如何将Reader monad添加到Scotty的monad？

Question

我正在尝试使用Scotty来构建一个非常简单的API.我想扩展Scotty monad,以便我的路由处理程序操作能够访问一个不变的环境.我相信这样做的方法是Reader在堆栈中添加一个monad.现在我只想传递一些Text数据.

我把Scotty monads扩展如下:

type BrandyScottyM = ScottyT TL.Text (ReaderT T.Text IO)
type BrandyActionM = ActionT TL.Text (ReaderT T.Text IO)

https://github.com/stu-smith/Brandy/blob/0838a63537d7e396ac82d58d460c6529349303d3/src/Core.hs

所以我的第一个问题是,这是正确的方法吗？

我已经成功地改变了我的路由处理程序的类型,但我无法弄清楚如何使用这个堆栈启动Scotty.我尝试过以下方法:

runScotty :: Port -> Text -> BrandyScottyM () -> IO ()
runScotty port file = T.scottyT port ((\f -> runReader f file)) id

https://github.com/stu-smith/Brandy/blob/0838a63537d7e396ac82d58d460c6529349303d3/src/Main.hs

但我得到错误:

  Couldn't match type `IO' with `Data.Functor.Identity.Identity'
    Expected type: Reader Text (IO a)
      Actual type: ReaderT Text IO a
    In the first argument of `runReader', namely `f'
    In the expression: runReader f file
    In the second argument of `T.scottyT', namely
      `((\ f -> runReader f file))'
/home/stu/git/Brandy/src/Main.hs: line 36, column 65:
  Couldn't match type `ReaderT Text IO Network.Wai.Internal.Response'
                  with `IO Network.Wai.Internal.Response'
    Expected type: ReaderT Text IO Network.Wai.Internal.Response
                   -> IO Network.Wai.Internal.Response
      Actual type: ReaderT Text IO Network.Wai.Internal.Response
                   -> ReaderT Text IO Network.Wai.Internal.Response
    In the third argument of `T.scottyT', namely `id'
    In the expression: T.scottyT port ((\ f -> runReader f file)) id
    In an equation for `runScotty':
        runScotty port file = T.scottyT port ((\ f -> runReader f file)) id

所以我的第二个问题是,如何使用不同的monad堆栈启动Scotty？这是我第一次尝试使用monad变换器,我似乎无可救药地丢失了.

Answer 1

你的方法看起来没问题.类型错误是因为你应该使用runReaderT而不是runReader(runReader仅当你使用时Reader,它只使用它下面ReaderT的虚拟Identitymonad).