# The CFG API for D¶

The CFG reference implementation for D is written and tested using DMD >= 2.086.0. It’s implemented in a module called config.

## Installation¶

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

There’s a minimal example of a program and project that uses CFG here.

## Exploration¶

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 class. 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: 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  ### Loading a configuration¶ The configuration above can be loaded as shown below. In the REPL shell: D> import config; config D> Config cfg; shared static this() { cfg = null; } cfg D> cfg = new Config("tmp/test0.cfg") Config(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"] Hello, D> cfg["b"] world!  ### Access elements with paths¶ As well as simple keys, elements can also be accessed using path strings: D> cfg["c.d"] 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: D> cfg["f.g"] 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 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"]
/home/vinay


You can specify a default value to be used if an environment variable isn’t present using the $VARNAME|default-value form. Whatever string follows the pipe character (including the empty string) is returned if VARNAME is not a variable in the environment. D> cfg["foo"] bar  ### Access to computed values¶ Sometimes, it’s useful to have values computed declaratively in the configuration, rather than imperatively in the code that processes the configuration. For example, an overall time period may be specified and other configuration values are fractions thereof. It may also be desirable to perform other simple calculations declaratively, e.g. concatenation of numerous file names to a base directory to get a final pathname. test0a.cfg total_period : 100 header_time: 0.3 *${total_period}
steady_time: 0.5 * ${total_period} trailer_time: 0.2 *${total_period}
base_prefix: '/my/app/'
log_file: ${base_prefix} + 'test.log'  When this file is read in, the computed values can be accessed directly: D> cfg["header_time"] 30 D> cfg["steady_time"] 50 D> cfg["trailer_time"] 20 D> cfg["log_file"] /my/app/test.log  ### Including one configuration inside another¶ There are times when it’s useful to include one configuration inside another. For example, consider the following configuration files: logging.cfg layouts: { brief: { pattern: '%d [%t] %p %c - %m%n' } }, appenders: { file: { layout: 'brief', append: false, charset: 'UTF-8' level: 'INFO', filename: 'run/server.log', append: true, }, error: { layout: 'brief', append: false, charset: 'UTF-8' level: 'ERROR', filename: 'run/server-errors.log', }, debug: { layout: 'brief', append: false, charset: 'UTF-8' level: 'DEBUG', filename: 'run/server-debug.log', } } loggers: { mylib: { level: 'INFO' } 'mylib.detail': { level: 'DEBUG' } }, root: { handlers: ['file', 'error', 'debug'], level: 'WARNING' }  redirects.cfg cookies: { url: 'http://www.allaboutcookies.org/', permanent: false }, freeotp: { url: 'https://freeotp.github.io/', permanent: false }, 'google-auth': { url: 'https://play.google.com/store/apps/details?id=com.google.android.apps.authenticator2', permanent: false }  main.cfg secret: 'some random application secret', port: 8000, sitename: 'My Test Site', default_access: 'public', ignore_trailing_slashes: true, site_options: { want_ipinfo: false, show_form: true, cookie_bar: true }, connection: 'postgres+pool://db_user:db_pwd@localhost:5432/db_name', debug: true, captcha_length: 4, captcha_timeout: 5, session_timeout: 7 * 24 * 60 * 60, # 7 days in seconds redirects: @'redirects.cfg', email: { sender: 'no-reply@my-domain.com', host: 'smtp.my-domain.com:587', user: 'smtp-user', password: 'smtp-pwd' } logging: @'logging.cfg'  The main.cfg contents have been kept to the highest-level values, within logging and redirection configuration relegated to other files logging.cfg and redirects.cfg which are then included in main.cfg. This allows the high-level configuration to be more readable at a glance, and even allows the separate configuration files to be e.g. maintained by different people. The contents of the sub-configurations are easily accessible from the main configuration just as if they had been defined in the same file: D> cfg["logging.appenders.file.filename"] run/server.log D> cfg["redirects.freeotp.url"] https://freeotp.github.io/ D> cfg["redirects.freeotp.permanent"] false  ### Avoiding unnecessary repetition¶ Don’t Repeat Yourself (DRY) is a useful principle to follow. CFG can help with this. You may have noticed that the logging.cfg file above has some repetitive elements: logging.cfg (partial) appenders: { file: { layout: 'brief', append: false, charset: 'UTF-8' level: 'INFO', filename: 'run/server.log', append: true, }, error: { layout: 'brief', append: false, charset: 'UTF-8' level: 'ERROR', filename: 'run/server-errors.log', }, debug: { layout: 'brief', append: false, charset: 'UTF-8' level: 'DEBUG', filename: 'run/server-debug.log', } }  This portion could be rewritten as: logging.cfg (partial) defs: { base_appender: { layout: 'brief', append: false, charset: 'UTF-8' } }, appenders: { file:${defs.base_appender} + {
level: 'INFO',
filename: 'run/server.log',
append: true,
},
error: ${defs.base_appender} + { level: 'ERROR', filename: 'run/server-errors.log', }, debug:${defs.base_appender} + {
level: 'DEBUG',
filename: 'run/server-debug.log',
}
}


