treaty -


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use crate::{
    any::TypeName,
    effect::{Effect, Future},
    higher_ranked_type,
    hkt::Marker,
    protocol::{DynVisitor, DynWalker},
};

use super::VisitResult;

/// Protocol for requesting a hint from a visitor.
pub trait RequestHint<'ctx, E: Effect> {
    /// 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: DynWalker<'a, 'ctx>,
    ) -> Future<'a, VisitResult<DynWalker<'a, 'ctx>>, E>;
}

pub struct RequestHintProto<E: Effect>(Marker<E>);

higher_ranked_type! {
    impl TypeName {
        impl['a, 'ctx, E] type T['a, 'ctx] for RequestHintProto<E> =
            dyn RequestHint<'ctx, E> + Send + Sync + 'a
        where {
            E: Effect
        };

        impl['a, 'ctx, E] type HigherRanked['a, 'ctx] for dyn RequestHint<'ctx, E> + Send + Sync + 'a =
            RequestHintProto<E>
        where {
            E: Effect
        };
    }
}

/// 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>(
    visitor: DynVisitor<'a, 'ctx>,
    walker: DynWalker<'a, 'ctx>,
) -> Future<'a, VisitResult<DynWalker<'a, 'ctx>>, E> {
    if let Some(object) = visitor.0.upcast_mut::<RequestHintProto<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(VisitResult::Skipped(walker))
    }
}