I felt the need for an n-ary version of withFile. It works, but for something this simple, readFile + writeFile would've been enough

withFiles.hs

import System.Environment (getArgs)
import System.IO (withFile, IOMode(..), FilePath, Handle, hGetContents, hPutStr)
import Data.Functor ((<$>))

withFiles' :: [Handle] -> [(IOMode, FilePath)] -> ([Handle] -> IO r) -> IO r
withFiles' handles [] f = f $ reverse handles
withFiles' handles ((mode, fpath):xs) f = withFile fpath mode (\x -> withFiles' (x:handles) xs f)

withFiles = withFiles' []

cat [infile, outfile] = do
  text <- hGetContents infile
  hPutStr outfile text

main = do
  inputs <- zip [ReadMode, WriteMode] <$> getArgs
  withFiles inputs cat