How to response with HTTP status in custom servant handler?

Question

I created a custom servant handler

type ServiceSet = TVar (M.Map String [MicroService])
type LocalHandler = ReaderT ServiceSet IO

But i failed to find a way to response a 404-not-found status code to client in following function:

getService :: String -> LocalHandler MicroService
getService sn = do
  tvar <- ask
  ms <- liftIO $ do
    sl <- atomically $ do
      sm <- readTVar tvar
      return $ case M.lookup sn sm of
        Nothing -> []
        Just sl -> sl
    let n = length sl
    i <- randomRIO (0, n - 1)
    return $ if n == 0
      then Nothing
      else Just . head . drop i $ sl
  case ms of
    Nothing -> ??? -- throwError err404
    Just ms' -> return ms'

how to send 404 status code in ????

Answer 1

You will need to add ExceptT to your stack of monad transforms. Right now, with just ReaderT, there is no way to encode the notion of an error being thrown.

{-# LANGUAGE DataKinds     #-}
{-# LANGUAGE TypeOperators #-}

module Lib where

import Control.Monad.Except
import Control.Monad.Reader
import Data.Maybe
import Data.Map
import GHC.Conc
import Prelude hiding (lookup)
import Servant.API
import Servant.Server
import System.Random

type API =
  Capture "name" String :> Get '[JSON] Int

type World =
  TVar (Map String [Int])

type Effects =
  ExceptT ServantErr (ReaderT World IO)

server :: World -> Server API
server world =
  enter (Nat transform) get
  where
    transform :: Effects a -> ExceptT ServantErr IO a
    transform (ExceptT foo) =
      ExceptT $ runReaderT foo world

get :: String -> Effects Int
get sn = do
  tvar <- ask
  ms <- liftIO $ do
    sl <- atomically $ do
      sm <- readTVar tvar
      return (fromMaybe [] (lookup sn sm))
    let n = length sl
    i <- randomRIO (0, n - 1)
    return $ if n == 0
      then Nothing
      else Just . head . drop i $ sl
  case ms of
    Nothing ->
      throwError err404
    Just ms' ->
      return ms'

With ExceptT ServantErr . ReaderT (TVar ...) you can then throwError err404, which Servant will catch and use to return an HTTP 404. The natural transformation ExceptT ServantErr . ReaderT (TVar ...) :~> ExceptT ServantErr will then have to unwrap and rewrap in order to discharge the reader effect. All in all, not terribly more code.