15

我想创建一个打印“Hello”指定次数的宏。它的用法如下:

many_greetings!(3);  // expands to three `println!("Hello");` statements

创建该宏的简单方法是:

macro_rules! many_greetings {
    ($times:expr) => {{
        println!("Hello");
        many_greetings!($times - 1);
    }};
    (0) => ();
}

但是,这不起作用,因为编译器不计算表达式;$times - 1不是计算的,而是作为新表达式输入宏。

4

4 回答 4

8

虽然普通的宏系统不能让你多次重复宏展开,但在宏中使用 for 循环是没有问题的:

macro_rules! many_greetings {
    ($times:expr) => {{
        for _ in 0..$times {
            println!("Hello");
        }
    }};
}

如果你真的需要重复宏,你必须查看过程宏/编译器插件(从 1.4 开始,它们是不稳定的,并且更难编写)。

编辑:可能有更好的方法来实现这一点,但我今天在这方面花了足够长的时间,所以就这样吧。repeat!,一个实际上多次复制代码块的宏:

main.rs

#![feature(plugin)]
#![plugin(repeat)]

fn main() {
    let mut n = 0;
    repeat!{ 4 {
        println!("hello {}", n);
        n += 1;
    }};
}

库文件

#![feature(plugin_registrar, rustc_private)]

extern crate syntax;
extern crate rustc;

use syntax::codemap::Span;
use syntax::ast::TokenTree;
use syntax::ext::base::{ExtCtxt, MacResult, MacEager, DummyResult};
use rustc::plugin::Registry;
use syntax::util::small_vector::SmallVector;
use syntax::ast::Lit_;
use std::error::Error;

fn expand_repeat(cx: &mut ExtCtxt, sp: Span, tts: &[TokenTree]) -> Box<MacResult + 'static> {
    let mut parser = cx.new_parser_from_tts(tts);
    let times = match parser.parse_lit() {
        Ok(lit) => match lit.node {
            Lit_::LitInt(n, _) => n,
            _ => {
                cx.span_err(lit.span, "Expected literal integer");
                return DummyResult::any(sp);
            }
        },
        Err(e) => {
            cx.span_err(sp, e.description());
            return DummyResult::any(sp);
        }
    };
    let res = parser.parse_block();

    match res {
        Ok(block) => {
            let mut stmts = SmallVector::many(block.stmts.clone());
            for _ in 1..times {
                let rep_stmts = SmallVector::many(block.stmts.clone());
                stmts.push_all(rep_stmts);
            }
            MacEager::stmts(stmts)
        }
        Err(e) => {
            cx.span_err(sp, e.description());
            DummyResult::any(sp)
        }
    }
}

#[plugin_registrar]
pub fn plugin_registrar(reg: &mut Registry) {
    reg.register_macro("repeat", expand_repeat);
}

添加到 Cargo.toml

[lib]
name = "repeat"
plugin = true

请注意,如果我们真的不想进行循环,而是在编译时扩展,我们必须做一些事情,比如要求文字数字。毕竟,我们无法在编译时评估引用程序其他部分的变量和函数调用。

于 2015-11-17T18:30:43.127 回答
5

正如其他答案已经说过的那样:不,您不能像这样使用声明性宏 ( macro_rules!)来计算。


但是您可以将该many_greetings!示例实现为过程宏。程序宏在不久前已经稳定,因此该定义适用于稳定。但是,我们还不能将宏扩展为 stable 上的语句——这就是它的#![feature(proc_macro_hygiene)]用途。

这看起来代码很多,但大多数代码只是错误处理,所以并没有那么复杂!

examples/main.rs

#![feature(proc_macro_hygiene)]

use count_proc_macro::many_greetings;

fn main() {
    many_greetings!(3);
}

Cargo.toml

[package]
name = "count-proc-macro"
version = "0.1.0"
authors = ["me"]
edition = "2018"

[lib]
proc-macro = true

[dependencies]
quote = "0.6"

src/lib.rs

extern crate proc_macro;

use std::iter;
use proc_macro::{Span, TokenStream, TokenTree};
use quote::{quote, quote_spanned};


/// Expands into multiple `println!("Hello");` statements. E.g.
/// `many_greetings!(3);` will expand into three `println`s.
#[proc_macro]
pub fn many_greetings(input: TokenStream) -> TokenStream {
    let tokens = input.into_iter().collect::<Vec<_>>();

    // Make sure at least one token is provided.
    if tokens.is_empty() {
        return err(Span::call_site(), "expected integer, found no input");
    }

    // Make sure we don't have too many tokens.
    if tokens.len() > 1 {
        return err(tokens[1].span(), "unexpected second token");
    }

    // Get the number from our token.
    let count = match &tokens[0] {
        TokenTree::Literal(lit) => {
            // Unfortunately, `Literal` doesn't have nice methods right now, so
            // the easiest way for us to get an integer out of it is to convert
            // it into string and parse it again.
            if let Ok(count) = lit.to_string().parse::<usize>() {
                count
            } else {
                let msg = format!("expected unsigned integer, found `{}`", lit);
                return err(lit.span(), msg);
            }
        }
        other => {
            let msg = format!("expected integer literal, found `{}`", other);
            return err(other.span(), msg);
        }
    };

    // Return multiple `println` statements.
    iter::repeat(quote! { println!("Hello"); })
        .map(TokenStream::from)
        .take(count)
        .collect()
}

/// Report an error with the given `span` and message.
fn err(span: Span, msg: impl Into<String>) -> TokenStream {
    let msg = msg.into();
    quote_spanned!(span.into()=> {
        compile_error!(#msg);
    }).into()
}

运行cargo run --example main打印三个“Hello”。

于 2019-01-24T16:19:36.607 回答
4

据我所知,没有。宏语言基于模式匹配和变量替换,并且只评估宏。

现在,您可以通过评估来实现计数:这很无聊……看游戏围栏

macro_rules! many_greetings {
    (3) => {{
        println!("Hello");
        many_greetings!(2);
    }};
    (2) => {{
        println!("Hello");
        many_greetings!(1);
    }};
    (1) => {{
        println!("Hello");
        many_greetings!(0);
    }};
    (0) => ();
}

基于此,我很确定可以发明一组宏来“计数”并在每个步骤(使用计数)调用各种操作。

于 2015-11-17T08:22:39.503 回答
3

对于那些寻找方法的人,还有seq_macro crate

它相当容易使用,并且与稳定的 Rust 一起开箱即用。

use seq_macro::seq;

macro_rules! many_greetings {
    ($times:literal) => {
        seq!{ N in 0..$times {
            println!("Hello");
        }}
    };
}

fn main() {
    many_greetings!(3);
    many_greetings!(12);
}
于 2021-07-15T12:57:40.110 回答