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https://github.com/nrc/box-error

A library for error handling using boxed errors
https://github.com/nrc/box-error

Last synced: 17 days ago
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A library for error handling using boxed errors

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# box-error



A library for error handling using boxed errors.



This crate is work in progress, it is not ready for production use.



If errors are large and passed around a lot, then it can be more efficient to do so by reference. That usually means using `Box` since errors are usually passed up the stack. This crate aims to make such boxed errors ergonomic to use.



Goals:



* Support concrete and dynamic types for errors and make converting between the two easy.

* Interoperate with `std::Error` and `std::Result`.

* Don't require users to match using box patterns.

* Make it easy to work with errors from dependencies which do not implement `std::Error`.



In contrast to most error handling libraries, box-error provides a result type (a replacement for `std::Result`) rather than an error type. The intention is for users to use another crate (e.g., [thiserror](https://github.com/dtolnay/thiserror)) to help write and implement errors. Our result type (`BoxResult`) always keeps its error value as a boxed reference.



For dynamic error handling, we use [anyhow](https://github.com/dtolnay/anyhow) for the implementation, so we can easily support its features like backtraces, chaining, and downcasting. However, we wrap `anyhow::Error` in a new type `AnyError` and also wrap `std::Result` as `AnyResult`. That does not box its errors because `AnyError` is internally implemented using `Box`. We make inter-converting between `BoxResult`, `AnyResult`, and `std::Result` as easy as possible.



## Example



This example will not work today, but I'm pretty sure its possible and should work soon.



```rust

use box_error::prelude::*;

use thiserror::Error;



#[derive(Error, Debug)]

enum MyError {

    #[error("foo")]

    Foo,

    #[error("bar: {0}")]

    Bar(String),

    #[error("other")]

    Other(#[source] AnyError),

}



impl From<::std::num::TryFromIntError> for MyError {

    fn from(e: ::std::num::TryFromIntError) -> MyError {

        MyError::Other(e.into())

    }

}



pub fn exported() -> AnyResult {

    let i = internal()?;

    AnyResult::Ok(i.try_into()?)

}



fn internal() -> BoxResult {

    if ... {

        BoxResult::Ok(42)

    } else {

        BoxResult::Err(MyError::Foo)

    }

}



fn handle_bar_err(r: AnyResult) -> AnyResult {

    r.map_err(|e| if let MyError::Bar(s) = e.as_ref {

        println!("got a Bar: `{}`", s);

        AnyResult::Err(MyError::Foo)

    } else { e })

}

```



## Comparison to anyhow



We use anyhow to implement `AnyError`/`AnyResult`, so there are many similarities between the two libraries and we owe a massive debt of thanks to anyhow for the implementation and inspiration for box-error.



There are two main differences in the philosophy of anyhow and box-error:



* Anyhow provides a dynamic error type, but no concrete type. Box-error provides a dynamic error type and some parts of a concrete type.

* Box-error forces errors to be boxed whereas anyhow does not (they may be boxed, but are not forced to be). Because of this assumption, inter-conversion between boxed concrete and boxed dynamic errors is more ergonomic.



The use case for anyhow is applications, and for thiserror is library crates. The primary use case for box-error is large applications with crate-like modules and/or module-like crates where both dynamic and concrete errors have advantages in different places.



There are also a few minor improvements over anyhow - for example, being able to transparently match on boxed errors, easier handling of errors which don't implement `std::Error`, etc.



It is important to note that box-error is not yet production ready, but anyhow is. So if you're wandering which to use in production code, don't use box-error.



## Implementation notes



It is possible to use `BoxResult` to represent a dynamically typed box result. That representation makes for a cleaner implementation, however, it would require reimplementing most of the anyhow library and I prefer to reuse it.