if let

在某些情况下，当匹配枚举时，match 会显得笨拙。例如：

#![allow(unused)]
fn main() {
// Make `optional` of type `Option<i32>`
let optional = Some(7);

match optional {
    Some(i) => {
        println!("This is a really long string and `{:?}`", i);
        // ^ Needed 2 indentations just so we could destructure
        // `i` from the option.
    },
    _ => {},
    // ^ Required because `match` is exhaustive. Doesn't it seem
    // like wasted space?
};

}

对于这种用例，if let 更简洁，并且还可以指定各种失败选项。

fn main() {
    // All have type `Option<i32>`
    let number = Some(7);
    let letter: Option<i32> = None;
    let emoticon: Option<i32> = None;

    // The `if let` construct reads: "if `let` destructures `number` into
    // `Some(i)`, evaluate the block (`{}`).
    if let Some(i) = number {
        println!("Matched {:?}!", i);
    }

    // If you need to specify a failure, use an else:
    if let Some(i) = letter {
        println!("Matched {:?}!", i);
    } else {
        // Destructure failed. Change to the failure case.
        println!("Didn't match a number. Let's go with a letter!");
    }

    // Provide an altered failing condition.
    let i_like_letters = false;

    if let Some(i) = emoticon {
        println!("Matched {:?}!", i);
    // Destructure failed. Evaluate an `else if` condition to see if the
    // alternate failure branch should be taken:
    } else if i_like_letters {
        println!("Didn't match a number. Let's go with a letter!");
    } else {
        // The condition evaluated false. This branch is the default:
        println!("I don't like letters. Let's go with an emoticon :)!");
    }
}

同样，if let 可以用来匹配任何枚举值。

// Our example enum
enum Foo {
    Bar,
    Baz,
    Qux(u32)
}

fn main() {
    // Create example variables
    let a = Foo::Bar;
    let b = Foo::Baz;
    let c = Foo::Qux(100);
    
    // Variable a matches Foo::Bar
    if let Foo::Bar = a {
        println!("a is foobar");
    }
    
    // Variable b does not match Foo::Bar
    // So this will print nothing
    if let Foo::Bar = b {
        println!("b is foobar");
    }
    
    // Variable c matches Foo::Qux which has a value
    // Similar to Some() in the previous example
    if let Foo::Qux(value) = c {
        println!("c is {}", value);
    }

    // Binding also works with `if let`
    if let Foo::Qux(value @ 100) = c {
        println!("c is one hundred");
    }
}

另一个好处是 if let 允许我们匹配非参数化枚举变体。即使在枚举没有实现或派生 PartialEq 的情况下也是如此。在这种情况下，if Foo::Bar == a 将无法编译，因为枚举的实例无法进行相等比较，但是 if let 将继续有效。

想要挑战一下吗？请修改以下示例以使用 if let。

// This enum purposely neither implements nor derives PartialEq.
// That is why comparing Foo::Bar == a fails below.
enum Foo {Bar}

fn main() {
    let a = Foo::Bar;

    // Variable a matches Foo::Bar
    if Foo::Bar == a {
    // ^-- this causes a compile-time error. Use `if let` instead.
        println!("a is foobar");
    }
}

另请参阅

enum、Option 和 RFC