Macro cond 

macro_rules! cond {
    ($i:expr, $cond:expr, $submac:ident!( $($args:tt)* )) => { ... };
    ($i:expr, $cond:expr, $f:expr) => { ... };
}

Conditionally execute the given parser.

If you are familiar with nom, this is nom’s cond_with_error parser.

• Syntax: cond!(CONDITION, THING)
• Output: Some(THING) if the condition is true, else None

extern crate syn;
#[macro_use] extern crate synom;

use syn::parse::boolean;

// Parses a tuple of booleans like `(true, false, false)`, possibly with a
// dotdot indicating omitted values like `(true, true, .., true)`. Returns
// separate vectors for the booleans before and after the dotdot. The second
// vector is None if there was no dotdot.
named!(bools_with_dotdot -> (Vec<bool>, Option<Vec<bool>>), do_parse!(
    punct!("(") >>
    before: separated_list!(punct!(","), boolean) >>
    after: option!(do_parse!(
        // Only allow comma if there are elements before dotdot, i.e. cannot
        // be `(, .., true)`.
        cond!(!before.is_empty(), punct!(",")) >>
        punct!("..") >>
        after: many0!(preceded!(punct!(","), boolean)) >>
        // Only allow trailing comma if there are elements after dotdot,
        // i.e. cannot be `(true, .., )`.
        cond!(!after.is_empty(), option!(punct!(","))) >>
        (after)
    )) >>
    // Allow trailing comma if there is no dotdot but there are elements.
    cond!(!before.is_empty() && after.is_none(), option!(punct!(","))) >>
    punct!(")") >>
    (before, after)
));

fn main() {
    let input = "(true, false, false)";
    let parsed = bools_with_dotdot(input).expect("bools with dotdot");
    assert_eq!(parsed, (vec![true, false, false], None));

    let input = "(true, true, .., true)";
    let parsed = bools_with_dotdot(input).expect("bools with dotdot");
    assert_eq!(parsed, (vec![true, true], Some(vec![true])));

    let input = "(.., true)";
    let parsed = bools_with_dotdot(input).expect("bools with dotdot");
    assert_eq!(parsed, (vec![], Some(vec![true])));

    let input = "(true, true, ..)";
    let parsed = bools_with_dotdot(input).expect("bools with dotdot");
    assert_eq!(parsed, (vec![true, true], Some(vec![])));

    let input = "(..)";
    let parsed = bools_with_dotdot(input).expect("bools with dotdot");
    assert_eq!(parsed, (vec![], Some(vec![])));
}