chumsky 0.3.0

A friendly parser combinator crate
Documentation 
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use super::*;

use std::rc::Rc;

// TODO: Remove when `OnceCell` is stable
struct OnceCell<T>(std::cell::RefCell<Option<T>>);
impl<T> OnceCell<T> {
    pub fn new() -> Self { Self(std::cell::RefCell::new(None)) }
    pub fn set(&self, x: T) -> Result<(), ()> {
        *self.0.try_borrow_mut().map_err(|_| ())? = Some(x);
        Ok(())
    }
    pub fn get(&self) -> Option<std::cell::Ref<T>> { Some(std::cell::Ref::map(self.0.borrow(), |x| x.as_ref().unwrap())) }
}

type ParserFn<'a, I, O, E> = dyn Fn(&mut dyn Stream<I, <E as Error<I>>::Span>, &mut Vec<E>) -> (usize, Result<(O, Option<E>), E>) + 'a;

/// See [`recursive()`].
pub struct Recursive<'a, I, O, E: Error<I>>(Rc<OnceCell<Box<ParserFn<'a, I, O, E>>>>);

impl<'a, I, O, E: Error<I>> Clone for Recursive<'a, I, O, E> {
    fn clone(&self) -> Self { Self(self.0.clone()) }
}

impl<'a, I, O, E: Error<I>> Parser<I, O> for Recursive<'a, I, O, E> {
    type Error = E;

    fn parse_inner<S: Stream<I, <Self::Error as Error<I>>::Span>>(&self, stream: &mut S, errors: &mut Vec<Self::Error>) -> (usize, Result<(O, Option<E>), E>) where Self: Sized {
        (self.0
            .get()
            .expect("Recursive parser used prior to construction"))(stream, errors)
    }
}

/// Construct a recursive parser (i.e: a parser that may contain itself as part of its pattern).
///
/// The given function must create the parser. The parser must not be used to parse input before this function returns.
pub fn recursive<'a, I, O, P: Parser<I, O, Error = E> + 'a, F: FnOnce(Recursive<'a, I, O, E>) -> P, E: Error<I>>(f: F) -> Recursive<'a, I, O, E> {
    let rc = Rc::new(OnceCell::new());
    let parser = f(Recursive(rc.clone()));
    rc.set(Box::new(move |mut stream: &mut dyn Stream<I, E::Span>, errors| parser.parse_inner(&mut stream, errors)))
        .unwrap_or_else(|_| unreachable!());
    Recursive(rc)
}