haskell - Scotty 帖子类型不允许效果

Question

和：

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

main = scotty 3000 $ post "/mail" mailHandler

我收到以下类型错误：

    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

花了几个小时后，我仍然不知道如何解决这个问题。如果我使用 Scotty 内部的函数，没问题，但如果我想在外部处理任何事情，我会收到上面的错误。我怎样才能克服这个Web.Scotty.Internal.Types.ActionT T.Text IO ()

Answer 1

Web.Scotty.Internal.Types.ActionT T.Text IO ()是一个单子IO变压器。这些通常实现MonadIO类，以便您可以使用该liftIO函数（从Control.Monad.IO.Class，如果尚未导入）将IO操作“提升”到它们中：

liftIO . renderSendMail $ forwardMail name email