Anyhow ¯\\\_(ツ)\_/¯
=========================
[](https://travis-ci.com/dtolnay/anyhow)
[](https://crates.io/crates/anyhow)
[](https://docs.rs/anyhow)
This library provides [`anyhow::Error`][Error], a trait object based error type
for easy idiomatic error handling in Rust applications.
[Error]: https://docs.rs/anyhow/1.0/anyhow/struct.Error.html
```toml
[dependencies]
anyhow = "1.0"
```
*Compiler support: requires rustc 1.34+*
## Details
- Use `Result`, or equivalently `anyhow::Result`, 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 anyhow::Result;
fn get_cluster_info() -> Result {
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.
```rust
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))?;
...
}
```
```console
Error: failed to read instrs from ./path/to/instrs.jsox
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::() {
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.
- 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].
[thiserror]: https://github.com/dtolnay/thiserror
```rust
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`.
```rust
return Err(anyhow!("missing attribute: {}", missing));
```
## 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].
[RFC 2504]: https://github.com/rust-lang/rfcs/blob/master/text/2504-fix-error.md
## Acknowledgements
The implementation of the `anyhow::Error` type is originally forked from
`fehler::Exception` (https://github.com/withoutboats/fehler). This library
exposes it under the more standard `Error` / `Result` terminology rather than
the `throw!` / `#[throws]` / `Exception` language of exceptions.
#### License
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.