Adding Context to Errors
The widely used anyhow crate can help you add contextual information to your errors and allows you to have fewer custom error types:
use std::{fs, io};
use std::io::Read;
use anyhow::{Context, Result, bail};
fn read_username(path: &str) -> Result<String> {
let mut username = String::with_capacity(100);
fs::File::open(path)
.context(format!("Failed to open {path}"))?
.read_to_string(&mut username)
.context("Failed to read")?;
if username.is_empty() {
bail!("Found no username in {path}");
}
Ok(username)
}
fn main() {
//fs::write("config.dat", "").unwrap();
match read_username("config.dat") {
Ok(username) => println!("Username: {username}"),
Err(err) => println!("Error: {err:?}"),
}
}
anyhow::Result<V>
is a type alias forResult<V, anyhow::Error>
.anyhow::Error
is essentially a wrapper aroundBox<dyn Error>
. As such it’s again generally not a good choice for the public API of a library, but is widely used in applications.- Actual error type inside of it can be extracted for examination if necessary.
- Functionality provided by
anyhow::Result<T>
may be familiar to Go developers, as it provides similar usage patterns and ergonomics to(T, error)
from Go.