food_reflection.swift

import Cocoa

enum Nutrients: String  {
    case fat, calcium
}

protocol NutrientsType {
    var calcium: Int { get }
    var fat: Int { get }
}

struct Meal: NutrientsType {
    var calcium: Int
    var fat: Int
}

extension NutrientsType {
    static func total(meals: [Self], nutrient: Nutrients) -> Int {
        return meals.reduce(0) { (sum, meal) -> Int in
            for child in Mirror(reflecting: meal).children where child.label == nutrient.rawValue {
                let valueMirror = Mirror(reflecting: child.value)
                if valueMirror.subjectType == Optional<Int>.self {
                    return sum + ((valueMirror.children.first?.1 as? Int) ?? 0)
                } else {
                    return sum + (child.value as! Int)
                }
            }
            return sum
        }
    }
}

extension Array where Element: NutrientsType {
    func total(nutrition: Nutrients) -> Int {
        return Element.total(self, nutrient: nutrition)
    }
}

let meal1 = Meal(calcium: 3, fat: 30)
let meal2 = Meal(calcium: 13, fat: 20)

Meal.total([meal1, meal2], nutrient: .fat)

[meal1, meal2].total(.fat)
[meal1, meal2].total(.calcium)