A Dart library for working with the CFG configuration format.


The package can be installed by adding cfg_lib to your list of dependencies in pubspec.yaml:

cfg_lib: ^0.1.0


The CFG configuration format is a text format for configuration files which is similar to, and a superset of, the JSON format. It dates from before its first announcement in 2008 and has the following aims:

  • Allow a hierarchical configuration scheme with support for key-value mappings and lists.
  • Support cross-references between one part of the configuration and another.
  • Provide a string interpolation facility to easily build up configuration values from other configuration values.
  • Provide the ability to compose configurations (using include and merge facilities).
  • Provide the ability to access real application objects safely, where supported by the platform.
  • Be completely declarative.

It overcomes a number of drawbacks of JSON when used as a configuration format:

  • JSON is more verbose than necessary.
  • JSON doesn’t allow comments.
  • JSON doesn’t provide first-class support for dates and multi-line strings.
  • JSON doesn’t allow trailing commas in lists and mappings.
  • JSON doesn’t provide easy cross-referencing, interpolation, or composition.

A simple example

With the following configuration file, test0.cfg:

a: 'Hello, '
b: 'world!'
c: {
  d: 'e'
'f.g': 'h'
christmas_morning: `2019-12-25 08:39:49`
home: `$HOME`
foo: `$FOO|bar`

You can load and query the above configuration using iex:

Loading a configuration

The configuration above can be loaded as shown below. In the REPL shell:

iex(1)> alias CFG.Config
iex(2)> {:ok, cfg} = Config.from_file("test0.cfg")
{:ok, #PID<0.218.0>}

The successful call returns a Config which can be used to query the configuration.

Access elements with keys

Accessing elements of the configuration with a simple key is not much harder than using a map:

iex(3)> Config.get(cfg, "a")
{:ok, "Hello, "}
iex(4)> Config.get(cfg, "b")
{:ok, "world!"}

Access elements with paths

As well as simple keys, elements can also be accessed using path strings:

iex(5)> Config.get(cfg, "c.d")
{:ok, "e"}

Here, the desired value is obtained in a single step, by (under the hood) walking the path c.d – first getting the mapping at key c, and then the value at d in the resulting mapping.

Note that you can have simple keys which look like paths:

iex(6)> Config.get(cfg, "f.g")
{:ok, "h"}

If a key is given that exists in the configuration, it is used as such, and if it is not present in the configuration, an attempt is made to interpret it as a path. Thus, f.g is present and accessed via key, whereas c.d is not an existing key, so is interpreted as a path.

Access to date/time objects

You can also get native Elixir date/time objects from a configuration, by using an ISO date/time pattern in a backtick-string:

iex(7)> Config.get(cfg, "christmas_morning")
{:ok, ~U[2019-12-25 08:39:49.000000Z]}

Access to other Elixir/Erlang objects

Access to other Elixir/Erlang objects is also possible using the backtick-string syntax, provided that they are one of:

  • Environment variables
  • Public functions in public modules which take no arguments
iex(8)> {:ok, dt} = Config.get(cfg, "now")
{:ok, ~U[2021-10-16 12:37:37.781391Z]}
iex(9)> DateTime.diff(DateTime.utc_now, dt)

Access to environment variables

To access an environment variable, use a backtick-string of the form $VARNAME:

iex(10)> elem(Config.get(cfg, "home"), 1) == System.get_env("HOME")

You can specify a default value to be used if an environment variable isn’t present using the $VARNAME|default-value form. Whatever string follows the pipe character (including the empty string) is returned if the VARNAME is not a variable in the environment.

iex(11)> Config.get(cfg, "foo")
{:ok, "bar"}

For more information, see the CFG documentation.


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