纯函数式编程：如何将元素添加到列表中？ [英] pure functional programming: how to add an element to a list?

问题描述

在纯函数式编程中，我们不修改任何函数的参数。那么，我们如何设计一个函数来为其参数添加一个元素，一个列表？例如，函数列表添加（elem，list）。此问题与此主题类似：功能性编程：状态与重新分配。

我的解决方案是深层复制输入列表，然后使用一些破坏性操作，如 append 来操纵新的。我是正确的吗？

追加

我从图表-copy算法：

  //在节点
中public节点deepCopy（Map< Node，Node> isomorphism）{
节点拷贝= isomorphism.get（this）; 
 if（copy == null）{
 copy = new Node（）; 
 isomorphism.put（this，copy）; 
 for（Node connection：connections）{
 copy.connections.add（connection.deepCopy（isomorphism））; 
} 
} 
返回副本; 
} 
  

要深度复制图形，必须跟踪每个正在复制的节点。在宇宙中，我们使用同构参数来做到这一点。我认为使这个 deepCopy 操作的纯函数版本的唯一方法是不仅返回变量 copy ，而且还有一个标志，指示返回的节点是否是新节点。对吗？

解决方案

是的，您必须返回一个添加元素的新列表。

但是，这不需要深层复制。功能性语言通常具有持久性数据结构，可以让部分结构在各个版本之间共享，因此你可以例如在O（1）时间前添加一个元素到列表中。

In pure functional programming, we do not modify any arguments of a function. Then, how can we design a function that adds an element to its argument, a list? For example, function list add (elem, list). This question is similar to this thread: Functional programming: state vs. reassignment .

My guess solution is to deep-copy the input list and then to use some destructive operations like append to manipulate the new one. Am I right?

appending

I copy the following code from a graph-copy algorithm:

// in Node
public Node deepCopy(Map<Node, Node> isomorphism) {
    Node copy = isomorphism.get(this);
    if (copy == null) {
        copy = new Node();
        isomorphism.put(this, copy);
        for (Node connection: connections) {
            copy.connections.add(connection.deepCopy(isomorphism));
        }
    }
    return copy;
}

To deep-copy a graph, one must track every node that is being copied. In the universe, we use the isomorphism argument to do that. I think the only way to make a pure functional version of this deepCopy operation is to return not only the variable copy but also a flag indicating whether the returned node is new one. Right?

解决方案

Yes, you have to return a new list with the element added.

However, this doesn't have to require a deep copy. Functional languages typically feature persistent data structures which allow parts of the structure to be shared between versions, so that you can e.g. prepend an element to a list in O(1) time.

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