最近我读了很多关于函数式编程的东西,大部分我都能理解,但有一件事我就是搞不懂,那就是无状态编码。在我看来,通过删除可变状态来简化编程就像通过删除仪表盘来“简化”一辆汽车:最终产品可能更简单,但希望它能与最终用户交互。
几乎我能想到的每个用户应用程序都将状态作为核心概念。如果你写了一个文档(或一个SO post),状态会随着每一个新的输入而改变。或者如果你玩电子游戏,会有大量的状态变量,从所有角色的位置开始,这些角色往往会不断移动。如果不跟踪不断变化的值,您怎么可能做任何有用的事情呢?
每次我发现一些讨论这个问题的东西,它都是用真正的技术函数语言写的,假设我没有浓厚的FP背景。有谁知道如何向那些对命令式编码有很好的、扎实的理解,但在函数方面完全是n00b的人解释这一点吗?
编辑:到目前为止,一堆回复似乎试图让我相信不可变值的优点。我懂你的意思。这很有道理。我不明白的是,在没有可变变量的情况下,如何跟踪必须不断变化的值。
For highly interactive applications such as games, Functional Reactive Programming is your friend: if you can formulate the properties of your game's world as time-varying values (and/or event streams), you are ready! These formulae will be sometimes even more natural and intent-revealing than mutating a state, e.g. for a moving ball, you can directly use the well-known law x = v * t. And what's better, the game's rules written such way compose better than object-oriented abstractions. For example, in this case, the ball's speed can be also a time-varying value, which depends on the event stream consisting of the ball's collisions. For more concrete design considerations, see Making Games in Elm.
For highly interactive applications such as games, Functional Reactive Programming is your friend: if you can formulate the properties of your game's world as time-varying values (and/or event streams), you are ready! These formulae will be sometimes even more natural and intent-revealing than mutating a state, e.g. for a moving ball, you can directly use the well-known law x = v * t. And what's better, the game's rules written such way compose better than object-oriented abstractions. For example, in this case, the ball's speed can be also a time-varying value, which depends on the event stream consisting of the ball's collisions. For more concrete design considerations, see Making Games in Elm.
简单的回答是:你不能。
那么不变性有什么好大惊小怪的呢?
If you're well-versed in imperative language, then you know that "globals are bad". Why? Because they introduce (or have the potential to introduce) some very hard-to-untangle dependencies in your code. And dependencies are not good; you want your code to be modular. Parts of program not influence other parts as little as possible. And FP brings you to the holy grail of modularity: no side effects at all. You just have your f(x) = y. Put x in, get y out. No changes to x or anything else. FP makes you stop thinking about state, and start thinking in terms of values. All of your functions simply receive values and produce new values.
这有几个优点。
首先,没有副作用意味着程序更简单,更容易推理。不用担心引入程序的新部分会干扰并使现有的正在工作的部分崩溃。
其次,这使得程序的可并行性微不足道(有效的并行化是另一回事)。
第三,有一些可能的性能优势。假设你有一个函数:
double x = 2 * x
现在你输入一个3的值,得到一个6的值。每一次。但是在祈使句中也可以这样做,对吧?是的。但问题是,在命令式中,你可以做更多的事情。我可以:
int y = 2;
int double(x){ return x * y; }
但我也可以
int y = 2;
int double(x){ return x * (y++); }
命令式编译器不知道我是否会有副作用,这使得优化更加困难(即double 2不必每次都是4)。函数函数知道我不会——因此,它可以在每次看到“double 2”时进行优化。
现在,即使每次创建新值对于复杂类型的值在计算机内存方面看起来是难以置信的浪费,但它不必如此。因为,如果你有f(x) = y,并且x和y的值“基本相同”(例如,只有少数叶子不同的树),那么x和y可以共享部分内存——因为它们都不会突变。
如果这个不可变的东西这么好,为什么我说没有可变状态就不能做任何有用的事情。如果没有可变性,整个程序就是一个巨大的f(x) = y函数。同样的道理也适用于程序的所有部分:只是函数,而且是“纯粹”意义上的函数。我说过,这意味着每次都是f(x) = y。因此,例如readFile("myFile.txt")每次都需要返回相同的字符串值。不是很有用。
因此,每个FP都提供了一些突变状态的方法。“纯”函数语言(例如Haskell)使用一些可怕的概念(如单子)来做到这一点,而“不纯”函数语言(例如ML)则直接允许这样做。
当然,函数式语言还带来了许多其他优点,使编程更加高效,比如一类函数等。