A Functional Approach to Optionals

mapFTW.swift

// Scenario: Parsing a JSON object that represents an item in a store
// JSON: { "name": "Bucket", "price": "19.95" }

// The following functions take in a price as a string, and output the result as an Int of cents
// cause I don't trust floating point operations when dealing with other people's money

struct Item {
  let name: String
  let price: Int
  
  init(json: JSON)
  {
    guard let name = json["name"].string,
              price = parsePrice(json["price"].string)
      else { return nil }
      
    self.name = name
    self.price = price
  }
}

// If let approach
func parsePrice(price: String?) -> Int?
{
  if let price = price,
         double = Double(price),
         cents = Int(double * 100) {
    return cents
  }
  
  return nil
}

// Guard let approach
func parsePrice(price: String?) -> Int?
{
  guard let price = price,
         double = Double(price),
         cents = Int(double * 100)
    else { return nil }
    
  return cents
}

// The functional approach
func parsePrice(price: String?) -> Int?
{
  return price.flatMap { price in Double(price) }
              .map { double in Int(double * 100) }
}