Make the graph generic over whether it's sync/async

crates/graph/Cargo.toml

@@ -7,3 +7,6 @@ edition = "2021"
 
 [dependencies]
 petgraph = "0.6.5"
+
+[dev-dependencies]
+tokio = { version = "1.41.0", features = ["rt", "macros"] }

crates/graph/src/lib.rs

@@ -1,24 +1,117 @@
 #![feature(let_chains)]
+#![feature(async_closure)]
 
 mod util;
 
 use petgraph::visit::{IntoEdgeReferences, NodeIndexable};
 use petgraph::{graph::NodeIndex, stable_graph::StableDiGraph, visit::EdgeRef};
-use std::cell::{Cell, RefCell};
+use std::any::Any;
+use std::cell::{Cell, Ref, RefCell, RefMut};
 use std::collections::HashMap;
+use std::collections::VecDeque;
+use std::future::Future;
+use std::ops::{Deref, DerefMut};
+use std::pin::Pin;
 use std::rc::Rc;
-use std::{any::Any, collections::VecDeque};
 
-type NodeGraph = StableDiGraph<ErasedNode, (), u32>;
+// TODO: consider using a struct for this, because generic bounds of type aliases aren't enforced
+type NodeGraph<S: Synchronicity> = StableDiGraph<ErasedNode<S>, (), u32>;
 
-pub struct Graph<Output> {
-    // we treat this as a StableGraph, since nodes are never removed
-    node_graph: Rc<RefCell<NodeGraph>>,
+pub trait Synchronicity: 'static {
+    type AnyStorage;
+    fn make_any_storage<T: Any>(value: T) -> Self::AnyStorage;
+    fn unbox_any_storage<T: 'static>(storage: &Self::AnyStorage) -> impl Deref<Target = T>;
+    fn unbox_any_storage_mut<T: 'static>(
+        storage: &mut Self::AnyStorage,
+    ) -> impl DerefMut<Target = T>;
+
+    type UpdateFn;
+    fn make_update_fn<V: 'static>() -> Self::UpdateFn;
+
+    type UpdateResult<'a>;
+    fn make_update_result<'a>() -> Self::UpdateResult<'a>;
+}
+
+pub enum Synchronous {}
+
+impl Synchronicity for Synchronous {
+    type AnyStorage = Box<dyn Any>;
+
+    fn make_any_storage<T: Any>(value: T) -> Self::AnyStorage {
+        Box::new(value)
+    }
+
+    fn unbox_any_storage<T: 'static>(storage: &Self::AnyStorage) -> impl Deref<Target = T> {
+        storage.downcast_ref().unwrap()
+    }
+
+    fn unbox_any_storage_mut<T: 'static>(
+        storage: &mut Self::AnyStorage,
+    ) -> impl DerefMut<Target = T> {
+        storage.downcast_mut().unwrap()
+    }
+
+    type UpdateFn = Box<dyn Fn(&mut Box<dyn Any>) -> ()>;
+
+    fn make_update_fn<V: 'static>() -> Self::UpdateFn {
+        Box::new(|any| {
+            let x = any.downcast_mut::<Box<dyn Node<V, Self>>>().unwrap();
+            x.update();
+        })
+    }
+
+    type UpdateResult<'a> = ();
+
+    fn make_update_result<'a>() -> Self::UpdateResult<'a> {}
+}
+
+pub enum Asynchronous {}
+
+impl Synchronicity for Asynchronous {
+    type AnyStorage = Rc<RefCell<Box<dyn Any>>>;
+
+    fn make_any_storage<T: Any>(value: T) -> Self::AnyStorage {
+        Rc::new(RefCell::new(Box::new(value)))
+    }
+
+    fn unbox_any_storage<T: 'static>(storage: &Self::AnyStorage) -> impl Deref<Target = T> {
+        Ref::map(storage.borrow(), |any| any.downcast_ref().unwrap())
+    }
+
+    fn unbox_any_storage_mut<T: 'static>(
+        storage: &mut Self::AnyStorage,
+    ) -> impl DerefMut<Target = T> {
+        RefMut::map(storage.borrow_mut(), |any| any.downcast_mut().unwrap())
+    }
+
+    type UpdateFn = Box<dyn Fn(Rc<RefCell<Box<dyn Any>>>) -> Pin<Box<dyn Future<Output = ()>>>>;
+
+    fn make_update_fn<V: 'static>() -> Self::UpdateFn {
+        Box::new(|any| Box::pin(Asynchronous::do_async_update::<V>(any)))
+    }
+
+    type UpdateResult<'a> = Pin<Box<dyn Future<Output = ()> + 'a>>;
+
+    fn make_update_result<'a>() -> Self::UpdateResult<'a> {
+        Box::pin(std::future::ready(()))
+    }
+}
+
+impl Asynchronous {
+    async fn do_async_update<V: 'static>(any: Rc<RefCell<Box<dyn Any>>>) {
+        let mut any_ = any.borrow_mut();
+        let x = any_.downcast_mut::<Box<dyn Node<V, Self>>>().unwrap();
+        x.update().await;
+    }
+}
+
+pub struct Graph<Output, Synch: Synchronicity> {
+    node_graph: Rc<RefCell<NodeGraph<Synch>>>,
     output: Option<NodeIndex<u32>>,
     output_type: std::marker::PhantomData<Output>,
 }
 
