Binary search with Scala

BinarySearchSuite.scala

import org.junit.runner.RunWith
import org.scalatest.junit.JUnitRunner
import org.scalatest.FunSuite
import scala.annotation.tailrec

@RunWith(classOf[JUnitRunner])
class BinarySearchSuite extends FunSuite {

  trait TestBinarySearch {
    val v = Vector(1, 2, 4)
  }

  test("find the first existent element")(
    new TestBinarySearch {
      assert(binarySearch(v, 1) === 0)
    })

  test("find the second existent element")(
    new TestBinarySearch {
      assert(binarySearch(v, 2) === 1)
    })

  test("find the third existent element")(
    new TestBinarySearch {
      assert(binarySearch(v, 4) === 2)
    })

  test("find a smaller non-existent element")(
    new TestBinarySearch {
      assert(binarySearch(v, 0) === -1)
    })

  test("find a bigger non-existent element")(
    new TestBinarySearch {
      assert(binarySearch(v, 5) === -4)
    })

  test("find an in-between non-existent element")(
    new TestBinarySearch {
      assert(binarySearch(v, 3) === -3)
    })

  def binarySearch(a: Vector[Int], needle: Int): Int = {
    @tailrec
    def binarySearch(low: Int, high: Int): Int = {
      if (low <= high) {
        val middle = low + (high - low) / 2

        if (a(middle) == needle)
          middle
        else if (a(middle) < needle)
          binarySearch(middle + 1, high)
        else
          binarySearch(low, middle - 1)
      } else
        -(low + 1)
    }

    binarySearch(0, a.length - 1)
  }

}