# Macro synom::epsilon [−] [src]

    macro_rules! epsilon {
        ($i:expr,) => { ... };
    }

Parses nothing and always succeeds.

This can be useful as a fallthrough case in `alt!`.

- Syntax: `epsilon!()`
- Output: `()`

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

    use syn::Mutability;

    named!(mutability -> Mutability, alt!(
        keyword!("mut") => { |_| Mutability::Mutable }
        |
        epsilon!() => { |_| Mutability::Immutable }
    ));

    fn main() {
        let input = "mut";
        let parsed = mutability(input).expect("mutability");
        assert_eq!(parsed, Mutability::Mutable);

        let input = "";
        let parsed = mutability(input).expect("mutability");
        assert_eq!(parsed, Mutability::Immutable);
    }

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