-impl<Output: Clone + 'static> Graph<Output> {
+impl<Output: Clone + 'static> Graph<Output, Synchronous> {
     pub fn new() -> Self {
         Self {
             node_graph: Rc::new(RefCell::new(StableDiGraph::new())),
@@ -26,13 +119,25 @@ impl<Output: Clone + 'static> Graph<Output> {
             output_type: std::marker::PhantomData,
         }
     }
+}
 
-    pub fn set_output<R: Rule<Output> + 'static>(&mut self, rule: R) {
+impl<Output: Clone + 'static> Graph<Output, Asynchronous> {
+    pub fn new_async() -> Self {
+        Self {
+            node_graph: Rc::new(RefCell::new(StableDiGraph::new())),
+            output: None,
+            output_type: std::marker::PhantomData,
+        }
+    }
+}
+
+impl<O: Clone + 'static, S: Synchronicity> Graph<O, S> {
+    pub fn set_output<R: Rule<O> + 'static>(&mut self, rule: R) {
         let input = self.add_rule(rule);
         self.output = Some(input.node_idx);
     }
 
-    fn add_node<V: Clone + 'static>(&mut self, node: impl Node<V> + 'static) -> Input<V> {
+    fn add_node<V: Clone + 'static>(&mut self, node: impl Node<V, S> + 'static) -> Input<V> {
         let value = node.value_rc();
         let erased = ErasedNode::new(node);
         let idx = self.node_graph.borrow_mut().add_node(erased);
@@ -43,7 +148,7 @@ impl<Output: Clone + 'static> Graph<Output> {
     }
 
     pub fn add_value<V: Clone + 'static>(&mut self, value: V) -> Input<V> {
-        return self.add_node(ConstNode(value.clone()));
+        return self.add_node(ConstNode::new(value.clone()));
     }
 
     pub fn add_rule<R: Rule<V> + 'static, V: Clone + 'static>(&mut self, rule: R) -> Input<V> {
@@ -54,7 +159,7 @@ impl<Output: Clone + 'static> Graph<Output> {
     where
         R: Rule<V> + 'static,
         V: Clone + 'static,
-        F: FnMut(InvalidationSignal) -> R,
+        F: FnMut(InvalidationSignal<S>) -> R,
     {
         let node_idx = Rc::new(Cell::new(None));
         let signal = InvalidationSignal {
@@ -66,7 +171,7 @@ impl<Output: Clone + 'static> Graph<Output> {
         input
     }
 
