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#568 in Rust patterns

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Used in surrealdb_migration_engin…

Apache-2.0

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error_set

A concise way to define errors and ergonomically coerce a subset into a superset with with just .into() or ?, and coerce between intersecting sets with coerce!.

error_set was inspired by zig's error set and works functionally the same.

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

use error_set::error_set;

error_set! {
    MediaError = {
        IoError(std::io::Error)
    } || BookParsingError || DownloadError || ParseUploadError;
    BookParsingError = {
        MissingBookDescription,
        CouldNotReadBook(std::io::Error),
    } || BookSectionParsingError;
    BookSectionParsingError = {
        MissingName,
        NoContents,
    };
    DownloadError = {
        InvalidUrl,
        CouldNotSaveBook(std::io::Error),
    };
    ParseUploadError = {
        MaximumUploadSizeReached,
        TimedOut,
        AuthenticationFailed,
    };
}

Cargo Expand

#![feature(prelude_import)]
#[prelude_import]
use std::prelude::rust_2021::*;
#[macro_use]
extern crate std;
pub enum MediaError {
    IoError(std::io::Error),
    MissingBookDescription,
    MissingName,
    NoContents,
    InvalidUrl,
    MaximumUploadSizeReached,
    TimedOut,
    AuthenticationFailed,
}
#[automatically_derived]
impl ::core::fmt::Debug for MediaError {
    #[inline]
    fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
        match self {
            MediaError::IoError(__self_0) => {
                ::core::fmt::Formatter::debug_tuple_field1_finish(
                    f,
                    "IoError",
                    &__self_0,
                )
            }
            MediaError::MissingBookDescription => {
                ::core::fmt::Formatter::write_str(f, "MissingBookDescription")
            }
            MediaError::MissingName => {
                ::core::fmt::Formatter::write_str(f, "MissingName")
            }
            MediaError::NoContents => ::core::fmt::Formatter::write_str(f, "NoContents"),
            MediaError::InvalidUrl => ::core::fmt::Formatter::write_str(f, "InvalidUrl"),
            MediaError::MaximumUploadSizeReached => {
                ::core::fmt::Formatter::write_str(f, "MaximumUploadSizeReached")
            }
            MediaError::TimedOut => ::core::fmt::Formatter::write_str(f, "TimedOut"),
            MediaError::AuthenticationFailed => {
                ::core::fmt::Formatter::write_str(f, "AuthenticationFailed")
            }
        }
    }
}
impl error_set::ErrorSetMarker for MediaError {}
#[allow(unused_qualifications)]
impl std::error::Error for MediaError {
    fn source(&self) -> Option<&(dyn std::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 {
        let variant_name = match *self {
            MediaError::IoError(_) => "MediaError::IoError",
            MediaError::MissingBookDescription => "MediaError::MissingBookDescription",
            MediaError::MissingName => "MediaError::MissingName",
            MediaError::NoContents => "MediaError::NoContents",
            MediaError::InvalidUrl => "MediaError::InvalidUrl",
            MediaError::MaximumUploadSizeReached => {
                "MediaError::MaximumUploadSizeReached"
            }
            MediaError::TimedOut => "MediaError::TimedOut",
            MediaError::AuthenticationFailed => "MediaError::AuthenticationFailed",
        };
        f.write_fmt(format_args!("{0}", variant_name))
    }
}
impl From<BookParsingError> for MediaError {
    fn from(error: BookParsingError) -> Self {
        match error {
            BookParsingError::MissingBookDescription => {
                MediaError::MissingBookDescription
            }
            BookParsingError::CouldNotReadBook(source) => MediaError::IoError(source),
            BookParsingError::MissingName => MediaError::MissingName,
            BookParsingError::NoContents => MediaError::NoContents,
        }
    }
}
impl From<BookSectionParsingError> for MediaError {
    fn from(error: BookSectionParsingError) -> Self {
        match error {
            BookSectionParsingError::MissingName => MediaError::MissingName,
            BookSectionParsingError::NoContents => MediaError::NoContents,
        }
    }
}
impl From<DownloadError> for MediaError {
    fn from(error: DownloadError) -> Self {
        match error {
            DownloadError::InvalidUrl => MediaError::InvalidUrl,
            DownloadError::CouldNotSaveBook(source) => MediaError::IoError(source),
        }
    }
}
impl From<ParseUploadError> for MediaError {
    fn from(error: ParseUploadError) -> Self {
        match error {
            ParseUploadError::MaximumUploadSizeReached => {
                MediaError::MaximumUploadSizeReached
            }
            ParseUploadError::TimedOut => MediaError::TimedOut,
            ParseUploadError::AuthenticationFailed => MediaError::AuthenticationFailed,
        }
    }
}
impl From<std::io::Error> for MediaError {
    fn from(error: std::io::Error) -> Self {
        MediaError::IoError(error)
    }
}
pub enum BookParsingError {
    MissingBookDescription,
    CouldNotReadBook(std::io::Error),
    MissingName,
    NoContents,
}
#[automatically_derived]
impl ::core::fmt::Debug for BookParsingError {
    #[inline]
    fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
        match self {
            BookParsingError::MissingBookDescription => {
                ::core::fmt::Formatter::write_str(f, "MissingBookDescription")
            }
