closure本身是指 将 一个处于open状态（包含自由变量）的函数转换为closed状态
（所有变量都被绑定） 所需要的 外部环境。
从编程实践角度，closure是一个对象，包含 一段代码 以及 它被创建时所处的环境。
$ sdk i groovy
$ groovysh
groovy:000> :set interpreterMode true
groovy:000> def myadd = { x, y -> x + y + 2 }
===> groovysh_evaluate$_run_closure1@7fb95505
groovy:000> def l1 = [1,2,3]
===> [1, 2, 3]
groovy:000> def l2 = [10,20,30]
===> [10, 20, 30]
groovy:000> res = [l1, l2].transpose().collect(myadd)
===> [13, 24, 35]
groovy:000> res.class
===> class java.util.ArrayList
groovy:000> exit
$ ipython
In [1]: myadd = lambda x, y: x + y + 2
In [2]: res = map(myadd, [1,2,3], [10,20,30])
In [3]: list(res)
Out[3]: [13, 24, 35]
In [4]: exit
$ sdk i kotlin
$ kotlinc-jvm
Ref:
http://stackoverflow.com/a/36878651/701420
http://stackoverflow.com/questions/220658/what-is-the-difference-between-a-closure-and-a-lambda
https://en.wikipedia.org/wiki/Closure_(computer_programming)
https://en.wikipedia.org/wiki/Map_(higher-order_function)#Language_comparison
http://stackoverflow.com/questions/35973196/which-kotlin-features-are-not-available-in-statically-compiled-groovy