# hello

The smallest spec: one message and the rules that answer it. A rule's last `payload` goes back to whoever sent the message.

$when hello.greet (name: String) {
  payload = "Hello, ${name}!"
}

When several rules match a message, they all run, in order, each starting from the payload the one before left. This one only matches when its guard (the `if` after the head) is true.

$when hello.greet (name: String) if (name is "world") {
  payload = payload + " Nice to meet everyone at once."
}

Try it: [[Story:hello]], or open both in the [Fiddle](/fiddle?tab=stories&topic=Story:hello).