configuration.rb

c = Configuration.new
c[:foo][:bar][:baz] = [1,2,3]
c['foo']['bar']['baz'] # => [1,2,3]

c[:foo] = {bar: :baz}
c[:foo][:another][:brick][:in][:the] = :wall

c # => {:foo => {:bar => :baz, :another => {:brick => {:in => {:the => :wall}}}}}

class Configuration < ::Hash
  def initialize(hash={})
    configure_from_hash(hash)
  end

  def configure_from_hash(hash)
    hash.each do |key, value|
      if value.is_a?(Hash)
        self[key] = Configuration.new(value)
      else
        self[key] = value
      end
    end
  end

  def [](key)
    super(key.to_sym)
  end

  def []=(key, value)
    if value.is_a?(Hash) && !value.is_a?(self.class)
      self[key].configure_from_hash(value)
    else
      super(key.to_sym, value)
    end
  end

  def default(key)
    self[key] = Configuration.new
  end

  def default=(*args)
    raise RuntimeError, "#{self.class} does not allow changing the default hash value"
  end
end

configuration_spec.rb

describe Configuration do
  it { should be_a(Hash) }

  describe '#[]' do
    it 'creates keys on the fly' do
      subject[:foo]
      subject.should have_key(:foo)
    end

    it 'uses our Configuration class when creating keys' do
      subject[:foo].should be_a(described_class)
    end

    it 'normalizes keys to symbols' do
      subject['foo']
      subject.should have_key(:foo)
      subject.should_not have_key('foo')
    end
  end

  describe '#[]=' do
    it 'normalizes keys to symbols' do
      subject['foo'] = :bar
      subject.should have_key(:foo)
      subject.should_not have_key('foo')
    end

    context 'when a hash is passed as the value' do
      it 'should be converted to a Configuration hash' do
        subject[:foo] = {bar: :baz}
        subject[:foo].should be_a(described_class)
      end
    end
  end

  describe '#default=' do
    it 'raises an error' do
      expect { subject.default = proc { } }.to raise_error
    end
  end
end