treaty -


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use crate::{
    effect::{Effect, Future},
    nameable,
    never::Never,
    protocol::{Visitor, Walker},
    Flow,
};

/// Protocol for requesting a hint from a visitor.
pub trait RequestHint<'ctx, E: Effect<'ctx>> {
    /// Call this to request a hint.
    ///
    /// `walker` is what the visitor (`self`) will call to give a hint using the
    /// [`Hint`][crate::builtins::walker::Hint] protocol.
    fn request_hint<'a>(&'a mut self, walker: Walker<'a, 'ctx>) -> Future<'a, 'ctx, Flow, E>;
}

pub type DynRequestHint<'a, 'ctx, E> = dyn RequestHint<'ctx, E> + Send + 'a;

nameable! {
    pub struct Name['a, 'ctx, E];
    impl [E] for DynRequestHint<'a, 'ctx, E> where {
        E: Effect<'ctx>,
        'ctx: 'a
    }
}

/// Visit using the [`RequestHint`] protocol.
///
/// If `true` is returned then the walker should stop.
/// This is either because of an error or the visitor is done.
pub fn visit_request_hint<'a, 'ctx, E: Effect<'ctx>>(
    visitor: Visitor<'a, 'ctx>,
    walker: Walker<'a, 'ctx>,
) -> Future<'a, 'ctx, bool, E> {
    if let Some(object) = visitor.upcast_mut::<DynRequestHint<'_, 'ctx, E>>() {
        // Allow the visitor to give a hint if it wants.
        E::map(object.request_hint(walker), |flow| match flow {
            Flow::Continue => {
                // The visitor wants the walker to continue to it's normal
                // walking.
                false
            }
            _ => {
                // The visitor is done (either because of an error or because
                // it already used a hint).
                true
            }
        })
    } else {
        // If the visitor doesn't support request hint then we continue.
        E::ready(false)
    }
}