//
// simple four-function calculator
// 
// maintains a "current value" and repeatedly:
//   prompts for operation (+, -, *, /) and value
//   calculates and displayed new "current value"
//
// illustrates use of "match" and also using a function as a parameter to 
// another function
//

// the next line is needed to avoid warnings with the newest Scala
// "comment it out" (insert a //) if you compile with an earlier version
import scala.io.StdIn._

calcStep(0.0)

//
// perform one step:
// prompt for operator and value
// compute and display new value
//
def calcStep(current:Double) {
  println("\nenter one of +, -, *, /, q")
  val in = readChar
  in match {
    case '+' => { calculate(current, add) }
    case '-' => { calculate(current, sub) }
    case '*' => { calculate(current, mult) }
    case '/' => { calculate(current, div) }
    case 'q' => { println("done") }
    case _ => { println("invalid") ; calcStep(current) }
  }
}

//
// perform the part of "calcStep" common to all four operators:
// prompt for value, calculate and print new value
//
def calculate(current:Double, fcn:(Double,Double)=>Double ) {
  println("enter value")
  val in = readDouble
  val newValue = fcn(current, in)
  println("new value " + newValue)
  calcStep(newValue)
}

//
// functions for four operators
//
def add(x:Double,y:Double) : Double = x + y
def sub(x:Double,y:Double) : Double = x - y
def mult(x:Double,y:Double) : Double = x * y
def div(x:Double,y:Double) : Double = x / y