rust 46 lines · 7 steps

Loading typed config from environment variables in Rust

A Config struct is populated from env vars with typed parsing, defaults, and clear errors.

Explained by highlit

Intermediate configuration error-handling generics environment-variables

1use std::env;
2use std::time::Duration;
3 
4#[derive(Debug, Clone)]
5pub struct Config {
6    pub host: String,
7    pub port: u16,
8    pub database_url: String,
9    pub max_connections: u32,
10    pub request_timeout: Duration,
11    pub debug: bool,
12}
13 
14impl Config {
15    pub fn from_env() -> Result<Self, String> {
16        Ok(Config {
17            host: env::var("HOST").unwrap_or_else(|_| "127.0.0.1".into()),
18            port: parse_var("PORT", 8080)?,
19            database_url: env::var("DATABASE_URL")
20                .map_err(|_| "DATABASE_URL must be set".to_string())?,
21            max_connections: parse_var("MAX_CONNECTIONS", 16)?,
22            request_timeout: Duration::from_secs(parse_var("REQUEST_TIMEOUT_SECS", 30)?),
23            debug: parse_bool("DEBUG", false),
24        })
25    }
26}
27 
28fn parse_var<T>(key: &str, default: T) -> Result<T, String>
29where
30    T: std::str::FromStr,
31{
32    match env::var(key) {
33        Ok(raw) => raw
34            .trim()
35            .parse()
36            .map_err(|_| format!("invalid value for {key}: {raw:?}")),
37        Err(_) => Ok(default),
38    }
39}
40 
41fn parse_bool(key: &str, default: bool) -> bool {
42    match env::var(key) {
43        Ok(raw) => matches!(raw.trim().to_ascii_lowercase().as_str(), "1" | "true" | "yes" | "on"),
44        Err(_) => default,
45    }
46}

01 / 01

STEP 01

Walkthrough

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Three takeaways

1Centralizing config in one struct with a single loader keeps environment access out of the rest of the codebase.
2A generic helper over FromStr lets one function parse any target type while supplying defaults and consistent errors.
3Returning Result with descriptive messages surfaces misconfiguration at startup instead of failing mysteriously later.

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