Functional Programming

I've been enjoying learning ocaml, but I found it very easy to write code riddled with side effects and imperative-ness.

Take this example:

let some_func arg = 
  let input = format_input_from_stdin ()
  let read_content = read_file "some/file/path.txt"
  let to_write = get_write_file_content input read_content
  let () = write_file "some/other/path.txt" to_write 
  let output = run_external_command_with_output 
  (output, read_content)

As you can see, many side effects and imperative steps in the code. Is there a better practice for coding this in a functional manner?