treaty -


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use crate::{
    any::{MaybeSized, TypeName},
    bijective_higher_ranked_type,
    effect::{Effect, Future},
    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>;
}

bijective_higher_ranked_type! {
    pub type DynRequestHint['ctx][E]: MaybeSized['ctx][]
    for<'a>
    (dyn RequestHint<'ctx, E> + Send + 'a)
    where {
        E: Effect<'ctx>
    }
}

bijective_higher_ranked_type! {
    pub type [][E]: TypeName[][]
    for<'ctx>
    (DynRequestHint<'ctx, E>)
    where {
        E: Effect<'ctx>
    }
}

/// Visit using the [`RequestHint`] protocol.
///
/// If [`Flow::Continue`] is returned then the visitor wants/needs more information it didn't get
/// from the hints.
/// If [`Flow::Done`] is returned then the visitor doesn't need any more information and the walker
/// should stop walking.
/// If [`Flow::Break`] is returned then there was an error and the walker should stop walking.
pub fn visit_request_hint<'a, 'ctx, E: Effect<'ctx>>(
    visitor: Visitor<'a, 'ctx>,
    walker: Walker<'a, 'ctx>,
) -> Future<'a, 'ctx, Flow, E> {
    if let Some(object) = visitor.upcast_mut::<DynRequestHint<'ctx, E>>() {
        // Allow the visitor to give a hint if it wants.
        object.request_hint(walker)
    } else {
        // If the visitor doesn't support request hint then we continue.
        E::ready(Flow::Continue)
    }
}