anchors 0.6.0

async incremental computations
Documentation 
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use crate::expert::{
    Anchor, AnchorHandle, AnchorInner, Engine, OutputContext, Poll, UpdateContext,
};
use std::panic::Location;

/// An Anchor type for immutable values.
pub struct Constant<T> {
    val: T,
    first_poll: bool,
    location: &'static Location<'static>,
}

impl<T: 'static> Constant<T> {
    /// Creates a new Constant Anchor from some value.
    #[track_caller]
    pub fn new<E: Engine>(val: T) -> Anchor<T, E> {
        E::mount(Self {
            val,
            first_poll: true,
            location: Location::caller(),
        })
    }

    #[cfg(test)]
    pub fn new_raw_testing(val: T) -> Constant<T> {
        Self {
            val,
            first_poll: true,
            location: Location::caller(),
        }
    }
}

impl<T: 'static, E: Engine> AnchorInner<E> for Constant<T> {
    type Output = T;
    fn dirty(&mut self, _child: &<E::AnchorHandle as AnchorHandle>::Token) {
        panic!("Constant never has any inputs; dirty should not have been called.")
    }
    fn poll_updated<G: UpdateContext<Engine = E>>(&mut self, _ctx: &mut G) -> Poll {
        let res = if self.first_poll {
            Poll::Updated
        } else {
            Poll::Unchanged
        };
        self.first_poll = false;
        res
    }
    fn output<'slf, 'out, G: OutputContext<'out, Engine = E>>(
        &'slf self,
        _ctx: &mut G,
    ) -> &'out Self::Output
    where
        'slf: 'out,
    {
        &self.val
    }

    fn debug_location(&self) -> Option<(&'static str, &'static Location<'static>)> {
        Some(("constant", self.location))
    }
}