Availability

Availability is the state-based precondition for an action.

Use available when when an action should only be available in some current domain states.

Gate an Action

domain TodoApp {
  type Todo = {
    id: string,
    title: string,
    completed: boolean
  }

  state {
    todos: Array<Todo> = []
  }

  computed completedCount = len(filter(todos, $item.completed))
  computed hasCompleted = completedCount > 0

  action clearCompleted() available when hasCompleted {
    onceIntent {
      patch todos = filter(todos, !$item.completed)
    }
  }
}

available when reads current state and computed values. It is a good fit for rules like “only clear completed todos when at least one completed item exists.”

Keep Input-Specific Gates Separate

action toggleTodo(id: string)
  available when len(todos) > 0
  dispatchable when len(filter(todos, $item.id == id)) > 0 {
  onceIntent {
    patch todos = map(todos,
      $item.id == id
        ? { id: $item.id, title: $item.title, completed: !$item.completed }
        : $item
    )
  }
}

Use dispatchable when for input-specific checks that need the submitted action parameters. Treat it as the next step after available when is comfortable.

Common Mistake

Do not reference action parameters in available when. If the gate depends on the specific submitted input, use dispatchable when and confirm the current MEL reference.

Next

Put the essential pieces together in Building a Todo App, then follow the hands-on Todo tutorial. For exact syntax, read the MEL Syntax Cookbook.