小编Ole*_*osh的帖子

我有一个枚举：

enum Operation {
    Add,
    Subtract,
}

impl Operation {
    fn from(s: &str) -> Result<Self, &str> {
        match s {
            "+" => Ok(Self::Add),
            "-" => Ok(Self::Subtract),
            _ => Err("Invalid operation"),
        }
    }
}

我想确保在编译时在from函数中处理每个枚举变量。

我为什么需要这个？例如，我可能添加一个Product操作而忘记在from函数中处理这种情况：

enum Operation {
    // ...
    Product,
}

impl Operation {
    fn from(s: &str) -> Result<Self, &str> {
        // No changes, I forgot to add a match arm for `Product.
        match s {
            "+" => Ok(Self::Add),
            "-" => Ok(Self::Subtract),
            _ …

Original question

I need an utility type Subtract<A, B> where A and B are numbers. For example:

type Subtract<A extends number, B extends number> = /* implementation */ 
const one: Subtract<2, 1> = 1
const two: Subtract<4, 2> = 2
const error: Subtract<2, 1> = 123 // Error here: 123 is not assignable to type '1'.

Arguments to the Subtract<A, B> are always number literals or compile time constants. I do not need

let foo: Subtract<number, number> // 'foo' may …