#![feature(assert_matches)]
#![allow(dead_code)] //TODO eventually turn this off


type ParseResult<I, O, E> = Result<(O, I), E>;

trait Parser<I, O, E> {
    fn parse(&self, input: I) -> ParseResult<I, O, E>;
}

impl<I, O, E, F> Parser<I, O, E> for F where F: Fn(I) -> ParseResult<I, O, E> {
    fn parse(&self, input: I) -> ParseResult<I, O, E> {
        self(input)
    }
}

fn literal(expected: &'static str) -> impl Fn(&str) -> ParseResult<&str, (), &str> {
    move |input| match input.get(0..expected.len()) {
        Some(next) if next == expected => 
            Ok(((), &input[expected.len()..])),
        _  => Err(input)
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::assert_matches::assert_matches;

    #[test]
    fn parsing() {
        let output = literal("a")("a yolo");
        assert_matches!(output.unwrap(), ((), " yolo"));
    }
}