Add a proof of concept that we can match over different types using t…

…he MessageHandler

src/bastion/examples/message_handler_multiple_types.rs

@@ -0,0 +1,63 @@
+use bastion::message::MessageHandler;
+use bastion::prelude::*;
+use std::fmt::Debug;
+use tracing::error;
+
+// This example shows that it is possible to use the MessageHandler to match
+// over different types of message.
+
+async fn child_task(ctx: BastionContext) -> Result<(), ()> {
+    loop {
+        MessageHandler::new(ctx.recv().await?)
+            .on_question(|n: i32, sender| {
+                if n == 42 {
+                    sender.reply(101).expect("Failed to reply to sender");
+                } else {
+                    error!("Expected number `42`, found `{}`", n);
+                }
+            })
+            .on_question(|s: &str, sender| {
+                if s == "marco" {
+                    sender.reply("polo").expect("Failed to reply to sender");
+                } else {
+                    panic!("Expected string `marco`, found `{}`", s);
+                }
+            })
+            .on_fallback(|v, addr| panic!("Wrong message from {:?}: got {:?}", addr, v))
+    }
+}
+
+async fn request<T: 'static + Debug + Send + Sync>(
+    child: &ChildRef,
+    body: T,
+) -> std::io::Result<()> {
+    let answer = child
+        .ask_anonymously(body)
+        .expect("Couldn't perform request")
+        .await
+        .expect("Couldn't receive answer");
+
+    MessageHandler::new(answer)
+        .on_tell(|n: i32, _| assert_eq!(n, 101))
+        .on_tell(|s: &str, _| assert_eq!(s, "polo"))
+        .on_fallback(|_, _| panic!("Unknown message"));
+
+    Ok(())
+}
+
+fn main() {
+    env_logger::init();
+
+    Bastion::init();
+    Bastion::start();
+
+    let children =
+        Bastion::children(|c| c.with_exec(child_task)).expect("Failed to spawn children");
+
+    let child = &children.elems()[0];
+
+    run!(request(child, 42)).unwrap();
+    run!(request(child, "marco")).unwrap();
+
+    // run!(request(child, "foo")).unwrap();
+}