Error Set

Error Set simplifies error management by providing a streamlined method for defining errors and easily converting between them.

Error Set is inspired by Zig's error set, and offers similar functionality.

Instead of defining various enums/structs for errors and hand rolling relations, use an error set:

use error_set::error_set;

error_set! {
    /// The syntax below aggregates the referenced error variants
    MediaError = DownloadError || BookParsingError;

    /// Since all variants in [DownloadError] are in [MediaError], a
    /// [DownloadError] can be turned into a [MediaError] with just `.into()` or `?`. 
    DownloadError = {
        #[display("Easily add custom display messages")]
        InvalidUrl,
        /// The `From` trait for `std::io::Error` will also be automatically derived
        #[display("Display messages work just like the `format!` macro {0}")]
        IoError(std::io::Error),
    };

    /// Traits like `Debug`, `Display`, `Error`, and `From` are all automatically derived
    #[derive(Clone)]
    BookParsingError = { MissingBookDescription, } || BookSectionParsingError;

    BookSectionParsingError = {
        /// Inline structs are also supported
        #[display("Display messages can also reference fields, like {field}")]
        MissingField {
            field: String
        },
        NoContent,
    };
}

Cargo Expand

#[doc = " The syntax below aggregates the referenced "]
#[doc = " errors into the generated enum"]
#[derive(Debug)]
pub enum MediaError {
    InvalidUrl,
    #[doc = " The `From` trait for `std::io::Error` will also be automatically derived"]
    IoError(std::io::Error),
    MissingBookDescription,
    #[doc = " Inline structs are also supported"]
    MissingField {
        field: String,
    },
    NoContent,
}
#[allow(unused_qualifications)]
impl core::error::Error for MediaError {
    fn source(&self) -> Option<&(dyn core::error::Error + 'static)> {
        match self {
            MediaError::IoError(ref source) => source.source(),
            #[allow(unreachable_patterns)]
            _ => None,
        }
    }
}
impl core::fmt::Display for MediaError {
    #[inline]
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        match *self {
            MediaError::InvalidUrl => f.write_fmt(core::format_args!(
                "{}",
                "Easily add custom display messages"
            )),
            MediaError::IoError(ref source) => f.write_fmt(core::format_args!(
                "Display messages work just like the `format!` macro {0}",
                source
            )),
            MediaError::MissingBookDescription => f.write_fmt(core::format_args!(
                "{}",
                concat!(
                    stringify!(MediaError),
                    "::",
                    stringify!(MissingBookDescription)
                )
            )),
            MediaError::MissingField { ref field } => f.write_fmt(core::format_args!(
                "Display messages can also reference fields, like {field}"
            )),
            MediaError::NoContent => f.write_fmt(core::format_args!(
                "{}",
                concat!(stringify!(MediaError), "::", stringify!(NoContent))
            )),
        }
    }
}
impl From<DownloadError> for MediaError {
    fn from(error: DownloadError) -> Self {
        match error {
            DownloadError::InvalidUrl => MediaError::InvalidUrl,
            DownloadError::IoError(source) => MediaError::IoError(source),
        }
    }
}
impl From<BookParsingError> for MediaError {
    fn from(error: BookParsingError) -> Self {
        match error {
            BookParsingError::MissingBookDescription => MediaError::MissingBookDescription,
            BookParsingError::MissingField { field } => MediaError::MissingField { field },
            BookParsingError::NoContent => MediaError::NoContent,
        }
    }
}
impl From<BookSectionParsingError> for MediaError {
    fn from(error: BookSectionParsingError) -> Self {
        match error {
            BookSectionParsingError::MissingField { field } => MediaError::MissingField { field },
            BookSectionParsingError::NoContent => MediaError::NoContent,
        }
    }
}
impl From<std::io::Error> for MediaError {
    fn from(error: std::io::Error) -> Self {
        MediaError::IoError(error)
    }
}
#[doc = " Since this all of the variants in [DownloadError] are in [MediaError],"]
#[doc = " this can be turned into a [MediaError] with just `.into()` or `?`. "]
#[doc = " Note restating variants directly, instead of using `||`, also works"]
#[derive(Debug)]
pub enum DownloadError {
    InvalidUrl,
    #[doc = " The `From` trait for `std::io::Error` will also be automatically derived"]
    IoError(std::io::Error),
}
#[allow(unused_qualifications)]
impl core::error::Error for DownloadError {
    fn source(&self) -> Option<&(dyn core::error::Error + 'static)> {
        match self {
            DownloadError::IoError(ref source) => source.source(),
            #[allow(unreachable_patterns)]
            _ => None,
        }
    }
}
impl core::fmt::Display for DownloadError {
    #[inline]
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        match *self {
            DownloadError::InvalidUrl => f.write_fmt(core::format_args!(
                "{}",
                "Easily add custom display messages"
            )),
            DownloadError::IoError(ref source) => f.write_fmt(core::format_args!(
                "Display messages work just like the `format!` macro {0}",
                source
            )),
        }
    }
}
impl From<std::io::Error> for DownloadError {
    fn from(error: std::io::Error) -> Self {
        DownloadError::IoError(error)
    }
}
#[doc = " Traits like `Debug`, `Display`, `Error`, and `From` are all automatically"]
#[doc = " derived, but one can always add more like below"]
#[derive(Clone, Debug)]
pub enum BookParsingError {
    MissingBookDescription,
    #[doc = " Inline structs are also supported"]
    MissingField {
        field: String,
    },
    NoContent,
}
#[allow(unused_qualifications)]
impl core::error::Error for BookParsingError {}