-    pub fn freeze(self) -> Result<FrozenGraph<Output>, GraphFreezeError> {
+    pub fn freeze(self) -> Result<FrozenGraph<O, S>, GraphFreezeError> {
         let output: NodeIndex<u32> = match self.output {
             None => return Err(GraphFreezeError::NoOutput),
             Some(idx) => idx,
@@ -78,7 +183,7 @@ impl<Output: Clone + 'static> Graph<Output> {
         let mut edges = vec![];
         for idx in indices {
             let node = &mut self.node_graph.borrow_mut()[idx];
-            (node.visit_inputs)(&mut node.any, &mut |input_idx| {
+            node.visit_inputs(&mut |input_idx| {
                 edges.push((input_idx, idx));
             });
         }
@@ -108,57 +213,53 @@ impl<Output: Clone + 'static> Graph<Output> {
     }
 }
 
+impl<O: Clone + 'static> Graph<O, Asynchronous> {
+    pub fn set_async_output<R: AsyncRule<O> + 'static>(&mut self, rule: R) {
+        let input = self.add_async_rule(rule);
+        self.output = Some(input.node_idx);
+    }
+
+    pub fn add_async_rule<R: AsyncRule<V> + 'static, V: Clone + 'static>(
+        &mut self,
+        rule: R,
+    ) -> Input<V> {
+        self.add_node(AsyncRuleNode::new(rule))
+    }
+
+    pub fn add_invalidatable_async_rule<R, V, F>(&mut self, mut f: F) -> Input<V>
+    where
+        R: AsyncRule<V> + 'static,
+        V: Clone + 'static,
+        F: FnMut(InvalidationSignal<Asynchronous>) -> R,
+    {
+        let node_idx = Rc::new(Cell::new(None));
+        let signal = InvalidationSignal {
+            node_idx: Rc::clone(&node_idx),
+            graph: Rc::clone(&self.node_graph),
+        };
+        let input = self.add_async_rule(f(signal));
+        node_idx.set(Some(input.node_idx));
+        input
+    }
+}
+
 #[derive(Debug)]
 pub enum GraphFreezeError {
     NoOutput,
     Cyclic(Vec<NodeIndex<u32>>),
 }
 
-pub struct FrozenGraph<Output> {
-    node_graph: Rc<RefCell<NodeGraph>>,
+pub struct FrozenGraph<Output, Synch: Synchronicity> {
+    node_graph: Rc<RefCell<NodeGraph<Synch>>>,
     output: NodeIndex<u32>,
     output_type: std::marker::PhantomData<Output>,
 }
 
-impl<Output: Clone + 'static> FrozenGraph<Output> {
-    fn update_node(&mut self, idx: NodeIndex<u32>) {
-        let graph = self.node_graph.borrow();
-        let node = &graph[idx];
-        let is_valid = (node.is_valid)(&node.any);
-        drop(graph);
-        if !is_valid {
-            // collect all the edges into a vec so that we can mutably borrow the graph to update the nodes
-            let edge_sources = self
-                .node_graph
-                .borrow()
-                .edges_directed(idx, petgraph::Direction::Incoming)
-                .map(|edge| edge.source())
-                .collect::<Vec<_>>();
-
-            // Update the dependencies of this node.
-            // TODO: iterating/recursing here seems less than efficient
-            // instead, in evaluate, topo sort the graph and update invalid nodes?
-            for source in edge_sources {
-                self.update_node(source);
-            }
-
-            let node = &mut self.node_graph.borrow_mut()[idx];
-            // Actually update the node's value.
-            (node.update)(&mut node.any);
-        }
-    }
-
+impl<O: Clone + 'static, S: Synchronicity> FrozenGraph<O, S> {
     pub fn is_output_valid(&self) -> bool {
         let graph = self.node_graph.borrow();
         let node = &graph[self.output];
-        (node.is_valid)(&node.any)
-    }
-
-    pub fn evaluate(&mut self) -> Output {
-        self.update_node(self.output);
-        let graph = self.node_graph.borrow();
-        let node = &graph[self.output].expect_type::<Output>();
-        node.value_rc().borrow().clone().unwrap()
+        node.is_valid()
     }
 
     pub fn node_count(&self) -> usize {
@@ -167,7 +268,7 @@ impl<Output: Clone + 'static> FrozenGraph<Output> {
 
     pub fn modify<F>(&mut self, mut f: F) -> Result<(), GraphFreezeError>
     where
-        F: FnMut(&mut Graph<Output>) -> (),
+        F: FnMut(&mut Graph<O, S>) -> (),
     {
         // Copy all the current edges so we can check if any change.
         let graph = self.node_graph.borrow();
@@ -206,12 +307,74 @@ impl<Output: Clone + 'static> FrozenGraph<Output> {
                 to_invalidate.push_back(edge.target());
             }
         }
-        invalidate_nodes(&mut graph, to_invalidate);
+        invalidate_nodes::<S>(&mut graph, to_invalidate);
 
