Read from an enum without pattern matching

问题

The Rust documentation gives this example where we have an instance of Result<T, E> named some_value:

match some_value {
    Ok(value) => println!("got a value: {}", value),
    Err(_) => println!("an error occurred"),
}

Is there any way to read from some_value without pattern matching? What about without even checking the type of the contents at runtime? Perhaps we somehow know with absolute certainty what type is contained or perhaps we're just being a bad programmer. In either case, I'm just curious to know if it's at all possible, not if it's a good idea.

It strikes me as a really interesting language feature that this branch is so difficult (or impossible?) to avoid.

回答1:

At the lowest level, no, you can't read enum fields without a match¹.

Methods on an enum can provide more convenient access to data within the enum (e.g. Result::unwrap), but under the hood, they're always implemented with a match.

If you know that a particular case in a match is unreachable, a common practice is to write unreachable!() on that branch (unreachable!() simply expands to a panic!() with a specific message).

---

¹ If you have an enum with only one variant, you could also write a simple let statement to deconstruct the enum. Patterns in let statements must be exhaustive, and a pattern matching the single variant from an enum is exhaustive. But enums with only one variant are pretty much never used; a struct would do the job just fine. And if you intend to add variants later, you're better off writing a match right away.

enum Single {
    S(i32),
}

fn main() {
    let a = Single::S(1);
    let Single::S(b) = a;
    println!("{}", b);
}

On the other hand, if you have an enum with more than one variant, you can also use if let and while let if you're interested in the data from a single variant only. While let requires an exhaustive pattern, if let and while let require a non-exhaustive pattern. You'll often see them used with Option:

fn main() {
    if let Some(x) = std::env::args().len().checked_add(1) {
        println!("{}", x);
    } else {
        println!("too many args :(");
    }
}

来源：https://stackoverflow.com/questions/45473656/read-from-an-enum-without-pattern-matching