函数式编程的例子-谈side effect

在读《Functional Programming in Scala》一书，
关于移除副作用带来的好处，可以用一个简单的场景来模拟。

场景：咖啡店买咖啡

• 场景构成
  • 信用卡
  • 咖啡店
  • 咖啡
  • 外部API（支付时调用）

一个有副作用的函数

class Cafe:
    def buyCoffee(cc: CreditCard):Coffee = 
        val cup = new Coffee()
        cc.charge(cup.price)
        cup
class CreditCard:
    def charge(price: Double): UInt = 
        println('charging: '+price)
class Coffee:
    val price : Double = 2.0

一个没有副作用的函数

√ 单元测试可行

class Cafe:
    def buyCoffee(cc: CreditCard): (Coffee, Charge) =
        val cup = new Coffee()
        (cup, Charge(cc, cup.price)
case Class Charge(cc: CreditCard, amount: Double):
    def combine(other: Charge): Charge = 
        if cc = other.cc then
            Charge(cc, amount + other.amount)
        else 
            throw new Exception("Can't combine with different card")

√ 可复用、可组合

class Cafe:
    def buyCoffee(cc: CreditCard): (Coffee, Charge) =
        val cup = new Coffee()
        (cup, Charge(cc, cup.price)
    def buyCoffees(cc: CreditCard, n: Int): (List[Coffee], Charge) = 
        val purchases: List[(Coffee, Charge)] = List.fill(n)(buyCoffee(cc))
        val (coffees, charges) = purchases.unzip
        val reduced = charges.reduce((c1, c2)=>c1.combine(c2))
        (coffees, reduced)