Crate error_chain [−] [src]
A library for consistent and reliable error handling
error-chain makes it easy to take full advantage of Rust's powerful error handling features without the overhead of maintaining boilerplate error types and conversions. It implements an opinionated strategy for defining your own error types, as well as conversions from others' error types.
Quick start
If you just want to set up your new project with error-chain, follow the quickstart.rs template, and read this intro to error-chain.
Why error chain?
- error-chain is easy to configure. Handle errors robustly with minimal effort.
- Basic error handling requires no maintenance of custom error types
nor the
From
conversions that make?
work. - error-chain scales from simple error handling strategies to more rigorous. Return formatted strings for simple errors, only introducing error variants and their strong typing as needed for advanced error recovery.
- error-chain makes it trivial to correctly manage the cause of the errors generated by your own code. This is the "chaining" in "error-chain".
Principles of error-chain
error-chain is based on the following principles:
- No error should ever be discarded. This library primarily
makes it easy to "chain" errors with the
chain_err
method. - Introducing new errors is trivial. Simple errors can be introduced at the error site with just a string.
- Handling errors is possible with pattern matching.
- Conversions between error types are done in an automatic and
consistent way -
From
conversion behavior is never specified explicitly. - Errors implement
Sync
andSend
. - Errors can carry backtraces.
Similar to other libraries like error-type and quick-error,
this library introduces the error chaining mechanism originally
employed by Cargo. The error_chain!
macro declares the types
and implementation boilerplate necessary for fulfilling a
particular error-handling strategy. Most importantly it defines a
custom error type (called Error
by convention) and the From
conversions that let the try!
macro and ?
operator work.
This library differs in a few ways from previous error libs:
- Instead of defining the custom
Error
type as an enum, it is a struct containing anErrorKind
(which defines thedescription
anddisplay
methods for the error), an opaque, optional, boxedstd::error::Error + Sync + Send + 'static
object (which defines thecause
, and establishes the links in the error chain), and aBacktrace
. - The macro also defines a
ResultExt
trait that defines achain_err
method. This method on allstd::error::Error + Sync + Send + 'static
types extends the error chain by boxing the current error into an opaque object and putting it inside a new concrete error. - It provides automatic
From
conversions between other error types defined by theerror_chain!
that preserve type information, and facilitate seamless error composition and matching of composed errors. - It provides automatic
From
conversions between any other error type that hides the type of the other error in thecause
box. - If
RUST_BACKTRACE
is enabled, it collects a single backtrace at the earliest opportunity and propagates it down the stack throughFrom
andResultExt
conversions.
To accomplish its goals it makes some tradeoffs:
- The split between the
Error
andErrorKind
types can make it slightly more cumbersome to instantiate new (unchained) errors, requiring anInto
orFrom
conversion; as well as slightly more cumbersome to match on errors with another layer of types to match. - Because the error type contains
std::error::Error + Sync + Send + 'static
objects, it can't implementPartialEq
for easy comparisons.
Declaring error types
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!
:
mod other_error { error_chain! {} } error_chain! { // The type defined for this error. These are the conventional // and recommended names, but they can be arbitrarily chosen. // // It is also possible to leave this section out entirely, or // leave it empty, and these names will be used automatically. types { Error, ErrorKind, ResultExt, Result; } // Without the `Result` wrapper: // // types { // Error, ErrorKind, ResultExt; // } // Automatic conversions between this error chain and other // error chains. In this case, it will e.g. generate an // `ErrorKind` variant called `Another` which in turn contains // the `other_error::ErrorKind`, with conversions from // `other_error::Error`. // // Optionally, some attributes can be added to a variant. // // This section can be empty. links { Another(other_error::Error, other_error::ErrorKind) #[cfg(unix)]; } // Automatic conversions between this error chain and other // error types not defined by the `error_chain!`. These will be // wrapped in a new error with, in the first case, the // `ErrorKind::Fmt` variant. The description and cause will // forward to the description and cause of the original error. // // Optionally, some attributes can be added to a variant. // // This section can be empty. foreign_links { Fmt(::std::fmt::Error); Io(::std::io::Error) #[cfg(unix)]; } // 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) } } }
Each section, types
, links
, foreign_links
, and errors
may
be omitted if it is empty.
This populates the module with a number of definitions,
the most important of which are the Error
type
and the ErrorKind
type. An example of generated code can be found in the
example_generated module.
Returning new errors
Introducing new error chains, with a string message:
fn foo() -> Result<()> { Err("foo error!".into()) }
Introducing new error chains, with an ErrorKind
:
error_chain! { errors { FooError } } fn foo() -> Result<()> { Err(ErrorKind::FooError.into()) }
Note that the return type 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, the explicit conversion isn't needed; try!
will
automatically convert Err(ErrorKind)
to Err(Error)
. So the
below is equivalent to the previous:
fn foo() -> Result<()> { Ok(try!(Err(ErrorKind::FooError))) } fn bar() -> Result<()> { Ok(try!(Err("bogus!"))) }
The bail!
macro
The above method of introducing new errors works but is a little
verbose. Instead we can use the bail!
macro, which, much like try!
and ?
, performs an early return with conversions. With bail!
the
previous examples look like:
fn foo() -> Result<()> { bail!(ErrorKind::FooError); Ok(()) } fn bar() -> Result<()> { bail!("bogus!"); Ok(()) }
Chaining errors
To extend the error chain:
let res: Result<()> = 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 + Sync + 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.
Matching errors
error-chain error variants are matched with simple patterns.
Error
is a tuple struct and its first field is the ErrorKind
,
making dispatching on error kinds relatively compact:
error_chain! { errors { InvalidToolchainName(t: String) { description("invalid toolchain name") display("invalid toolchain name: '{}'", t) } } } match Error::from("error!") { Error(ErrorKind::InvalidToolchainName(_), _) => { } Error(ErrorKind::Msg(_), _) => { } }
Chained errors are also matched with (relatively) compact syntax
mod utils { error_chain! { errors { BadStuff { description("bad stuff") } } } } mod app { error_chain! { links { Utils(::utils::Error, ::utils::ErrorKind); } } } match app::Error::from("error!") { app::Error(app::ErrorKind::Utils(utils::ErrorKind::BadStuff), _) => { } _ => { } }
Foreign links
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. Error
s 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 ErrorKind
s
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.
Backtraces
If the RUST_BACKTRACE
environment variable is set to anything
but 0
, 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.
Backtrace generation can be disabled by turning off the backtrace
feature.
Iteration
The iter
method returns an iterator over the chain of error boxes.
Modules
example_generated |
These modules show an example of code generated by the macro. IT MUST NOT BE USED OUTSIDE THIS CRATE. |
Macros
bail |
Exits a function early with an error |
error_chain |
This macro is used for handling of duplicated and out-of-order fields. For
the exact rules, see |
error_chain_processed |
Prefer to use |
quick_error | |
quick_main |
Convenient wrapper to be able to use |
Structs
Backtrace |
Representation of an owned and self-contained backtrace. |
Display |
A struct which formats an error for output. |
ErrorChainIter |
Iterator over the error chain using the |
Traits
ChainedError |
This trait is implemented on all the errors generated by the |
ExitCode |
Represents a value that can be used as the exit status of the process.
See |