Package flat_error

Simple Error wrapper to ensure Clone, Debug, and PartialEq.

In general it is recommended that error types implement not just Error and it's required Display, but also Clone, Debug, and PartialEq although [Eq] is optional. Beyond these additional traits are added as normal such as Copy, PartialOrd, Ord, Hash, etc.

However, there are a number of error types in the standard library, and many more in commonly used traits that do not meet this requirement. This simple crate introduces a new trait, ExtendedError, which provides these additional requirements and a new concrete type, FlatError which provides a way to capture existing errors such that they meet these requirements.

Example

The following demonstrates a new type that meets the requirements of ExtendedError.

use std::{
    error::Error,
    fmt::{Display, Formatter, Result as FmtResult},
};

#[derive(Clone, Debug, PartialEq)]
pub struct MyError;

impl Display for MyError {
    fn fmt(&self, f: &mut Formatter<'_>) -> FmtResult {
        write!(f, "MyError")
    }
}

impl Error for MyError {}

However, the following fails because the error in std::io does not implement either Clone or PartialEq.

use std::{
    error::Error,
    fmt::{Display, Formatter, Result as FmtResult},
    io::Error as IoError,
};

#[derive(Clone, Debug, PartialEq)]
pub enum MyError {
    Io(IoError), // <= this doesn't work.
}

FlatError allows the capture and flattening of these errors into MyError, as shown below. This does however lose the ability to access any specific methods on the flattened error such as last_os_error on std::io::Error.

use flat_error::FlatError;
use std::{
    error::Error,
    fmt::{Display, Formatter, Result as FmtResult},
    io::Error as IoError,
};

#[derive(Clone, Debug, PartialEq)]
pub enum MyError {
    Io(FlatError),
}

Features

License(s)

The contents of this repository are made available under the following licenses:

Apache-2.0

Copyright 2025 Simon Johnston <johnstonskj@gmail.com>

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.

See the enclosed file LICENSE-Apache.

MIT

Copyright 2025 Simon Johnston <johnstonskj@gmail.com>

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the “Software”), to deal
in the Software without restriction, including without limitation the rights to
use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
the Software, and to permit persons to whom the Software is furnished to do so,
subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

See the enclosed file LICENSE-MIT.

Contributions

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

For information on contributing to this project, see the following.

1. Project Code of Conduct.
2. Project Contribution Guidelines.
3. Project TODO Items in Issues.
4. Repository Change Log.