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A better Box<dyn Error>
Rust
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README.md

Anyhow ¯\_(ツ)_/¯

Build Status Latest Version Rust Documentation

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 1.34+


Details

  • Use Result<T, anyhow::Error>, or equivalently anyhow::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.

    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.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.

    // 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.

  • 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.

    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.

    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.


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.
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