How to specify that a parameter is optional

I am new to Twirl, and am trying to create a TODO application with play, the way I have it right now, I am trying to send two objects(an ArrayList, and a String). I have wrapped some code in the controller in a try catch. If an exception is caught the template should be rendered with a String, but not the ArrayList, and if no exception is caught, It will return an ArrayList, but not a String. I have tried just passing one as null, but I feel there is a better way to do it. Is there? Here is my Twirl:

@(message: String, tasks: ArrayList[Task])

<!DOCTYPE html>
<html>
<head>
<title>TODO</title>
</head>
<body>
    <h1>TODOs</h1>
    <p>@message</p>
    @for(task <- tasks) {
    <li><@task.task</li> }
</body>
</html>

Solution

You could use Options:

@(maybeMessage: Option[String], maybeTasks: Option[Seq[Task]])

<!DOCTYPE html>
<html>
<head>
    <title>TODO</title>
</head>
<body>
    <h1>TODOs</h1>
    @for(message <- maybeMessage) {
        <p>@message</p>
    }
    @for(tasks <- maybeTasks) {
        @for(task <- tasks) {
            <li>@task.task</li>
        }
    }
</body>
</html>

And then from your controller:

Ok(views.html.foo(None, Some(Seq(Task("task 1"), Task("task 2")))))

Ok(views.html.foo(Some("Something went wrong."), None))