anyhow & thiserror

thiserror defines typed errors for libraries. anyhow provides ergonomic Result handling for applications with context chains and downcasting.

Recipe

# Library crate
thiserror = "2"
 
# Binary / service crate
anyhow = "1"
thiserror = "2"  # if you expose typed errors internally

// library
use thiserror::Error;
 
#[derive(Debug, Error)]
pub enum ConfigError {
    #[error("missing key: {0}")]
    Missing(&'static str),
    #[error(transparent)]
    Io(#[from] std::io::Error),
}

// binary
use anyhow::{Context, Result};
 
fn load() -> Result<String> {
    std::fs::read_to_string("config.toml")
        .context("reading config.toml")?;
    Ok(String::new())
}

When to reach for this:

• Libraries: thiserror enums with #[from]
• CLIs and services: anyhow::Result in main and top-level handlers

Working Example

use anyhow::{Context, Result};
use thiserror::Error;
 
#[derive(Debug, Error)]
enum ParseError {
    #[error("invalid port: {0}")]
    BadPort(u16),
}
 
fn parse_port(s: &str) -> Result<u16, ParseError> {
    let n: u16 = s.parse().map_err(|_| ParseError::BadPort(0))?;
    if n == 0 {
        return Err(ParseError::BadPort(n));
    }
    Ok(n)
}
 
fn main() -> Result<()> {
    let port = parse_port("8080").context("parsing PORT env")?;
    println!("{port}");
    Ok(())
}

What this demonstrates:

• Library boundary returns typed ParseError
• Application adds context with .context()
• main returns anyhow::Result<()>

Deep Dive

Split by Crate Type

Context Chains

use anyhow::Context;
 
db_query().context("loading users for billing run")?;

Printed backtrace shows the full chain when RUST_BACKTRACE=1.

Gotchas

• anyhow in public library APIs - callers cannot match errors. Fix: export thiserror types from lib crates.
• Stringly map_err(|e| e.to_string()) - loses source. Fix: #[from] or .context().
• Too many error enums - fragmentation. Fix: one enum per crate module with #[from].
• Panicking in From impls - hides bugs. Fix: only ? on real From conversions.
• Mixing Box<dyn Error> - outdated pattern in new code. Fix: standardize on anyhow/thiserror.

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Alternatives

FAQs

Related

• Error handling basics
• thiserror
• anyhow
• Error-Handling Skill

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.