Exception handing by catch in Servant

servant_sample_02.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 FileRead02 = "sample02.txt" :> Get '[PlainText] String 

myAPI02 :: Proxy FileRead02 
myAPI02 = Proxy

app02 :: Application
app02 = serve myAPI02 server02

server02 :: Server FileRead02
server02 = do 
  f <- liftIO (readF "sample02.txt")
  return f

readF :: String -> IO String 
readF x = (readFile x) `E.catch` (\(SomeException x) -> return "error!!")
-- catch :: Exception e => IO a -> (e -> IO a) -> IO a

main02 :: IO ()
main02 = run 8080 app02