vsajip / dart-cfg-lib

A Dart library for working with the CFG configuration format.

CFG.Config

A Dart library for working with the CFG configuration format.

Installation

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

cfg_lib: ^0.1.1

Usage

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 Repl.it:

Loading a configuration

The configuration above can be loaded as shown below:

var cfg = Config.fromFile('test0.cfg');

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:

print('a is "${cfg['a']}"');
print('b is "${cfg['b']}"');

which prints:

a is "Hello, "
b is "world!"

Access elements with paths

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

print('c.d is "${cfg['c.d']}"');

which prints:

c.d is "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:

print('f.g is "${cfg['f.g']}"');

which prints:

f.g is "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:

print('Christmas morning is ${cfg['christmas_morning']} (${cfg['christmas_morning'].runtimeType})');

which prints:

Christmas morning is 2019-12-25 08:39:49.000Z (DateTime)

As Dart doesn’t currently support timezone-aware date/times out of the box, currently the approach used is to compute the offset and add to the UTC time to yield the result. Although there are some third-party timezone-aware libraries around, they don’t allow computing an offset and setting it on the date/time - they work from timezone names.

Access to environment variables

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

print(cfg['home'] == Platform.environment['HOME']);

which prints:

true

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.

print('foo is "${cfg['foo']}"');

which prints:

foo is "bar"

For more information, see the CFG documentation.