在Ruby中使用to_enum创建可枚举对象的好处是什么？ [英] What is the advantage of creating an enumerable object using to_enum in Ruby?

问题描述

为什么要通过使用to_enum方法而不是直接使用对象来在Ruby中创建对对象的代理引用？我想不出任何实际用途，试图理解这个概念。有人可以使用它，但是我看到的所有示例似乎都很琐碎。

Why would you create a proxy reference to an object in Ruby, by using the to_enum method rather than just using the object directly? I cannot think of any practical use for this, trying to understand this concept & where someone might use it, but all the examples I have seen seem very trivial.

例如，为什么使用：

"hello".enum_for(:each_char).map {|c| c.succ }

而不是

"hello".each_char.map {|c| c.succ }

我知道这是一个非常简单的示例，没有人有任何真实示例？

I know this is a very simple example, does anyone have any real-world examples?

推荐答案

在没有提供块的情况下，大多数接受块的内置方法将返回枚举数（例如 String＃each_char ）。对于这些，没有理由使用 to_enum ；

Most built-in methods that accept a block will return an enumerator in case no block is provided (like String#each_char in your example). For these, there is no reason to use to_enum; both will have the same effect.

不过，有些方法不会返回Enumerator。在这种情况下，您可能需要使用 to_enum 。

A few methods do not return an Enumerator, though. In those case you might need to use to_enum.

# How many elements are equal to their position in the array?
[4, 1, 2, 0].to_enum(:count).each_with_index{|elem, index| elem == index} #=> 2

另一个例子是 Array＃product ， #uniq 和 #uniq！并没有接受块。在1.9.2版中，已对此进行了更改，但是为了保持兼容性，没有块的表单不能返回 Enumerator 。仍然可以手动使用 to_enum 来获取枚举数：

As another example, Array#product, #uniq and #uniq! didn't use to accept a block. In 1.9.2, this was changed, but to maintain compatibility, the forms without a block can't return an Enumerator. One can still "manually" use to_enum to get an enumerator:

require 'backports/1.9.2/array/product' # or use Ruby 1.9.2+
# to avoid generating a huge intermediary array:
e = many_moves.to_enum(:product, many_responses)
e.any? do |move, response|
  # some criteria
end 

to_enum 是在实现自己的迭代方法时。通常，您通常将其作为第一行：

The main use of to_enum is when you are implementing your own iterative method. You typically will have as a first line:

def my_each
  return to_enum :my_each unless block_given?
  # ...
end

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