我想创建一个宏,打印“Hello”一个指定的次数。它的使用,如:
many_greetings!(3); // expands to three `println!("Hello");` statements
创建宏用简单的方式是:
macro_rules! many_greetings {
($times:expr) => {{
println!("Hello");
many_greetings!($times - 1);
}};
(0) => ();
}
然而,这并不工作,因为编译器不计算表达式; $times - 1
不计算,而是供给作为新表达成宏。
而普通的宏系统不允许您重复宏扩展了很多次,还有与使用在宏观循环没有问题:
macro_rules! many_greetings {
($times:expr) => {{
for _ in 0..$times {
println!("Hello");
}
}};
}
如果你真的需要重复宏,你不得不考虑程序的宏/ compiler plugins(其为1.4是不稳定的,并且有点难以写)。
编辑:有可能实现这个更好的方式,但我已经花了足够长的时间对这个在今天,所以这里去。 repeat!
,实际复制的代码的次数的块的宏:
#![feature(plugin)]
#![plugin(repeat)]
fn main() {
let mut n = 0;
repeat!{ 4 {
println!("hello {}", n);
n += 1;
}};
}
#![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);
}
[lib]
name = "repeat"
plugin = true
需要注意的是,如果我们真的不想做循环,但在编译时扩大,我们必须做的事情一样,需要字面数字。毕竟,我们无法评估变量,在编译时参考该计划的其他部分函数调用。
据我所知,没有。宏语言是基于模式匹配和变量替换,并且仅评估宏。
现在,你可以实现与评价统计:它只是无聊......看the playpen
macro_rules! many_greetings {
(3) => {{
println!("Hello");
many_greetings!(2);
}};
(2) => {{
println!("Hello");
many_greetings!(1);
}};
(1) => {{
println!("Hello");
many_greetings!(0);
}};
(0) => ();
}
在此基础上,我敢肯定一个能发明一套宏“计数”,并在每一步(与计)调用各种操作。
至于其他的答案已经说了:不,你不能指望像这样用声明宏(macro_rules!
)。
但是你可以实现many_greetings!
例如,作为程序的宏。宏程序前一阵子被稳定下来,所以定义适用于稳定。然而,我们还不能扩展到宏在稳定的声明 - 这是#![feature(proc_macro_hygiene)]
是什么。
这看起来像一个大量的代码,但大多数代码只是错误处理,所以它不是那么复杂!
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3“你好” S。