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Add a section on recursive macros

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Keegan McAllister 2015-02-20 12:12:25 -08:00
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src/doc/trpl/macros.md
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@ -357,6 +357,45 @@ fn main() {
[items]: ../reference.html#items
# Recursive macros
A macro's expansion can include more macro invocations, including invocations
of the very same macro being expanded. These recursive macros are useful for
processing tree-structured input, as illustrated by this (simplistic) HTML
shorthand:
```rust
# #![allow(unused_must_use)]
macro_rules! write_html {
($w:expr, ) => (());
($w:expr, $e:tt) => (write!($w, "{}", $e));
($w:expr, $tag:ident [ $($inner:tt)* ] $($rest:tt)*) => {{
write!($w, "<{}>", stringify!($tag));
write_html!($w, $($inner)*);
write!($w, "</{}>", stringify!($tag));
write_html!($w, $($rest)*);
}};
}
fn main() {
# // FIXME(#21826)
use std::fmt::Write;
let mut out = String::new();
write_html!(&mut out,
html[
head[title["Macros guide"]]
body[h1["Macros are the best!"]]
]);
assert_eq!(out,
"<html><head><title>Macros guide</title></head>\
<body><h1>Macros are the best!</h1></body></html>");
}
```
# Further reading
The [advanced macros chapter][] goes into more detail about macro syntax. It