         Ok(())
     }
 }
 
+impl<Output: Clone + 'static> FrozenGraph<Output, Synchronous> {
+    fn update_node(&mut self, idx: NodeIndex<u32>) {
+        let graph = self.node_graph.borrow();
+        let node = &graph[idx];
+        if !node.is_valid() {
+            // collect all the edges into a vec so that we can mutably borrow the graph to update the nodes
+            let edge_sources = graph
+                .edges_directed(idx, petgraph::Direction::Incoming)
+                .map(|edge| edge.source())
+                .collect::<Vec<_>>();
+            drop(graph);
+
+            // Update the dependencies of this node.
+            // TODO: iterating/recursing here seems less than efficient
+            // instead, in evaluate, topo sort the graph and update invalid nodes?
+            for source in edge_sources {
+                self.update_node(source);
+            }
+
+            let node = &mut self.node_graph.borrow_mut()[idx];
+            // Actually update the node's value.
+            (node.update)(&mut node.any);
+        }
+    }
+
+    pub fn evaluate(&mut self) -> Output {
+        self.update_node(self.output);
+        let graph = self.node_graph.borrow();
+        let node = &graph[self.output].expect_type::<Output>();
+        node.value_rc().borrow().clone().unwrap()
+    }
+}
+
+impl<Output: Clone + 'static> FrozenGraph<Output, Asynchronous> {
+    async fn update_node_async(&mut self, idx: NodeIndex<u32>) {
+        // TODO: same note about recursing as above, and consider doing this in parallel
+        let graph = self.node_graph.borrow();
+        let node = &graph[idx];
+        if !node.is_valid() {
+            let edge_sources = graph
+                .edges_directed(idx, petgraph::Direction::Incoming)
+                .map(|edge| edge.source())
+                .collect::<Vec<_>>();
+            drop(graph);
+
+            for source in edge_sources {
+                Box::pin(self.update_node_async(source)).await;
+            }
+
+            let node = &self.node_graph.borrow()[idx];
+            (node.update)(Rc::clone(&node.any)).await;
+        }
+    }
+
+    pub async fn evaluate_async(&mut self) -> Output {
+        self.update_node_async(self.output).await;
+        let graph = self.node_graph.borrow();
+        let node = &graph[self.output].expect_type::<Output>();
+        node.value_rc().borrow().clone().unwrap()
+    }
+}
+
 #[derive(Clone, Debug)]
 pub struct Input<T> {
     node_idx: NodeIndex<u32>,
@@ -219,7 +382,7 @@ pub struct Input<T> {
 }
 
 impl<T: Clone + 'static> Input<T> {
-    fn value(&self) -> T {
+    pub fn value(&self) -> T {
         self.value
             .as_ref()
             .borrow()
@@ -229,24 +392,27 @@ impl<T: Clone + 'static> Input<T> {
 }
 
 // TODO: there's a lot happening here, make sure this doesn't create a reference cycle
-pub struct InvalidationSignal {
+pub struct InvalidationSignal<Synch: Synchronicity> {
     node_idx: Rc<Cell<Option<NodeIndex<u32>>>>,
-    graph: Rc<RefCell<NodeGraph>>,
+    graph: Rc<RefCell<NodeGraph<Synch>>>,
 }
 
-impl InvalidationSignal {
+impl<S: Synchronicity> InvalidationSignal<S> {
     pub fn invalidate(&self) {
         let mut queue = VecDeque::new();
         queue.push_back(self.node_idx.get().unwrap());
-        invalidate_nodes(&mut *self.graph.borrow_mut(), queue);
+        invalidate_nodes::<S>(&mut *self.graph.borrow_mut(), queue);
     }
 }
 
