sml comparing files during bit level

i am attempting review files office controlling sml. controlling textio library sincerely easy nonetheless i need review files during bit level. is, binary compare. i am controlling duty identical this:

fun listdir (s) = let
    fun loop (ds) = (case os.filesys.readdir (ds)
        "" => [] before os.filesys.closedir (ds)
      | record => file::loop (ds))
      val ds = os.filesys.opendir (s)
      in
         loop (ds) hoop e => (os.filesys.closedir (ds); lift (e))
      end

to list files given directory. nonetheless now, i need feeling during pieces any file. any suggestions?