Scala Play类型推论的怪异情况

Question

@(css: Option[String])(content: Html)(customJS: Option[Array[String]])

 @for(js <- customJS if !customJS.isEmpty) {
            <script src="@routes.Assets.at(js)"></script>
        }

I was writing for this small for-comprehension that will loop through and print out the uri String. Since it is possible to be none, or an array of Strings, I used Option .

Then the complier from typesafe activator told me that @routes.Assets.at() is supposed to take a String not an Array[String] . I'm in shock because js is supposed to be String right?

Then I tried to write like this:

 @for(js <- customJS if !customJS.isEmpty) {
      @var uri = js
            <script src="@routes.Assets.at(js)"></script>
        }

Then the complier said that added line is illegal start of "simple expression".

It is not a simple expression! It is a full for-comprehension with {} ! Also I was eventually forced to use js.asInstanceOf[String] to fool the complier. I'm not glad with this because once I was told that I should never use asInstanceOf . So what's wrong with my original code?

Answer 1

Note that you should not use Array - use Seq .

There is no need in Option[Seq[String]] - use Seq[String] . Instead of None you could always use an empty Seq .

In case you still want to use Option[Seq[String]] you should rewrite your for like this:

@for(jsArray <- customJS; js <- jsArray) {
  <script src="@routes.Assets.at(js)"></script>
}

You don't need if !customJS.isEmpty . Option is something like collection with 1 or 0 elements. Element of Option[Array[String]] is Array[String] .