Retrieve a value from a Hash. If the key is not in the hash, return the given default value and save it to the hash

hash_evoke.rb

class Hash
  # Pass either a default value or a block to return it (a la Hash#fetch()). 
  # If both are passed, the block will take precedence over the default value
  def evoke(key, default = nil)
    if include?(key)
      self[key]
    else
      self[key] = block_given? ? yield : default
    end
  end
end


class TestHashExtensions < Test::Unit::TestCase
  def test_evoke_found_block
    hash = {:test => "test"}
    assert_equal "test", hash.evoke(:test) {"not test"}
  end
  
  def test_evoke_found_default
    hash = {:test => "test"}
    assert_equal "test", hash.evoke(:test, "not test")
  end

  def test_evoke_returns_default
    hash = {}
    assert_equal "not test", hash.evoke(:test, "not test")
  end
  
  def test_evoke_returns_block
    hash = {}
    assert_equal "test", hash.evoke(:test) {"test"}
  end

  def test_evoke_found_default_does_not_add
    hash = {:test => "test"}
    hash.evoke(:test, "not test")
    assert_equal "test", hash[:test]
  end
  
  def test_evoke_found_block_does_not_add
    hash = {:test => "test"}
    hash.evoke(:test) {"not test"}
    assert_equal "test", hash[:test]
  end

  def test_evoke_default_adds
    hash = {}
    hash.evoke(:test, "test")
    assert_equal "test", hash[:test]
  end

  def test_evoke_block_adds
    hash = {}
    hash.evoke(:test) { "test" }
    assert_equal "test", hash[:test]
  end
end