This library provides anyhow::Error
, a trait object based error type
for easy idiomatic error handling in Rust applications.
[dependencies]
anyhow = "1.0"
Compiler support: requires rustc 1.39+
Use Result<T, anyhow::Error>
, or equivalently anyhow::Result<T>
, as the
return type of any fallible function.
Within the function, use ?
to easily propagate any error that implements the
std::error::Error
trait.
use anyhow::Result;
fn get_cluster_info() -> Result<ClusterMap> {
let config = std::fs::read_to_string("cluster.json")?;
let map: ClusterMap = serde_json::from_str(&config)?;
Ok(map)
}
Attach context to help the person troubleshooting the error understand where things went wrong. A low-level error like "No such file or directory" can be annoying to debug without more context about what higher level step the application was in the middle of.
use anyhow::{Context, Result};
fn main() -> Result<()> {
...
it.detach().context("Failed to detach the important thing")?;
let content = std::fs::read(path)
.with_context(|| format!("Failed to read instrs from {}", path))?;
...
}
Error: Failed to read instrs from ./path/to/instrs.json
Caused by:
No such file or directory (os error 2)
Downcasting is supported and can be by value, by shared reference, or by mutable reference as needed.
// If the error was caused by redaction, then return a
// tombstone instead of the content.
match root_cause.downcast_ref::<DataStoreError>() {
Some(DataStoreError::Censored(_)) => Ok(Poll::Ready(REDACTED_CONTENT)),
None => Err(error),
}
If using Rust ≥ 1.65, a backtrace is captured and printed with the error if
the underlying error type does not already provide its own. In order to see
backtraces, they must be enabled through the environment variables described
in std::backtrace
:
RUST_BACKTRACE=1
;RUST_LIB_BACKTRACE=1
;RUST_BACKTRACE=1
and
RUST_LIB_BACKTRACE=0
.Anyhow works with any error type that has an impl of std::error::Error
,
including ones defined in your crate. We do not bundle a derive(Error)
macro
but you can write the impls yourself or use a standalone macro like
thiserror.
use thiserror::Error;
#[derive(Error, Debug)]
pub enum FormatError {
#[error("Invalid header (expected {expected:?}, got {found:?})")]
InvalidHeader {
expected: String,
found: String,
},
#[error("Missing attribute: {0}")]
MissingAttribute(String),
}
One-off error messages can be constructed using the anyhow!
macro, which
supports string interpolation and produces an anyhow::Error
.
return Err(anyhow!("Missing attribute: {}", missing));
A bail!
macro is provided as a shorthand for the same early return.
bail!("Missing attribute: {}", missing);
In no_std mode, almost all of the same API is available and works the same way. To depend on Anyhow in no_std mode, disable our default enabled "std" feature in Cargo.toml. A global allocator is required.
[dependencies]
anyhow = { version = "1.0", default-features = false }
With versions of Rust older than 1.81, no_std mode may require an additional
.map_err(Error::msg)
when working with a non-Anyhow error type inside a
function that returns Anyhow's error type, as the trait that ?
-based error
conversions are defined by is only available in std in those old versions.
The anyhow::Error
type works something like failure::Error
, but unlike
failure ours is built around the standard library's std::error::Error
trait
rather than a separate trait failure::Fail
. The standard library has adopted
the necessary improvements for this to be possible as part of RFC 2504.
Use Anyhow if you don't care what error type your functions return, you just want it to be easy. This is common in application code. Use thiserror if you are a library that wants to design your own dedicated error type(s) so that on failures the caller gets exactly the information that you choose.
Licensed under either of Apache License, Version 2.0 or MIT license at your option.
Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this crate by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.