如何讓ReaderT與另一個monad變壓器一起工作？

Question

我想嵌入ReaderT到另一個monad變壓器。我該怎麼做呢？下面的例子使用Scotty，但我認爲它會與任何其他monad相同。

{-# LANGUAGE OverloadedStrings #-} 

import qualified Web.Scotty 
import Web.Scotty.Trans 

import Data.Text.Lazy 
import Control.Monad.IO.Class (liftIO) 

import Control.Monad.Trans.Reader 
import Control.Monad.Trans 

data Config = Config Text 

main :: IO() 
main = do 
    let config = Config "Hello World" 
    -- how to I make this line work? 
    scottyT 3000 id id routes 

routes :: ScottyT Text (ReaderT Config IO)() 
routes = do 
    get "/" info 

info :: ActionT Text (ReaderT Config IO)() 
info = do 
    -- this part seems like it works! 
    Config message <- lift ask 
    text $ "Info: " `append` message 

上線scottyT 3000 id id routes這樣的錯誤，因爲scottyT期望一個ScottyT Text IO()。我如何完成這項工作？以下是當前的錯誤：

Server.hs: line 24, column 24: 
    Couldn't match type `ReaderT Config IO' with `IO' 
    Expected type: ScottyT Text IO() 
     Actual type: ScottyT Text (ReaderT Config IO)() 

Answer 1

你必須改變你作爲id所提供的是那些有類型分別forall a. m a -> n a和m Response -> IO Response的參數。爲什麼？我不知道，但我發現here的範例顯示某人運行它類似於

main = do 
    let config = Config "Hello, world" 
     runner = flip runReaderT config 
    scottyT 3000 runner runner routes 

我測試過它，它至少起作用。我不知道這是否是最佳實踐。如果有人有更好的方法，隨時發佈。