Rust’s `std::error::Error` trait marks a type as an official error type. It requires no methods of its own, but an error must also implement `Debug` and `Display` so programs can show useful information. section 13.2.3
A custom error can be a struct or enum. Add `#[derive(Debug)]`, implement `Display` with `fmt`, and then write `impl std::error::Error {}` for the type. `Display` controls user-facing output, while `Debug` supports diagnostic output. section 13.2.3
Returning different error types
A function that can fail in more than one way can return `Result<String, Box<dyn Error>>`. Each concrete error is placed in a `Box`, allowing the function to return different types through one trait object. The `?` operator can then propagate parsing errors, while `unwrap()` will panic if the final result is `Err`. section 13.2.3
use std::error::Error;
use std::fmt;
#[derive(Debug)]
struct ErrorOne;
impl Error for ErrorOne {} #1
impl fmt::Display for ErrorOne {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
write!(f, "You got the first error!")
}
}
#[derive(Debug)]
struct ErrorTwo;
impl Error for ErrorTwo {}
impl fmt::Display for ErrorTwo {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
write!(f, "You got the second error!")
}
}
fn returns_errors(input: u8) -> Result<String, Box<dyn Error>> { #2
match input {
0 => Err(Box::new(ErrorOne)), #3
1 => Err(Box::new(ErrorTwo)),
_ => Ok("Looks fine to me".to_string()), #4
}
}
fn main() {
let vec_of_u8s = vec![0_u8, 1, 80]; #5
for number in vec_of_u8s {
match returns_errors(number) {
Ok(input) => println!("{}", input),
Err(message) => println!("{}", message),
}
}
}
Shows two custom error types implementing Error and Display, then returns either error through Box<dyn Error>. section 13.2.3
The trait becomes useful as the error type inside `Result<T, E>`. `Result` carries either a successful `Ok` value or an `Err` value containing information about what went wrong; using an error type instead of a plain string gives that failure a defined interface. section 5.2.2 section 13.2.3
