typed-env

Describe the requirements of environment variables in a type-safe and ergonomic way.

Features

• Type Safety: Parse environment variables directly into Rust types
• Flexible Loading: Choose between on-demand and startup loading strategies
• Globals: LazyLock-like globals for easy access to environment variables.
• Rich Types: Support for primitives, booleans, strings, lists and your custom types.
• Default Values: Support for factory-based fallback/default values.
• Error Handling: Comprehensive error types with helpful messages
• Specialized Support:
  • List: Parse comma-separated (or custom delimiter) lists with filtering options.
  • Boolean: Flexible boolean parsing with multiple accepted formats: true, 1, yes, y, on, enabled (case insensitive) for true, and false, 0, no, n, off, disabled (case insensitive) for false.
  • Custom Types: Parse custom types with custom parsing and error handling.

Quick Start

Add to your Cargo.toml:

[dependencies]
typed-env = "0.2"

Basic Usage

use typed_env::{Envar, EnvarDef};

// Define typed environment variables
static PORT: Envar<u16> = Envar::on_demand("PORT", || EnvarDef::Default(8080));
static DEBUG: Envar<bool> = Envar::on_demand("DEBUG", || EnvarDef::Default(false));
static DATABASE_URL: Envar<String> = Envar::on_demand("DATABASE_URL", || EnvarDef::Unset);

fn main() -> Result<(), Box<dyn std::error::Error>> {
    // Use the values - they're parsed automatically
    let port = PORT.value()?;           // u16
    let debug = DEBUG.value()?;         // bool
    let db_url = DATABASE_URL.value()?; // String

    println!("Server running on port {} (debug: {})", port, debug);
    println!("Database: {}", db_url);
    Ok(())
}

Loading Strategies

On-Demand Loading

Variables are parsed when first accessed and cached thereafter. If the environment variable is changed, the new value will be loaded on the next access.

static API_KEY: Envar<String> = Envar::on_demand("API_KEY", || EnvarDef::Unset);

// Parsed and cached on first call
let key1 = API_KEY.value()?;
// Returns cached value
let key2 = API_KEY.value()?;
// ...
unsafe { std::env::set_var("API_KEY", "1234567890"); }
// ...
let key3 = API_KEY.value()?; // Loads the new value
// ...

Startup Loading

Variables are parsed once at startup and never re-read:

static MAX_CONNECTIONS: Envar<u32> = Envar::on_startup("MAX_CONNECTIONS", || EnvarDef::Default(100));

// Always returns the same value, even if env var changes
let max_conn = MAX_CONNECTIONS.value()?;

WARNING: Envar::on_startup does not load the environment variables at the actual startup time, but at the time of the first access.

Supported Types

Primitives

All standard integer and float types are supported:

static TIMEOUT_MS: Envar<u64> = Envar::on_demand("TIMEOUT_MS", || EnvarDef::Default(5000));
static RATE_LIMIT: Envar<f64> = Envar::on_demand("RATE_LIMIT", || EnvarDef::Default(10.5));

Booleans

Flexible boolean parsing with multiple accepted formats:

static ENABLE_LOGS: Envar<bool> = Envar::on_demand("ENABLE_LOGS", || EnvarDef::Default(true));

Accepted values:

• True: true, 1, yes, y, on, enabled (case insensitive)
• False: false, 0, no, n, off, disabled (case insensitive)
• Empty string: treated as false

Lists

Parse delimited lists with configurable separators and filtering:

use typed_env::{ListEnvar, ListEnvarConfig};

// Define a custom list configuration
struct CommaList;
impl ListEnvarConfig for CommaList {
    const SEP: &'static str = ",";
    const FILTER_EMPTY_STR: bool = true;
    const FILTER_WHITESPACE: bool = true;
}

static ALLOWED_ORIGINS: Envar<ListEnvar<String, CommaList>> =
    Envar::on_demand("ALLOWED_ORIGINS", || EnvarDef::Unset);

// ALLOWED_ORIGINS="localhost,127.0.0.1,example.com"
let origins = ALLOWED_ORIGINS.value()?;
for origin in origins.iter() {
    println!("Allowed origin: {}", origin);
}

Custom Types

use typed_env::{
    Envar,
    EnvarDef,
    EnvarError,
    EnvarParse,
    EnvarParser,
    ErrorReason
};

// accept: LEVEL="vvv"
static LEVEL: Envar<Level> = Envar::on_demand("LEVEL", || EnvarDef::Unset);

#[derive(Clone, Debug)]
pub struct Level(pub usize);

impl EnvarParse<Level> for EnvarParser<Level> {
    fn parse(varname: std::borrow::Cow<'static, str>, value: &str) -> Result<Level, typed_env::EnvarError> {
        let value = value.trim();
        let mut count = 0;
        for c in value.chars() {
            if c == 'v' {
                count += 1;
            }
            else {
                return Err(EnvarError::ParseError {
                    varname,
                    typename: std::any::type_name::<Level>(),
                    value: value.to_string(),
                    reason: ErrorReason::new(move || format!("invalid character: {}", c)),
                });
            }
        }
        Ok(Level(count))
    }
}

Default Values

Set Defaults

Provide a fallback value when the environment variable is not set:

static WORKER_THREADS: Envar<usize> = Envar::on_demand("WORKER_THREADS", || EnvarDef::Default(4));

// Returns 4 if WORKER_THREADS is not set
let threads = WORKER_THREADS.value()?;

Unset (Required)

Require the environment variable to be present:

static SECRET_KEY: Envar<String> = Envar::on_demand("SECRET_KEY", || EnvarDef::Unset);

// Returns Err(EnvarError::NotSet) if SECRET_KEY is not set
let secret = SECRET_KEY.value()?;

Error Handling

The library provides detailed error information:

use typed_env::EnvarError;

match DATABASE_URL.value() {
    Ok(url) => println!("Database URL: {}", url),
    Err(EnvarError::NotSet(name)) => {
        eprintln!("Required environment variable {} is not set", name);
    }
    Err(EnvarError::ParseError { varname, typename, value, .. }) => {
        eprintln!("Failed to parse {} as {}: {:?}", varname, typename, value);
    }
}

API Reference

Core Types

• Envar<T>: The main environment variable container
• EnvarDef<T>: Defines default behavior (Default(value) or Unset)
• ListEnvar<T, C>: Container for list-type environment variables
• ListEnvarConfig: Trait for configuring list parsing behavior

Methods

• Envar::on_demand(name, default_factory): Create an on-demand loaded variable
• Envar::on_startup(name, default_factory): Create a startup-loaded variable
• envar.value(): Get the parsed value, returns Result<T, EnvarError>
• envar.name(): Get the environment variable name

Error Types

• EnvarError::NotSet(name): Environment variable is not set and no default provided
• EnvarError::ParseError { varname, typename, value, reason }: Failed to parse the value
• EnvarError::TryDefault(varname): A "soft" error, indicating that the environment variable could fallback to the default value, if specified with EnvarDef::Default.

Contributing

Contributions are welcome! Please feel free to submit a Pull Request.

License

This project is licensed under the MIT License - see the LICENSE file for details.