            BookParsingError::CouldNotReadBook(__self_0) => {
                ::core::fmt::Formatter::debug_tuple_field1_finish(
                    f,
                    "CouldNotReadBook",
                    &__self_0,
                )
            }
            BookParsingError::MissingName => {
                ::core::fmt::Formatter::write_str(f, "MissingName")
            }
            BookParsingError::NoContents => {
                ::core::fmt::Formatter::write_str(f, "NoContents")
            }
        }
    }
}
impl error_set::ErrorSetMarker for BookParsingError {}
#[allow(unused_qualifications)]
impl std::error::Error for BookParsingError {
    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
        match *self {
            BookParsingError::CouldNotReadBook(ref source) => source.source(),
            #[allow(unreachable_patterns)]
            _ => None,
        }
    }
}
impl core::fmt::Display for BookParsingError {
    #[inline]
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        let variant_name = match *self {
            BookParsingError::MissingBookDescription => {
                "BookParsingError::MissingBookDescription"
            }
            BookParsingError::CouldNotReadBook(_) => "BookParsingError::CouldNotReadBook",
            BookParsingError::MissingName => "BookParsingError::MissingName",
            BookParsingError::NoContents => "BookParsingError::NoContents",
        };
        f.write_fmt(format_args!("{0}", variant_name))
    }
}
impl From<BookSectionParsingError> for BookParsingError {
    fn from(error: BookSectionParsingError) -> Self {
        match error {
            BookSectionParsingError::MissingName => BookParsingError::MissingName,
            BookSectionParsingError::NoContents => BookParsingError::NoContents,
        }
    }
}
impl From<std::io::Error> for BookParsingError {
    fn from(error: std::io::Error) -> Self {
        BookParsingError::CouldNotReadBook(error)
    }
}
pub enum BookSectionParsingError {
    MissingName,
    NoContents,
}
#[automatically_derived]
impl ::core::fmt::Debug for BookSectionParsingError {
    #[inline]
    fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
        ::core::fmt::Formatter::write_str(
            f,
            match self {
                BookSectionParsingError::MissingName => "MissingName",
                BookSectionParsingError::NoContents => "NoContents",
            },
        )
    }
}
impl error_set::ErrorSetMarker for BookSectionParsingError {}
#[allow(unused_qualifications)]
impl std::error::Error for BookSectionParsingError {}
impl core::fmt::Display for BookSectionParsingError {
    #[inline]
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        let variant_name = match *self {
            BookSectionParsingError::MissingName => {
                "BookSectionParsingError::MissingName"
            }
            BookSectionParsingError::NoContents => "BookSectionParsingError::NoContents",
        };
        f.write_fmt(format_args!("{0}", variant_name))
    }
}
pub enum DownloadError {
    InvalidUrl,
    CouldNotSaveBook(std::io::Error),
}
#[automatically_derived]
impl ::core::fmt::Debug for DownloadError {
    #[inline]
    fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
        match self {
            DownloadError::InvalidUrl => {
                ::core::fmt::Formatter::write_str(f, "InvalidUrl")
            }
            DownloadError::CouldNotSaveBook(__self_0) => {
                ::core::fmt::Formatter::debug_tuple_field1_finish(
                    f,
                    "CouldNotSaveBook",
                    &__self_0,
                )
            }
        }
    }
}
impl error_set::ErrorSetMarker for DownloadError {}
#[allow(unused_qualifications)]
impl std::error::Error for DownloadError {
    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
        match *self {
            DownloadError::CouldNotSaveBook(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 {
        let variant_name = match *self {
            DownloadError::InvalidUrl => "DownloadError::InvalidUrl",
            DownloadError::CouldNotSaveBook(_) => "DownloadError::CouldNotSaveBook",
        };
        f.write_fmt(format_args!("{0}", variant_name))
    }
}
impl From<std::io::Error> for DownloadError {
    fn from(error: std::io::Error) -> Self {
        DownloadError::CouldNotSaveBook(error)
    }
}
pub enum ParseUploadError {
    MaximumUploadSizeReached,
    TimedOut,
    AuthenticationFailed,
}
#[automatically_derived]
impl ::core::fmt::Debug for ParseUploadError {
    #[inline]
    fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
        ::core::fmt::Formatter::write_str(
            f,
            match self {
                ParseUploadError::MaximumUploadSizeReached => "MaximumUploadSizeReached",
                ParseUploadError::TimedOut => "TimedOut",
                ParseUploadError::AuthenticationFailed => "AuthenticationFailed",
            },
        )
    }
}
impl error_set::ErrorSetMarker for ParseUploadError {}
#[allow(unused_qualifications)]
impl std::error::Error for ParseUploadError {}
impl core::fmt::Display for ParseUploadError {
    #[inline]
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        let variant_name = match *self {
            ParseUploadError::MaximumUploadSizeReached => {
                "ParseUploadError::MaximumUploadSizeReached"
            }
            ParseUploadError::TimedOut => "ParseUploadError::TimedOut",
            ParseUploadError::AuthenticationFailed => {
                "ParseUploadError::AuthenticationFailed"
            }
        };
        f.write_fmt(format_args!("{0}", variant_name))
    }
}

