Macro epsilon

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

Parses nothing and always succeeds.

This can be useful as a fallthrough case in alt!, as shown below. Also useful for parsing empty delimiters using parens! or brackets! or braces! by parsing for example braces!(epsilon!()) for an empty {}.

• Syntax: epsilon!()
• Output: ()

#[macro_use]
extern crate syn;

use syn::synom::Synom;

enum Mutability {
    Mutable(Token![mut]),
    Immutable,
}

impl Synom for Mutability {
    named!(parse -> Self, alt!(
        keyword!(mut) => { Mutability::Mutable }
        |
        epsilon!() => { |_| Mutability::Immutable }
    ));
}

This macro is available if Syn is built with the "parsing" feature.