include module in ruby

Question

Hi All,

I am a starter with ruby, I searched that if someone else has asked similar question but was not able to find any. so I am asking it here.

I am trying my hand at modules in ruby.

I created a folder Project

inside Project folder, created a class One

class Project::One

  include Project::Rest

end

inside Project folder, created a module Rest

module Project::Rest

 def display
   puts "in display"
 end

end

but when I try to run the program(ruby one.rb) I get the

uninitialized constant Project (NameError)

Please help me

Answer 1

The issue is that you're not nesting your classes/modules correctly. You have to declare a module with the module keyword, not merely by writing class Project::Class. Assuming you have this structure:

Project/
    one.rb
    rest.rb

then your files should look something like this:

# one.rb
require 'rest'

module Project
  class One
    include Project::Rest
  end
end

# rest.rb
module Project
  module Rest
    def display
      puts 'in display'
    end
  end
end

Note how the modules are nested in these examples.

Answer 2

If you have code in multiple files, you have to load those files before you can access what's in them. This is usually done with a require statement. I think what you want to do should look like this:

# one.rb
require 'rest'
module Project
  class One
    include Rest
  end
end


# rest.rb
module Project
  module Rest
    def display
      puts "in display"
    end
  end
end

Answer 3

The problem is that you never actually define the Project constant. You have to define it before you can use it. Example:

# root.rb
module Project
end
require "project/test"

# project/test.rb
class Project::Test
end

You should then be able to run ruby root.rb. Another approach is to state the module in the namespace.

# root.rb
require "project/test"

# project/test.rb
module Project
  class Test
  end
end

With this example, you are able to run ruby project/test.rb as well, since the Project module is defined in that file.

And if you have multiple files defining the Project module, that's not a problem either. It won't be re-defined, it will always be the same module.

Both of these methods will define the Project module. Simply going Project::Test will not, however, define the module.

As a sidenote, Rails has a auto loader. If you're in a rails app, and use a certain folder structure, these kind of intermediate modules will be defined for you. Without Rails, though, you have to define them yourself.