A Rust library for working with the CFG configuration format

2 releases

0.1.1 Sep 13, 2021
0.1.0 Mar 25, 2020

#32 in Configuration


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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, for example, the evcxr REPL:

$ evcxr
>> :dep cfg-lib
>> use cfg_lib::*;

Loading a configuration

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

>> let cfg = Config::from_file("test0.cfg").unwrap();

The successful from_file() call returns a Config instance 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 HashMap:

>> cfg.get("a")
Ok(Base(String("Hello, ")))
>> cfg.get("b")

The values returned are of type Value.

Access elements with paths

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

>> cfg.get("c.d")

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:

>> cfg.get("f.g")

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 Rust date/time objects from a configuration, by using an ISO date/time pattern in a backtick-string:

>> cfg.get("christmas_morning")

You get either NaiveDate objects, if you specify the date part only, or else DateTime objects, if you specify a time component as well. If no offset is specified, it is assumed to be zero.

Access to environment variables

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

>> cfg.get("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.

>> cfg.get("foo")

For more information, see the CFG documentation.


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