Creating a ReasonML Native App: Configuration Files

This post is part of a series. Related posts:

Creating a ReasonML Native App: What We Are Building
Creating a ReasonML Native App: Installing, Editing, Configuring
Creating a ReasonML Native App: Building and Running
Creating a ReasonML Native App: Command Line Arguments
Creating a ReasonML Native App: Curses Terminal Interface
Creating a ReasonML Native App: Python Plugins
Creating a ReasonML Native App: Files I/Os
Creating a ReasonML Native App: Native Promises
Creating a ReasonML Native App: Web Requests
Creating a ReasonML Native App: Shell Commands
Creating a ReasonML Native App: Handling CSV Files
Creating a ReasonML Native App: Native JSON Handling
Creating a ReasonML Native App: Memoizing DNS queries

This is going to be a fairly short post, as it focuses on using the config-file library. As previously mentioned, when I mention a library in RasonML’s native context, chances are pretty good that I am referring to one of OCaml’s many available libraries.

Our configuration files are not going to be json-formatted of xml-based; instead they are a tad…OCaml-ish, which of course is instantly betrayed by our use of comments:

host_info = {
  (* Server ip address or host name *)
  ip = "127.0.0.1"
  (* User name (or none if no login is required) *)
  user = user
...
}

This library’s usage is pretty straightforward: after opening the library, we instantiate a new group object. The first thing we will do is create a default configuration file with sane values. Thus far, we have:

open Config_file;
let group = new group;
...
group#write("config/default.cfg");

Classes’ members are accessed using objectname#member and not using dot notation. This behavior is inherited from OCaml.

To be clear, this code does not do much of anything: it is writing an empty configuration file. What we need to do, before writing the file, is create a few entries:

let _ip = (new string_cp)(
  ~group,
  ["host_info", "ip"],
  "127.0.0.1",
  "Server ip address or host name");
...

The library supports several types of configuration objects:

int_cp, float_cp, bool_cp, string_cp, list_cp, option_cp, enumeration_cp, tuple2_cp, tuple3_cp, tuple4_cp, string2_cp, font_cp, filename_cp

We construct an object with:

which group this preference belongs to, the key’s hierarchy, a default value and a description.

You can use a shortand when passing a variable named after the option being targeted: myname = myname => ~myname

But, what if you want to make a preference optional?

This is the option_cp type. Use a wrapper to turn objects into these option types. For instance, to provide an optional string:

let _start_at = (new option_cp)(
  string_wrappers,
  ~group,
  ["run_info", "start_at"],
  None,
  "Only start test when matching this record timestamp/duration");

Rather than None obviously our default value could have been Some("my-time")

Reading

Reading a configuration file is just as straightforward:

let read_config_file = (dir_path) => {
  group#read(
    ~on_type_error=
      (groupable_cp, raw_cp, output, filename, in_channel) =>
        {
          /*
           * Handle read error. Parameter names can be retrieved using
           *  String.concat(".", groupable_cp#get_name),
           */
        },
    sprintf("%s/default.cfg", dir_path));
};

Bonus: reading a .ignore file line-by-line

This is helpful for quick prototyping: how do you read an arbitrary collection of strings from a file?

let read_ignore_list = (dir_path, name) =>
  List.fold_left((accu, item) =>
    StringSet.add(item, accu),
    StringSet.empty,
    Str.split(
      Str.regexp("\n"),
      my_read_file_function(sprintf("%s/ignored.%s", dir_path, name)))
  );