From: "David A. Black" <dblack@...>
Date: 2008-10-10T00:54:31+09:00
Subject: Re: Enumerating constants in anonymous modules

Hi --

On Fri, 10 Oct 2008, Xavier Shay wrote:

> How can I enumerate all the constants inside the module in the second
> example?
> Notice that #constants array is empty

See comments in code, below.

> For bonus points, which is the "correct" eval method to use to define a
> class inside the module (they all seem to work)

module_eval and class_eval are synonyms for each other. I'd use
module_eval over instance_eval, unless you specifically need instance
methods to be singleton methods on the module object (which is
unlikely, and can be achieved by other means under module_eval).

> module Foo
>  class A
>  end
> end
>
> puts Foo.constants.inspect # => ["A"]
>
> [:module_eval, :instance_eval, :class_eval].each do |method|
>  m = Module.new
>  m.send(method) do
>    class B

The constant B is resolved in the usual "semi-static", eager way,
which means it does not belong to the new module: it's actually being
defined at the top level.

>    end
>  end
>  puts m.constants.inspect      # => [] (I would expect ['B'])
>  puts m.const_get('B').inspect # => B
> end

Try this:

   m = Module.new
   m.module_eval do
     class self::B    # forces Ruby to wait to bind the constant
     end
   end
   puts m.constants.inspect         #   ["B"]
   puts m.const_get('B').inspect    #   #<Module:0x220164>::B


David

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