Library for working with the CFG configuration format

library, development, data_structures
dub fetch cfg-lib --version 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 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 the ability to compose configurations (using include and merge facilities).
  • Provide the ability to access real application objects safely.

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 allow trailing commas in lists and mappings.


The package can be installed for use from the D package registry using the package name cfg-lib.


To explore CFG functionality for D, we use the drepl Read-Eval-Print-Loop (REPL), which is available from here. Once installed, you can invoke a shell using

$ drepl

Getting Started with CFG in D

A configuration is represented by an instance of the Config struct. The constructor for this class can be passed a filename or a stream which contains the text for the configuration. The text is read in, parsed and converted to an object that you can then query. A simple example:

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

Loading a configuration

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

D> import config;
D> Config cfg; shared static this() { cfg = null; }
D> cfg = new Config("tmp/test0.cfg")

The one-time-per-session dance with shared static this() is currently needed due to a limitation of drepl.

Access elements with keys

Accessing elements of the configuration with a simple key is just like using an associative array:

D> cfg["a"]
D> cfg["b"]

You can see the types and values of the returned objects are as expected.

Access elements with paths

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

D> cfg["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:

D> cfg["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 date/time objects from a configuration, by using an ISO date/time pattern in a backtick-string:

D> cfg["christmas_morning"]
2019-Dec-25 08:39:49

Access to environment variables

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

D> cfg["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 VARNAME is not a variable in the environment.

D> cfg["foo"]

For more information, see the CFG documentation.