Haskell Scotty post类型不允许使用效果

与：

mailHandler=do
name
Web.Scotty.Internal.Types.ActionT.Text IO（）
是
IO
上的monad转换器。它们通常实现
MonadIO
类，因此您可以使用
liftIO
函数（从
Control.Monad.IO.class
，如果尚未导入）将
IO
操作“提升”到它们中：

    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

如果您不介意，请告诉我要安装以获取
Control.Monad.IO.Class
的cabal包的名称。如果你能解释整个monad transformer的概念，那就更好了好吧，我安装了
transformers
，事情又恢复了活力，但我还是希望能解释一下这个概念。@Freshyeball
scotty
似乎已经依赖于
transformers
，所以我怀疑这是多余的？（因为scotty定义了一个变压器本身，所以它有点像。）@Freshyeball Monad transformers是一个有点大的主题，所以我想我会在这里继续解释它，但请参见例如…@Freshyeball这篇文章：
    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

liftIO . renderSendMail $ forwardMail name email