{-# LANGUAGE TypeApplications, OverloadedLabels, PolyKinds, DataKinds, ScopedTypeVariables, Rank2Types #-}
import Data.Extensible

import Prelude hiding (readFile)
import Control.Exception
import Data.ByteString (ByteString)

data FileSystem r where
  ReadFile
    :: FilePath
    -> FileSystem (Either SomeException ByteString)

readFile
  :: forall xs
  . (Associate "fs" FileSystem xs
  , Associate "fs_error" (EitherEff SomeException) xs)
  => FilePath
  -> Eff xs ByteString
readFile fp = liftEff #fs (ReadFile fp)
           >>= either (throwEff #fs_error) pure

foo :: forall xs. Associate "fs" FileSystem xs => Eff xs (Either SomeException ByteString)
foo = castEff (runEitherEff @ "fs_error" (readFile "foo") :: Eff '["fs" >: FileSystem] (Either SomeException ByteString))