Summary - Advanced error handling

Combine thiserror and From to build a config reader error.

Advanced error handling summary

Error types implement Debug + Display + Error. Libraries use concrete enums (thiserror), applications a flexible type (anyhow or Box<dyn Error>). impl From enables automatic conversion with ?. Context (with_context or map_err) makes errors understandable. source() gives the underlying cause of an error and can be walked as a chain.

The error chain

An error type implementing source can give a high-level message without losing the cause:

#[derive(Debug)]
struct UstHata {
    kaynak: ParseIntError,
}

impl fmt::Display for UstHata {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "yapılandırma yüklenemedi")
    }
}

impl Error for UstHata {
    fn source(&self) -> Option<&(dyn Error + 'static)> {
        Some(&self.kaynak)
    }
}

The chain is walked with while let Some(k) = s { ..; s = k.source(); }.

Exercise Print the chain

Write the UstHata type above. Create an instance with "x".parse::<i32>().unwrap_err(); print the error first, then walk the source chain as neden: MESSAGE.

Show example solution

use std::error::Error;
use std::fmt;
use std::num::ParseIntError;

#[derive(Debug)]
struct UstHata {
    kaynak: ParseIntError,
}

impl fmt::Display for UstHata {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "yapılandırma yüklenemedi")
    }
}

impl Error for UstHata {
    fn source(&self) -> Option<&(dyn Error + 'static)> {
        Some(&self.kaynak)
    }
}

fn main() {
    let e = UstHata { kaynak: "x".parse::<i32>().unwrap_err() };
    println!("{}", e);
    let mut s = e.source();
    while let Some(k) = s {
        println!("  neden: {}", k);
        s = k.source();
    }
}

Test yourself

What does <code>Error::source</code> return by default?
- The Display message
- It panics
- None
- The error itself
Show answer

None — Error types with an underlying cause override source to return Some(&inner_error).

Give the cause

impl Error for UstHata {
    fn ____(&self) -> Option<&(dyn Error + 'static)> {
        Some(&self.kaynak)
    }
}

Show answer

impl Error for UstHata {
    fn source(&self) -> Option<&(dyn Error + 'static)> {
        Some(&self.kaynak)
    }
}

Group summary

impl Error

Error contract
impl From<E>

Conversion with ?
with_context

Adds context
source()

Underlying cause

Next lesson: Defining modules

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