来自编程Ruby 1.9

We’ll make a couple of points about the include statement before we go on. First, it has nothing to do with files. C programmers use a preprocessor directive called #include to insert the contents of one file into another during compilation. The Ruby include statement simply makes a reference to a module. If that module is in a separate file, you must use require (or its less commonly used cousin, load) to drag that file in before using include. Second, a Ruby include does not simply copy the module’s instance methods into the class. Instead, it makes a reference from the class to the included module. If multiple classes include that module, they’ll all point to the same thing. If you change the definition of a method within a module, even while your program is running, all classes that include that module will exhibit the new behavior.

您曾经尝试过要求一个模块吗?结果如何?试试:

MyModule = Module.new
require MyModule # see what happens

模块不是必需的，只是包含的!

What's the difference between "include" and "require" in Ruby? Answer: The include and require methods do very different things. The require method does what include does in most other programming languages: run another file. It also tracks what you've required in the past and won't require the same file twice. To run another file without this added functionality, you can use the load method. The include method takes all the methods from another module and includes them into the current module. This is a language-level thing as opposed to a file-level thing as with require. The include method is the primary way to "extend" classes with other modules (usually referred to as mix-ins). For example, if your class defines the method "each", you can include the mixin module Enumerable and it can act as a collection. This can be confusing as the include verb is used very differently in other languages.

源

所以如果你只是想使用一个模块，而不是扩展它或做一个混合，那么你会想要使用require。

奇怪的是，Ruby的require类似于C的include，而Ruby的include几乎与C的include完全不同。

例如:当你使用require 'math'时，你必须写math::PI。 但是当你使用include math时，你可以简单地写成PI。

如果你正在使用一个模块，这意味着你要把所有的方法都带入你的类中。 如果你用一个模块扩展一个类，这意味着你把模块的方法作为类方法“引入”了。 如果你在模块中包含了一个类，这意味着你将模块的方法作为实例方法“引入”了。

EX:

 module A
   def say
     puts "this is module A"
   end
 end

 class B
   include A
 end

 class C
   extend A
 end

B.say =>未定义的方法'说'的B:类

B.new.say 这是模块A

C.say 这是模块A

C.new.say =>未定义的方法'说' C:类

Ruby的require更像其他语言(比如C语言)中的“include”。它告诉Ruby你想引入另一个文件的内容。在其他语言中类似的机制有: 在c#中使用<namespace>指令。 在Java中导入<package>。 Ruby include是用于mixin的面向对象继承机制。

这里有一个很好的解释:

简单的答案是require和include本质上是不相关的。 “require”类似于C中的include，这可能会让新手感到困惑。 (一个显著的区别是所需文件中的局部变量“蒸发”了 当require完成时。) Ruby包含与C包含完全不同。include语句将一个模块“混合”到一个类中。这是有限形式 多重继承。一个被包含的模块字面上赋予了一个“is-a” 对事物的关系包括它。

重点补充道。