anyhow-and-thiserror.md

Table of contents

• Table of contents
• Thiserror and Anyhow
• Anyhow

Thiserror and Anyhow

There are 2 useful crates to handle errors in Rust:

• Anyhow
• Thiserror

Anyhow just wraps any error type E with Box<dyn Error + Send + Sync>, i.e., anyhow abstracts from any kind of returned error and simplifies writing of code.
Thiserror is a macro to derive std::error::Error, std::fmt::Display, std::convert::From traits.
Anything that you write with thiserror could sensibly be written by hand.

Recommendations:

• use anyhow if you don't care what error type your functions return. This is common in application code.
• use thiserror if you writes 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.

Anyhow

The most useful feature of anyhow is its Result<T> type.
Also it provides anyhow! macro. It will convert your &str or any type implementing Debug and Display to an error.
anyhow provides a Context trait that gives access to a context method you can use on Result (and Option) types. This method will add the information you specify in the error.

use anyhow::{Result, anyhow};

enum Kind {
    String,
    IO,
    Any
}

fn string_error() -> Result<()> {
    Err(anyhow!("string_error").context("string_error"))
}

fn io_error() -> Result<()> {
    Err(anyhow!("io_error").context("io_error"))
}

fn any_error(kind: Kind) -> Result<()> {
    match kind {
        Kind::String => Ok(string_error()?),
        Kind::IO => Ok(io_error()?),
        Kind::Any => Err(anyhow!("fizzbazz").context("any_error")),
    }
}

fn main() -> Result<()> {
    Ok(any_error(Kind::Any)?)
}