Macro for inline definition of anonymous functions, and examples of passing functions.

l.rs

fn once(f: Box<Fn()>) { f() }

fn twice(f: fn()) { f(); f() }

fn thrice<F: Fn()>(f: F) { f(); f(); f() }

// Inline definition of anonymous functions. Examples:
// l!(42;)
// l!(i32 := 42)
// l!(i: i32 := i + 42)
// l!(i: i32, j: i32 -> i32 := i + j)
macro_rules! l {
    ($body: expr) => ({ fn f() { $body } f });
    ($res: ty := $body: expr) => ({ fn f() -> $res { $body } f });
    ($($n: ident: $t: ty),+ := $body: expr) => ({
        fn f($($n: $t),+) { $body } f
    });
    ($($n: ident: $t: ty),+ -> $res: ty := $body: expr) => ({
        fn f($($n: $t),+) -> $res { $body } f
    })
}

fn main() {
    once(Box::new(|| println!("Once!")));
    twice(l!(println!("Twice!")));
    thrice(|| println!("Thrice!"));
    
    let inc = l!(i: i32 -> i32 := i + 1);
    println!("{}", inc(inc(40)));
}