rust - 为什么将枚举变体与其名称和类型匹配不起作用？

Question

考虑以下示例：

pub enum DigitalWallet {
    WithMemo {
        currency: String,
        address: String,
        tag: String,
    },
    WithoutMemo {
        currency: String,
        address: String,
    },
}

impl<'a> DigitalWallet {
    fn getCurrency(self: &'a Self) -> &'a String {
        match self {
            DigitalWallet::WithMemo {
                currency: String, ..
            } => currency,
            DigitalWallet::WithoutMemo {
                currency: String, ..
            } => currency,
        }
    }
}

为什么会导致以下结果？

error[E0425]: cannot find value `currency` in this scope
  --> src/lib.rs:18:18
   |
18 |             } => currency,
   |                  ^^^^^^^^ not found in this scope

error[E0425]: cannot find value `currency` in this scope
  --> src/lib.rs:21:18
   |
21 |             } => currency,
   |                  ^^^^^^^^ not found in this scope

标签： rust

Answer 1

你有语法错误。类型永远不会在模式中重复。

字段模式的语法是field_name: binding_name（并且field_name: field_name可以简化为field_name）：

struct Foo {
    foo: i32,
}

fn main() {
    let foo = Foo { foo: 42 };

    match foo {
        Foo {
            foo: local_name_for_that_foo,
        } => {
            println!("{}", local_name_for_that_foo);
        }
    }
}

（永久链接到操场）

因此你需要

match self {
    DigitalWallet::WithMemo { currency, .. } => currency,
    DigitalWallet::WithoutMemo { currency, .. } => currency,
}