Module snafu::guide::comparison::failure

SNAFU vs. Failure

This comparison was made against the examples in the guide for failure 0.1.5.

“Strings as errors”

It’s unclear what benefit Failure provides here. If you are using this functionality, we recommend using the standard library’s Box<dyn Error>:

fn example() -> Result<(), Box<dyn std::error::Error>> {
    Err(format!("Something went bad: {}", 1 + 1))?;
    Ok(())
}

If you wanted to do something similar with SNAFU, you can create a single-variant error enum with String data:

use snafu::Snafu;
use std::ops::Range;

#[derive(Debug, Snafu)]
enum Error {
    Any { detail: String },
}

fn check_range(x: usize, range: Range<usize>) -> Result<usize, Error> {
    if x < range.start {
        return Any {
            detail: format!("{} is below {}", x, range.start),
        }
        .fail();
    }
    if x >= range.end {
        return Any {
            detail: format!("{} is above {}", x, range.end),
        }
        .fail();
    }
    Ok(x)
}

This could be enhanced in a few ways:

• create methods on your Error type
• create a custom macro
• add a Backtrace to the enum variant

For example:

use snafu::{Backtrace, Snafu};
use std::ops::Range;

#[derive(Debug, Snafu)]
enum Error {
    Any {
        detail: String,
        backtrace: Backtrace,
    },
}

macro_rules! format_err {
    ($($arg:tt)*) => { Any { detail: format!($($arg)*) }.fail() }
}

fn check_range(x: usize, range: Range<usize>) -> Result<usize, Error> {
    if x < range.start {
        return format_err!("{} is below {}", x, range.start);
    }
    if x >= range.end {
        return format_err!("{} is above {}", x, range.end);
    }
    Ok(x)
}

Please see the next section for the recommended pattern for this error.

“A Custom Fail type” and “Using the Error type”

These two idioms from Failure are combined into one primary use case in SNAFU. Additionally, SNAFU avoids the downsides listed in the Failure guide.

You can represent multiple types of errors, allocation is not required, and you can include any extra information relevant to the error:

use snafu::{ensure, Snafu};
use std::ops::Range;

#[derive(Debug, Snafu)]
enum Error {
    #[snafu(display("{} is below {}", value, bound))]
    Below { value: usize, bound: usize },

    #[snafu(display("{} is above {}", value, bound))]
    Above { value: usize, bound: usize },
}

fn check_range(value: usize, range: Range<usize>) -> Result<usize, Error> {
    ensure!(value >= range.start, Below { value, bound: range.start });
    ensure!(value < range.end, Above { value, bound: range.end });
    Ok(value)
}

You do not have to have a one-to-one relationship between an underlying error and an error variant:

use snafu::{ResultExt, Snafu};
use std::num::ParseIntError;

#[derive(Debug, Snafu)]
enum Error {
    #[snafu(display(r#"Could not parse the area code from "{}": {}"#, value, source))]
    AreaCodeInvalid {
        value: String,
        source: ParseIntError,
    },

    #[snafu(display(r#"Could not parse the phone exchange from "{}": {}"#, value, source))]
    PhoneExchangeInvalid {
        value: String,
        source: ParseIntError,
    },
}

fn two_errors_from_same_underlying_error(
    area_code: &str,
    exchange: &str,
) -> Result<(i32, i32), Error> {
    let area_code: i32 = area_code
        .parse()
        .context(AreaCodeInvalid { value: area_code })?;
    let exchange: i32 = exchange
        .parse()
        .context(PhoneExchangeInvalid { value: exchange })?;
    Ok((area_code, exchange))
}

“An Error and ErrorKind pair”

If you choose to make your error type opaque, you can implement methods on the opaque type, allowing you to selectively choose what your public API is.

This includes the ability to return a different public enum that users can match on without knowing the details of your error implementation.

use snafu::Snafu;

#[derive(Debug, Snafu)]
enum InnerError {
    MyError1 { username: String },
    MyError2 { username: String },
    MyError3 { address: String },
}

#[derive(Debug, Snafu)]
pub struct Error(InnerError);

#[derive(Debug, Copy, Clone, PartialEq, Eq)]
pub enum ErrorKind {
    Authorization,
    Network,
}

impl Error {
    pub fn kind(&self) -> ErrorKind {
        use InnerError::*;

        match self.0 {
            MyError1 { .. } | MyError2 { .. } => ErrorKind::Authorization,
            MyError3 { .. } => ErrorKind::Network,
        }
    }

    pub fn username(&self) -> Option<&str> {
        use InnerError::*;

        match &self.0 {
            MyError1 { username } | MyError2 { username } => Some(username),
            _ => None,
        }
    }
}