where the common elements are separated out and just referenced where they are needed. We find it useful to put all things which will be reused like this in one place in the condiguration, so we always know where to go to make changes. The key used is conventionally defs or base, though it can be anything you like.

Access is just as before, and provides the same results:

D> cfg["logging.appenders.file.level"]
INFO
D> cfg["logging.appenders.file.layout"]
brief
D> cfg["logging.appenders.file.append"]
true
D> cfg["logging.appenders.file.filename"]
run/server.log
D> cfg["logging.appenders.error.append"]
false
D> cfg["logging.appenders.error.filename"]
run/server-errors.log


The definition of logging.appenders.file as ${defs.base_appender} + { level: 'INFO', filename: 'run/server.log', append: true } has resulted in an evaluation which first fetches the defs.base_appender value, which is a mapping, and “adds” to that the literal mapping which defines the level, filename and append keys. The + operation for mappings is implemented as a copy of the left-hand side merged with the right-hand side. Note that the append value for logging.appenders.file is overridden by the right-hand side to true, whereas that for e.g. logging.appenders.error is unchanged as false. We could do some further refinement by factoring out the common location for the log files: logging.cfg (partial) defs: { base_appender: { layout: 'brief', append: false, charset: 'UTF-8' } log_prefix: 'run/', }, layouts: { brief: { pattern: '%d [%t] %p %c - %m%n' } }, appenders: { file:${defs.base_appender} + {
level: 'INFO',
filename: ${defs.log_prefix} + 'server.log', append: true, }, error:${defs.base_appender} + {
level: 'ERROR',
filename: ${defs.log_prefix} + 'server-errors.log', }, debug:${defs.base_appender} + {
level: 'DEBUG',
filename: \${defs.log_prefix} + 'server-debug.log',
}
}


with the same result as before. It is slightly more verbose than before, but the location of all files can be changed in just one place now, as opposed to three, as it was before.

## Classes¶

### The Config class¶

This class implements a CFG configuration. You’ll generally interface to CFG files using this class. When you pass in a stream or file path to the constructor or the Config.load / Config.loadFile methods, the CFG source in the stream or file is parsed and converted into an internal form that can be queried.

class Config

Variables

var noDuplicates : bool = true

Whether duplicates keys are allowed when parsing a mapping or mapping body. If not allowed and duplicates are seen, a ConfigException is thrown.

var strictConversions : bool = true

If true, a ConfigException is thrown if a string can’t be converted. It’s intended to help catch typos in backtick-strings.

var context : Variant[string]

A mapping within which to look up identifiers during evaluation of AST nodes.

var cached : bool = false

If true, an internal cache is used.

var includePath : string[]

A list of directories which is searched for included sub-configurations if the parent directory of path (or the current directory, if path isn’t set) doesn’t contain them.

var path : string = null

The path to the file from which the configuration has been read. You won’t usually need to set this, unless you want to load a configuration from text that references included files. In that case, set the path to a value whose parent directory contains the included files, or specify relevant directories using includePath. When a configuration is loaded from a file, this attribute is automatically set.

Constructors

fun this()

Constructs an instance with no actual configuration loaded. A call to load or loadFile would be needed to actually make a usable instance.

this(InputRange!(ubyte) r)

Construct this instance with the configuration read from the provided range.

this(string path)

Construct this instance with the configuration read from a file named by the provided path.

Methods

void load(InputRange!(ubyte) r)

void loadFile(string path)
Variant get(string key)
This method implements the key access operator, e.g. mapping[key]. The value of key can be a simple key (identifier) or a valid path string.