treaty -


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//! [`Protocol`] for giving a visitor an owned value.
//!
//! In some sense, this is the most basic protocol.

use crate::{
    any::TypeName,
    effect::{Effect, Future},
    higher_ranked_type,
    hkt::Marker,
    protocol::{
        walker::hint::{HintMeta, Meta},
        DynVisitor,
    },
};

use super::VisitResult;

/// Trait object for the [`Value`] protocol.
///
/// Types implementing the [`Value`] protocol will implement this trait.
pub trait Value<'ctx, T: ?Sized + TypeName::MemberType, E: Effect> {
    /// Visit a value of type `T`.
    ///
    /// Use this to give a value to a visitor. Its expected that a walker
    /// only calls this once per usage of the trait object, but that is not
    /// forced.
    ///
    /// If a [`ControlFlow::Break`] is returned then the walker
    /// should stop walking as soon as possible as there has likely been
    /// and error.
    fn visit<'a>(
        &'a mut self,
        value: TypeName::T<'a, 'ctx, T>,
    ) -> Future<'a, VisitResult<TypeName::T<'a, 'ctx, T>>, E>
    where
        TypeName::T<'a, 'ctx, T>: Send + Sized,
        'ctx: 'a;
}

pub struct ValueProto<T: ?Sized + TypeName::MemberType, E: Effect>(Marker<(*const T, E)>);

higher_ranked_type! {
    impl TypeName {
        impl['a, 'ctx, T, E] type T['a, 'ctx] for ValueProto<T, E> =
            dyn Value<'ctx, T, E> + Send + Sync + 'a
        where {
            T: ?Sized + TypeName::MemberType,
            E: Effect
        };

        impl['a, 'ctx, T, E] type HigherRanked['a, 'ctx] for dyn Value<'ctx, T, E> + Send + Sync + 'a =
            ValueProto<T, E>
        where {
            T: ?Sized + TypeName::MemberType,
            E: Effect
        };
    }
}

#[derive(Copy, Clone, PartialEq, Debug)]
pub struct ValueKnown<'a, T: ?Sized> {
    /// A preview of the value.
    ///
    /// This can be used to inspect the value before committing to a visit.
    pub preview: Option<&'a T>,
}

#[derive(Copy, Clone, Debug)]
pub struct ValueKnownHrt<T: ?Sized>(Marker<T>);

higher_ranked_type! {
    impl Meta {
        impl['a, 'ctx, T] type T['a, 'ctx] for ValueKnownHrt<T> =
            ValueKnown<'a, TypeName::T<'a, 'ctx, T>>
        where {
            T: ?Sized + TypeName::LowerForLt<'a, 'ctx, TypeName::Bound<'a, 'ctx>>,
        };

        impl['a, 'ctx, T] type HigherRanked['a, 'ctx] for ValueKnown<'a, T> =
            ValueKnownHrt<<T as TypeName::RaiseForLt<'a, 'ctx, TypeName::Bound<'a, 'ctx>>>::HigherRanked>
        where {
            T: ?Sized + TypeName::LowerType<'a, 'ctx>,
        };
    }
}

// This enrolls the Value protocol into the walker hint system.
impl<T: TypeName::MemberType, E: Effect> HintMeta for ValueProto<T, E> {
    type Known = ValueKnownHrt<T>;

    type Hint = ();

    type Effect = E;
}

pub fn visit_value<'a, 'ctx, T: Send + TypeName::LowerType<'a, 'ctx>, E: Effect>(
    visitor: DynVisitor<'a, 'ctx>,
    value: T,
) -> Future<'a, VisitResult<T>, E>
where
    TypeName::HigherRanked<'a, 'ctx, T>: TypeName::MemberType,
{
    if let Some(object) = visitor
        .0
        .upcast_mut::<ValueProto<TypeName::HigherRanked<'a, 'ctx, T>, E>>()
    {
        // Allow the visitor to give a hint if it wants.
        object.visit(value)
    } else {
        // If the visitor doesn't support request hint then we continue.
        E::ready(VisitResult::Skipped(value))
    }
}