Exception handling example Freer monad (with freer-simple)

Freer monad file reading exception.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE MonoLocalBinds #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE ScopedTypeVariables #-}

module Main where

import Data.String
import qualified Data.ByteString.Char8 as B
import Control.Exception
import Data.List
import Data.Text (Text, pack, unpack)
import Data.Text.IO
import Data.Text.Encoding (decodeUtf8)
import Control.Natural (type (~>))
import qualified  Control.Monad.Freer.Error as ER
import Control.Monad.Freer
  (
    Eff
  , LastMember
  , Member
  , Members
  , interpret
  , send
  , sendM
  , runM
  )

readFileUTF8 :: String -> IO Text
readFileUTF8 path = decodeUtf8 <$> B.readFile path

ioToAppErr :: IOException -> AppError
ioToAppErr ioe = AppError (displayException ioe)

newtype AppError = AppError String deriving Show

data FileSystem r where
  ReadFile :: FilePath -> FileSystem Text

readFile :: Members '[FileSystem, ER.Error AppError] effs => FilePath -> Eff effs Text
readFile fpath = if (elem 'x' fpath)
                     then (ER.throwError  (AppError "No Xs allowed in file name"))
                 else(send $ ReadFile fpath)

runAppError :: Eff (ER.Error AppError ': r) a -> Eff r (Either AppError a)
runAppError = ER.runError

fileSystemIOInterpreter
  :: (Members '[ER.Error AppError] effs, LastMember IO effs)
  => Eff (FileSystem ': effs) a -> Eff effs a
fileSystemIOInterpreter = interpret $ \case
    ReadFile path -> do
        r <- sendM (try (readFileUTF8 path))
        case r of
            Left (e :: IOException) -> ER.throwError (ioToAppErr e)
            Right f -> pure f

application :: Members '[FileSystem, ER.Error AppError] effs => FilePath -> Eff effs Text
application = Main.readFile

ioApp :: FilePath -> IO (Either AppError Text)
ioApp path = runM
              $ runAppError
              $ fileSystemIOInterpreter
              $ application path

main :: IO ()
main = do
  let pathX = "C:\\text.info"
  let pathNoX = "C:\\simple.t"
  let pathNoSuchAFile = "C:\\habrahabr.bib"
  result <- ioApp pathX
  Data.Text.IO.putStrLn $ pack (show result)

{-
dependencies:
- base >= 4.7 && < 5
- text
- freer-simple
- natural-transformation
- bytestring
-}