error-stack: Use along with or replacing thiserror?

[dpc]

I'm trying to understand how does error-stack fit with other error libraries. Currently there are two types of libraries; application error handling (boxed) like anyhow and eyre and library level (unboxed, enum-based) like thiserror.

Which layer does error-stack trying to fit (app, lib, both?) and why does it fit there?

[TimDiekmann]

Hi @dpc!

The main difference between library-type errors and application-type errors is how it's exposed. thiserror does not appear in the public API of a crate, which makes it incredibly useful for libraries (NB: derive(Error) from thiserror is not limited to enums). Downstream crates depending on thiserror can use the returned error like any other Error. Application-type errors like anyhow or eyre do appear in the public API. Additionally, error types from application-type errors usually don't implement Error (but usually implement a way returning a &dyn Error like using AsRef<&dyn Error>).

error_stack::Report does not (and will not) implement Error. However, we plan to implement a way to convert Report into an Error. This is an important step so crates depending on libraries using error-stack can easily use the Report without the need of using error-stack themselves. However, crates that are using error-stack and depend on libraries using error-stack as well directly benefit from it and its features.

Using error-stack in a public API requires downstream crates to deal with Report. This means, that this crate is not suited as well for libraries as thiserror. Given the planned features, however, error-stack is suited for libraries and provides dependent crates a lot of information about the error, which is hard to encapsulate if not using types outside of Error.
For applications, error-stack was designed to provide useful error messages, thus making this crate a great fit.

error-stack relies on a Context trait to change the context. A blanket implementation for Error is provided, so thiserror is an easy way to create a Context. error-stack can be built on top of any existing Error type.