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 core::ops::ControlFlow;

use crate::{
    any::{TypeName, TypeNameable},
    effect::{Effect, Future},
    hkt::AnySend,
    nameable,
    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: AnySend::Trait<'ctx>;

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

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

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

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

pub type DynHint<'a, 'ctx, Protocol, E> = dyn Hint<'a, 'ctx, Protocol, E> + Send + 'a;

nameable! {
    pub struct Name['ctx, Protocol, E] for<'a>;

    impl [Protocol::Name, E] for DynHint<'a, 'ctx, Protocol, E> where {
        Protocol: TypeNameable<'ctx> + ?Sized,
        E: Effect<'ctx>,
    }

    impl [Protocol, E] where DynHint<'a, 'ctx, Protocol::Nameable, E> {
        Protocol: TypeName<'ctx> + ?Sized,
        E: Effect<'ctx>,
    }
}

#[cfg(test)]
mod test {
    use core::ops::ControlFlow;

    use crate::{
        any::TypeNameable,
        effect::{BlockOn, Blocking, Spin},
        higher_ranked_type,
    };

    use super::*;

    #[test]
    fn demo() {
        struct X<'ctx>(&'ctx mut i32);

        #[derive(Debug)]
        struct Y;

        nameable! {
            struct Name['a, 'ctx];
            impl for Y where {}
        }

        impl<'a, 'ctx: 'a, E: Effect<'ctx>> Hint<'a, 'ctx, Y, E> for X<'ctx> {
            fn hint(
                &'a mut self,
                _visitor: Visitor<'a, 'ctx>,
                _hint: <Y as HintMeta<'ctx>>::Hint,
            ) -> Future<'a, 'ctx, Flow, E> {
                todo!()
            }

            fn known(
                &'a mut self,
                _hint: &'a <Y as HintMeta<'ctx>>::Hint,
            ) -> Future<'a, 'ctx, Result<Known<'a, 'ctx, Y>, ()>, E> {
                E::ready(Ok(&mut *self.0))
            }
        }

        higher_ranked_type! {
            type KnownHkt['ctx]: (AnySend) = for<'lt> &'lt mut i32
        }

        impl<'ctx> HintMeta<'ctx> for Y {
            type Known = KnownHkt<'ctx>;

            type Hint = ();
        }

        let mut z = 42;
        let mut x = X(&mut z);
        let y: &mut DynHint<'_, '_, Y, Blocking> = &mut x;

        fn id<'a, 'ctx, T: ?Sized + TypeNameable<'a, 'ctx>>(_x: &T) {}
        id(y);

        let x = Spin::block_on(y.known(&()));
        match x {
            Ok(value) => *value += 1,
            Err(_) => todo!(),
        }
        assert_eq!(z, 43);
    }
}