Functional programming using Swift

Playground.swift

//: Playground - noun: a place where people can play

import UIKit

var result:Double?

var operandStack = Array<Double>()
operandStack.append(2)
operandStack.append(5)


func operate(sender:String)
{
    switch sender
    {
        case "+":
            
            // this forces to write four functions (add, subtract, etc...)
//            performOperation(add)
            
            performOperation(
                {
                    (operator1:Double, operator2:Double) in
                        return operator1 + operator2
                })
        
        
        case "-":
            
            // due to type inference we can skip the type declaration
            // and can also remove the return as it knows performOperation returns something
            performOperation({
                (operator1, operator2) in
                    operator2 - operator1
            })
        

        case "*":
            
            // shortening further the syntax by using argument notation
            performOperation({
                $0 * $1
            })

        
        case "/":
            
            // being the last argument we can even put the {} after the function
//            performOperation(){$1 / $0}
        
            // also being the only argument we can remove the ()
            performOperation{$1 / $0}
        
        default:
            break
    }
}


func performOperation(operation: (Double, Double) -> Double)
{
    result = operation(operandStack.removeLast(), operandStack.removeLast())
    print("The result is \(result!)")
}



// Bad use of functional programming! We need repeat all the code!!
// A better use is to apply the functions directly, as shown in the switch statement
func multiply(operator1:Double, operator2:Double) -> Double
{
    return operator1 * operator2
}


func divide(operator1:Double, operator2:Double) -> Double
{
    return operator2 / operator2
}


func add(operator1:Double, operator2:Double) -> Double
{
    return operator1 + operator2
}


func subtract(operator1:Double, operator2:Double) -> Double
{
    return operator2 - operator1
}



operate("*")
//operate("/")
//operate("+")
//operate("-")