Exception handling by try in Servant

servant_sample_03.hs

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

import Control.Exception as E

import Servant
import Control.Monad.IO.Class   -- liftIO
import Network.Wai.Handler.Warp -- run 

type FileRead03 = "sample03.txt" :> Get '[PlainText] String 

myAPI03 :: Proxy FileRead03 
myAPI03 = Proxy

app03 :: Application
app03 = serve myAPI03 server03

server03 :: Server FileRead03
server03 = do 
  ret <- liftIO (readF "sample03.txt")
  case ret of
    Left  e -> throwError err404 {errBody="file error"}
    Right s -> return s

readF :: String -> IO (Either SomeException String) 
readF x = E.try (readFile x) 

main03 :: IO ()
main03 = run 8080 app03