功能编程：副作用 [英] Functional Programming: Side Effects

问题描述

大多数教程/文章/书籍在介绍函数式编程时讨论副作用。拿这个Python代码：

  def isPrime（n）：
 k = 2 
 while k < n：
如果n％k == 0：
返回False 
k + = 1 
返回True 
  

文字说上述函数有局部副作用，我不明白。我看到变量k正在改变，我不明白它会带来什么坏处。

有人可以给出一个明显的不良副作用的例子，以及如何通过函数式编程避免它？

解决方案

您所指的文本是正确的，副作用。

并不是说这是一件坏事。这只是功能编程。在纯粹的函数式编程语言中，您将以递归方式编写循环，无需更改变量。

写这样的函数（没有可观察到的副作用）是任何语言的一个很好的练习，它只是一种功能性编程。

编辑：现在我看到您对不良副作用的评论。我不会说副作用不好。在大多数主流语言中，没有它们就很难进行编程，我认为很多程序员都会考虑副作用。但是在大型软件项目中，过度依赖副作用会让你的生活变得非常悲惨。 这是一个很好的例子，涉及到单身人士（导致副作用的最终方式）

在一种禁止副作用的语言中，作为程序员的你和作为编译器的人都不会感到惊讶。纯函数代码更易于分析和并行化，并且至少在理论上更容易被编译器优化。

Most tutorials/articles/books talk about side effects when presenting functional programming. Take this Python code:

def isPrime(n):
    k = 2
    while k < n:
        if n % k == 0:
            return False
        k += 1
    return True

The text says that the above function has local side effects, and I don't understand that. I see that the variable "k" is changing, I don't understand what bad comes out of it.

Can someone please give a clear example of bad side effect, and how it is avoided by functional programming?

解决方案

The text you're referring to is right, changing a local variable is considered a side effect.

It doesn't say that this is a bad thing necessarily. It is just not functional programming. In pure functional programming languages you would write the loop in a recursive way, eliminating the need for changing variables.

Writing functions like these (that have no observable side effects) is a fine practice in any language, it is just not functional programming.

Edit: Now I see your remark about "bad" side effects. I would not say that side effects are bad. In most mainstream languages, it is hard to program without them, and I think that many programmers think in terms of side effects. But in large software projects, relying too much on side effects can make your life pretty miserable. Here's a nice example involving singletons (the ultimate way to cause side effects)

In a language that forbids side effects, there are less surprises for you as a programmer, but also for the compiler. Pure functional code is easier to analyze and paralellize, and is, at least theoretically, easier to optimize by the compiler.

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