如何在 rust 宏中解析单个标记

Question

如何在 rust 宏中解析单个标记

3

我开始尝试使用Rust宏，来做这个小练习。我想定义一个宏，该宏扩展为初始化变量（名称不重要）类型为（例如，但不是真正重要）和对该变量的一系列操作，在这种情况下是var += 1或var -= 1，最后会调用println!("{}", var)。

该宏将基于+和-匹配上述操作的一系列标记。

例如：

operate_integer![+++---]

应该扩展为：

let mut var: i32 = 0;
var += 1;
var += 1;
var += 1;
var -= 1;
var -= 1;
var -= 1;
print!("{}", var);

我将使用两个宏来完成此操作，一个用于包装初始化和打印，另一个用于评估+-标记：

基本宏如下：

macro_rules! operate_integer {
    // $($all_tokens:tt)* should match everything, it will be forward to the helper macro
    ($($all_tokens:tt)*) => {
        let mut var : i32 = 0;
        operate_integer_helper![$($all_tokens:tt)*]
        print!("{}", var);
    }
}

辅助程序将扩展操作：

macro_rules! operate_integer_helper {
    // the idea is that it matches a `+` followed by more tokens
    (+$($t:tt)*) => {
        val += 1;
        operate_integer_helper![$($t:tt)*] // we recursively handle the remaining tokens
    }

    (-$($t:tt)*) => {
        val -= 1;
        operate_integer_helper![$($t:tt)*]
    }
}

当然，这并不起作用，它会因以下错误而编译失败(Playground)：

error: no rules expected the token `(`
   --> src/lib.rs:102:5
    |
102 |     (+$($t:tt)*) => {
    |     ^ no rules expected this token in macro call

我有些困惑。由于我刚开始接触，可能会错过许多概念，因此我真的很希望能够获得一些帮助，了解如何使用宏。

提前感谢您！

- Netwave

1个回答

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- Lukas Kalbertodt · Accepted Answer

你实际上已经非常接近成功了！只剩下一些小错误。(如果你想了解更多有关宏的内容，每次只读一个要点，并尝试自己逐步进展！)

When using (repeated) meta variables, you don't specify the meta-variable type again. So it's $($t:tt)* in the pattern of the macro, but if you want to use it, it's $($t)*!
If you have multiple rules in a macro definition, you need to end every rule with a semicolon.
```
macro_rules! {
    (+ $(t:tt)*) => { ... };
    (- $(t:tt)*) => { ... };
}
```
The Rust compiler always needs to know whether you want to expand your macro into an expression or statement(s). Since you are generating a list of statements and not a single expression, you have to add a semicolon to the invocation of your macros! That means, in main() but also all macro invocation of helper macros inside your macro definition.

Since yeah macro invocation creates a new syntax context and all identifiers (names) are only accessible in their syntax context, the helper macro cannot use var (even after fixing the typo val -> var). So instead, you have to pass that name to the helper macro:

macro_rules! operate_integer {
    ($($all_tokens:tt)*) => {
        let mut var: i32 = 0;
        operate_integer_helper![var $($all_tokens)*];  // <-- pass identifier 
        println!("{}", var);
    }
}

macro_rules! operate_integer_helper {
    ($var:ident +$($t:tt)*) => {              // <- accept identifier
        $var += 1;                            // <- use identifier
        operate_integer_helper![$var $($t)*]
    };

    ($var:ident -$($t:tt)*) => {
        $var -= 1;
        operate_integer_helper![$var $($t)*]
    };
}

Having done all that you get the error "unexpected end of macro invocation". This is because you don't have a recursion stop rule! So you have to add a new rule to your helper macro: ($var:ident) => {};. This rule is used when there is only the name and no + or - tokens left.

现在: 它已经工作了！

我还想改变最后一件事情:通常不建议使用第二个帮助宏，因为该宏可能不在调用主要宏的范围内。相反，通常使用内部规则。你可以在这里阅读更多关于内部规则的内容。

有了这个，这是生成的代码。