import javax.script.ScriptEngineManager

// I *could* write an evaluation engine by hand, since it's only + and - it would be dead easy, but... not this time...
val e = new ScriptEngineManager().getEngineByName("javascript")
val operators:Seq[String] = Seq("+","-","")

def operatorPermutations(position:Int):Seq[String] = {
  if (position > 1) {
    for {
      curOp <- operators
      choose1 = operatorPermutations(position-1)
      choice<-choose1
      strRes =  choice  + curOp + position.toString
    } yield {
      strRes
    }
  } else {
    Seq("1")
  }
}

for {
  option <- operatorPermutations(9)
  result = e.eval(option).asInstanceOf[Double].toInt
  if result == 100
} yield {
   option
}