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Documentation: Cookbook

Examples

There are tests and examples in the crate repository to get you started

Intro

To label a structure as a config, it is required to annotate it with #[config]:

use config_manager::config;

#[config]
struct Application {}

OBTAINING RESULTS
ConfigInit trait will be derived for the struct and one can invoke the initialization and obtain the result with <Application as ConfigInit>::parse() or <Application as ConfigInit>::parse_options(options) method.

All the sources of the value of a field must be specified explicitly. Fields that does not have at least one source specified are not allowed.

use config_manager::config;

#[config]
struct Application {
    #[source(clap(long = "cli_my_field"), env = "ENV_MY_FIELD")]
    my_field: i32,
}

In this example, it will be checked that cli_my_field is specified via the command line interface (i.e. ./your_binary --cli_my_field=42; see the clap documentation for more details). If cli_my_field is indeed specified, it will be parsed with serde and, if the parsing is successful, the value for my_field will be assigned from the result. In case of a parsing error, the error will be returned instead.

If cli_my_field is not specified, it will be checked that the ENV_MY_FIELD environment variable is present. If the ENV_MY_FIELD environment variable is present, its value will be parsed with serde and, if the parsing is successful, the value for my_field will be assigned from the result. In case of a parsing error, the error will be returned instead.

If the ENV_MY_FIELD environment variable is not found, an error will be returned, because this is the last source we can take the value from, and there was a failure.

Note

The order of the sources is important! The following example does NOT do the same thing as the previous:

use config_manager::config;

#[config]
struct Application {
    #[source(env = "ENV_MY_FIELD", clap(long = "cli_my_field"))]
    my_field: i32,
}

In this example, the env source will be checked first.

NOTES

  • The possible sources are: clap, env, config, default (see below)
  • Default value will be assigned the last (after the others were not found).
  • If the value is not found in any of the sources, an error will be returned
  • Field type must implement serde::de::Deserialize
  • All attributes except default must match either attribute = literal, or attribute(init_from = "...valid Rust code..."), or attribute. In the last case, the "key" value (the CLI argument name, the environment variable name, or the config file key name — depending on the source) will match the field name. For example, annotating my_field with #[clap] means that the value could be assigned to my_field by specifying --my_field=... via the CLI
  • Attribute default must match default = "...valid Rust code..." or default
  • expression from default = "expression" will be interpreted as a Rust expression (for example, expression could be a function call)
  • If the deserialize_with attribute is not set, values from command line, environment will be deserialized according to hjson syntax

Options

Parsing process may be run with a set of options by using the ConfigInit::parse_options(options). The key point here is the fact that the options take precedence over the corresponding attributes, that can be useful in testing and other cases.
More information can be found in the ConfigOption documentation.

Structure attributes

global name

If assigned, a global variable with the specified name will be created instead of deriving ConfigInit trait.

env_prefix

Prefix of the environment variables. The default prefix is the binary file name. Thus, the iter field in the example below will be searched in the environment by the demo_iter key.

#[config(
    env_prefix = "demo"
)]
struct AppConfig {
    #[source(env)]
    iter: i32,
}

Notes

  • The delimiter ('_') is placed automatically
  • If a prefix isn't required, set env_prefix = ""
  • env, env_prefix and similar attributes are case-insensitive. If both the demo_iter and DEMO_ITER environment variables are present, which of these two will be parsed is not defined

file

Description of the configuration file. Has the following nested attributes:

  • format: toml/json/yaml/ron/json5
  • env: environment key containing path to the configuration file (case-insensitive)
  • clap: clap attributes of the argument, responsible for the path to the configuration file
    Note: in this case, clap attribute must have the nested long attribute (clap(long = "..."))
  • default: default configuration file path
  • optional: boolean attribute: should the macro panic (false) or not (true)
    Note: It is allowed to specify multiple files: all of them will be merged. If there is a collision (the values of a particular key have been specified in two or more files), the value will be assigned from the file that has been described later (in the attribute list).
    Example
#[config(
    env_prefix = "",
    file(format = "toml", env = "demo_config")
)]
struct AppConfig {
#[source(clap(long), env, default = 5)]
    iter: i32,
}

In this case, the initialization order for the iter field is:

  • command line argument --iter
  • environment variable iter
  • variable iter from configuration file with path set by the demo_config environment variable
  • default value (5)

clap

Clap app attributes: name, version, author, about, long_about

table

Table of the configuration files to find fields of the structure.

Example

#[config(file(format = "toml", default = "./config.toml"), table = "input.data")]
struct Config {
    #[source(config)]
    frames: i32,
}

Field frames will be searched in the "input.data" table of the configuration file "config.toml".

