hoistServer is:exact package:servant-server

Hoist server implementation. Sometimes our cherished Handler monad isn't quite the type you'd like for your handlers. Maybe you want to thread some configuration in a Reader monad. Or have your types ensure that your handlers don't do any IO. Use hoistServer (a successor of now deprecated enter). With hoistServer, you can provide a function, to convert any number of endpoints from one type constructor to another. For example Note: Server Raw can also be entered. It will be retagged.
>>> import Control.Monad.Reader

>>> type ReaderAPI = "ep1" :> Get '[JSON] Int :<|> "ep2" :> Get '[JSON] String :<|> Raw :<|> EmptyAPI

>>> let readerApi = Proxy :: Proxy ReaderAPI

>>> let readerServer = return 1797 :<|> ask :<|> Tagged (error "raw server") :<|> emptyServer :: ServerT ReaderAPI (Reader String)

>>> let nt x = return (runReader x "hi")

>>> let mainServer = hoistServer readerApi nt readerServer :: Server ReaderAPI