# Macro synom::cond [−] [src]

    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![])));
    }

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