anyhow
The anyhow
crate provides a rich error type with
support for carrying additional contextual information, which can be used to
provide a semantic trace of what the program was doing leading up to the error.
This can be combined with the convenience macros from thiserror
to avoid
writing out trait impls explicitly for custom error types.
use anyhow::{bail, Context, Result}; use std::fs; use std::io::Read; use thiserror::Error; #[derive(Clone, Debug, Eq, Error, PartialEq)] #[error("Found no username in {0}")] struct EmptyUsernameError(String); fn read_username(path: &str) -> Result<String> { let mut username = String::with_capacity(100); fs::File::open(path) .with_context(|| format!("Failed to open {path}"))? .read_to_string(&mut username) .context("Failed to read")?; if username.is_empty() { bail!(EmptyUsernameError(path.to_string())); } Ok(username) } fn main() { //fs::write("config.dat", "").unwrap(); match read_username("config.dat") { Ok(username) => println!("Username: {username}"), Err(err) => println!("Error: {err:?}"), } }
This slide should take about 5 minutes.
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.anyhow::Result<V>
is a type alias forResult<V, anyhow::Error>
.- Functionality provided by
anyhow::Error
may be familiar to Go developers, as it provides similar behavior to the Goerror
type andResult<T, anyhow::Error>
is much like a Go(T, error)
(with the convention that only one element of the pair is meaningful). anyhow::Context
is a trait implemented for the standardResult
andOption
types.use anyhow::Context
is necessary to enable.context()
and.with_context()
on those types.
More to Explore
anyhow::Error
has support for downcasting, much likestd::any::Any
; the specific error type stored inside can be extracted for examination if desired withError::downcast
.