Load .env and convert it to struct genericly with helper function - rust

Question

I want to convert multiple env.variables to static struct.
I can do it mannually:
Env {
is_development: env::var("IS_DEVELOPMENT")
.unwrap()
.parse::<bool>()
.unwrap(),
server: Server {
host: env::var("HOST").unwrap(),
port: env::var("PORT")
.unwrap()
.parse::<u16>()
.unwrap(),
},
}
But when there is multiple values, it's became bloated. Is there a way to make generic helper function that will give me value that i specify or panic? Something like this (or another solution):
fn get_env_var<T>(env_var_name: String) -> T {
// panic is ok here
let var = env::var(env_var_name).unwrap();
T::from(var)
}
get_env_var<u16>("PORT") // here i got u16
get_env_var<bool>("IS_DEVELOPMENT") // here is my boolean
Full example
use crate::server::logger::log_raw;
use dotenv::dotenv;
use serde::Deserialize;
use std::env;
#[derive(Deserialize, Debug, Clone)]
pub struct Server {
pub host: String,
pub port: u16,
}
#[derive(Deserialize, Debug, Clone)]
pub struct Env {
pub is_development: bool,
pub server: Server,
}
impl Env {
pub fn init() -> Self {
dotenv().expect(".env loading fail");
// how can i specify what type i expect?
fn get_env_var<T>(env_var_name: String) -> T {
// panic is ok here
let var = env::var(env_var_name).unwrap();
T::from(var)
}
// instead this
Env {
is_development: env::var("IS_DEVELOPMENT")
.unwrap()
.parse::<bool>()
.unwrap(),
server: Server {
host: env::var("HOST").unwrap(),
port: env::var("PORT")
.unwrap()
.parse::<u16>()
.unwrap(),
},
}
// do something like this
/*
Env {
is_development: get_env_var<bool>("IS_DEVELOPMENT"),
server: Server {
host: get_env_var<String>("HOST"),
port: get_env_var<u16>("PORT"),
},
}
*/
}
}
lazy_static! {
pub static ref ENV: Env = Env::init();
}

Answer 1

Like in your manual version, where you use str::parse, you can have the same requirement as str::parse, which is FromStr. So if you include the T: FromStr requirement, then you'll be able to do var.parse::<T>().
use std::env;
use std::fmt::Debug;
use std::str::FromStr;
fn get_env_var<T>(env_var_name: &str) -> T
where
T: FromStr,
T::Err: Debug,
{
let var = env::var(env_var_name).unwrap();
var.parse::<T>().unwrap()
}
Then if you run the following by executing PORT=1234 IS_DEVELOPMENT=true cargo run.
fn main() {
println!("{}", get_env_var::<u16>("PORT"));
println!("{}", get_env_var::<bool>("IS_DEVELOPMENT"));
}
Then it will output:
1234
true
Alternatively, you might want to be able to handle VarError::NotPresent and fallback to a default.
use std::env::{self, VarError};
use std::fmt::Debug;
use std::str::FromStr;
fn get_env_var<T>(env_var_name: &str) -> Result<T, VarError>
where
T: FromStr,
T::Err: Debug,
{
let var = env::var(env_var_name)?;
Ok(var.parse().unwrap())
}
Now if you only executed PORT=1234 cargo run, then it would make it easier to do this:
let is_dev = get_env_var::<bool>("IS_DEVELOPMENT")
.map_err(|err| match err {
VarError::NotPresent => Ok(false),
err => Err(err),
})
.unwrap();
println!("{:?}", is_dev);
If you want to fallback to Default if VarError::NotPresent:
fn get_env_var<T>(env_var_name: &str) -> T
where
T: FromStr,
T::Err: Debug,
T: Default,
{
let var = match env::var(env_var_name) {
Err(VarError::NotPresent) => return T::default(),
res => res.unwrap(),
};
var.parse().unwrap()
}

Answer 2

Rust genericity, inspired by Haskell's works through traits and specifically trait bounds. This means when you write
fn get_env_var<T>(env_var_name: String) -> T
since there is no trait bound on T there are essentially no capabilities for it (this is rather unlike C++).
Therefore, as far as rustc is concerned, pretty much the only thing it can do with a T is... take one as parameter then return it as-is.
Thus to do anything useful with a T (including creating one, whether from something else or de novo) you need to use the correct trait and provide the correct trait bounds.
The From trait is entirely the wrong trait to involve here: it specifies total (never-failing) conversions e.g. converting a u16 to a u32, which can never fail.
Whether it's converting a String to a bool or a u16, the conversion is quite obviously less than total: there is an infinity of string values which are not sequences of decimal digits describing a number below 2^16.
In Rust, the signifier of failabibility tends to be Try. There is a TryFrom trait, however for historical reasons and as it documents in its signature the str::parse method is hooked on the FromStr trait.
This means in order to declare that your T can be created from a string (and use the parse method to create one), you need to bound T to FromStr. And of course indicate that it may fail, and will return whatever error T generates when it can't be parsed from a string:
fn get_env_var<T: FromStr>(env_var_name: String) -> Result<T, T::Err> {
let var = env::var(env_var_name).unwrap();
var.parse()
}
Incidentally, taking a String as input is usually avoided unless you really have to[0]. Usually you'd take an &str, that's a lot more flexible as it can be used e.g. with string literals (which are of type &'static str).
So
fn get_env_var<T: FromStr>(env_var_name: &str) -> Result<T, T::Err> {
let var = env::var(env_var_name).unwrap();
var.parse()
}
[0] or for efficiency purposes sometimes