# duang
Rust doesn't support default function arguments and named function arguments.
This crate generate an macro interface for a given function that can be invoked with named arguments and fill the default argument, meanwhile keep the old function intact.
## Generating macro interface
In order to generate macro for a function, we just need to wrap the definition with `duang!{...}`.
```rust
use duang::duang;
duang!(
pub fn foo<T>(a: T,b: f64 = 13.0, c: T = a*a) -> (T,f64,T)
where
T: std::ops::Mul<T, Output = T>,
T: std::fmt::Display,
T: Copy,
{
(a,b,c)
}
);
```
## Invoke
```rust
use demo_duang::foo;
// pass
assert_eq!(foo!(1, c = 30, b = -2.0), (1, -2.0, 30));
// pass
assert_eq!(foo!(a = 10), (10, 13.0, 100));
// fail
// foo!(1,c=30,c=2);
```
## Features
- Support generics, existensial type.
- Friendly error message.
## Common issues
### Use local variable in default value.
In order to use the generated macro in other crate, users should add `$crate` and path of the variable used.
Also, the variable should be visible(`pub`) for the scope where the macro is invoked.
```rust
mod bar {
use duang::duang;
pub static NUM: i32 = 42;
duang!(
pub fn foo(a: i32 = $crate::bar::NUM) -> i32 { a }
);
}
fn main() {
use bar::foo;
assert_eq!(foo!(), 42);
}
```
## Limitations
- Don't support associated function.
- Wildchar can not be used in pattern argument. For example `fn foo((a,_): (i32, i32))` is illegal.
## TODO
- Generate document for function or macro.
- After "Attributes in formal function parameter position"([#60406](https://github.com/rust-lang/rust/issues/60406)) stabilize, change function-like macros to attribute-like macros.
License: MIT