当两个选项包含可以测试相等的值时,测试两个选项是否相等的最惯用的方法是什么? [英] What's the most idiomatic way to test two Options for equality when they contain values which can be tested for equality?
问题描述
我有两种可以测试相等性的类型.然而,一旦我将它们包装在 Option
中,各种 PartialEq
实现所提供的细节就消失了.我必须使用 map
或以其他方式转换它们.
I have two types that can be tested for equality. However, once I wrap them in Option
s, the nicety provided by various PartialEq
implementations goes right out the window. I have to use map
or otherwise convert them.
例如,让我们使用 &str
和 String
:
As an example, let's use &str
and String
:
fn main() {
let a = "hello";
let b = "hello".to_owned();
assert_eq!(a, b); // Just fine
let a = Some(a);
let b = Some(b);
// error: mismatched types
assert_eq!(a, b);
// error: mismatched types
assert_eq!(a, b.as_ref());
// works, but highly tied to strings or slices,
// requires me to remember which is which
assert_eq!(a, b.as_ref().map(|x| &x[..]));
}
肯定有一种更简单或更直接的方法来做到这一点吗?
There surely must be a simpler or more straight-forward way to do this?
附带问题——是什么阻止了 Option
更广泛地实现 PartialEq
?我在猜测一致性,我的宿敌.
Side-question — what prevents Option
from implementing PartialEq
a bit more broadly? I'm guessing coherence, my old nemesis.
impl<T, U> PartialEq<Option<U>> for Option<T>
where
T: PartialEq<U>,
There's some chatter about this in the RFCs and Rust issues (1, 2).
推荐答案
从 Rust 1.40 开始,您可以使用 as_deref()
这样你就不必记住什么是什么:
As of Rust 1.40, you can use as_deref()
so you don't have to remember what is what:
assert_eq!(a.as_deref(), b.as_deref());
在 Rust 1.40 之前,我会这样做:
Before Rust 1.40, I would do something like this:
match (&a, &b) {
(Some(a), Some(b)) => assert_eq!(a, b),
(None, None) => (),
_ => panic!("a and b not equal"),
}
<小时>
另一个选项是自定义断言,基于 assert_eq!
:
macro_rules! cmp_eq_option {
($left:expr, $right:expr) => {{
match (&$left, &$right) {
(Some(left_val), Some(right_val)) => *left_val == *right_val,
(None, None) => true,
_ => false,
}
}};
}
#[macro_export]
macro_rules! assert_eq_option {
($left:expr, $right:expr) => ({
if !cmp_eq_option!($left, $right) {
panic!(r#"assertion failed: `(left == right)`
left: `{:?}`,
right: `{:?}`"#, $left, $right)
}
});
($left:expr, $right:expr,) => ({
assert_eq_option!($left, $right)
});
($left:expr, $right:expr, $($arg:tt)+) => ({
if !cmp_eq_option!($left, $right) {
panic!(r#"assertion failed: `(left == right)`
left: `{:?}`,
right: `{:?}`: {}"#, $left, $right, format_args!($($arg)+))
}
});
}
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