multiple_errors

Propagate multiple errors instead of just the first one.

Description

Rust's ? operator and Iterator::collect::<Result<_, _>> return early on the first encountered error. However, sometimes we want to execute multiple independent actions and then report all errors at once. Or turn all results into errors if there is at least one error.

This crate covers these use cases and aims to become an easily googlable "hub" for:

• more "mass error handling" fuctionality
• knowledge about related functionality in other crates

Think of multiple_errors as itertools. It's also a lightweight "pure logic" crate with no dependencies, and should be suitable for no_std and old MSRVs. I haven't worked on this yet, please open an issue or a pull request if you need this.

Example

use multiple_errors::{fail_all_vec, return_multiple_errors, CollectVecResult};
use multiple_errors::testing_prelude::*;

// We want to collect into a `Result`, but preserve all errors, not just the first one:
assert_eq!(
    [Err(ErrA), Ok(A), Err(ErrA)].into_iter().collect_vec_result(),
    // Collected all errors, not just the first one.
    Err(vec![ErrA, ErrA])
);

// We want to turn each `Result` into an error, if there's at least one error:
let err = fail_all_vec(
    vec![Ok(A), Err(ErrA), Ok(A)],
    // We provide a callback for "turning into an error".
    |res| res.err().map(HighLevelErr::from).unwrap_or(HighLevelErr::B(ErrB))
);
// Same length as the original, each element turned into an error.
assert_eq!(err, Err(vec![ErrB.into(), ErrA.into(), ErrB.into()]));

// We want to perform some independent fallible actions (with different return types)
// and then return a non-empty `Vec` of errors or proceed on the happy path:
fn a_b_c() -> Result<(A, B, C), Vec<HighLevelErr>> {
    return_multiple_errors!(
        // Annotate the "common" error type for the container of errors.
        let mut errors: Vec<HighLevelErr> = vec![];
        // Get some `Result`s.
        let a = action_a();
        let b = action_b();
        let c = action_c();
        // If there are any errors, they are implicitly converted and collected.
        if_there_are_errors {
            return Err(errors);
        }
    );
    // And here we can proceed on the happy path, with already-unwrapped `Ok` values.
    // `a`, `b`, and `c` are no longer `Result`s. They have been shadowed.
    Ok((a, b, c))
}

Similar crates

• lazy_errors

  Effortlessly create, group, and nest arbitrary errors, and defer error handling ergonomically.

  This crate is return_multiple_errors! done right! It appeared a few months later than mine. If it had existed, I would have probably just used it.

• frunk::validated::Validated

  A Validated is either an Ok holding an HList or an Err, holding a vector of collected errors.

  This is somewhat similar to lazy_errors and return_multiple_errors!, but seems more abstract and type-heavy, the docs are harder to follow.

• itertools::Itertools::partition_result and more general partition_map

  Partition a sequence of Results into one list of all the Ok elements and another list of all the Err elements.

  If you need both lists, just use itertools. If you discard Oks in case of errors, you can use CollectVecResult::collect_vec_result() that returns Result<Vec<T>, Vec<E>>. It's more precise and efficient.

License

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.