Beyond Closure in JS and Python

[xxleyi]

序

• 演示语言为 JS 和 Python
• 主题是闭包，但又不仅仅是闭包

大部分人对闭包的掌握程度：

能面试的程度 中级 我们用的插件里有用闭包的地方，如果我看源码，我能看出来

为啥有闭包？就是为了能在别的地方调用b函数用到函数a里的变量 a包着b

定场鸡汤

不要刻意去记住某件事，而要去寻找使其显而易见的解释。 -- 理查德·费曼

---

changA 不传参

let a = {a: 3}

function changeA(){
  a = {a: "change"}
  }

changeA()

a = {'a': 3}

def changeA():
  a = {'a': "change"}

changeA()

JS Tutor 小演示

Python3 Tutor 小演示

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changA 传参

let a = {a: 3}

function changeA(a){
  a = {a: "change"}
  }

changeA(a)

a = {'a': 3}

def changeA(a):
  a = {'a': "change"}

changeA(a)

JS Tutor 小演示

Python3 Tutor 小演示

[xxleyi]

Closure:

• In programming languages, a closure, also lexical closure or function closure, is a technique for implementing lexically scoped name binding in a language with first-class functions.
• Operationally, a closure is a record storing a function together with an environment.
• The environment is a mapping associating each free variable of the function (variables that are used locally, but defined in an enclosing scope) with the value or reference to which the name was bound when the closure was created.
• Unlike a plain function, a closure allows the function to access those captured variables through the closure's copies of their values or references, even when the function is invoked outside their scope.

---

Environment Diagrams: visual tool to keep track of bindings and state of a computer program.

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Python assumes that all variables in a function are local.

In Python local variable declaration is automatic/implicit while in JavaScript it is not (you have to use var or let or const).

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var funcs = [];
for (var i = 0; i < 3; i++) {
  funcs[i] = function() {
    console.log("My value: " + i);
  };
}
for (var j = 0; j < 3; j++) {
  funcs[j]();
}

var funcs = [];

function createfunc(i) {
    return function() { console.log("My value: " + i); };
}

for (var i = 0; i < 3; i++) {
    funcs[i] = createfunc(i);
}

for (var j = 0; j < 3; j++) {
    funcs[j]();
}

JS Tutor Closure 演示

function f() {
    var result = [];
    for (var i of [0, 1, 2]) {
        result.push(function () { return i });
    }
    return result;
}

var result = f();

console.log(result[0](), result[1](), result[2]());

var result = []

function f() {
  let y = result.length;
  result.push(function () { return y });
}

f();
f();
f();

console.log(result[0](), result[1](), result[2]());

JS Tutor Closure 演示

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# funcs = [];

# for i in range(3):
#     funcs.append(lambda: print("Myvalue: " + str(i)))

# for j in range(3):
#     funcs[j]()

funcs = [];

def createfunc(i):
    return lambda: print("Myvalue: " + str(i))

for i in range(3):
    funcs.append(createfunc(i))

for j in range(3):
    funcs[j]()

Python3 Tutor Closure 演示

def outer():
    result = []
    for i in range(3):
        def inner():
            return i
        result.append(inner)
    return result

result = outer()
print(result[0].__closure__)
print(result[1].__closure__)
print(result[2].__closure__)

print(result[0](), result[1](), result[2]())

# 另外一种形式
result = []
def outer():
    y = len(result)

    def inner():
        return y

    result.append(inner)

outer()
outer()
outer()

print(result[0].__closure__)
print(result[1].__closure__)
print(result[2].__closure__)

print(result[0](), result[1](), result[2]())

Python3 Tutor Closure 演示

[xxleyi]

Evaluation Model

• substitution model 没有赋值操作
• environment model 为赋值而生，使状态管理成为可能，为模块化铺平道路

环境模型干的事情：状态管理。 其数据结构不算复杂，就是一层层的 frame，每一层 frame 维护自己内部的映射关系，同时又有一个专门的link，指向自己所归属的 frame。

推荐文章：SICP Goodness - The Environment Model

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作用域: The scope of a variable are the locations where it is accessible.

JS 和 Python 共通大概有：

• 全局作用域
• 函数局部作用域
• 函数闭包作用域

JS 特有：

• 块级作用域
• 动态作用域（this）
  • Context in runtime（called）
  • how to control?
  • apply()
  • call()
  • bind()
  • 推荐文章：What You Should Already Know about JavaScript Scope
  • JS Tutor Control This 示范

Python 特有（可能）：

• 内建作用域（JS 其实也有类似的东西，但似乎不怎么提）

词法作用域：

• 全局
• 函数
• 闭包
• 块级（看语言自己的实现）

[xxleyi]

environment & scope

强烈推荐一篇文章（文章成篇较早，没涉及 let const 语法，但丝毫不影响这些概念）：Variables: Scopes, Environments, and Closures

[xxleyi]

应用举例：currying

let curriedAdd = a => b => a + b
let eleven = curriedAdd(5)(6)
console.log(eleven === 11)

// 进阶版
let curred2 = h => a => b => h(a, b)
let curriedAdd = curred2((a, b) => a + b)
let addFive = curriedAdd(5)
let eleven = addFive(6)
console.log(eleven === 11)

JS Tutor curry2 演示 推荐文章：Currying in JavaScript ES6

def curry2(h):
    def f(x):
        def g(y):
            return h(x, y)
        return g
    return f

make_adder = curry2(lambda x, y: x + y)
add_three = make_adder(3)
add_four = make_adder(4)
five = add_three(2)

# lambda expression
curry2 = lambda h: lambda x: lambda y: h(x, y)
make_adder = curry2(lambda x, y: x + y)
add_three = make_adder(3)
add_four = make_adder(4)
five = add_three(2)

Python3 Tutor curry2 演示

[xxleyi]

Python 中的 __closure__ 到底是个什么东西？

def outer():
    x = 100
    def inner():
        print(x)
    print(inner.__closure__)    # not None
    inner()

outer()

Python Tutor 演示

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现在回头来看，价值依旧。