Question

Hello there Stack Overflow,

I hope you’ll help me with my very first question here 🙂

So I’m having a problem with Scala type inference. Here is the code:

object Problem {

  def ok(fn: (String) => Unit) = fn("")

  // type mismatch; found: java.lang.String("") required: String
  def fail[String](fn: (String) => Unit) = fn("")

}

What kind of String does Scala expect here?

Note that this is a minimal example to explain my problem. The original issue appeared when I tried to implement a more complex interface (Play’s Iteratee, to be precise), so, no, leaving out the [String] is not an option. (If anyone thinks that the actual code would help, I’ll provide it.)

I tried def fail[java.lang.String] ... but then it says expected ], found ..

I did read Scala String vs java.lang.String – type inference which gives a good explanation on java.lang.String vs. scala.Predef.String, but I still could not come up with a solution for my specific problem.

Any ideas?

EDIT: So here is the original attempt how I tried to implement http://www.playframework.org/documentation/api/2.0/scala/play/api/libs/iteratee/Iteratee.html only that I wrote String instead of T. (With T it compiles, and it makes sense!) My fail; obviously I was a bit overwhelmed by all the type parameters:

val stream = WS.url("url").get({ headers =>
  (new Iteratee[Array[Byte], String] {
    def fold[T](done: (String, Input[Array[Byte]]) => Promise[T],
                cont: (Input[Array[Byte]] => Iteratee[Array[Byte], String]) => Promise[T],
                error: (String, Input[Array[Byte]]) => Promise[T]): Promise[T] =
    {
      done("something", Input.Empty)
    }
  })
})

Regards,
Hendrik

Answer 1

When you write:

def fail[String](fn: (String) => Unit) = fn("")

The type parameter between square brackets String is just an arbitrary name that, in your case, will hide scala or java string. It is fully equivalent to:

def fail[T](fn: (T) => Unit) = fn("")

If you want to constrain the type to string, you just have to write:

def fail(fn: (String) => Unit) = fn("")

And it will work for scala and java strings (since they are the same).