How to make this code more functional - scala

Question

I'm new to scala and functional programming. I'm trying out the usual beginner applications and scripts(Obviously using a bit of over-technology)
Anyways I have this code for a calculator that takes arguments and a switch to dictate the operation to use on the arguments.
object Main {
def main(args: Array[String]): Unit = {
var calc = calculate( "" , _:List[Int])
var values:List[Int] = List()
if(args.size < 1) println("No arguments supplied") else{
args collect {_ match{
case arg if arg.contains("-") => {
if(values.size>0){
calc(values)
values = List()}
calc = calculate( arg , _:List[Int])
}
case arg => {
try{
val value=arg.toInt
values = values ::: List(value)
}catch{case e:NumberFormatException=>println("\nError:Invalid argument:\""+arg+"\"\nCannot convert to Integer.\n")}
}
}
}
calc(values)
}
}
def sum(values:List[Int]) { println("The sum is:"+(values.foldLeft(0)((sum,value) => sum + value))) }
def subtract(values:List[Int]) {
val initial:Int = values(0)
var total:Int = 0
for(i <- 1 until values.size){
total = total + values(i)
}
val diff:Int = initial - total
println("The difference is:"+diff)
}
def calculate(operation:String,values:List[Int]) = operation match {
case "-sum" => sum(values)
case "-diff" => subtract(values)
case _ => println("Default operation \"Sum\" will be applied");sum(values)
}
}
Some points that id like to find if theres a better way to do is like removing the try catch statement.
A better way to compose this application would be very welcome.

Answer 1

How about this one?
object Main extends App {
require(args.size > 0, "Please, supply more arguments")
#annotation.tailrec
def parseArguments(arguments: List[String], operation: String, values: List[Int]() = Nil) {
if(values.nonEmpty) calculate(operation, values)
arguments match {
case op::unprocessed if op.startsWith("-") => parseArguments(unprocessed, op)
case maybeNumber::unprocessed => {
val newValues = try {
maybeNumber.toInt::values
} catch {
case _: NumberFormatException =>
println("\nError:Invalid argument:\""+maybeNumber+"\"\nCannot convert to Integer.\n")
values
}
parseArguments(unprocessed, operation, newValues)
}
case Nil => //done processing, exiting
}
}
parseArguments(args.toList, "")
def diff(values:List[Int]) = {
val initial::tail = values
val total = tail.sum
initial - total
}
def calculate(operation:String, values:List[Int]) = operation match {
case "-sum" => println("The sum is " + values.sum)
case "-diff" => println("The difference is: " + diff(values))
case _ =>
println("""Default operation "Sum" will be applied""")
sum(values)
}
}