2 releases
0.1.1 | Sep 13, 2021 |
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0.1.0 | Mar 25, 2020 |
#322 in Configuration
275KB
6.5K
SLoC
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")
Ok(Base(String("world!")))
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")
Ok(Base(String("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:
>> cfg.get("f.g")
Ok(Base(String("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 Rust date/time objects from a configuration, by using an ISO date/time pattern in a backtick-string:
>> cfg.get("christmas_morning")
Ok(Base(DateTime(2019-12-25T08:39:49+00:00)))
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")
Ok(Base(String("/home/vinay")))
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")
Ok(Base(String("bar")))
For more information, see the CFG documentation.
Dependencies
~3.5–4.5MB
~77K SLoC