什么是有和无? [英] What are Some and None?
问题描述
我在使用 Vec::get 时遇到了一些我不理解的输出.代码如下:
I came across some output I don't understand using Vec::get. Here's the code:
fn main() {
let command = [('G', 'H'), ('H', '5')];
for i in 0..3 {
print!(" {} ", i);
println!("{:?}", command.get(i));
}
}
输出是
0 Some(('G', 'H'))
1 Some(('H', '5'))
2 None
我之前接触过 Haskell，我的意思是在一个教程网站上看了 10 分钟，然后又跑回 C++，但我记得我读过一些关于 Some 和 None 用于 Haskell.我很惊讶在 Rust 中看到这个.有人能解释一下为什么 .get() 返回 Some 或 None 吗?
I've dabbled in Haskell before, and by that I mean looked at a tutorial site for 10 minutes and ran back to C++, but I remember reading something about Some and None for Haskell. I was surprised to see this here in Rust. Could someone explain why .get() returns Some or None?
推荐答案
get 的签名(对于 slice，not Vec，因为您正在使用数组/切片)是
The signature of get (for slices, not Vec, since you're using an array/slice) is
fn get(&self, index: usize) -> Option<&T>
也就是说，它返回一个 Option，这是一个像
That is, it returns an Option, which is an enum defined like
pub enum Option<T> {
None,
Some(T),
}
None 和 Some 是枚举的变体，即Option类型的值code> 可以是 None，也可以是包含 T 类型值的 Some.您也可以使用变体创建 Option 枚举:
None and Some are the variants of the enum, that is, a value with type Option<T> can either be a None, or it can be a Some containing a value of type T. You can create the Option enum using the variants as well:
let foo = Some(42);
let bar = None;
这个和核心一样data 也许a = Nothing |只是一个 Haskell 类型；两者都代表一个可选值，要么存在 (Some/Just)，要么不存在 (None/Nothing).
This is the same as the core data Maybe a = Nothing | Just a type in Haskell; both represent an optional value, it's either there (Some/Just), or it's not (None/Nothing).
这些类型通常用于表示失败的原因只有一种可能性，例如，.get 使用 Option 来提供类型安全的边界检查数组访问:当索引越界时返回None(即无数据)，否则返回包含请求指针的Some.
These types are often used to represent failure when there's only one possibility for why something failed, for example, .get uses Option to give type-safe bounds-checked array access: it returns None (i.e. no data) when the index is out of bounds, otherwise it returns a Some containing the requested pointer.
另见:
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