This library provides eyre::Report, 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. By default this crate does not
add any new features that anyhow doesn't already support, though it does rename
a number of the APIs to try to make the intended usage more obvious. The magic
of this crate is when you need to add extra context to a chain of errors beyond
what you can or should insert into the error chain. For an example of a
customized version of eyre check out
jane-eyre.
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.std::error::Error, and you'll notice that this trait
implementation is conspicuously absent on Report. Instead it contains
errors that it masqerades as, and provides helpers for creating new errors to
wrap those errors and for displaying those chains of errors, and the included
context, to the end user. The goal is to make it obvious that this type is
meant to be used when the only way you expect to handle errors is to print
them.anyhow::Context] trait to [eyre::WrapErr] to make it clear
that it is unrelated to the [eyre::EyreContext] trait and member, and is
only for inserting new errors into the chain of errors.member_ref/member_mut on EyreContext
and context/context_mut on Report for working with the custom
context and extracting forms of context based on their type independent of
the type of the custom context.These changes were made in order to facilitate the usage of
[tracing_error::SpanTrace] with anyhow, which is a Backtrace-like type for
rendering custom defined runtime context setup via tracing spans.
toml
[dependencies]
eyre = "0.3"
In order to insert your own custom context type you must first implement the
eyre::EyreContext trait for said type, which has two required methods and
three optional 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 debug(&self, error: &(dyn Error + 'static), f: &mut fmt::Formatter<'_>)
-> fmt Result and optionally display - For formatting the entire error
chain and the user provided context.When overriding the context it no longer makes sense for eyre::Report 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
fn member_ref(&self, typeid TypeID) -> Option<&dyn Any> - For extracting
arbitrary members from a context based on their type and member_mut for
getting a mutable reference in the same way.This method is like a flexible version of the fn backtrace(&self) method on
the Error trait. The main Report type provides versions of these methods
that use type inference to get the typeID that should be used by inner trait fn
to pick a member to return.
Note: The backtrace() fn on Report relies on the implementation of
this function to get the backtrace from the user provided context if one
exists. If you wish your type to guaruntee that it captures a backtrace for any
error it wraps you must implement member_ref and provide a path to return
a Backtrace type like below.
Here is how the eyre::DefaultContext type uses this to return Backtraces.
rust
fn member_ref(&self, typeid: TypeId) -> Option<&dyn Any> {
if typeid == TypeId::of::<Backtrace>() {
self.backtrace.as_ref().map(|b| b as &dyn Any)
} else {
None
}
}
Once you've defined a custom Context type you can use it throughout your application by defining a type alias.
```rust
type Report = eyre::Report
// And optionally...
type Result
Use Result<T, eyre::Report>, 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
Wrap a lower level error with a new error created from a message to help the person troubleshooting understand what the chain of failures that occured. A low-level error like "No such file or directory" can be annoying to debug without more information about what higher level step the application was in the middle of.
```rust use eyre::{WrapErr, Result};
fn main() -> Result<()> { ... it.detach().wrap_err("Failed to detach the important thing")?;
let content = std::fs::read(path) .wraperrwith(|| 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),
}
If using the nightly channel, 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.The tracking issue for this feature is [rust-lang/rust#53487].
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::Report.
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 = "0.3", 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(Report::msg) when working with a non-Eyre error
type inside a function that returns Eyre's error type.
The eyre::Report 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.
Beyond the fact that eyre renames many of the core APIs in anyhow the addition
of the type parameter makes the eyre! macro not work in certain places where
anyhow! does work. 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: Report = getoptionalval.okorelse(|| eyre!("failed to get value)).unwrap(); ```
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.