impl core::fmt::Display for BookParsingError {
    #[inline]
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        match *self {
            BookParsingError::MissingBookDescription => f.write_fmt(core::format_args!(
                "{}",
                concat!(
                    stringify!(BookParsingError),
                    "::",
                    stringify!(MissingBookDescription)
                )
            )),
            BookParsingError::MissingField { ref field } => f.write_fmt(core::format_args!(
                "Display messages can also reference fields, like {field}"
            )),
            BookParsingError::NoContent => f.write_fmt(core::format_args!(
                "{}",
                concat!(stringify!(BookParsingError), "::", stringify!(NoContent))
            )),
        }
    }
}
impl From<BookSectionParsingError> for BookParsingError {
    fn from(error: BookSectionParsingError) -> Self {
        match error {
            BookSectionParsingError::MissingField { field } => {
                BookParsingError::MissingField { field }
            }
            BookSectionParsingError::NoContent => BookParsingError::NoContent,
        }
    }
}
#[derive(Debug)]
pub enum BookSectionParsingError {
    #[doc = " Inline structs are also supported"]
    MissingField {
        field: String,
    },
    NoContent,
}
#[allow(unused_qualifications)]
impl core::error::Error for BookSectionParsingError {}

impl core::fmt::Display for BookSectionParsingError {
    #[inline]
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        match *self {
            BookSectionParsingError::MissingField { ref field } => f.write_fmt(core::format_args!(
                "Display messages can also reference fields, like {field}"
            )),
            BookSectionParsingError::NoContent => f.write_fmt(core::format_args!(
                "{}",
                concat!(
                    stringify!(BookSectionParsingError),
                    "::",
                    stringify!(NoContent)
                )
            )),
        }
    }
}

The above error set can also be written as the full expansion (without the || operator).

Full Expansion Representation

*Comments and messages removed for brevity*

error_set! {
    MediaError = {
        InvalidUrl,
        IoError(std::io::Error),
        MissingBookDescription,
        MissingField {
            field: String
        },
        NoContent,
    };
    DownloadError = {
        InvalidUrl,
        IoError(std::io::Error),
    };
    BookParsingError = {
        MissingBookDescription,
        MissingField {
            field: String
        },
        NoContent,
    };
    BookSectionParsingError = {
        MissingField {
            field: String
        },
        NoContent,
    };
}

Any above subset can be converted into a superset with .into() or ?. This makes correctly scoping and passing up call chains a breeze.

