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Interfaces for working with Errors. | ||
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# Error Handling In Rust | ||
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The Rust language provides two complementary systems for constructing / | ||
representing, reporting, propagating, reacting to, and discarding errors. | ||
These responsibilities are collectively known as "error handling." The | ||
components of the first system, the panic runtime and interfaces, are most | ||
commonly used to represent bugs that have been detected in your program. The | ||
components of the second system, `Result`, the error traits, and user | ||
defined types, are used to represent anticipated runtime failure modes of | ||
your program. | ||
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## The Panic Interfaces | ||
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The following are the primary interfaces of the panic system and the | ||
responsibilities they cover: | ||
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* [`panic!`] and [`panic_any`] (Constructing, Propagated automatically) | ||
* [`PanicInfo`] (Reporting) | ||
* [`set_hook`], [`take_hook`], and [`#[panic_handler]`][panic-handler] (Reporting) | ||
* [`catch_unwind`] and [`resume_unwind`] (Discarding, Propagating) | ||
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The following are the primary interfaces of the error system and the | ||
responsibilities they cover: | ||
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* [`Result`] (Propagating, Reacting) | ||
* The [`Error`] trait (Reporting) | ||
* User defined types (Constructing / Representing) | ||
* [`match`] and [`downcast`] (Reacting) | ||
* The question mark operator ([`?`]) (Propagating) | ||
* The partially stable [`Try`] traits (Propagating, Constructing) | ||
* [`Termination`] (Reporting) | ||
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## Converting Errors into Panics | ||
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The panic and error systems are not entirely distinct. Often times errors | ||
that are anticipated runtime failures in an API might instead represent bugs | ||
to a caller. For these situations the standard library provides APIs for | ||
constructing panics with an `Error` as it's source. | ||
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* [`Result::unwrap`] | ||
* [`Result::expect`] | ||
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These functions are equivalent, they either return the inner value if the | ||
`Result` is `Ok` or panic if the `Result` is `Err` printing the inner error | ||
as the source. The only difference between them is that with `expect` you | ||
provide a panic error message to be printed alongside the source, whereas | ||
`unwrap` has a default message indicating only that you unwraped an `Err`. | ||
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Of the two, `expect` is generally preferred since its `msg` field allows you | ||
to convey your intent and assumptions which makes tracking down the source | ||
of a panic easier. `unwrap` on the other hand can still be a good fit in | ||
situations where you can trivially show that a piece of code will never | ||
panic, such as `"127.0.0.1".parse::<std::net::IpAddr>().unwrap()` or early | ||
prototyping. | ||
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# Common Message Styles | ||
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There are two common styles for how people word `expect` messages. Using | ||
the message to present information to users encountering a panic | ||
("expect as error message") or using the message to present information | ||
to developers debugging the panic ("expect as precondition"). | ||
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In the former case the expect message is used to describe the error that | ||
has occurred which is considered a bug. Consider the following example: | ||
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```should_panic | ||
// Read environment variable, panic if it is not present | ||
let path = std::env::var("IMPORTANT_PATH").unwrap(); | ||
``` | ||
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In the "expect as error message" style we would use expect to describe | ||
that the environment variable was not set when it should have been: | ||
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```should_panic | ||
let path = std::env::var("IMPORTANT_PATH") | ||
.expect("env variable `IMPORTANT_PATH` is not set"); | ||
``` | ||
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In the "expect as precondition" style, we would instead describe the | ||
reason we _expect_ the `Result` should be `Ok`. With this style we would | ||
prefer to write: | ||
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```should_panic | ||
let path = std::env::var("IMPORTANT_PATH") | ||
.expect("env variable `IMPORTANT_PATH` should be set by `wrapper_script.sh`"); | ||
``` | ||
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The "expect as error message" style does not work as well with the | ||
default output of the std panic hooks, and often ends up repeating | ||
information that is already communicated by the source error being | ||
unwrapped: | ||
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```text | ||
thread 'main' panicked at 'env variable `IMPORTANT_PATH` is not set: NotPresent', src/main.rs:4:6 | ||
``` | ||
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In this example we end up mentioning that an env variable is not set, | ||
followed by our source message that says the env is not present, the | ||
only additional information we're communicating is the name of the | ||
environment variable being checked. | ||
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The "expect as precondition" style instead focuses on source code | ||
readability, making it easier to understand what must have gone wrong in | ||
situations where panics are being used to represent bugs exclusively. | ||
Also, by framing our expect in terms of what "SHOULD" have happened to | ||
prevent the source error, we end up introducing new information that is | ||
independent from our source error. | ||
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```text | ||
thread 'main' panicked at 'env variable `IMPORTANT_PATH` should be set by `wrapper_script.sh`: NotPresent', src/main.rs:4:6 | ||
``` | ||
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In this example we are communicating not only the name of the | ||
environment variable that should have been set, but also an explanation | ||
for why it should have been set, and we let the source error display as | ||
a clear contradiction to our expectation. | ||
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**Hint**: If you're having trouble remembering how to phrase | ||
expect-as-precondition style error messages remember to focus on the word | ||
"should" as in "env variable should be set by blah" or "the given binary | ||
should be available and executable by the current user". | ||
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[`panic_any`]: crate::panic::panic_any | ||
[`PanicInfo`]: crate::panic::PanicInfo | ||
[`catch_unwind`]: crate::panic::catch_unwind | ||
[`resume_unwind`]: crate::panic::resume_unwind | ||
[`downcast`]: crate::error::Error | ||
[`Termination`]: crate::process::Termination | ||
[`Try`]: crate::ops::Try | ||
[panic hook]: crate::panic::set_hook | ||
[`set_hook`]: crate::panic::set_hook | ||
[`take_hook`]: crate::panic::take_hook | ||
[panic-handler]: <https://doc.rust-lang.org/nomicon/panic-handler.html> | ||
[`match`]: ../../std/keyword.match.html | ||
[`?`]: ../../std/result/index.html#the-question-mark-operator- |
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