rust_errors.md

🦀 error handling in rust

• error handling in rust is different if you are coming from other languages, as it relies on Result and on returning instead of throwing:

enum Result<T, E> {
   Ok(T),
   Err(E),
}

ignoring the error

• use .unwrap():

use std::fs;

fn main() {
  let content = fs::read_to_string("./Cargo.toml").unwrap();
  println!("{}", content)
}

terminate the program

• use .unwrap() or .expect() to trigger panic!:

use std::fs;

fn main() {
  let content = fs::read_to_string("./Cargo.toml").expect("Can't read Cargo.toml");
  println!("{}", content)
}

use a fallback value

• use .unwrap() or .unwrap_or():

use std::env;

fn main() {
  let port = env::var("PORT").unwrap_or("3000".to_string());
  println!("{}", port);
}

bubble up the error

• bubble up (propagate) the error to the caller function using ? (similar to unwrap but instead of panicking, it propagates the error to the calling function).

use std::collections::HashMap;

fn main() {
  match get_current_date() {
    Ok(date) => println!("We've time travelled to {}!!", date),
    Err(e) => eprintln!("Oh noes, we don't know which era we're in! :( \n  {}", e),
  }
}

fn get_current_date() -> Result<String, reqwest::Error> {
  let url = "https://postman-echo.com/time/object";
  let res = reqwest::blocking::get(url)?.json::<HashMap<String, i32>>()?;
  let date = res["years"].to_string();

  Ok(date)
}

create custom errors

• using the debug and display traits:

// error.rs

// error.rs

use std::fmt;

#[derive(Debug)]
pub enum MyCustomError {
  HttpError,
  ParseError,
}

impl std::error::Error for MyCustomError {}

impl fmt::Display for MyCustomError {
  fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
    match self {
      MyCustomError::HttpError => write!(f, "HTTP Error"),
      MyCustomError::ParseError => write!(f, "Parse Error"),
    }
  }
}