DigBang: Safely unsafe hash traversal

dig_bang.rb

# `Hash#dig!`
#
# Like Ruby 2.3's `Hash#dig`, but raises an exception instead of returning `nil` 
# when a key isn't found.
#
# Ruby 2.3 introduces the new Hash#dig method for safe extraction of
# a nested value. See http://ruby-doc.org/core-2.3.0/Hash.html#method-i-dig.
# It is the equivalent of a safely repeated Hash#[].
#
# `#dig!`, on the other hand, is the equivalent of a repeated `Hash#fetch`. It's 
# 'safely unsafe', raising the appropriate `KeyError` anywhere down the line, or 
# returning the value if it exists.
#
# @example
#   places = { countries: { canada: true } }
#   places.dig  :countries, :canada  # => true
#   places.dig! :countries, :canada  # => true
#   places.dig  :countries, :canada, :ontario  # => nil
#   places.dig! :countries, :canada, :ontario  # => KeyError: Key not found: :ontario
#
class Hash
  def dig!(*keys)
    keys.reduce(self) { |a, e| a.fetch(e) }
  end
end

dig_bang_spec.rb

require 'spec_helper'
require 'ext/dig_bang'

DOG = {
  name: 'Phoebe',
  age: 7,
  vaccinations: {
    '2011-07-20' => :all
  }
}.freeze

describe Hash do
  describe '#dig!' do
    it 'returns an existing key/value' do
      expect( DOG.dig!(:name) ).to eq 'Phoebe'
    end

    it 'returns an existing key/value from a list' do
      expect( DOG.dig!(:vaccinations, '2011-07-20') ).to eq :all
    end

    it 'raises a KeyError if a key is not found' do
      expect { DOG.dig! :weight }.to raise_error(KeyError, /weight/)
    end

    it 'raises a KeyError if a sub-key is not found' do
      expect {
        DOG.dig! :vaccinations, '2012-01-01'
      }.to raise_error(KeyError, /2012-01-01/)
    end
  end
end