装饰模式在《Design Patterns: Elements of Reusable Object-Oriented Software》的定义如下：
Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.
对于装饰模式的优缺点或者代码样例可以参考电子工业出版社的《Java 与模式》和清华大学出版社的《Java设计模式》
下面将重点介绍在函数式编程中如何实现装饰模式
装饰模式主要是为了扩展功能，所以在下面的代码中可以看到，为add，subtract，multiply，divide四个函数“装饰”了一层打印结果的函数，使得这些函数有了新的功能
object DecoratorPattern {
def add(a: Int, b: Int) = a + b
def subtract(a: Int, b: Int) = a - b
def multiply(a: Int, b: Int) = a * b
def divide(a: Int, b: Int) = a / b
def makeLogger(calcFn: (Int, Int) => Int) =
(a: Int, b: Int) => {
val result = calcFn(a, b)
println("Result is: " + result)
result
}
val loggingAdd = makeLogger(add)
val loggingSubtract = makeLogger(subtract)
val loggingMultiply = makeLogger(multiply)
val loggingDivide = makeLogger(divide)
def main(args: Array[String]): Unit = {
loggingAdd(2, 3)
loggingSubtract(2, 3)
loggingMultiply(2, 3)
loggingDivide(6, 3)
}
}