// `coerce` macro omitted

which is also equivalent to writing:

error_set! {
    MediaError = {
        IoError(std::io::Error),
        MissingBookDescription,
        MissingName,
        NoContents,
        InvalidUrl,
        MaximumUploadSizeReached,
        TimedOut,
        AuthenticationFailed,
    };
    BookParsingError = {
        MissingBookDescription,
        CouldNotReadBook(std::io::Error),
        MissingName,
        NoContents,
    };
    BookSectionParsingError = {
        MissingName,
        NoContents,
    };
    DownloadError = {
        InvalidUrl,
        CouldNotSaveBook(std::io::Error),
    };
    ParseUploadError = {
        MaximumUploadSizeReached,
        TimedOut,
        AuthenticationFailed,
    };
}

As mentioned, any above subset can be converted into a superset with .into() or ?. Error enums and error variants can also accept doc comments and attributes like #[derive(...)].

`error_set!` Examples

Base Functionality In Action

fn main() {
    let book_section_parsing_error = BookSectionParsingError::MissingName;
    let book_parsing_error: BookParsingError = book_section_parsing_error.coerce(); // `.coerce()` == `.into()`
    assert!(matches!(book_parsing_error, BookParsingError::MissingName));
    let media_error: MediaError = book_parsing_error.coerce(); // `.coerce()` == `.into()`
    assert!(matches!(media_error, MediaError::MissingName));

    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(); // `.coerce()` == `.map_err(Into::into)`
    let result_media_error: Result<(), MediaError> = result_download_error.coerce(); // `.coerce()` == `.map_err(Into::into)`
    assert!(matches!(result_media_error, Err(MediaError::IoError(_))));
}

Here we can easily define all the different error states a function could exit with. Note this example is verbose as not all error states have downstream handlers that care about the error type, but imagine it so.

