Re-opened nested module anomaly in Ruby

module, nested, ruby

Solution

The `module` keyword sets a namespace context that is checked for references to existing names of Modules. These namespaces are then searched inner-to-outer to resolve references to Module (and Class) names.

In your first example, it looks like you may need to define `E.e` inside `module E` block, but in fact you don't:

module A
  module E
  end
end
module A
  def E.e
  end
end

What happens in both your examples is that Ruby looks at the current namespace, and tries `<namespace>::E` as a module name. So in both examples, the first thing it checks is in fact `A::E::E` which does not exist. Then it falls back to the next context. Which is where the examples differ: In the first example it is `A::E` which is valid, in the second example, it is just `E` which is not. The error that it then throws relates to the first name it checked.

Problem

Why does re-opening a nested module give different results depending on the syntax used? For example, this works fine: ``` module A module E end end module A module E def E.e end end end ``` But this: ``` module A module E end end module A::E def E.e end end ``` gives the error: ``` reopen.rb:6:in `<module:E>': uninitialized constant A::E::E (NameError) from reopen.rb:5:in `<main>' ``` (Before someone points this out, a workaround is to use `self` instead of the module name when defining E.e, but that's not really the point of this post.)

Original source