Haskell Servant passing custom data to auth handler

Question

I'm using custom monad (with reader) to easily pass data like DB pool to my handlers (before using custom monad I used to pass connection as fn argument).

This is how I've defined my custom monad :

newtype Controller a = Controller
    { runController :: ReaderT ServerEnvironment Handler a
    } deriving ( Functor, Applicative, Monad, MonadReader ServerEnvironment, 
                 MonadError ServantErr, MonadIO )

This ServerEnvironment is just custom datatype I use to carry my data.

Problem is that for my AuthHandler I have to specifically use function with:

r -> Handler usr

as authentication handler, I can't use my custom handler which would be :

r -> Controller usr

and I also have no way to pass in my ConnectionPool because signature can't be :

ConnPool -> r -> Handler usr

So, how does one pass extra data to authentication handler in servant without using global IO state?

Answer 1

The AuthHandler you put into the context doesn't have to be defined at the top-level! Generally, you'll want to do it in main so you have access to the database connections etc. which you've created:

type API = 
  ... :<|> (AuthProtect "myProtection" :> ...) :<|> ...

type instance AuthServerData (AuthProtect "myProtection") = User

server :: ServerEnvironment -> Server API
server env = ...

setupEnv :: IO ServerEnvironment
setupEnv = ..

-- This is essentially a 'Controller'.
authenticate :: ServerEnvironment -> Handler User
authenticate conn = ...

main :: IO ()
main = do
  env <- setupEnv
  -- Now, because we have access to the env, we can turn our
  -- 'authenticate' into the right type before putting it
  -- in the context
  let ctx = authenticate env :. EmptyContext
  run 8080 $ serveWithContext myAPI (server conn) ctx