使用枚举对结构进行分组

在 Rust 中，应该如何对相关结构进行分组，以便函数签名可以接受多种不同类型，同时引用方法体内的具体类型？

为了简单起见，设计了以下示例：

enum Command {
    Increment {quantity: u32},
    Decrement {quantity: u32},
}

fn process_command(command: Command) {
    match command {
        Command::Increment => increase(command),
        Command::Decrement => decrease(command),
    };
}

fn increase(increment: Command::Increment) {
    println!("Increasing by: {}.", increment.quantity);
}

fn decrease(decrement: Command::Decrement) {
    println!("Decreasing by: {}.", decrement.quantity);
}

fn main() {
    let input = "Add";
    let quantity = 4;

    let command: Command = match input {
        "Add" => Command::Increment { quantity: quantity },
        "Subtract" => Command::Decrement { quantity: quantity },
        _ => unreachable!(),
    };

    process_command(command);
}

编译结果出现以下两个错误：

src/main.rs:13:24: 13:42 error: found value name used as a type: DefVariant(DefId { krate: 0, node: 4 }, DefId { krate: 0, node: 5 }, true) [E0248]
src/main.rs:13 fn increase(increment: Command::Increment) {
                                      ^~~~~~~~~~~~~~~~~~
src/main.rs:17:24: 17:42 error: found value name used as a type: DefVariant(DefId { krate: 0, node: 4 }, DefId { krate: 0, node: 8 }, true) [E0248]
src/main.rs:17 fn decrease(decrement: Command::Decrement) {
                                      ^~~~~~~~~~~~~~~~~~
error: aborting due to 2 previous errors

如果我单独声明结构，并将结构包装在枚举中的元组结构（正确的术语？）中，那么我会得到预期的结果，但是由于冗长和类似的类型名称遍布各处，我怀疑我误解了某事：

struct Increment {
    quantity: u32,
}

struct Decrement {
    quantity: u32,
}

enum Command {
    Increment(Increment),
    Decrement(Decrement),
}

fn process_command(command: Command) {
    match command {
        Command::Increment(increment) => increase(increment),
        Command::Decrement(decrement) => decrease(decrement),
    };
}

fn increase(increment: Increment) {
    println!("Increasing by: {}.", increment.quantity);
}

fn decrease(decrement: Decrement) {
    println!("Decreasing by: {}.", decrement.quantity);
}

fn main() {
    let input = "Add";
    let quantity = 4;

    let command: Command = match input {
        "Add" => Command::Increment(Increment { quantity: quantity }),
        "Subtract" => Command::Decrement(Decrement { quantity: quantity }),
        _ => unreachable!(),
    };

    process_command(command);
}

运行输出：

Increasing by: 4.

将结构包装在共享相同名称的枚举类型（术语？）中真的是最好的解决方案吗？Command::Increment(Increment { quantity: 7 })

是的，这是您沿着这条实施路线所能得到的最好的结果。枚举只是一种类型；它的变体纯粹是变体，而不是类型。

另一种选择是使用特征和泛型：

struct Increment {
    quantity: u32,
}

struct Decrement {
    quantity: u32,
}

trait Command {
    fn process(self);
}

impl Command for Increment {
    fn process(self) {
        println!("Increasing by {}", self.quantity);
    }
}

impl Command for Decrement {
    fn process(self) {
        println!("Decreasing by {}", self.quantity);
    }
}

当然，这并不是直接的类比。如果你想存储一个command您需要更改的可能不同的类型process采取self: Box<Self> or &self，并且您需要使用Box<Command> or &Command，但这是另一种可能适合您要求的做事方式。就定义而言，它更纯粹。