Question

I am trying to learn Scala and am a newbie. I know that this is not optimal functional code and welcome any advice that anyone can give me, but I want to understand why I keep getting true for this function.

  def balance(chars: List[Char]): Boolean = {
    val newList = chars.filter(x => x.equals('(') || x.equals(')'));
    return countParams(newList, 0)
  }                                               

  def countParams(xs: List[Char], y: Int): Boolean = {
    println(y + " right Here")
    if (y < 0) {
      println(y + " Here")
      return false
    } else {
      println(y + " Greater than 0")
      if (xs.size > 0) {
        println(xs.size + " this is the size")
        xs match {
          case xs if (xs.head.equals('(')) => countParams(xs.tail, y + 1)
          case xs if (xs.head.equals(')')) => countParams(xs.tail, y - 1)
          case xs => 0
        }
      }
    }
    return true;
  }
  balance("()())))".toList)

I know that I am hitting the false branch of my if statement, but it still returns true at the end of my function. Please help me understand. Thanks.

Answer 1

You either must be more explicit in what you are returning or make it more explicit for the compiler. This works:

def countParams(xs: List[Char], y: Int): Boolean = {
    println(y + " right Here")
    if (y < 0) {
      println(y + " Here")
      false
    } else {
      println(y + " Greater than 0")
      if (xs.size > 0) {
        println(xs.size + " this is the size")
        xs match {
          case xs if (xs.head.equals('(')) => countParams(xs.tail, y + 1)
          case xs if (xs.head.equals(')')) => countParams(xs.tail, y - 1)
          case xs => false
        }
      } else {
        true
      }
    }
}

In the code above each branch of if returns some value so the compiler assumes it’s a value to be returned. BTW version without logging and much more idiomatic:

def countParams(xs: List[Char], y: Int) =
    xs match {
        case Nil => y == 0
        case '(' :: rest => countParams(rest, y + 1)
        case ')' :: rest if(y > 0) => countParams(rest, y - 1)
        case _ => false  //must be closing parens but y < 0
    }