Scala `mkString` tail-recursion version

mkstr.scala

import scala.annotation.tailrec
import scala.collection.mutable.StringBuilder

def mkStr(elems: Seq[_], sep: String = ""): String = {
/* invoke java.lang.StackOverflowError
  elems match {
    case Seq() => ""
    case Seq(head) => head.toString
    case _ => elems.head + sep + mkStr(elems.tail, sep)
  }
*/
  @tailrec
  def worker(buf: StringBuilder, seq: Seq[_]): StringBuilder = seq match {
    case Seq() => buf
    case Seq(head) => buf.append(head)
    case _ => worker(buf.append(seq.head).append(sep), seq.tail)
  }
  worker(new StringBuilder, elems).toString
}

// test
assert{ mkStr(1 to 0, ", ") == ("") }
assert{ mkStr(1 to 1, ", ") == ("1") }
assert{ mkStr(1 to 2, ", ") == ("1, 2") }
assert{ mkStr(1 to 3, ", ") == ("1, 2, 3") }

// benchmark
val fixture = (0 to 1000000)
val sep = ", "
val count = 10

val mkstring = new scala.testing.Benchmark {
  def run = fixture.mkString(sep)
}.runBenchmark(count)
println("standard `mkString` => " + mkstring)

val mkstr = new scala.testing.Benchmark {
  def run = mkStr(fixture, sep)
}.runBenchmark(count)
println("tail recursion `mkString` => " + mkstr)