Basic Example

use error_set::error_set;

error_set! {
    MediaError = DownloadError || BookParsingError;
    DownloadError = {
        InvalidUrl,
        IoError(std::io::Error),
    };
    BookParsingError = { MissingBookDescription, } || BookSectionParsingError;
    BookSectionParsingError = {
        MissingField {
            field: String
        },
        NoContent,
    };
}

fn main() {
    let book_section_parsing_error: BookSectionParsingError =
        BookSectionParsingError::MissingField {
            field: "author".to_string(),
        };
    let book_parsing_error: BookParsingError = book_section_parsing_error.into();
    assert!(matches!(
        book_parsing_error,
        BookParsingError::MissingField { field: _ }
    ));
    let media_error: MediaError = book_parsing_error.into();
    assert!(matches!(media_error, MediaError::MissingField { field: _ }));

    let io_error = std::io::Error::new(std::io::ErrorKind::OutOfMemory, "oops out of memory");
    let result_download_error: Result<(), DownloadError> = Err(io_error).coerce(); // == .map_err(Into::into);
    let result_media_error: Result<(), MediaError> = result_download_error.map_err(Into::into);
    assert!(matches!(result_media_error, Err(MediaError::IoError(_))));
}

The typical project approach is to have one errors.rs file with a single error_set. This keeps all the errors in one place and allows your IDE to autocomplete crate::errors:: with of all errors. But error_set! can also be used for quick errors "unions", no longer requiring users to hand write From<..> or use .map_err(..) for these simple cases. e.g.

error_set! {
    JwtVerifierCreationError = {
        Reqwest(reqwest::Error),
        Jwt(jsonwebtoken::errors::Error),
    };
}

impl JwtVerifier {
    pub async fn new(project_id: String) -> Result<Self, JwtVerifierCreationError> {
        let public_keys = Self::fetch_public_keys().await?; // Err is `reqwest::Error`
        let decoding_keys = public_keys
            .into_iter()
            .map(|(key, value)| {
                DecodingKey::from_rsa_pem(value.as_bytes()).map(|decoding_key| (key, decoding_key))
            })
            .collect()?; // Err is `jsonwebtoken::errors::Error`
        ...
    }
}

More Details

Source Variants

Error sets that have source variants (aka wrapped variants), will delegate the Error trait's source() method to the correct source branch's wrapped error. From traits are also automatically generated from the inner type to the Error enum.

Source Tuple Variants

Source tuple variants are the most common source variant. Declared like

error_set! {
    ErrorEnum = {
        IoError(std::io::Error),
        FmtError(std::fmt::Error),
    };
}

Which has the generated enum

pub enum ErrorEnum {
    IoError(std::io::Error),
    FmtError(std::fmt::Error),
}

Source Struct Variants

Source struct variants are also supported, declared like so

error_set! {
    ErrorEnum = {
        IoError(std::io::Error) {} // Note the `{}`
    };
}

Which has the generated enum

pub enum ErrorEnum {
    IoError {
        source: std::io::Error,
    }
}

Source structs become useful when you want to attach additional fields to an error

error_set! {
    ErrorEnum = {
        IoError(std::io::Error) {
            field1: String,
            field2: &'static str,
        }
    };
}

Which has the generated enum

pub enum ErrorEnum {
    IoError {
        source: std::io::Error,
        field1: String,
        field2: &'static str,
    }
}

A From implementation for the inner source is not automatically generated for source struct variants that have fields, like above.

Multiple Source Variants Of The Same Type

Error sets can have multiple source variants of the same type. e.g.

error_set! {
    ErrorEnum3 = {
        IoError1(std::io::Error),
        IoError2(std::io::Error),
    };
}

But a From implementation will not be automatically generated for these cases.

