error_def

error_def: A Rust syntax extension for generating error-handling boilerplate code.

Quick Example: The following code:

error_def! ExampleError {
    AVariant
        => "Unit-like variant",
    AVariantWithALongDescription
        => "Unit-like variant" ("A more verbose description"),
    AVariantWithArgs { flim: u32, flam: u32 }
        => "Variant with args" ("This is a format string. flim is {}. flam is {}.", flim, flam),
    AVariantWithACause { blah: bool, #[from] cause: io::Error }
        => "Variant with a cause" ("self.cause() would return Some({})", cause)
    AVariantWithJustACause { #[from] blah: io::Error }
        => "This variant can be made `From` an `io::Error`"
}

Expands (roughly) to:

pub enum ExampleError {
    /// Unit-like variant
    AVariant,

    /// Unit-like variant
    AVariantWithALongDescription,

    /// Variant with args
    AVariantWithArgs {
        flim: u32,
        flam: u32,
    },

    /// Variant with a cause
    AVariantWithACause {
        blah: bool,
        cause: io::Error,
    },

    /// This variant can be made `From` an `io::Error`
    AVariantWithJustACause {
        blah: io::Error,
    },
}

impl fmt::Debug for ExampleError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result<(), fmt::Error> {
        match self {
            &ExampleError::AVariant
                => write!(f, "AVariant /* {} */", self),
            &ExampleError::AVariantWithALongDescription
                => write!(f, "AVariantWithALongDescription /* {} */", self),
            &ExampleError::AVariantWithArgs { ref flim, ref flam }
                => write!(f, "AVariantWithArgs {{ flim: {:?}, flam: {:?} }} /* {} */", flim, flim, self),
            &ExampleError::AVariantWithACause { ref blah, ref cause }
                => write!(f, "AVariantWithACause {{ blah: {:?}, cause: {:?} }} /* {} */", blah, cause, self),
            &ExampleError::AVariantWithJustACause { ref blah }
                => write!(f, "AVariantWithJustACause {{ blah: {:?} }} /* {} */", blah, self),
        }
    }
}

impl fmt::Display for ExampleError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result<(), fmt::Error> {
        match self {
            &ExampleError::AVariant                                => {
                try!(write!(f, "Unit-like variant."));
                Ok(())
            },
            &ExampleError::AVariantWithALongDescription            => {
                try!(write!(f, "Unit-like variant"));
                try!(write!(f, "A more verbose description"));
                Ok(())
            },
            &ExampleError::AVariantWithArgs { ref flim, ref flam } => {
                try!(write!(f, "Variant with args"));
                try!(write!(f, "This is a format string. flim is {}. flam is {}.", flim, flam));
                Ok(())
            },
            &ExampleError::AVariantWithACause { ref cause, .. }    => {
                try!(write!(f, "Variant with a cause"));
                try!(write!(f, "self.cause() would return Some({})", cause));
                Ok(())
            },
            &ExampleError::AVariantWithJustACause { .. }           => {
                try!(write!(f, "This variant can be made `From` an `io::Error`"));
                Ok(())
            },
        }
    }
}

impl Error for ExampleError {
    fn description(&self) -> &str {
        match self {
            &ExampleError::AVariant                            => "Unit-like variant",
            &ExampleError::AVariantWithALongDescription { .. } => "Unit-like variant",
            &ExampleError::AVariantWithArgs { .. }             => "Variant with args",
            &ExampleError::AVariantWithACause { .. }           => "Variant with a cause",
            &ExampleError::AVariantWithJustACause { .. }       => "This variant can be made `From` an `io::Error`",
        }
    }

    fn cause(&self) -> Option<&Error> {
        match self {
            &ExampleError::AVariant                                => None,
            &ExampleError::AVariantWithALongDescription { .. }     => None,
            &ExampleError::AVariantWithArgs { .. }                 => None,
            &ExampleError::AVariantWithACause { ref cause, .. }    => Some(cause as &Error),
            &ExampleError::AVariantWithJustACause { ref blah, .. } => Some(blah as &Error),
        }
    }
}

impl From<io::Error> for ExampleError {
    fn from(e: io::Error) -> ExampleError {
        ExampleError::AVariantWithJustACause { blah: e }
    }
}

Explanation: error_def defines an enum where each variant is paired with a description of the variant.

error_def! SomeError {
    AVariant       => "A description",
    AnotherVariant => "Another description",
}

This description is added as a doc-comment to the variant and is returned by calls to Error::description.

assert!(SomeError::AVariant.description() == "A description")

Variants can be struct-like.

error_def! SomeError {
    AVariant { an_i32: i32 }  => "I'm a variant",
}

Variants can also have an optional long-description which consists of a format string and a sequence of arguments. The long description is placed in parenthesis after the short-description. If the variant is a struct, the arguments to the format string can refer to it's members.

error_def! SomeError {
    Io { cause: io::Error }
        => "I/O error occured!" ("Error: {}", cause),
}

error_def! uses the short and long descriptions to provide impls of fmt::Display and fmt::Debug. In the above case, SomeError::Io would be formatted as

I/O error occured. Error: <insert fmt::Display(cause) here>

For fmt::Display and

SomeError::Io { cause: <insert fmt::Debug(cause) here> } /* I/O error occured. Error: <insert fmt::Display(cause) here> */

For fmt::Debug.

Members of a struct-variant can be marked with an optional #[from] pseudo-attribute.

error_def! SomeError {
    Io {
        foo: u32,
        #[from] cause: io::Error,
    } => "Io error"
}

This causes the member to be returned by calls to Error::cause. In the above example, calling Error::cause on a SomeError::Io will return an Option<&Error> where the &Error points to an io::Error.

If a struct variant has only one member and it is marked #[from] then From will be implemented to cast the type of that member to the type of the error.

For example, if we define an error like this:

error_def! SomeError {
    Io { #[from] cause: io::Error } => "I/O error",
}

Then error_def! will define an impl:

impl std::convert::From<io::Error> for SomeError {
    fn from(e: io::Error) -> SomeError {
        SomeError::Io {
            cause: e,
        }
    }
}