More Intricate Example

error_set::error_set! {
    MediaError = {
        IoError(std::io::Error)
    } || BookParsingError || DownloadError || UploadError;
    BookParsingError = {
        MissingContent,
        BookAccess(std::io::Error),
    };
    DownloadError = {
        InvalidUrl,
        CouldNotSaveBook(std::io::Error),
    };
    UploadError = {
        MaximumUploadSizeReached,
        TimedOut,
        AuthenticationFailed,
    };
}

fn parse_book(file_path: &str) -> Result<String, BookParsingError> {
    let mut file = File::open(file_path).coerce::<BookParsingError>()?;
    let mut content = String::new();
    file.read_to_string(&mut content).coerce::<BookParsingError>()?;
    if content.is_empty() {
        Err(BookParsingError::MissingContent)
    } else {
        Ok(content)
    }
}

fn download_book(url: &str, save_path: &str) -> Result<(), DownloadError> {
    if url.is_empty() {
        Err(DownloadError::InvalidUrl)
    } else {
        let simulated_book_content = "This is a downloaded book content.";
        let mut file = File::create(save_path).coerce::<DownloadError>()?;
        file.write_all(simulated_book_content.as_bytes()).coerce::<DownloadError>()?;
        Ok(())
    }
}

fn upload_content(content: &str) -> Result<(), UploadError> {
    let auth = true;
    if !auth { // Simulate auth
        return Err(UploadError::AuthenticationFailed);
    }
    let time_out = false;
    if !time_out { // Simulate timeout uploading
        return Err(UploadError::TimedOut);
    }
    if content.len() > 1024 { // Simulate an upload size limit
        Err(UploadError::MaximumUploadSizeReached)
    } else {
        println!("Book uploaded successfully.");
        Ok(())
    }
}

fn process_book(download_path: &str, download_url: &str) -> Result<String, MediaError> {
    download_book(download_url, download_path).coerce::<MediaError>()?;
    let content = parse_book(download_path).coerce::<MediaError>()?;
    const MAX_RETRIES: u8  = 3;
    let mut current_retries = 0;
    match upload_content(&content) {
        Err(UploadError::TimedOut) => {
            while current_retries < MAX_RETRIES {
                current_retries += 1;
                if let Ok(_) = upload_content(&content) {
                    break;
                }
            }
        }
        Err(e) => return Err(e.coerce()),
        _ => (),
    }
    fs::remove_file(download_path).coerce::<MediaError>()?;
    Ok(content)
}

fn main() {
    match process_book("downloaded_book.txt", "http://example.com/book") {
        Ok(content) => println!("Book processed successfully: {}", content),
        Err(e) => eprintln!("An error occurred: {:?}", e),
    }
}

The `coerce!` Macro

The coerce! macro handles coercing between intersecting sets (sets where some of the error types are in common). This allows only being explicit where relevant, such as the disjointedness. e.g. given:

error_set! {
   SetX = {
       X
   } || Common;
   SetY = {
       Y
   } || Common;
   Common = {
       A,
       B,
       C,
       D,
       E,
       F,
       G,
       H,
   };
}

rather than writting:

fn setx_result_to_sety_result() -> Result<(), SetY> {
   let _ok = match setx_result() {
       Ok(ok) => ok,
       Err(SetX::X) => {} // handle disjointedness
       Err(SetX::A) => {
           return Err(SetY::A);
       }
       Err(SetX::B) => {
           return Err(SetY::B);
       }
       Err(SetX::C) => {
           return Err(SetY::C);
       }
       Err(SetX::D) => {
           return Err(SetY::D);
       }
       Err(SetX::E) => {
           return Err(SetY::E);
       }
       Err(SetX::F) => {
           return Err(SetY::F);
       }
       Err(SetX::G) => {
           return Err(SetY::G);
       }
       Err(SetX::H) => {
           return Err(SetY::H);
       }
   };
   Ok(())
}

one can write this, which compiles to the match statement above:

fn setx_result_to_sety_result() -> Result<(),SetY> {
   let _ok = coerce!(setx_result() => {
       Ok(ok) => ok,
       Err(SetX::X) => {}, // handle disjointedness
       { Err(SetX) => return Err(SetY) } // terminal coercion
   });
   Ok(())
}

The coerce! macro is a flat fast (no tt muncher 🦫) declarative macro created by the error_set! macro for the set. coerce! behaves like a regular match statement, except it allows a terminal coercion statement between sets. e.g.

{ Err(SetX) => return Err(SetY) }
{ Err(SetX) => Err(SetY) }
{ SetX => return SetY }
{ SetX => SetY }

With coerce!, one can concisely handle specific variants of errors as they bubble up the call stack and propagate the rest.

Dependencies

~315–770KB
~18K SLoC