treaty -


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

use effectful::{
    bound::Dynamic, effective::{Canonical, Effective}, environment::Environment, higher_ranked::{for_lt, Rank1}, DynBind, SendSync
};

use crate::{
    any::type_name,
    hkt::Marker,
    protocol::{walker::hint::HintMeta, 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 + type_name::Static, E: Environment>: DynBind<E> {
    /// 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<'this: 'value, 'value: 'e, 'e>(
        &'this mut self,
        value: type_name::Lowered<'value, 'ctx, T>,
    ) -> Canonical<'e, VisitResult<Dynamic<type_name::Lowered<'value, 'ctx, T>>>, E>
    where
        type_name::Lowered<'value, 'ctx, T>: Sized,
        Dynamic<type_name::Lowered<'value, 'ctx, T>>: DynBind<E>,
        'ctx: 'this + 'value;
}

const _: () = {
    pub struct ValueProto<T: ?Sized, E>(Marker<(*const T, E)>);

    impl<'a, 'ctx, T: ?Sized, E> type_name::Lower<'a, 'ctx, &'a &'ctx ()> for ValueProto<T, E>
    where
        E: Environment,
        T: type_name::Static,
    {
        type Lowered = dyn Value<'ctx, T, E> + 'a;
    }

    impl<'a, 'ctx, T: ?Sized, E> type_name::Raise<'a, 'ctx, &'a &'ctx ()> for dyn Value<'ctx, T, E> + 'a
    where
        E: Environment,
        T: type_name::Static,
    {
        type Raised = ValueProto<T, E>;
    }

    // This enrolls the Value protocol into the walker hint system.
    impl<T, E: Environment> HintMeta for ValueProto<T, E>
    where
        T: ?Sized + type_name::Static,
    {
        type Known = for_lt!(<'b> ValueKnown<'b, T>);

        type Hint = Rank1<()>;
    }

    impl<T: ?Sized, E: Environment> effectful::environment::InEnvironment for ValueProto<T, E> {
        type Env = E;
    }
};

#[derive(Copy, Clone, PartialEq, Debug, SendSync)]
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<Dynamic<&'a T>>,
}

pub fn visit_value<
    'ctx: 'visitor,
    'visitor: 'e,
    'lt: 'e,
    'e,
    T: type_name::WithLt<'e, 'ctx>,
    E: Environment,
>(
    visitor: DynVisitor<'visitor, 'lt, 'ctx, E>,
    value: T,
) -> Canonical<'e, VisitResult<Dynamic<T>>, E>
where
    Dynamic<T>: DynBind<E>,
    type_name::Raised<'e, 'ctx, T>: type_name::Static,
{
    if let Some(object) = visitor
        .0
        .cast_mut()
        .upcast_mut::<dyn Value<'ctx, type_name::Raised<'e, '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::value(VisitResult::Skipped(Dynamic(value))).cast()
    }
}