-fn invalidate_nodes(graph: &mut NodeGraph, mut queue: VecDeque<NodeIndex<u32>>) {
+fn invalidate_nodes<S: Synchronicity>(
+    graph: &mut NodeGraph<S>,
+    mut queue: VecDeque<NodeIndex<u32>>,
+) {
     while let Some(idx) = queue.pop_front() {
         let node = &mut graph[idx];
-        if (node.is_valid)(&node.any) {
-            (node.invalidate)(&mut node.any);
+        if node.is_valid() {
+            node.invalidate();
             let dependents = graph
                 .edges_directed(idx, petgraph::Direction::Outgoing)
                 .map(|edge| edge.target());
@@ -257,62 +423,74 @@ fn invalidate_nodes(graph: &mut NodeGraph, mut queue: VecDeque<NodeIndex<u32>>)
 
 // TODO: i really want Input to be able to implement Deref somehow
 
-struct ErasedNode {
-    any: Box<dyn Any>,
-    is_valid: Box<dyn Fn(&Box<dyn Any>) -> bool>,
-    invalidate: Box<dyn Fn(&mut Box<dyn Any>) -> ()>,
-    visit_inputs: Box<dyn Fn(&mut Box<dyn Any>, &mut dyn FnMut(NodeIndex<u32>) -> ()) -> ()>,
-    update: Box<dyn Fn(&mut Box<dyn Any>) -> ()>,
+pub struct ErasedNode<Synch: Synchronicity> {
+    any: Synch::AnyStorage,
+    is_valid: Box<dyn Fn(&Synch::AnyStorage) -> bool>,
+    invalidate: Box<dyn Fn(&mut Synch::AnyStorage) -> ()>,
+    visit_inputs: Box<dyn Fn(&mut Synch::AnyStorage, &mut dyn FnMut(NodeIndex<u32>) -> ()) -> ()>,
+    update: Synch::UpdateFn,
 }
 
-impl ErasedNode {
-    fn new<N: Node<V> + 'static, V: 'static>(base: N) -> Self {
+impl<S: Synchronicity> ErasedNode<S> {
+    fn new<N: Node<V, S> + 'static, V: 'static>(base: N) -> Self {
         // i don't love the double boxing, but i'm not sure how else to do this
-        let thing: Box<dyn Node<V>> = Box::new(base);
-        let any: Box<dyn Any> = Box::new(thing);
+        let thing: Box<dyn Node<V, S>> = Box::new(base);
         Self {
-            any,
+            any: S::make_any_storage(thing),
             is_valid: Box::new(|any| {
-                let x = any.downcast_ref::<Box<dyn Node<V>>>().unwrap();
+                let x = S::unbox_any_storage::<Box<dyn Node<V, S>>>(any);
                 x.is_valid()
             }),
             invalidate: Box::new(|any| {
-                let x = any.downcast_mut::<Box<dyn Node<V>>>().unwrap();
+                let mut x = S::unbox_any_storage_mut::<Box<dyn Node<V, S>>>(any);
                 x.invalidate();
             }),
             visit_inputs: Box::new(|any, visitor| {
-                let x = any.downcast_mut::<Box<dyn Node<V>>>().unwrap();
+                let mut x = S::unbox_any_storage_mut::<Box<dyn Node<V, S>>>(any);
                 x.visit_inputs(visitor);
             }),
-            update: Box::new(|any| {
-                let x = any.downcast_mut::<Box<dyn Node<V>>>().unwrap();
-                x.update();
-            }),
+            update: S::make_update_fn::<V>(),
         }
     }
 
