Refer to a class in Ruby

Question

In Ruby I create a module like this:

Mod = Module.new do
  class MyClass
    attr_reader :a
    def initialize(a)
      @a = a
    end
  end
end

And I'm trying to create an instance of the class:

puts Mod::MyClass.new(1).a

But I get an error:

(irb):10:in `<main>': uninitialized constant Mod::MyClass (NameError)

puts Mod::MyClass.new(1).a

Although that's how it works:

Mod.module_eval { puts MyClass.new(1).a }
1
 => nil

I'm also trying to make the class public:

Mod.instance_eval do
  public_constant :MyClass
end

But I also get an error:

(irb):16:in `public_constant': constant Mod::MyClass not defined (NameError)

  public_constant :MyClass

But if you create it like this, then everything works:

module Mod2
  class MyClass
    attr_reader :a
    def initialize(a)
      @a = a
    end
  end
end
puts Mod2::MyClass.new(1).a
1
 => nil

Is there a way to access Mod::MyClass without eval?

puts Mod::MyClass.new(1).a

MyClass is defined in the outer scope, so referring to it as MyClass will work. This occurs because Module.new do does not create a proper "scope gate" — engineersmnky
– engineersmnky, Commented Jun 17 at 23:58
"access Mod::MyClass without eval" – even with eval there’s no Mod::MyClass. You’re merely referring to the top level class from within the module. — Stefan
– Stefan, Commented Jun 18 at 6:01
Just wondering because of puts Mod::MyClass.new(1).a – why are you creating an anonymous module in the first place if you're referring to it via a constant anyway? Or is this more of a conceptual question? — Stefan
– Stefan, Commented Jun 25 at 13:12

Stefan · Accepted Answer · 2025-06-18 07:38:03Z

6

The module keyword creates a namespace:

Module.nesting #=> []       # <- top level

module Mod
  Module.nesting #=> [Mod]  # <- module namespace

  class MyClass
    Module.nesting #=> [Mod::MyClass, Mod]
  end
end

whereas Module.new doesn't:

Module.nesting #=> []       # <- top level

Mod = Module.new do
  Module.nesting #=> []     # <- still top level

  class MyClass
    Module.nesting #=> [MyClass]
  end
end

Which is why your class is defined on the top level despite being created from inside the module. Namespace-wise, your code is equivalent to:

Mod = Module.new do
  # ...
end

class MyClass
  # ...
end

You can use the scope resolution operator :: to explicitly define a class under an existing module. (see the docs on Nesting)

Within the Module.new block this would either be self:

Mod = Module.new do
  class self::MyClass
    # ...
  end
end

or the block's argument which refers to the module object:

Mod = Module.new do |m|
  class m::MyClass
    # ...
  end
end

or the constant you're assigning the anonymous module to: (note that the constant becomes available only after the block has been evaluated)

Mod = Module.new do
  # ...
end

class Mod::MyClass
   # ...
end

... depending on your use case.

edited Jun 18 at 7:38

answered Jun 18 at 7:22

Stefan

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