问题

在这里，description 和 prompt 是可选的，因此它们的默认值是 None。

#[derive(Debug, Default)]
pub struct Keymap {
    pub key: char,
    pub command: String,
    pub description: Option<String>,
    pub prompt: Option<String>,
}

impl Keymap {
    pub fn new<S: AsRef<str>>(key: char, command: S) -> Self {
        Self {
            key,
            command: command.as_ref().to_owned(),
            description: None, // 默认情况下 description 是 None
            prompt: None, // 默认情况下 prompt 是 None
        }
    }

    pub fn with_prompt<S: AsRef<str>>(mut self, prompt: S) -> Self {
        self.prompt = Some(prompt.as_ref().to_owned());
        self
    }

    pub fn with_description<S: AsRef<str>>(mut self, description: S) -> Self {
        self.description = Some(description.as_ref().to_owned());
        self
    }
}

fn main() {
    let keymap = Keymap::new('a', "echo Hello, World!")
        .with_prompt("Enter your name:")
        .with_description("Prints 'Hello, World!'");

    println!("{:?}", keymap);
}

但我想要 description 的回退值是 command 的值。如果提供了 description，那么应该使用该值。

如何实现这一点？

#[derive(Debug, Default)]
pub struct Keymap {
    pub key: char,
    pub command: String,
    pub description: Option<String>,
    pub prompt: Option<String>,
}

impl Keymap {
    pub fn new<S: AsRef<str>>(key: char, command: S) -> Self {
        Self {
            key,
            command: command.as_ref().to_owned(),
            ..Default::default()
        }
    }

    pub fn with_prompt<S: AsRef<str>>(mut self, prompt: S) -> Self {
        self.prompt = Some(prompt.as_ref().to_owned());
        self
    }

    pub fn with_description<S: AsRef<str>>(mut self, description: S) -> Self {
        self.description = Some(description.as_ref().to_owned());
        self
    }
}

fn main() {
    let keymap = Keymap::new('a', "echo Hello, World!")
        .with_prompt("Enter your name:")
        .with_description("Prints 'Hello, World!'");

    println!("{:?}", keymap);
}

答案1

得分: 2

> 但我希望 `description` 的默认值是 `command` 的值。

```rust
impl Keymap {
    pub fn new<S: AsRef<str>>(key: char, command: S) -> Self {
        let command = command.as_ref().to_string();
        Self {
            key,
            description: Some(command.clone()),
            command,
            ..Default::default()
        }
    }
}

不确定为什么 description 仍然是一个 Option。

或者，就在消费者方面回退。

如果你在问是否有办法使 description 在没有显式设置的情况下自动成为 command 的副本，那么答案是否定的。不过，你可以通过将 description 保持私有并通过方法访问来模拟这种情况，例如：

pub fn description(&self) -> &str {
    self.description.as_deref().unwrap_or(&self.command)
}

在这种情况下，你可能需要手动实现 Debug。

<details>
<summary>英文:</summary>

&gt; But I want `description`&#39;s default value to be the value of `command`.

```rust
impl Keymap {
    pub fn new&lt;S: AsRef&lt;str&gt;&gt;(key: char, command: S) -&gt; Self {
        let command = command.as_ref().to_string();
        Self {
            key,
            description: Some(command.clone()),
            command,
            ..Default::default()
        }
    }
}

?

Not sure why description would still be an Option tho.

Alternatively, just fallback on the consumer side.

If you're asking if there's a way to make description magically be a copy of command unless it's explicitly set, then no. Though you could emulate that by keeping description private and accessing it via a method e.g.

    pub fn description(&self) -> &str {
        self.description.as_deref().unwrap_or(&self.command)
    }

in which case you may want to impl Debug by hand.