Awesome
Anyhow ¯\_(°ペ)_/¯
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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+
<br>Details
-
Use
Result<T, anyhow::Error>
, or equivalentlyanyhow::Result<T>
, as the return type of any fallible function.Within the function, use
?
to easily propagate any error that implements thestd::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
:- If you want panics and errors to both have backtraces, set
RUST_BACKTRACE=1
; - If you want only errors to have backtraces, set
RUST_LIB_BACKTRACE=1
; - If you want only panics to have backtraces, set
RUST_BACKTRACE=1
andRUST_LIB_BACKTRACE=0
.
- If you want panics and errors to both have backtraces, set
-
Anyhow works with any error type that has an impl of
std::error::Error
, including ones defined in your crate. We do not bundle aderive(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 ananyhow::Error
.return Err(anyhow!("Missing attribute: {}", missing));
A
bail!
macro is provided as a shorthand for the same early return.bail!("Missing attribute: {}", missing);
No-std support
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 }
Since the ?
-based error conversions would normally rely on the
std::error::Error
trait which is only available through std, no_std mode will
require an explicit .map_err(Error::msg)
when working with a non-Anyhow error
type inside a function that returns Anyhow's error type.
Comparison to failure
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.
Comparison to thiserror
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.
<br>