功能规划：状态与重新分配 [英] Functional programming: state vs. reassignment

问题描述

我需要帮助了解我当前的OOP状态概念与在Haskell或Clojure之类的函数式语言中所做的区别。

I need help getting my head around the difference between my current OOP notion of state, and the way it would be done in a functional language like Haskell or Clojure.

为了使用一个哈希的例子，让我们说，我们处理简化的银行帐户对象/结构/任何。在OOP语言中，我会有一些类来保存对BankAccount的引用，BankAccount将包含利率等实例变量，以及setInterestRate（）等方法，它们改变对象的状态，通常不返回任何内容。在Clojure中，我有一个银行帐户结构（一个荣耀的hashmap）和特殊功能，需要银行帐户参数和其他信息，并返回一个新的结构。因此，不是改变原始对象的状态，我现在有一个新的返回与所需的修改。

To use a hackneyed example, let's say we're dealing with simplified bank account objects/structs/whatever. In an OOP language, I'd have some class holding a reference to a BankAccount, which would have instance variables for things like interest rate, and methods like setInterestRate() which change the object's state and generally return nothing. In say Clojure, I'd have a bank-account struct (a glorified hashmap), and special functions that take a bank-account parameter and other info, and return a new struct. So instead of changing the state of the original object, I now have a new one being returned with the desired modifications.

那么...我该怎么办呢？覆盖任何引用旧银行帐户的变量？如果是这样，那比国家变化的OOP方法有优势吗？最后，在这两种情况下，似乎有一个变量引用对象与必要的更改。像我一样，我只有一个模糊的概念，发生了什么。

So... what do I do with it? Overwrite whatever variable was referencing the old bank-account? If so, does that have advantages over the state-changing OOP approach? In the end, in both cases it seems one has a variable that references the object with the necessary changes. Retarded as I am, I have only a vague concept of what's going on.

我希望有意义，感谢您的帮助！

I hope that made sense, thanks for any help!

推荐答案

在纯粹的功能风格中，你永远不会覆盖任何变量。

In a pure functional style, you'll never overwrite any variable.

类比是物理学中的时空。如果你认为世界是3d的，那么对象没有固定的位置 - 它们随着时间移动。为了使物理世界上的数学，我们因此添加一个时间维度，并考虑在特定时间的各种属性的价值。在这样做的时候，我们将我们的研究对象变成了常量。类似地，在编程中，通过使用不可变值有一个概念上的简单性。在现实世界中具有身份的对象可以被建模为不可变值的序列（对象在增加的时间的状态），而不是作为改变的单个值。

An analogy would be to spacetime in physics. If you consider the world as 3d, then objects don't have fixed positions - they move over time. To bring math to bear on the physical world, we therefore add a time dimension and consider the values of various properties at particular times. In doing so, we've made the objects of our study into constants. Similarly, in programming, there is a conceptual simplicity to be had by working with immutable values. Objects with an identity in the real world can be modeled as a sequence of immutable values (the states of the object at increasing times) rather than as a single value that changes.

当然，如何将值序列与对象标识相关联的细节可能有点毛。 Haskell有 Monads ，可让您建模状态。 功能反应性编程是一个更加直接的尝试，在世界上用纯功能更新建模对象，我认为这是一个非常有前途的方向编程。

Of course the details of how to associate the sequence of values to an "object identity" can be a little hairy. Haskell has Monads that let you model state. Functional Reactive Programming is a more literal attempt at modeling objects in the world with pure functional updates, that I think is a very promising direction for programming.

我会注意到，Clojure，不像Haskell，不是纯粹的，你可以更新变量，你建议。如果你只是在高级别更新几个变量，你仍然可能享受许多功能编程的概念简单的好处。

I will note that Clojure, unlike Haskell, isn't pure, and you can update variables as you suggested. If you're only updating a few variables at a high level, you'll still probably enjoy many of the conceptual simplicity benefits of functional programming.

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