Aggregations And Conversions

Error set uses || (or) for aggregation, which performs an "or" operation on the set space. Note, || is not needed, just a convenience -

error_set! {
    ErrorEnum1 = {
        Variant1,
        Variant2
    } || ErrorEnum2;
    ErrorEnum2 = {
        Variant3
    };
}

is equivalent to

error_set! {
    ErrorEnum1 = {
        Variant1,
        Variant2,
        Variant3,
    };
    ErrorEnum2 = {
        Variant3
    };
}

For one type to be converted into another it needs to be considered a subset of the target type. Thus in the example above, ErrorEnum2 can be converted into ErrorEnum1 with .into() or ?.

Display

The #[display(...)] attribute provides a custom display message for variant. If a custom display is not provided for a wrapped error type like IoError(std::io::Error), it will directly delegate its display to the inner type (std::io::Error). If it is desired to prevent this, provide a custom display message, like in the below example, or add #[display(opaque)]. The default display for other variant types is ErrorName::VariantName.

error_set! {
    AuthError = {
        #[display("User `{name}` with role `{role}` does not exist")] // Shorthand for `#[display("User `{}` with role `{}` does not exist", name, role)]`
        UserDoesNotExist {
            name: String,
            role: u32,
        },
        #[display("The provided credentials are invalid")]
        InvalidCredentials
    };
    LoginError = {
        #[display("Io Error: {0}")] // Shorthand for `#[display("Io Error: {}", source)]`
        IoError(std::io::Error),
    } || AuthError;
}

Usage

fn main() {
    let x: AuthError = AuthError::UserDoesNotExist {
        name: "john".to_string(),
        role: 30,
    };
    assert_eq!(x.to_string(), "User `john` with role `30` does not exist".to_string());
    let y: LoginError = x.into();
    assert_eq!(y.to_string(), "User `john` with role `30` does not exist".to_string());
    let x = AuthError::InvalidCredentials;
    assert_eq!(x.to_string(), "The provided credentials are invalid".to_string());
}

Redeclaring the same variant in a different set and changing the display message, does not effect the conversion between sets.

Disable

error_set auto-implements From, Display, Debug, and Error for a set. If it is ever desired to disable this. Add #[disable(..)] to the set. e.g.

error_set! {
    #[disable(Display,Debug)]
    X = {
        A,
    };
}

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

impl Debug for X {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "X")
    }
}

From also accepts arguments to only disable certain From implementations. e.g.

error_set! {
    U = {
        IoError(std::io::Error),
    };
    V = {
        FmtError(std::fmt::Error),
        IoError(std::io::Error),
    };
    #[disable(From(std::io::Error, U))]
    W = V || U;
}

Generics

error_set supports generics. e.g.

error_set! {
    X<G: Debug> = {
        A {
            a: G
        }
    };
    Y<H: Debug> = {
        B {
            b: H
        }
    };
    Z<T: Debug> = X<T> || Y<T>;
}

In Z<T: Debug> = X<T> || Y<T>; T will replace G in X - X<T: Debug>. Thus this statement is the same as writing

error_set! {
    ...

    Z<T: Debug> = {
        A {
            a: T
        },
        B {
            b: T
        }
    };
}

Feature Flags

tracing / log / defmt : Enables support for the tracing or log or defmt crates. Methods are added to Result and are executed when the Result is an Err for logging purposes. They work similarly to anyhow's .context(..) method. e.g.

let result: Result<(), &str> = Err("operation failed");

let value: Result<(), &str> = result.error("If `Err`, this message is logged as error via tracing/log/defmt");
let value: Result<(), &str> = result.warn("If `Err`, this message is logged as warn via tracing/log/defmt");
let value: Result<(), &str> = result.with_debug(|err| format!("If `Err`, this message is logged as debug via tracing/log/defmt: {}", err));
let value: Option<()> = result.consume_info(); // If `Err`, the `Err` is logged as info via tracing/log/defmt
let value: Option<()> = result.consume_with_trace(|err| format!("If `Err`, this message is logged as trace via tracing/log/defmt: {}", err));
// ...etc.

