rust-parser-combinator/src/choice.rs

use crate::{ParseResult, Parser};

pub trait Choice<I, O, E> {
    fn parse_choice(&self, input: I) -> Result<(O, I), (E, I)>;
}

pub fn choice<C: Choice<I, O, E>, I, O, E>(choices: C) -> impl Parser<I, O, E> {
    move |input| choices.parse_choice(input)
}

fn choice_loop<'a, I, O, E>(
    mut input: I,
    parsers: &'a [&'a dyn Parser<I, O, E>],
) -> ParseResult<I, O, E> {
    //TODO need a more principled way to return an error when no choices work
    let mut err = None;

    for parser in parsers.iter() {
        match parser.parse(input) {
            Ok(res) => return Ok(res),
            Err((e, rest)) => {
                err = Some(e);
                input = rest;
            }
        }
    }
    Err((err.unwrap(), input))
}

impl<P1, P2, I, O, E> Choice<I, O, E> for (P1, P2)
where
    P1: Parser<I, O, E>,
    P2: Parser<I, O, E>,
{
    fn parse_choice(&self, input: I) -> Result<(O, I), (E, I)> {
        choice_loop(input, &[&self.0, &self.1])
    }
}

impl<P1, P2, P3, I, O, E> Choice<I, O, E> for (P1, P2, P3)
where
    P1: Parser<I, O, E>,
    P2: Parser<I, O, E>,
    P3: Parser<I, O, E>,
{
    fn parse_choice(&self, input: I) -> Result<(O, I), (E, I)> {
        choice_loop(input, &[&self.0, &self.1, &self.2])
    }
}