【codewar-7kyu】Unary function chainer

## 题目描述：

Your task is to write a higher order function for chaining together a list of unary functions. In other words, it should return a function that does a left fold on the given functions. 

chained([a,b,c,d])(input)

Should yield the same result as

d(c(b(a(input))))

## 思路分析：

返回值是由原参数构成的函数，解析成多层函数的嵌套。input 是内层的参数。

## 代码解析：

- Python

def chained(functions):
    def apply(param):
        result = param
        for f in functions:
            result = f(result)
        return result
    return apply

param 可是换成 result, 省略第三行（如下），不过不省略会语义上更清晰易读。注意 result 的作用域，别写成 for 的局部变量了

完整可运行代码如下：

#!/usr/bin/python
# -*- coding: utf-8 -*-

__author__ = 'pudding'


def chained(functions):
    def apply(result):
        for f in functions:
            result = f(result)
        return result
    return apply


def f1(x): return x*2


def f2(x): return x+2


def f3(x): return x**2

def f4(x): return x.split()


def f5(xs): return [x[::-1].title() for x in xs]


def f6(xs): return "_".join(xs)


if __name__ == '__main__':
    print chained([f1, f2, f3])(0)

输出结果：4 即：((0**2)+2)*2

apply一开始的参数（result 的初始值）为 chained([f1, f2, f3])(0)中的0

一定要注意 return apply 而不是 return apply()