OCaml Higher Order Functions Dispose system resources even when an exception is raised


Higher-order functions can be used to ensure that system resources are disposed, even when a treatment raises an exception. The pattern used by with_output_file allows a clean separation of concerns: the higher-order with_output_file functions takes care of managing the system resources bound to file manipulation while the treatment f only consumes the output channel.

let with_output_file path f =
  let c = open_out path in
    let answer = f c in
    (close_out c; answer)
  with exn -> (close_out c; raise exn)

Let us use this higher-order function to implement a function writing a string to a file:

let save_string path s =
  (with_output_file path) (fun c -> output_string c s)

Using more advanced functions than fun c -> output_string c s it is possible to save more complex values. See for instance the Marshal module in the standard library or the Yojson library by Martin Jambon.