use crate::parser::{Parser, ParserInput, Representation};

pub fn optional<P, I, O, E>(parser: P) -> impl Parser<I, Option<O>, E>
where
    P: Parser<I, O, E>,
    I: ParserInput + Clone,
{
    let rep = Representation::from_choice(
        &mut [parser.representation(), Representation::new("ε")].into_iter(),
    );
    let p = move |input: I| match parser.parse(input.clone()) {
        Ok((output, rest)) => Ok((Some(output), rest)),
        Err(_e) => Ok((None, input)),
    };

    (p, rep)
}