treaty -


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//! Protocol for giving a hint to a walker.
//!
//! Sometimes a walker has multiple protocols it could use,
//! this module gives a protocol by which a visitor can give a hint
//! to the walker about what it is expecting.

use crate::{
    any::{TypeName, WithContextLt},
    bijective_higher_ranked_type,
    effect::{Effect, Future},
    hkt::AnySizedSend,
    protocol::Visitor,
    Flow,
};

/// Meta information for the hint.
///
/// This gives the visitor more information to work from when selecting a hint.
pub trait HintMeta<'ctx> {
    /// Information known by the walker.
    ///
    /// This should be information easy to get without changing the state of the walker
    /// in an irreversible way.
    type Known: AnySizedSend::MemberType;

    /// Extra information the visitor can give to the walker about what it is expecting.
    type Hint;
}

pub type Known<'a, 'ctx, Protocol> = AnySizedSend::T<'a, <Protocol as HintMeta<'ctx>>::Known>;

/// Object implementing the [`Hint`] protocol.
pub trait Hint<'ctx, Protocol: ?Sized + HintMeta<'ctx>, E: Effect> {
    /// Hint to the walker to use the `P` protocol.
    ///
    /// This should only be called once per [`RequestHint`].
    fn hint<'a>(
        &'a mut self,
        visitor: Visitor<'a, 'ctx>,
        hint: <Protocol as HintMeta<'ctx>>::Hint,
    ) -> Future<'a, Flow, E>;

    /// Ask the walker for information about it's support of the protocol.
    fn known<'a>(
        &'a mut self,
        hint: &'a <Protocol as HintMeta<'ctx>>::Hint,
    ) -> Future<'a, Result<Known<'a, 'ctx, Protocol>, ()>, E>;
}

bijective_higher_ranked_type! {
    pub type DynHint['ctx][Protocol, E]: WithContextLt['ctx][]
    for<'a>
    (dyn Hint<'ctx, Protocol, E> + Send + 'a)
    where {
        E: Effect,
        Protocol: ?Sized + WithContextLt::MemberType<'ctx> + 'ctx,
    }
}

bijective_higher_ranked_type! {
    pub type [][E][Protocol[][]]: TypeName[][]
    for<'ctx>
    (DynHint<'ctx, TypeName::T<'ctx, Protocol>, E>)
    (DynHint<'ctx, Protocol, E>)
    where {
        E: Effect,
        Protocol: ?Sized,
    }
}

// #[cfg(test)]
// mod test {
//     use crate::{
//         effect::{BlockOn, Blocking, Spin},
//     };
//
//     use super::*;
//
//     #[test]
//     fn demo() {
//         struct X<'ctx>(&'ctx mut i32);
//
//         #[derive(Debug)]
//         struct Y;
//
//         bijective_higher_ranked_type! {
//             type DynY['ctx][]: WithContextLt['ctx][] for<'a> (Y)
//         }
//
//         bijective_higher_ranked_type! {
//             type [][]: TypeName[][] for<'ctx> (DynY<'ctx>)
//         }
//
//         impl<'ctx, E: Effect<'ctx>> Hint<'ctx, DynY<'ctx>, E> for X<'ctx> {
//             fn hint<'a>(
//                 &'a mut self,
//                 _visitor: Visitor<'a, 'ctx>,
//                 _hint: <DynY<'ctx> as HintMeta<'ctx>>::Hint,
//             ) -> Future<'a, 'ctx, Flow, E> {
//                 todo!()
//             }
//
//             fn known<'a>(
//                 &'a mut self,
//                 _hint: &'a <DynY<'ctx> as HintMeta<'ctx>>::Hint,
//             ) -> Future<'a, 'ctx, Result<Known<'a, 'ctx, DynY<'ctx>>, ()>, E> {
//                 E::ready(Ok(&mut *self.0))
//             }
//         }
//
//         higher_ranked_type! {
//             type KnownHkt['ctx]: (AnySend) = for<'lt> &'lt mut i32
//         }
//
//         impl<'ctx> HintMeta<'ctx> for DynY<'ctx> {
//             type Known = KnownHkt<'ctx>;
//
//             type Hint = ();
//         }
//
//         let mut z = 42;
//         let mut x = X(&mut z);
//         let y: &mut WithContextLt::T<'_, '_, DynHint<'_, DynY<'_>, Blocking>> = &mut x;
//
//         fn id<'a, 'ctx, T: ?Sized + TypeName::LowerType<'ctx>>(_x: &WithContextLt::T<'a, 'ctx, T>) {
//         }
//         id::<DynHint<'_, DynY<'_>, Blocking>>(y);
//
//         let x = Spin::block_on(y.known(&()));
//         match x {
//             Ok(value) => *value += 1,
//             Err(_) => todo!(),
//         }
//         assert_eq!(z, 43);
//     }
// }