Rust 宏规则中的 S 表达式

Question

我正在编写自己的语言编译器，我想用我的宏将 AST 描述为 S 表达式。

以下是不起作用的最小示例代码。

#[derive(Debug, PartialEq)]
pub enum Expression {
    BinOP(Box<Expression>, OP, Box<Expression>),
    Number(f64),
}

#[derive(Debug, PartialEq)]
pub enum OP {
    Add,
}

macro_rules! ast {
    (+ $left:tt $right:tt) => {
        Expression::BinOP(Box::new(ast!($left)), OP::Add, Box::new(ast!($right)))
    };
    ($other:tt) => {
        Expression::from($other)
    };
}

impl From<usize> for Expression {
    fn from(u: usize) -> Self {
        Expression::Number(u as f64)
    }
}

fn main() {
    dbg!(ast!(+ 1 2)); // this works.
    dbg!(ast!(+ (+ 3 4) 2)); // error: expected expression, found `+`
              // ^ expected expression
}

Answer 1

您需要一个单独的宏来解析参数。试试这个代码：

#[derive(Debug, PartialEq)]
pub enum Expression {
    BinOP(Box<Expression>, OP, Box<Expression>),
    Number(f64),
}

#[derive(Debug, PartialEq)]
pub enum OP {
    Add,
}

macro_rules! ast {
    (+ $left:tt $right:tt) => {
        Expression::BinOP(Box::new(ast_arg!($left)), OP::Add, Box::new(ast_arg!($right)))
    };
}

#[macro_export]
macro_rules! ast_arg {
    ( ( $e:tt ) ) => (ast!($e));
    ( ( $($e:tt)* ) ) => ( ast!( $($e)* ) );
    ($e:expr) => (Expression::from($e));
}

impl From<usize> for Expression {
    fn from(u: usize) -> Self {
        Expression::Number(u as f64)
    }
}

fn main() {
    dbg!(ast!(+ 1 2)); // this works
    dbg!(ast!(+ (+ 3 4) 2)); // also works now
}

游乐场链接：https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=e35fcd9aa7b126cd34aea6b33857e6c9

如果您想使用宏创建类 Lisp 语言，请查看此项目：https://github.com/JunSuzukiJapan/macro-lisp