为其中一些依赖于配置）和处理实际请求时，我都必须使用配置。

后者在 ActionT 中工作得很好，但无论我尝试什么，我都无法在 ScottyT 中获得正确的类型。

这是我从 Scotty GitHub 存储库的 ReaderT 示例编译的最小示例：

{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE OverloadedStrings #-}

module Main where

import Control.Applicative
import Control.Monad.Reader (MonadIO, MonadReader, ReaderT, asks, lift, runReaderT)
import Data.Default.Class (def)
import Data.Text.Lazy (Text, pack)
import Prelude
import Web.Scotty.Trans (ScottyT, get, scottyOptsT, text, capture)

data Config = Config
  { environment :: String
  } deriving (Eq, Read, Show)

newtype ConfigM a = ConfigM
  { runConfigM :: ReaderT Config IO a
  } deriving (Applicative, Functor, Monad, MonadIO, MonadReader Config)

application :: ScottyT Text ConfigM ()
application = do
  get "/" $ do
    e <- lift $ asks environment
    text $ pack $ show e

  path <- lift $ asks environment
  get (capture path) $ do
    text $ pack $ "Hello, custom path"

main :: IO ()
main = scottyOptsT def runIO application where
  runIO :: ConfigM a -> IO a
  runIO m = runReaderT (runConfigM m) config

  config :: Config
  config = Config
    { environment = "Development"
    }

我得到的错误是：

• No instance for (Control.Monad.Trans.Class.MonadTrans
                     (ScottyT Text))
    arising from a use of ‘lift’
• In a stmt of a 'do' block: path <- lift $ asks environment

我查看了概述 ScottyT 类型的代码，确实似乎没有为其定义的 MonadTrans 实例。

但是，我觉得我的法力和 Haskell 经验都不够，无法找到解决方法，非常感谢任何帮助！

谢谢！

Answer 1

我们集思广益，找到了当前可行的问题解决方案。

ScottyT 类型是一个 monad 转换器，后 https://github.com/scotty-web/scotty/pull/167 got merged, therefore there's currently no way of using it this way. There was a PR https://github.com/scotty-web/scotty/pull/181 旨在恢复该功能，但据我所知它从未被合并。

因为它不是 monad 转换器，我们只能再次包装它：

{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE OverloadedStrings #-}

module Main where

import Control.Applicative
import Control.Monad.Reader (MonadIO, MonadReader, ReaderT, asks, lift, runReaderT)
import Data.Default.Class (def)
import Data.Text.Lazy (Text, pack)
import Prelude
import Web.Scotty.Trans (ScottyT, get, scottyOptsT, text, capture)

data Config = Config
  { environment :: String
  } deriving (Eq, Read, Show)

newtype ConfigM a = ConfigM
  { runConfigM :: ReaderT Config IO a
  } deriving (Applicative, Functor, Monad, MonadIO, MonadReader Config)

application :: ConfigM (ScottyT Text ConfigM ())
application = do
  path <- asks environment

  return $
    get "/" $ do
      e <- lift $ asks environment
      text $ pack $ show e

    get (capture path) $          
      text $ pack $ "Hello, custom path"

runIO :: Config -> ConfigM a -> IO a
runIO c m = runReaderT (runConfigM m) c

main :: IO ()
main = do
  let config = Config { environment = "/path" }
  app <- runIO config application
  scottyOptsT def (runIO config) app

感谢大家帮助我，希望这能帮助另一个像我一样流浪的斯科蒂 :)。

在 ScottyT 中使用 ReaderT 转换器（对比 ActionT）

Using ReaderT transformer in ScottyT (vs ActionT)

monads

haskell

monad-transformers

scotty