-    // TODO: revisit if these are necessary
-    fn expect_type<'a, V: 'static>(&'a self) -> &'a dyn Node<V> {
-        let res = self
-            .any
-            .downcast_ref::<Box<dyn Node<V>>>()
-            .expect("matching node type");
-        res.as_ref()
+    fn expect_type<'a, V: 'static>(&'a self) -> impl Deref<Target = Box<dyn Node<V, S>>> + 'a {
+        S::unbox_any_storage::<Box<dyn Node<V, S>>>(&self.any)
+    }
+
+    fn is_valid(&self) -> bool {
+        (self.is_valid)(&self.any)
+    }
+    fn invalidate(&mut self) {
+        (self.invalidate)(&mut self.any);
+    }
+    fn visit_inputs(&mut self, f: &mut dyn FnMut(NodeIndex<u32>) -> ()) {
+        (self.visit_inputs)(&mut self.any, f);
     }
 }
 
-trait Node<Value> {
+trait Node<Value: 'static, Synch: Synchronicity> {
     fn is_valid(&self) -> bool;
     fn invalidate(&mut self);
     fn visit_inputs(&mut self, visitor: &mut dyn FnMut(NodeIndex<u32>) -> ());
-    fn update(&mut self);
-    // TODO: are these both necessary?
+    fn update(&mut self) -> Synch::UpdateResult<'_>;
     fn value_rc(&self) -> Rc<RefCell<Option<Value>>>;
 }
 
-struct ConstNode<V>(V);
+struct ConstNode<V, S> {
+    value: V,
+    synchronicity: std::marker::PhantomData<S>,
+}
 
-impl<V: Clone + 'static> Node<V> for ConstNode<V> {
+impl<V, S> ConstNode<V, S> {
+    fn new(value: V) -> Self {
+        Self {
+            value,
+            synchronicity: std::marker::PhantomData,
+        }
+    }
+}
+
+impl<V: Clone + 'static, S: Synchronicity> Node<V, S> for ConstNode<V, S> {
     fn is_valid(&self) -> bool {
         true
     }
@@ -321,30 +499,34 @@ impl<V: Clone + 'static> Node<V> for ConstNode<V> {
 
     fn visit_inputs(&mut self, _visitor: &mut dyn FnMut(NodeIndex<u32>) -> ()) {}
 
-    fn update(&mut self) {}
+    fn update(&mut self) -> <S as Synchronicity>::UpdateResult<'_> {
+        unreachable!()
+    }
 
     fn value_rc(&self) -> Rc<RefCell<Option<V>>> {
-        Rc::new(RefCell::new(Some(self.0.clone())))
+        Rc::new(RefCell::new(Some(self.value.clone())))
     }
 }
 
-struct RuleNode<R, V> {
+struct RuleNode<R, V, S> {
     rule: R,
     value: Rc<RefCell<Option<V>>>,
     valid: bool,
+    synchronicity: std::marker::PhantomData<S>,
 }
 
-impl<R: Rule<V>, V> RuleNode<R, V> {
+impl<R: Rule<V>, V, S> RuleNode<R, V, S> {
     fn new(rule: R) -> Self {
         Self {
             rule,
             value: Rc::new(RefCell::new(None)),
             valid: false,
+            synchronicity: std::marker::PhantomData,
         }
     }
 }
 
-impl<R: Rule<V> + 'static, V: Clone + 'static> Node<V> for RuleNode<R, V> {
+impl<R: Rule<V> + 'static, V: Clone + 'static, S: Synchronicity> Node<V, S> for RuleNode<R, V, S> {
     fn is_valid(&self) -> bool {
         self.valid
     }
@@ -363,10 +545,11 @@ impl<R: Rule<V> + 'static, V: Clone + 'static> Node<V> for RuleNode<R, V> {
         self.rule.visit_inputs(&mut InputIndexVisitor(visitor));
     }
 
-    fn update(&mut self) {
-        self.valid = true;
+    fn update(&mut self) -> <S as Synchronicity>::UpdateResult<'_> {
         let new_value = self.rule.evaluate();
+        self.valid = true;
         *self.value.borrow_mut() = Some(new_value);
+        S::make_update_result()
     }
 
     fn value_rc(&self) -> Rc<RefCell<Option<V>>> {
@@ -374,12 +557,70 @@ impl<R: Rule<V> + 'static, V: Clone + 'static> Node<V> for RuleNode<R, V> {
     }
 }
 
