## error_def: A Rust syntax extension for generating error-handling boilerplate code.
**Quick Example:** The following code:
```rust
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:
```rust
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.
```rust
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`.
```rust
assert!(SomeError::AVariant.description() == "A description")
```
Variants can be struct-like.
```rust
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.
```rust
error_def! SomeError {
Io { cause: io::Error }
=> "I/O error occured!" ("Error: {}", cause),
}
```
`error_def!` uses the short and long descriptions to provide `impl`s 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.
```rust
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:
```rust
error_def! SomeError {
Io { #[from] cause: io::Error } => "I/O error",
}
```
Then `error_def!` will define an `impl`:
```rust
impl std::convert::From<io::Error> for SomeError {
fn from(e: io::Error) -> SomeError {
SomeError::Io {
cause: e,
}
}
}
```