Allow to early stop

[andylokandy]

Allow to early stop and allow to return error. Maybe utilizing https://doc.rust-lang.org/std/ops/enum.ControlFlow.html.

[tisonkun]

Sounds like to change visit method's signature to:

fn visit(&mut self, this: &dyn core::any::Any, event: Event) -> ControlFlow<Result<(), E>>;

Or define our own Try impl that has a three-way exits:

enum VisitResult<E> {
  Continue,
  Break,
  Error(E)
}

[tisonkun]

@andylokandy Could you share some examples where a ControlFlow::Break or Error can be useful?

I can understand the Error case, but ControlFlow::Break looks too generic, so we may overdesign the interface.

That said, we can still have visitor_fn which always return ControlFlow::Continue and with Error = Infaillible, but when derived Traversable would looks like:

struct Data {
    payload: u32,
}

impl ::visitor::TraversableMut for Data {
    fn traverse_mut<V: ::visitor::VisitorMut>(&mut self, visitor: &mut V) -> Result<ControlFlow<()>, Err> {
        ::visitor::VisitorMut::visit_mut(visitor, self, ::visitor::Event::Enter)??;
        ::visitor::TraversableMut::traverse_mut(&mut self.payload, visitor)??;
        ::visitor::VisitorMut::visit_mut(visitor, self, ::visitor::Event::Exit)??;
        Ok(ControlFlow::Continue(()))
    }
}

[andylokandy]

I'd prefer patterns like this:

trait TraversableMut {
    fn enter_mut(&mut self, this: &dyn core::any::Any) -> ControlFlow<Result<(), E>>;
    fn exit_mut(&mut self, this: &dyn core::any::Any) -> ControlFlow<Result<(), E>>;
}

[tisonkun]

Yes. This is the first version of the design. Later I noticed that to have a visitor_fn utility an Event interface would be helpful, but it can be also:

pub fn visitor_fn<T, F1: FnMut(&T), F2: FnMut(&T)>(enter: F1, exit: F2) -> FnVisitor<T, F1, F2> { .. }

[andylokandy]

Ok, reasonable.

[tisonkun]

I'd prefer patterns like this:

trait TraversableMut {
    fn enter_mut(&mut self, this: &dyn core::any::Any) -> ControlFlow<Result<(), E>>;
    fn exit_mut(&mut self, this: &dyn core::any::Any) -> ControlFlow<Result<(), E>>;
}

Instead of TraversableMut, I guess you mean VisitorMut here?

[tisonkun]

Also I just notice that we can parameterize ControlFlow::Break's type to be optionally Error to return an error.