This library provides eyre::ErrReport, a trait object based
error handling type for easy idiomatic error handling and reporting in Rust
applications.
This crate is a fork of anyhow by @dtolnay. My goal in writing this crate is
to explore new ways to associate context with errors, to cleanly separate the
concept of an error and context about an error, and to more clearly communicate
the intended usage of this crate via changes to the API.
The main changes this crate brings to anyhow are
eyre::EyreContext] trait and a type parameter on the core error
handling type which users can use to insert custom forms of context into
their catch all error handling type.anyhow::Context] trait to [eyre::WrapErr] to make it clear
that it is unrelated to the [eyre::EyreContext] and the context member, and
is only for inserting new errors into the chain of errors.context function on [eyre::ErrReport] to assist with
extracting members from the inner Context, which is used by
[eyre::ErrReport] to extract [std::backtrace::Backtrace]'s from generic
contexts types.These changes were made in order to facilitate the usage of
[tracing::SpanTrace] with anyhow, which is a Backtrace-like type for
rendering custom defined runtime context.
toml
[dependencies]
eyre = "0.2"
Note: The way the eyre! macro works in practice differs from how
anyhow! works due to the addition of the generic type parameter. In anyhow
the following is valid.
rust
// Works
let val = get_optional_val.ok_or_else(|| anyhow!("failed to get value)).unwrap();
Where as with eyre! this will fail due to being unable to infer the type for
the Context parameter. The solution to this problem, should you encounter it,
is to give the compiler a hint for what type it should be resolving to, either
via your return type or a type annotation.
```rust // Broken let val = getoptionalval.okorelse(|| eyre!("failed to get value)).unwrap();
// Works
let val: ErrReport = getoptionalval.okorelse(|| eyre!("failed to get value)).unwrap();
```
In order to insert your own custom context type you must first implement the
eyre::EyreContext trait for said type, which has four required methods.
fn default(error: &Error) -> Self - For constructing default context while
allowing special case handling depending on the content of the error you're
wrapping.This is essentially Default::default but more flexible, for example, the
eyre::DefaultContext type provide by this crate uses this to only capture a
Backtrace if the inner Error does not already have one.
```rust fn default(error: &(dyn StdError + 'static)) -> Self { let backtrace = backtraceifabsent!(error);
Self { backtrace }
} ```
fn context_raw(&self, typeid TypeID) -> Option<&dyn Any> - For extracting
arbitrary members from a context based on their type.This method is like a flexible version of the fn backtrace(&self) method on
the Error trait. In the future we will likely support extracting Backtraces
and SpanTraces by default by relying on the implementation of context_raw
provided by the user.
Here is how the eyre::DefaultContext type uses this to return Backtraces.
rust
fn context_raw(&self, typeid: TypeId) -> Option<&dyn Any> {
if typeid == TypeId::of::<Backtrace>() {
self.backtrace.as_ref().map(|b| b as &dyn Any)
} else {
None
}
}
fn debug(&self, error: &(dyn Error + 'static), f: &mut fmt::Formatter<'_>) -> fmt Result
it's companion display version. - For formatting the entire error chain and
the user provided context.When overriding the context it no longer makes sense for eyre::ErrReport to
provide the Display and Debug implementations for the user, becase we
cannot predict what forms of context you will need to display along side your
chain of errors. Instead we forward the implementations of Display and
Debug to these methods on the inner EyreContext type.
This crate does provide a few helpers to assist in writing display
implementations, specifically the Chain type, for treating an error and its
sources like an iterator, and the Indented type, for indenting multi line
errors consistently without using heap allocations.
Note: best practices for printing errors suggest that {} should only
print the current error and none of its sources, and that the primary method of
displaying an error, its sources, and its context should be handled by the
Debug implementation, which is what is used to print errors that are returned
from main. For examples on how to implement this please refer to the
implementations of display and debug on eyre::DefaultContext
Once you've defined a custom Context type you can use it throughout your application by defining a type alias.
```rust
type ErrReport = eyre::ErrReport
// And optionally...
type Result
Use Result<T, eyre::ErrReport>, or equivalently eyre::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.
```rust use eyre::Result;
fn getclusterinfo() -> Result
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.
```rust use eyre::{WrapErr, 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))?; ... } ```
```console 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.
rust
// 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),
}
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, the
RUST_LIB_BACKTRACE=1 environment variable must be defined.
Eyre 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].
```rust use thiserror::Error;
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 eyre! macro, which
supports string interpolation and produces an eyre::ErrReport.
rust
return Err(eyre!("Missing attribute: {}", missing));
NOTE: tests are currently broken for no_std so I cannot guaruntee that
everything works still. I'm waiting for upstream fixes to be merged rather than
fixing them myself, so bear with me.
In nostd mode, the same API is almost all available and works the same way. To depend on Eyre in nostd mode, disable our default enabled "std" feature in Cargo.toml. A global allocator is required.
toml
[dependencies]
eyre = { version = "0.2", 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(ErrReport::msg) when working with a non-Eyre error
type inside a function that returns Eyre's error type.
The eyre::ErrReport 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 Eyre 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.