Add mutator

src/bastion/examples/with_data.rs

@@ -6,11 +6,12 @@ async fn child_task(ctx: BastionContext) -> Result<(), ()> {
     loop {
         MessageHandler::new(ctx.recv().await?)
             .on_tell(|_: DummyMessage, _| {
-                ctx.read_data(|child_usize: Option<&usize>| {
-                    let child_usize = child_usize.unwrap();
-
-                    println!("My state says my usize is `{}`", child_usize);
-                })
+                ctx.write_data(|value| {
+                    value.map(|v: &usize| {
+                        println!("My counter is at `{}`. Let's increment it!", v);
+                        v + 1
+                    })
+                });
             })
             .on_fallback(|_, _| panic!("Unhandled message"));
     }
@@ -19,6 +20,10 @@ async fn child_task(ctx: BastionContext) -> Result<(), ()> {
 #[derive(Debug)]
 struct DummyMessage;
 
+// $ cargo r --example with_data
+// My counter is at `42`. Let's increment it!
+// My counter is at `43`. Let's increment it!
+// My counter is at `44`. Let's increment it!
 fn main() {
     env_logger::init();
 
@@ -30,6 +35,8 @@ fn main() {
 
     let child = &children.elems()[0];
 
+    child.tell_anonymously(DummyMessage).unwrap();
+    child.tell_anonymously(DummyMessage).unwrap();
     child.tell_anonymously(DummyMessage).unwrap();
 
     thread::sleep(Duration::from_secs(1));

src/bastion/src/context.rs

@@ -778,6 +778,14 @@ impl BastionContext {
     pub fn read_data<T: Send + Sync + 'static, O>(&self, f: impl FnOnce(Option<&T>) -> O) -> O {
         self.state.data.read(|t| f(t))
     }
+
+    // TODO(scrabsha): add doc, document same type collision
+    pub fn write_data<T: std::fmt::Debug + Send + Sync + 'static>(
+        &self,
+        f: impl FnOnce(Option<&T>) -> Option<T>,
+    ) {
+        self.state.data.write(f)
+    }
 }
 
 impl ContextState {

src/bastion/src/global_state.rs

@@ -54,9 +54,9 @@ impl GlobalState {
     }
 
     /// Invokes a function with the requested data type.
-    pub fn write<T: std::fmt::Debug + Send + Sync + 'static, F>(&mut self, f: F)
+    pub fn write<T: std::fmt::Debug + Send + Sync + 'static, F>(&self, f: F)
     where
-        F: Fn(Option<&T>) -> Option<T>,
+        F: FnOnce(Option<&T>) -> Option<T>,
     {
         let mut hm = self.table.write().unwrap();
         let stuff_to_insert = match hm.entry(TypeId::of::<T>()) {