Types
- Rust course
- Advanced
- Lesson 44/81
- 4 min
Write macros with type parameters like $ty:ty.
Fragment types
The fragment type says which code piece the macro expects:
$x:expr // expression: 1 + 2, f(x)
$x:ident // identifier: ad, Kutu
$x:ty // type: u32, Vec<String>
$x:pat // pattern: Some(_), 1..=5
$x:block // block: { .. }
$x:item // item: fn, struct, impl
$x:literal // literal: 42, "metin"
$x:tt // a single token tree (anything)ident and ty let macros generate function and type definitions; this is impossible with functions.
An item-generating macro
To reduce repetitive definitions to a single line:
macro_rules! sabit_fn {
($ad:ident, $tur:ty, $deger:expr) => {
fn $ad() -> $tur {
$deger
}
};
}
sabit_fn!(surum, &'static str, "1.0");
sabit_fn!(port, u16, 8080);
println!("{} {}", surum(), port());The standard library generates dozens of trait implementations for number types with macros like this.
Practice Function-generating macro
Write the sabit_fn! macro above and generate the functions surum (&'static str, "1.0") and port (u16, 8080). Print the results of both functions separated by a space.
Starter code
fn main() {
// buraya yaz
}Sample solution
macro_rules! sabit_fn {
($ad:ident, $tur:ty, $deger:expr) => {
fn $ad() -> $tur {
$deger
}
};
}
sabit_fn!(surum, &'static str, "1.0");
sabit_fn!(port, u16, 8080);
fn main() {
println!("{} {}", surum(), port());
}Expected output
1.0 8080
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which fragment is needed for a macro to generate a new function name?
- literal
- ident Correct answer
- expr
- ty
Explanation: A name taken with $ad:ident is used in the body as fn $ad().
2 Type fragment
Question
macro_rules! sabit_fn {
($ad:ident, $tur:____, $deger:expr) => {
fn $ad() -> $tur { $deger }
};
}
Answer
macro_rules! sabit_fn {
($ad:ident, $tur:ty, $deger:expr) => {
fn $ad() -> $tur { $deger }
};
}Explanation: The ty fragment matches a type.
3 Fragment and example
| Term | Meaning |
|---|---|
ident |
surum |
ty |
&'static str |
expr |
"1.0" |
pat |
Some(_) |