本文承接自上一篇《iOS的block与链式编程思想》
函数式编程
First-class and higher-order functions：这样拆成两部分：First-class functions 和 higher-order functions，点进去分别的定义如下。
The distinction between the two is subtle: "higher-order" describes a mathematical concept of functions that operate on other functions, while "first-class" is a computer science term that describes programming language entities that have no restriction on their use (thus first-class functions can appear anywhere in the program that other first-class entities like numbers can, including as arguments to other functions and as their return values).
区别就是高阶函数描述的是数学概念上的函数操作，而一级函数是计算机科学中的专业名词。。
未完待续。。最近有点忙