def sum(input: Array[Int]): Int = {
  var i=0;
  while(i<input.length) {
    sum=i+input(i);
    i=i+1;
  }
  sum
}

<console>:17: error: reassignment to val
  sum= (i+input(i))
     ^
<console>:21: error: missing argument list for method
sum Unapplied methods are only converted to functions when a function type is expected.
You can make this conversion explicit by writing `sum _` or `sum(_)` instead of `sum`.

<console>:17: error: reassignment to val
   sum=i+input(i);
      ^
<console>:20: error: not enough arguments for method sum: (input: Array[Int])Int.
Unspecified value parameter input.sum()

def sum(input: Array[Int]): Int = input.sum

val l = List(1,2,3,4,5)

l.reduce((a,b) => a + b)
l.foldLeft(0)((a,b) => a + b)

def sum(l: List[Int]): Int = l match {
  case Nil => 0
  case head :: tail => head + sum(tail)
}

sum(l)