+struct AsyncRuleNode<R, V> {
+    rule: R,
+    value: Rc<RefCell<Option<V>>>,
+    valid: bool,
+}
+
+impl<R: AsyncRule<V>, V> AsyncRuleNode<R, V> {
+    fn new(rule: R) -> Self {
+        Self {
+            rule,
+            value: Rc::new(RefCell::new(None)),
+            valid: false,
+        }
+    }
+}
+
+impl<R: AsyncRule<V> + 'static, V: Clone + 'static> Node<V, Asynchronous> for AsyncRuleNode<R, V> {
+    fn is_valid(&self) -> bool {
+        self.valid
+    }
+
+    fn invalidate(&mut self) {
+        self.valid = false;
+    }
+
+    fn visit_inputs(&mut self, visitor: &mut dyn FnMut(NodeIndex<u32>) -> ()) {
+        struct InputIndexVisitor<'a>(&'a mut dyn FnMut(NodeIndex<u32>) -> ());
+        impl<'a> InputVisitor for InputIndexVisitor<'a> {
+            fn visit<T>(&mut self, input: &mut Input<T>) {
+                self.0(input.node_idx);
+            }
+        }
+        self.rule.visit_inputs(&mut InputIndexVisitor(visitor));
+    }
+
+    fn update(&mut self) -> <Asynchronous as Synchronicity>::UpdateResult<'_> {
+        Box::pin(self.do_update())
+    }
+
+    fn value_rc(&self) -> Rc<RefCell<Option<V>>> {
+        Rc::clone(&self.value)
+    }
+}
+
+impl<R: AsyncRule<V>, V> AsyncRuleNode<R, V> {
+    async fn do_update(&mut self) {
+        let new_value = self.rule.evaluate().await;
+        self.valid = true;
+        *self.value.borrow_mut() = Some(new_value);
+    }
+}
+
 pub trait Rule<Output> {
     fn visit_inputs(&mut self, visitor: &mut impl InputVisitor);
 
     fn evaluate(&mut self) -> Output;
 }
 
+pub trait AsyncRule<Output> {
+    fn visit_inputs(&mut self, visitor: &mut impl InputVisitor);
+
+    async fn evaluate(&mut self) -> Output;
+}
+
 pub trait InputVisitor {
     fn visit<T>(&mut self, input: &mut Input<T>);
 }
@@ -390,7 +631,7 @@ mod tests {
 
     #[test]
     fn erase_node() {
-        let node = ErasedNode::new(ConstNode(1234 as i32));
+        let node = ErasedNode::<Synchronous>::new(ConstNode::new(1234 as i32));
         let unwrapped = node.expect_type::<i32>();
         assert_eq!(unwrapped.value_rc().borrow().unwrap(), 1234);
     }
@@ -512,7 +753,7 @@ mod tests {
 
     #[test]
     fn cant_freeze_no_output() {
-        let graph = Graph::<i32>::new();
+        let graph = Graph::<i32, Synchronous>::new();
         match graph.freeze() {
             Err(GraphFreezeError::NoOutput) => (),
             Err(e) => assert!(false, "unexpected error {:?}", e),
@@ -579,4 +820,27 @@ mod tests {
         assert!(!frozen.is_output_valid());
         assert_eq!(frozen.evaluate(), 2);
     }
+
+    #[tokio::test]
+    async fn async_graph() {
+        let mut graph = Graph::new_async();
+        graph.set_output(ConstantRule(42));
+        let mut frozen = graph.freeze().unwrap();
+        assert_eq!(frozen.evaluate_async().await, 42);
+    }
+
+    #[tokio::test]
+    async fn async_rule() {
+        struct AsyncConst(i32);
+        impl AsyncRule<i32> for AsyncConst {
+            fn visit_inputs(&mut self, _visitor: &mut impl InputVisitor) {}
+            async fn evaluate(&mut self) -> i32 {
+                self.0
+            }
+        }
+        let mut graph = Graph::new_async();
+        graph.set_async_output(AsyncConst(42));
+        let mut frozen = graph.freeze().unwrap();
+        assert_eq!(frozen.evaluate_async().await, 42);
+    }
 }