Anyhow ¯\_(ツ)_/¯

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.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),
  }
  

• 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.

Comparison to thiserror

Use Anyhow 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.

License