Add dynamic defaults to a Hash

hash_extensions.rb

require 'rubygems'
require 'shoulda'

class Hash
  def with_defaults( default_blocks )
    new_hash = Hash.new do |hash, key|
      value = default_blocks[key]
      if value.respond_to? :call
        value = value.call
      end
      value
    end
    new_hash.merge!(self)
  end
end

class TestHashExtensions < Test::Unit::TestCase
  def self.should_return_original_values
    should "return the original value for a non-defaulted key" do
      assert_equal "original value", subject[:original_key]        
    end
  
    should "return the original value for a defaulted key" do
      assert_equal "original value to be defaulted", subject[:to_be_defaulted]        
    end
  
    should "return nil for a never-set key, as normal" do
      assert_nil subject[:never_set]
    end
  end

  context "A hash" do
    setup do
      @hash = {:original_key => "original value", 
        :to_be_defaulted => "original value to be defaulted" }
    end
    
    context "with a value-based default" do      
      subject do
        @hash.with_defaults( { 
          :defaulted => "default value", 
          :to_be_defaulted => "default value for to_be_defaulted"}
        )
      end

      should "return that default" do
        assert_equal "default value", subject[:defaulted]
      end
      
      should_return_original_values
    end
    
    context "with a closure-based default" do
      subject do
        @hash.with_defaults( { 
          :defaulted => lambda { "value of default block" }, 
          :to_be_defaulted => lambda{"value of default block for to_be_defaulted"} 
        })
      end

      should "return the value of the default block" do
        assert_equal "value of default block", subject[:defaulted]
      end
      
      should_return_original_values
    end
  end
end