Field attributes

Only fields can be annotated with the following attributes and only one of them can be assigned to a field.

Source

If a field is annotated with the source attribute, at least one of the following nested attributes must be present.

default

Numeric literal or valid Rust code. If the field's type implement std::default::Default, the attribute can be set without value.

Example

#[config]
struct AppConfig {
    #[source(default = "Vec::new()")]
    buf: Vec<String>,
    #[source(default)]
    opt: Option<String>
    // Option::<String>::default() will be assigned (None)
}

env

Name of the environment variable to set the value from. If present, env_prefix (see above) is ignored. The case is ignored.

config

Name of the configuration file field to set the value from. It can contain dots: in this case the name will be parsed as a path to the field.
Example

#[config(file(format = "toml", default = "./config.toml"), table = "input.data")]
struct Config {
    #[source(config = "images.frame_rate")]
    rate: i32,
}

Field rate will be searched in the "input.data.images" table of the "config.toml" configuration file by the frame_rate key.

clap

Clap-crate attributes. Available nested attributes: help, long_help, short, long, flatten, subcommand. Note: the default long and short values (#[clap(long)] and #[clap(short)]) is the field name and it's first letter.

deserialize_with

Custom deserialization of the field. The deserialization function must have the signature

fn fn_name<'de, D>(de: D) -> Result<FieldType, D::Error>
    where D: serde::Deserializer<'de>

Example

use std::time::Duration;

#[config]
struct MethodConfig {
    #[source(clap(long), deserialize_with = "deser_duration")]
    a: Duration,
}

fn deser_duration<'de, D>(de: D) -> Result<Duration, D::Error>
where
    D: Deserializer<'de>,
{
    u64::deserialize(de).map(Duration::from_millis)
}

Flatten

If a field is annotated with the flatten attribute, it will be parsed as a nested structure and its fields will be initiated like fields of the primal config. In this case, the field's type must implement config_manager::Flatten (it is highly discouraged to implement this trait manually, use derive macro: #[derive(Flatten)]) and serde::Deserialize

Example

use config_manager::{config, Flatten};

#[config]
struct PrimalConfig {
    #[flatten]
    child: NestedConfig,
}

#[derive(Deserialize, Flatten)]
struct NestedConfig {
    #[source(env = "recharge")]
    recharge_time: f32,
    #[source(default = 0.0)]
    capacity: f32,
}

Notes:

  • Nested configs can also contain flatten fields
  • env_prefix will be inherited from the initial struct

Flatten attributes

Flatten struct may have the following helper attributes: table, flatten, source (they work the same way as the described above ones).

Subcommand

If a field is annotated with the flatten attribute, it will be taken as a clap subcommand (see clap documentation for more info). The field's type must implement clap::Subcommand and serde::Deserialize.

Example

#[config]
struct Cargo {
    #[subcommand]
    sub: CargoCommands,
}

#[derive(Deserialize, clap::Subcommand)]
enum CargoCommands {
    #[clap(about = "Compile the current package")]
    Build {
    // ...
    },
    #[clap(about = "Analyze the current package and report errors, but don't build object files")]
    Check {
    // ...
    },
    #[clap(about = "Build this package's and its dependencies' documentation")]
    Doc,
    #[clap(about = "Create a new cargo package")]
    New,
    // ...
}

Notes:

  • Value for the subcommand enumeration will be searched only in command line, so the source and the flatten attributes are forbidden (flatten subcommand attribute is allowed due to clap documentation).
  • Multiple subcommand fields are forbidden.
  • subcommand field in nested(flatten) structures are forbidden.
  • subcommand field can be optional (Option<T>, T: clap::Subcommand + serde::Deserialize), so if no subcommand is found in the command line, the None will be assigned.

get_command

ConfigInit trait has the get_command method that builds Command that can initialize the structure.
By using this method along with the ClapSource::Matches option, one can initialize the structure as a subcommand, so settings of the application and the configuration can be divided, like:

binary [APPLICATION'S SETTINGS] configuration [CONFIGURATION'S SETTINGS]

Example

use config_manager::*;
use clap::*;

#[config(clap(name = "configuration", version, author))]
struct Config {
#[source(clap(long, short))]
    a: i32,
}

fn init_from_app() -> Option<Config> {
    let app = Command::new("binary")
        .arg(Arg::new("some_field").long("field"))
        .subcommand(Config::get_command())
        .get_matches();

    if let Some(subcommand) = app.subcommand_matches(Config::get_command().get_name()) {
        let opts = ConfigOption::ExplicitSource(Source::Clap(ClapSource::Matches(subcommand.clone())));
        Some(Config::parse_options(HashSet::from([opts])).unwrap())
    } else {
        None
    }
}