let opt_err err = function
  | None -> Error err
  | Some x -> Ok x

let ( let* ) = Result.bind

let get_post course_id_opt =
  let* course_id = course_id_opt |> opt_err "invalid null course ID" in
  let* post_id = CommonDb.get_post_id course_id |> opt_err
    (sprintf "unable to find post ID for course ID %d and post type '%s'" course_id (post_type ()))
  in
  let* post = get_post post_id |> opt_err
    (sprintf "unable to find post ID %d for post type '%s'" post_id (post_type ()))
  in post