如何将Reader monad添加到Scotty的monad？

Question

I'm trying to use Scotty to build a very simple API. I'd like to extend the Scotty monads such that my route handler actions are able to access an unchanging environment. I believe the way to do this would be to add a Reader monad to the stack. For now I just want to pass some Text data around.

I've extended the Scotty monads as follows:

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

So my first question is, is this the correct approach?

I've successfully changed the types of my route handlers, but I can't work out how to launch Scotty using this stack. I've attempted the following:

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

But I get the error:

  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

So my second question is, how do I launch Scotty with a different monad stack? This is my first attempt at using monad transformers and I seem to be hopelessly lost.

Answer 1

Your approach looks okay. The type error is because you should use runReaderT rather than runReader ( runReader is only for when you use Reader , which is ReaderT with just the dummy Identity monad below it).