🌏 梦开始的地方

题目

用你找到的三门语言（注意，某语言的方言和该语言算作同一种语言）写出下文 Scheme 语言的等价代码：

(define (make-withdraw balance)
  (lambda (amount)
    (if (>= balance amount)
        (begin (set! balance (- balance amount))
               balance)
        "v我50，撫慰我支離破碎的心")))

(define W1 (make-withdraw 100))
(define W2 (make-withdraw 100))

(W1 50)
; 50
(W2 70)
; 30
(W1 20)
; 30

附加题：

• 20%：在用法不变的前提下，用你用过的其中一种语言或者 Lisp 及其方言增加以下功能：

  (v50 W1)

  使得余额增加 50 元。

说明

这道题会让人不由自主的写一个类出来当伪函数，然而出题人的本意是使用闭包，因此问题关键点在于如何捕获。

题解

下面给出四种语言的解

C++

#include <iostream>
#include <functional>

std::function<void(int)> withdraw(int balance) {
    return [balance] (int amount) mutable {
        if (balance >= amount) {
            balance -= amount;
            if (amount >= 0)
                std::cout << balance << std::endl;
        } else {
            std::cout << "v我50，撫慰我支離破碎的心" << std::endl;
        }
    };
}

void v50(std::function<void(int)>& w) {
    w(-50);
}


int main() {
    auto w1 = withdraw(100), w2 = withdraw(100);
    w1(50);
    w2(70);
    w2(50);
    v50(w2);
    w2(50);
}

Python

我提交的版本：

def withdraw(balance):
    def apply(amount, balance = [balance]):
        if balance[0] >= amount:
            balance[0] -= amount
            if amount >= 0:
                print(balance[0])
        else:
            print('v我50，撫慰我支離破碎的心')
    return apply


def v50(w):
    w(-50)


w1 = withdraw(100)
w2 = withdraw(100)
w1(50)
w2(70)
w2(50)
v50(w2)
w2(50)

更好的版本：

def withdraw(balance):
    def apply(amount):
        nonlocal balance
        if balance >= amount:
            balance -= amount
            if amount >= 0:
                print(balance)
        else:
            print('v我50，撫慰我支離破碎的心')
    return apply


def v50(w):
    w(-50)


w1 = withdraw(100)
w2 = withdraw(100)
w1(50)
w2(70)
w2(50)
v50(w2)
w2(50)

JavaScript

function withdraw(balance) {
    return function(amount) {
        if (balance >= amount) {
            balance -= amount
            if (amount >= 0) console.log(balance)
        } else {
            console.log('v我50，撫慰我支離破碎的心')
        }
    }
}

function v50(w) {
    w(-50)
}

var w1 = withdraw(100)
var w2 = withdraw(100)
w1(50)
w2(70)
w2(50)
v50(w2)
w2(50)

Kotlin

fun withdraw(balance: Int): (Int)->Unit {
    var balance = balance
    return { amount: Int->
        if (balance >= amount) {
            balance -= amount
            if (amount >= 0)
                println(balance)
        } else {
            println("v我50，撫慰我支離破碎的心")
        }
    }
}

fun v50(w: (Int)->Unit) = w(-50)


fun main() {
    val w1 = withdraw(100)
    val w2 = withdraw(100)
    w1(50)
    w2(70)
    w2(50)
    v50(w2)
    w2(50)
}