#error #error-handling #caller #location #line-column #file-line #tracking

no-std wallee

Flexible concrete Error type built on std::error::Error with caller location tracking

8 releases

0.2.1 Feb 29, 2024
0.2.0 Feb 28, 2024
0.1.7 Feb 20, 2024

#396 in Rust patterns

Download history 355/week @ 2024-02-16 210/week @ 2024-02-23 94/week @ 2024-03-01 23/week @ 2024-03-08 27/week @ 2024-03-15 29/week @ 2024-03-29 1/week @ 2024-04-05

60 downloads per month




This library provides wallee::Error, a trait object based error type for easy idiomatic error handling in Rust applications.

This crate is a fork of anyhow with support for caller location tracking. This is useful when debug information is not included in the build. The caller location attached to wallee::Error includes the file, line and column where the error originated.

wallee = "0.1"

Compiler support: requires rustc 1.76+


  • Use Result<T, wallee::Error>, or equivalently wallee::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 wallee::Result;
    fn get_cluster_info() -> Result<ClusterMap> {
        let config = std::fs::read_to_string("cluster.json")?;
        let map: ClusterMap = serde_json::from_str(&config)?;
  • 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 wallee::{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, they must be enabled through the environment variables described in std::backtrace:

    • If you want panics and errors to both have backtraces, set RUST_BACKTRACE=1;
    • If you want only errors to have backtraces, set RUST_LIB_BACKTRACE=1;
    • If you want only panics to have backtraces, set RUST_BACKTRACE=1 and RUST_LIB_BACKTRACE=0.
  • Wallee 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}")]
  • One-off error messages can be constructed using the wallee! macro, which supports string interpolation and produces an wallee::Error.

    return Err(wallee!("Missing attribute: {}", missing));

    A bail! macro is provided as a shorthand for the same early return.

    bail!("Missing attribute: {}", missing);

Comparison to failure

The wallee::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 Wallee 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.

No runtime deps