deserialize env vars into typesafe structs

Add the following to your Cargo.toml fails_with_invalid_type

[dependencies]
envy = "0.2"

assuming your rust program looks something like this.

#[macro_use]
extern crate serde_derive;
extern crate envy;

use std::env;

#[derive(Deserialize, Debug)]
struct Config {
  foo: u16,
  bar: bool,
  baz: String,
  boom: Option<u64>
}

fn main() {
    match envy::from_env::<Config>() {
       Ok(config) => println!("{:#?}", config)
       Err(error) => panic!("{:#?}", error)
    }
}

export some environment variables

$ FOO=8080 BAR=true BAZ=hello yourapp

You should be able to access a completely typesafe config struct deserialized from env vars.

Envy assumes an env var exists for each struct field with a matching name in all uppercase letters. i.e. A struct field foo_bar would map to an env var named FOO_BAR

Structs with Option type fields will successfully be deserialized when their associated env var is absent.

Envy also supports deserializing Vecs from comma separated env var values.

Because envy is built on top of serde, you take use all of serde's attributes to your advantage

For instance let's say you're app requires a field but would like a sensible default when one is not provided.

/// provides default value for zoom if ZOOM env var is not set
fn default_zoom() -> {
  32
}

#[derive(Deserialize, Debug)]
struct Config {
  foo: u16,
  bar: bool,
  baz: String,
  boom: Option<u64>,
  #[serde(default="default_zoom")]
  zoom: u16
}

The following will yield an application configured with a zoom of 32

$ FOO=8080 BAR=true BAZ=hello yourapp

The following will yield an application configured with a zoom of 10

$ FOO=8080 BAR=true BAZ=hello ZOOM=10 yourapp

• deserializing enums

• error handling/reporting

Doug Tangren (softprops) 2016