The error-chain crate (docs) is a new crate for dealing with Rust error boilerplate. It provides a few unique features:
chain_err method.I think the lack of the above are widespread problems with Rust error handling, so I'm interested to hear what people think about this solution. It is inspired by quick-error (and in fact includes a hacked up version of it for internal use) as well as Cargo's internal error handling techniques. This library is used by rustup for error handling.
One note about usage that isn't included in the docs: the
chain_error! macro recurses deeply, so you'll probably need to use
the little-known #![recursion_limit = "1024"] macro on crates that
import it.
For detailed usage information read the docs, which are reproduced in part below.
Generally, you define one family of error types per crate, though it's also perfectly fine to define error types on a finer-grained basis, such as per module.
Assuming you are using crate-level error types, typically you will
define an errors module and inside it call error_chain!:
```rust // Define the error types and conversions for this crate error_chain! { // The type defined for this error. These are the conventional // and recommended names, but they can be arbitrarily chosen. types { Error, ErrorKind, ChainErr, Result; }
// Automatic conversions between this error chain and other
// error chains. In this case, it will e.g. generate an
// `ErrorKind` variant called `Dist` which in turn contains
// the `rustup_dist::ErrorKind`, with conversions from
// `rustup_dist::Error`.
//
// This section can be empty.
links {
rustup_dist::Error, rustup_dist::ErrorKind, Dist;
rustup_utils::Error, rustup_utils::ErrorKind, Utils;
}
// Automatic conversions between this error chain and other
// error types not defined by the `error_chain!`. These will be
// boxed as the error cause and wrapped in a new error with,
// in this case, the `ErrorKind::Temp` variant.
//
// This section can be empty.
foreign_links {
temp::Error, Temp,
"temporary file error";
}
// Define additional `ErrorKind` variants. The syntax here is
// the same as `quick_error!`, but the `from()` and `cause()`
// syntax is not supported.
errors {
InvalidToolchainName(t: String) {
description("invalid toolchain name")
display("invalid toolchain name: '{}'", t)
}
}
} ```
This populates the the module with a number of definitions, the most
important of which are the Error type and the ErrorKind type. They
look something like the following:
```rust use std::error::Error as StdError; use std::sync::Arc;
pub struct Error(pub ErrorKind,
pub Option
impl Error { pub fn kind(&self) -> &ErrorKind { ... } pub fn intokind(self) -> ErrorKind { ... } pub fn iter(&self) -> errorchain::ErrorChainIter { ... } pub fn backtrace(&self) -> &error_chain::Backtrace { ... } }
impl StdError for Error { ... } impl Display for Error { ... }
pub enum ErrorKind { Msg(String), Dist(rustupdist::ErrorKind), Utils(rustuputils::ErrorKind), Temp, InvalidToolchainName(String), } ```
This is the basic error structure. You can see that ErrorKind has
been populated in a variety of ways. All ErrorKinds get a Msg
variant for basic errors. When strings are converted to ErrorKinds
they become ErrorKind::Msg. The "links" defined in the macro are
expanded to Dist and Utils variants, and the "foreign links" to
the Temp variant.
Both types come with a variety of From conversions as well: Error
can be created from ErrorKind, from &str and String, and from
the "link" and "foreign_link" error types. ErrorKind can be created
from the corresponding ErrorKinds of the link types, as wall as from
&str and String.
into() and From::from are used heavily to massage types into the
right shape. Which one to use in any specific case depends on the
influence of type inference, but there are some patterns that arise
frequently.
This is the focus of the crate's design. To extend the error chain:
use errors::ChainErr;
try!(do_something().chain_err(|| "something went wrong"));
chain_err can be called on any Result type where the contained
error type implements std::error::Error + Send + 'static. If the
Result is an Err then chain_err evaluates the closure, which
returns some type that can be converted to ErrorKind, boxes the
original error to store as the cause, then returns a new error
containing the original error.
The above example turns a string into an error, but you could also write e.g.
try!(do_something().chain_err(|| ErrorKind::Foo));
Introducing new error chains, with a string message:
rust
fn foo() -> Result<()> {
Err("foo error!".into())
}
Introducing new error chains, with an ErrorKind:
rust
fn foo() -> Result<()> {
Err(ErrorKind::FooError.into())
}
Note that the return type is is the typedef Result, which is defined
by the macro as pub type Result<T> = ::std::result::Result<T,
Error>. Note that in both cases .into() is called to convert a type
into the Error type: both strings and ErrorKind have From
conversions to turn them into Error.
When the error is emitted inside a try! macro or behind the ?
operator, then the explicit conversion isn't needed, since the
behavior of try! will automatically convert Err(ErrorKind) to
Err(Error). So the below is equivalent to the previous:
```rust fn foo() -> Result<()> { Ok(try!(Err(ErrorKind::FooError))) }
fn bar() -> Result<()> { Ok(try!(Err("bogus!"))) } ```
Errors that do not conform to the same conventions as this library can
still be included in the error chain. They are considered "foreign
errors", and are declared using the foreign_links block of the
error_chain! macro. Errors are automatically created from foreign
errors by the try! macro.
Foreign links and regular links have one crucial difference: From
conversions for regular links do not introduce a new error into the
error chain, while conversions for foreign links always introduce a
new error into the error chain. So for the example above all errors
deriving from the temp::Error type will be presented to the user as
a new ErrorKind::Temp variant, and the cause will be the original
temp::Error error. In contrast, when rustup_utils::Error is
converted to Error the two ErrorKinds are converted between each
other to create a new Error but the old error is discarded; there is
no "cause" created from the original error.
The earliest non-foreign error to be generated creates a single
backtrace, which is passed through all From conversions and
chain_err invocations of compatible types. To read the backtrace
just call the backtrace() method.
The iter method returns an iterator over the chain of error
boxes. See how rustup uses this during error
reporting.
MIT/Apache-2.0