A Short Guide on How to Use Include, Require, and Modules in Ruby


Having become accustomed to thinking of include as a preprocessor directive as in C (i.e., #include), I have been a little confused about Ruby's own use of the statement. In addition, the require keyword is also common in Ruby. From a C perspective, this all seems quite strange. At least that was how I felt until I finally got fed up with not understanding the difference between include and require and finally made sense of it all.

The explanation from Programming Ruby 1.9 (i.e., the "pickaxe book") is especially good for those coming to Ruby from C:

[The include statement] 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.

Let's look at a basic example. If we have a file called hello.rb which simply defines a method say_hello, then we can gain access to the function in a second file by using the following line. Note that the file must exist within Ruby's $LOAD_PATH.

require 'hello'

In the second file, we can call the function directly with say_hello as we would in the original file. Of course, this is an ugly way to bring in our function because it pollutes the global namespace.

The answer to this problem is to wrap up our function into a module. Let's call it Greeter. Now when we require our original file, we can call the say_hello function by invoking it through the module name: Greeter.say_hello.

The include statement lets us take things one step further. Let's say we have a Person class and want to include the say_hello function from the Greeter module. Basically, we want to be able to call the say_hello function on an instance of Person without bothering to define a new method. The include handles all this for us.

class Person
  include Greeter

In effect, the include statement pulls in all the methods defined in Greeter, which at this point consists of only the say_hello function. Now we can call the method directly on an instance of Person as bob.say_hello.

As a side note, there is a minor difference in syntax in the module depending on how it will be used. To use methods outside of a class, each method within the module must be defined on the module, e.g., Greeter.say_hello. Otherwise, if the methods within a module will be included within a class, then methods should be defined without any prefix, e.g., say_hello.

Sytanx details aside, let's return to look at Ruby itself now with a sense of how modules and include works.

There are a number of "built-in" modules which are included on the foundational Object class. The Kernel module is a good example. It provides all sorts of methods used throughout Ruby. For instance, Kernel defines puts and major classes like Array and String. Because Object brings in the Kernel module using include (that's the effect, at least), we can simply call puts directly without any prefix.

Meanwhile, in addition to the built-in modules, Ruby likewise ships with the Standard Library which includes more than one hundred additional modules. A particular libray module is loaded in the same way as seen above. For example, to use FileUtils, we first pull it in with a require statement. The file name should be all lowercase:

require 'fileutils'

Then, if we don't want to prefix each method call with the module name, we can use include in the appropriate place and all the module's methods will be available locally.

In short, require and include are quite simple: require pulls in a file's modules and include localizes the module to the current namespace. It's stuff like this that makes Ruby so much fun.