A Rust library for modifying structs via environment variables.

Unlike envy, this function does not create a new object. It is used to reconfigure an existing structure (after having parsed it from a config file, for example).

Ignored fields

If a certain field should not be configurable via environment variables, mark it with #[env(ignore)].

Nested fields

By default, fields are parsed using the FromStr trait. This can be a problem when you have a nested struct and only want to change one of its fields. To mark a field as nested, first #[derive(Environment)] on the sub-structure. Then mark the field as #[env(nested)].

Extendable fields

If a field implements the Extend trait, like Vec or VecDeque, you can use the #[env(extendable)] annotation to configure the field by index. If the collection contains a nested field, you can use #[env(nested, extendable)] together. Note that types are constructed in-place, and some fields may be missing from the environment. Because of this, the contents of the collection must implement the Default trait. You can derive it with #[derive(Default)].

Examples

Creating a config file:

``` use derive_environment::Environment;

[derive(Environment)]

[env(prefix = "HL7_")] // or whatever you want

pub struct Config { // ... } ```

Nesting fields:

``` use derive_environment::Environment;

[derive(Environment)]

struct ServerConfig { port: u16, }

[derive(Environment)]

[env(prefix = "MYCONFIG")]

pub struct Config { #[env(nested)] server: ServerConfig, } ```

Generates: - MYCONFIGSERVER:PORT - MYCONFIGSERVER__PORT

Vector of Nested fields:

``` use derive_environment::Environment;

[derive(Environment)]

struct ServerConfig { port: u16, }

[derive(Environment)]

[env(prefix = "MYCONFIG")]

pub struct Config { #[env(nested, extendable)] server: Vec, } ```

Generates: - MYCONFIGSERVER:0:PORT - MYCONFIGSERVER0PORT