斯科蒂柱式不會允許效果

Question

有了：

mailHandler = do 
    name <- param "name" 
    email <- param "email" 
    renderSendMail $ forwardMail name email 

main = scotty 3000 $ post "/mail" mailHandler 

我獲得下列類型的錯誤：

Couldn't match type `IO' 
        with `Web.Scotty.Internal.Types.ActionT T.Text IO' 
    Expected type: Web.Scotty.Internal.Types.ActionT T.Text IO() 
     Actual type: IO() 
    In a stmt of a 'do' block: renderSendMail $ forwardMail name email 
    In the expression: 
     do { name <- param "name"; 
      email <- param "email"; 
      renderSendMail $ forwardMail name email } 
    In an equation for `mailHandler': 
     mailHandler 
      = do { name <- param "name"; 
       email <- param "email"; 
       renderSendMail $ forwardMail name email } 
Failed to install server-0.0.1 
cabal: Error: some packages failed to install: 
server-0.0.1 failed during the building phase. The exception was: 
ExitFailure 1 

花費幾個小時後，我還沒有看到如何解決這個問題。如果我使用Scotty內部的函數，沒有問題，但如果我想從外部處理任何事情，我會得到上面的錯誤。我怎樣才能突破這個Web.Scotty.Internal.Types.ActionT T.Text IO()

Answer 1

Web.Scotty.Internal.Types.ActionT T.Text IO()是單子轉換超過IO。這些通常實施MonadIO類，這樣就可以使用liftIO功能（從Control.Monad.IO.Class，如果它沒有被已經導入），以「提升」 IO動作融入其中：

liftIO . renderSendMail $ forwardMail name email