don't repeat the wheel, use fold/foldLeft/foldRight

testAccS.scala

object TestAccS extends App {

 def add(i: Int, j: Int) = i + j
 def accS[A](acc: A, s: Stream[A], fn: (A,A) => A) : A = {
  if ( s.isEmpty ) return acc
  accS( fn(acc, s.head), s.tail, fn) 
 }
 val iS = Stream.iterate(1, 100){_ + 1} // the unbound parameter's draws the value from the immediate closure i.e. 1
 println("Sum of elements: " + accS(0, iS, add) )

}