From: Phrogz <gavin@...>
Date: 2006-02-27T03:18:35+09:00
Subject: Re: Scope of constants in instance_eval

For those interested in doing the same, I've hacked up a quick method
for promoting the constants of a class/module to the main Object scope,
and then restoring the original values.

You can:
  * Automatically promote all constants of the class/module
  * Promote only specific constants
  * Choose to explicitly restore the constants or
  * Supply a block that will be yielded to before the constants are
automatically restored

class Module
  # Push the constants of this class/module up to global space
  # saving any original values in Object for later restoration
  #
  # If no names are supplied, all constants in this class/module
  # are promoted.
  #
  # If a block is supplied, it will be yielded to before automatically
  # restoring the global scope to its pristine state.
  def promote_constants( *const_names )
    const_names.flatten!
    const_names = self.constants if const_names.empty?
    @_original_constant_values = {}
    const_names.each { |name|
      if self.const_defined?( name )
        if Object.const_defined?( name )
          @_original_constant_values[ name ] = Object.const_get( name )

        end
        Object.const_set( name, self.const_get( name ) )
      else
        warn "Cannot promote non-existent constant '#{name}'"
      end
    }
    if block_given?
      yield
      restore_constants
    end
  end

  # Restore Object's constants to undo the effects of
#promote_constants
  def restore_constants( *const_names )
    return unless @_original_constant_values
    const_names.flatten!
    const_names = self.constants if const_names.empty?
    const_names.each { |name|
      if val=@_original_constant_values[ name ]
        Object.const_set( name, val )
      else
        Object.instance_eval{
          remove_const( name )
        }
      end
    }
    @_original_constant_values = nil
  end
end