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。
当前回答
只是做同一件事的不同方式。
Consider a simple example such as adding the numbers 3, 5, and 10. Imagine thinking about doing that by first changing the value of 3 by adding 5 to it, then adding 10 to that "3", then outputting the current value of "3" (18). This seems patently ridiculous, but it is in essence the way that state-based imperative programming is often done. Indeed, you can have many different "3"s that have the value 3, yet are different. All of this seems odd, because we have been so ingrained with the, quite enormously sensible, idea that the numbers are immutable.
现在，当您将值设置为不可变时，考虑添加3、5和10。3加5得到另一个值8，然后再加10得到另一个值18。
这是做同样事情的等价方法。所有必要的信息都存在于这两种方法中，只是形式不同。其中，信息以状态的形式存在，并存在于改变状态的规则中。在另一种情况下，信息存在于不可变数据和函数定义中。
其他回答
我现在才开始讨论这个问题，但是我想为那些正在与函数式编程作斗争的人补充几点。
函数式语言维护与命令式语言完全相同的状态更新，但它们是通过将更新后的状态传递给后续的函数调用来实现的。这是一个沿着数轴移动的简单例子。您的状态是您当前的位置。
首先是命令式方式(在伪代码中)
moveTo(dest, cur):
while (cur != dest):
if (cur < dest):
cur += 1
else:
cur -= 1
return cur
现在是函数式的方式(在伪代码中)。我非常依赖三元运算符，因为我希望有命令式背景的人能够读懂这段代码。所以如果你不经常使用三元运算符(我总是避免它在我的命令式的日子)下面是它是如何工作的。
predicate ? if-true-expression : if-false-expression
您可以通过将一个新的三元表达式放在假表达式的位置来连接三元表达式
predicate1 ? if-true1-expression :
predicate2 ? if-true2-expression :
else-expression
考虑到这一点，下面是函数版本。
moveTo(dest, cur):
return (
cur == dest ? return cur :
cur < dest ? moveTo(dest, cur + 1) :
moveTo(dest, cur - 1)
)
这是一个简单的例子。如果这是在游戏世界中移动人，你就必须引入一些副作用，如在屏幕上绘制对象的当前位置，并根据对象移动的速度在每次调用中引入一些延迟。但你仍然不需要可变状态。
The lesson is that functional languages "mutate" state by calling the function with different parameters. Obviously this doesn't really mutate any variables, but that's how you get a similar effect. This means you'll have to get used to thinking recursively if you want to do functional programming. Learning to think recursively is not hard, but it does take both practice and a toolkit. That small section in that "Learn Java" book where they used recursion to calculate factorial does not cut it. You need a toolkit of skills like making iterative processes out of recursion (this is why tail recursion is essential for functional language), continuations, invariants, etc. You wouldn't do OO programming without learning about access modifiers, interfaces etc. Same thing for functional programming.
我的建议是做小Schemer(注意我说的是“做”而不是“读”)，然后做SICP的所有练习。当你完成时，你的大脑会和刚开始时不一样。
Functional programming avoids state and emphasizes functionality. There's never any such thing as no state, though the state might actually be something that's immutable or baked into the architecture of what you're working with. Consider the difference between a static web server that just loads up files off the filesystem versus a program that implements a Rubik's cube. The former is going to be implemented in terms of functions designed to turn a request into a file path request into a response from the contents of that file. Virtually no state is needed beyond a tiny bit of configuration (the filesystem 'state' is really outside the scope of the program. The program works the same way regardless of what state the files are in). In the latter though, you need to model the cube and your program implementation of how operations on that cube change its state.
除了别人给出的很好的答案，想想Java中的Integer和String类。这些类的实例是不可变的，但这并不意味着仅仅因为它们的实例不可更改，这些类就毫无用处。不可变性给了你一定的安全性。您知道，如果使用String或Integer实例作为Map的键，则该键不能更改。将其与Java中的Date类进行比较:
Date date = new Date();
mymap.put(date, date.toString());
// Some time later:
date.setTime(new Date().getTime());
你已经无声地改变了地图中的一个键!使用不可变对象(如在函数式编程中)要干净得多。更容易推断会发生什么副作用——没有!这意味着程序员更容易，优化器也更容易。
JavaScript provides very clear examples of the different ways of approaching mutable or immutable state\values within its core because the ECMAScript specifications were not able to settle on a universal standard so one must continue to memorize or doublecheck which functions create a new object that they return or modify the original object passed to it. If your entire language is immutable then you know you are always getting a new (copied & possibly modified) result and never have to worry about accidentally modifying the variable before passing it into a function.
你知道哪个会返回一个新的对象，哪个会改变下面例子中的原始对象吗?
Array.prototype.push()
String.prototype.slice()
Array.prototype.splice()
String.prototype.trim()
你不可能有一种有用的纯函数式语言。总会有一定程度的可变性需要处理，IO就是一个例子。
将函数式语言视为您使用的另一种工具。它对某些事情有好处，但对其他事情没有好处。你给出的游戏例子可能不是使用函数语言的最佳方式，至少屏幕会有一个可变的状态，你不能用FP做任何事情。使用FP思考问题的方式和解决问题的类型将不同于使用命令式编程所习惯的方式。
