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:

  1. use std::{fs, io};
  2. use std::io::Read;
  3. use anyhow::{Context, Result, bail};
  5. fn read_username(path: &str) -> Result<String> {
  6.     let mut username = String::with_capacity(100);
  7.     fs::File::open(path)
  8.         .context(format!("Failed to open {path}"))?
  9.         .read_to_string(&mut username)
  10.         .context("Failed to read")?;
  11.     if username.is_empty() {
  12.         bail!("Found no username in {path}");
  13.     }
  14.     Ok(username)
  15. }
  17. fn main() {
  18.     //fs::write("config.dat", "").unwrap();
  19.     match read_username("config.dat") {
  20.         Ok(username) => println!("Username: {username}"),
  21.         Err(err)     => println!("Error: {err:?}"),
  22.     }
  23. }
  • anyhow::Result<V> is a type alias for Result<V, anyhow::Error>.
  • anyhow::Error is essentially a wrapper around Box<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.