Expand description
This library provides anyhow::Error
, a trait object based error
type for easy idiomatic error handling in Rust applications.
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 the nightly channel, or stable with
features = ["backtrace"]
, 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 instd::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
.
The tracking issue for this feature is rust-lang/rust#53487.
- 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, the same API is almost all 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.
Re-exports
pub use anyhow as format_err;
Macros
Construct an ad-hoc error from a string or existing non-anyhow
error
value.
Return early with an error.
Return early with an error if a condition is not satisfied.
Structs
Iterator of a chain of source errors.
The Error
type, a wrapper around a dynamic error type.
Traits
Provides the context
method for Result
.
Functions
Equivalent to Ok::<_, anyhow::Error>(value).
Type Definitions
Result<T, Error>