treaty -


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pub mod walkers;

use crate::{
    effect::{Effect, Future},
    protocol::Visitor,
    Flow,
};

/// A type that can be walked.
pub trait Walk<'ctx, M, E: Effect<'ctx>>: WalkerTypes + Sized {
    /// The walker for the type.
    type Walker: Walker<'ctx, E, Error = Self::Error, Output = Self::Output>;

    fn into_walker(self) -> Self::Walker;
}

pub trait WalkerTypes {
    type Error: Send;

    /// An arbitrary type the walker is left with after walking.
    ///
    /// Its recommended that this is `Self` if the walker is repeatable.
    type Output: Send;
}

/// Walker for a type.
///
/// The `'ctx` lifetime is some lifetime that is longer than `Self`.
/// Data from the value may borrow using `'ctx`.
///
/// The way to use a walker is as follows.
/// - Call [From::from()] with a value to be walked to make a walker.
/// - Call [Self::walk()] to walk the value. Data will be sent to the provided
///     visitor.
pub trait Walker<'ctx, E: Effect<'ctx>>: WalkerTypes + Send {
    /// Walk the value.
    ///
    /// The walker should send data to the `visitor` as it walks the value.
    fn walk<'a>(
        self,
        visitor: Visitor<'a, 'ctx>,
    ) -> Future<'a, 'ctx, Result<Self::Output, Self::Error>, E>
    where
        Self: 'a;
}

pub trait WalkerObjSafe<'ctx, E: Effect<'ctx>>: Send {
    fn walk<'a>(&'a mut self, visitor: Visitor<'a, 'ctx>) -> Future<'a, 'ctx, Flow, E>
    where
        Self: 'a;

    fn skip(&mut self);
}

pub type DynWalker<'a, 'ctx, E> = &'a mut (dyn WalkerObjSafe<'ctx, E> + Send + 'a);

enum DynWalkerState<W: WalkerTypes> {
    Skipped,
    Walking,
    Pending(W),
    Done(W::Output),
    Err(W::Error),
}

pub enum DynWalkerError<W: WalkerTypes> {
    NeverWalked(W),

    /// This can only happen if a panic happens furing the walk and is then caught before calling
    /// finish..
    WalkNeverFinished,

    Walker(W::Error),
}

pub struct DynWalkerAdapter<W: WalkerTypes> {
    state: DynWalkerState<W>,
}

impl<W: WalkerTypes> DynWalkerAdapter<W> {
    pub fn new(walker: W) -> Self {
        Self {
            state: DynWalkerState::Pending(walker),
        }
    }

    pub fn finish(self) -> Result<Option<W::Output>, DynWalkerError<W>> {
        match self.state {
            DynWalkerState::Skipped => Ok(None),
            DynWalkerState::Walking => Err(DynWalkerError::WalkNeverFinished),
            DynWalkerState::Pending(walker) => Err(DynWalkerError::NeverWalked(walker)),
            DynWalkerState::Done(value) => Ok(Some(value)),
            DynWalkerState::Err(err) => Err(DynWalkerError::Walker(err)),
        }
    }
}

impl<'ctx, W: Walker<'ctx, E>, E: Effect<'ctx>> WalkerObjSafe<'ctx, E> for DynWalkerAdapter<W> {
    fn walk<'a>(&'a mut self, visitor: Visitor<'a, 'ctx>) -> Future<'a, 'ctx, Flow, E>
    where
        Self: 'a,
    {
        E::wrap(async {
            if let DynWalkerState::Pending(walker) =
                core::mem::replace(&mut self.state, DynWalkerState::Walking)
            {
                // Walk the walker.
                match walker.walk(visitor).await {
                    Ok(value) => {
                        self.state = DynWalkerState::Done(value);
                        Flow::Continue
                    }
                    Err(err) => {
                        self.state = DynWalkerState::Err(err);

                        // Signal that control flow should stop as soon as possible as we
                        // are in an error state.
                        Flow::Break
                    }
                }
            } else {
                // Can't do anything if the walker has already been walked.
                Flow::Continue
            }
        })
    }

    fn skip(&mut self) {
        self.state = DynWalkerState::Skipped;
    }
}