This is useful tracing context around errors. e.g.

let val = func().warn("`func` failed, here is some extra context like variable values")?;
let val = func().consume_warn();

rather than

let val = func().inspect_err(|err| tracing::warn!("`func` failed, here is some extra context like variable values"))?;
let val = func().inspect_err(|err| tracing::warn!("{}", err)).ok();

Note: a context_stub feature flag also exists to be used by libraries. This allows the api's to be used in libraries while a downstream binary can ultimately decide the implementation. If no implementations is selected, since all the above methods are inlined, the code will be optimized away during compilation.

Why Choose error_set Over thiserror or anyhow

error_set is a unique approach with some of the same features of thiserror and anyhow, while solving a few more problems common to Rust developers.

Like thiserror, error_set allows you define errors, their display messages, and conversions between errors. However error_set is more maintainable and approximately 50% more concise:

example

// thiserror
#[derive(Error)]
enum Error1 {
    a,
    b,
}
#[derive(Error)]
enum Error2 {
    c,
    d,
}
#[derive(Error)]
enum Error3 {
    Error1(#[from] Error1),
    Error2(#[from] Error2),
}

// error_set
error_set! {
    Error1 = {
        a,
        b
    };
    Error2 = {
        c,
        d
    };
  Error3 = Error1 || Error2;
  // `Error3` above is equivalent to writing
  // ```
  // Error3 = {
  //    a,
  //    b,
  //    c,
  //    d
  // };
  // ```
}

With error_set there is no need to maintain a web of nested wrapped enums (with #[from]), since there is no nesting, and all the From implementations are automatically generated if one error type is a subset of another.

Like anyhow, error_set attempts to capture the context around errors. To accomplish this, it uses the help of tracing/log crate. See the feature flags section for more info. However, if your project doesn't require handling specific error types and you just need to propagate errors up the call stack, then anyhow is likely a good choice for you. It's straightforward and skips the need to define error types all together.

For libraries and general projects that require precise error handling and differentiation, error management can often become complex and unwieldy as projects grow. This may even result in "mega enums". error_set can help here where others can't.

What is a Mega Enum?

A mega enum, or mega error enum, is an enumeration that consolidates various error types into one large enum, whereas the code would be more precise if split into multiple enums. These often arise due to refactors or developers opting for less intrusive programming approach. This method can lead to inefficiencies and confusion because it includes error variants that are not relevant in certain scopes.

Example Scenario:

Consider the following functions and their respective error types:

• func1 can produce errors a and b, represented by enum1.
• func2 can produce errors c and d, represented by enum2.
• func3 calls both func1 and func2.

If func3 does not handle the errors from func1 and func2, it must return an error enum that encompasses variants a, b, c, and d. Without a tool like error_set, developers might skip defining enum1 and enum2 due to the complexity and instead create a mega enum with all possible error variants (a, b, c, d). This means that any caller of func1 or func2 would have to handle all these cases, even those that are not possible in that specific context. error_set being so concise and simple, developers actually want to scope their errors to the correct context and join them when needed with a simple || operation. No need to ever think about a web of nested wrapped error types.

How error_set Simplifies Error Management:

error_set allows you to define errors quickly and precisely. Correctly scoping errors is easy and no wrapping of various error enum types is necessary. Conversions/Propagation up the stack are as simple as .into() or ? (or coerce! macro). error_set also makes display messages and tracking context easy. By using error_set, your project can maintain clear and precise error definitions, enhancing code readability and maintainability without the tedious process of manually defining and managing error relations.

no_std

This crate supports #![no_std].

Cavets:

• tracing/log features are not supported, but defmt is supported.