Optional identity for Enumerable#sum defaults to zero. Closes #5657.

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@4599 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

activesupport/CHANGELOG

@@ -1,5 +1,7 @@
 *SVN*
 
+* Optional identity for Enumerable#sum defaults to zero. #5657 [gensym@mac.com]
+
 * HashWithIndifferentAccess shouldn't confuse false and nil. #5601 [shugo@ruby-lang.org]
 
 * Fixed HashWithIndifferentAccess#default #5586 [chris@seagul.co.uk]

activesupport/lib/active_support/core_ext/enumerable.rb

@@ -30,7 +30,14 @@ module Enumerable #:nodoc:
   #
   # Also calculates sums without the use of a block:
   #   [5, 15, 10].sum # => 30
-  def sum(&block)
+  #
+  # The default identity (sum of an empty list) is zero. 
+  # However, you can override this default:
+  #
+  # [].sum(Payment.new(0)) { |i| i.amount } # => Payment.new(0)
+  #
+  def sum(identity = 0, &block)
+    return identity unless size > 0
     if block_given?
       map(&block).sum
     else

activesupport/test/core_ext/enumerable_test.rb

@@ -44,7 +44,13 @@ class EnumerableTests < Test::Unit::TestCase
     assert_raise(TypeError) { payments.sum(&:price) }
     assert_equal 60, payments.sum { |p| p.price.to_i * 2 }
   end
-
+  
+  def test_empty_sums
+    assert_equal 0, [].sum
+    assert_equal 0, [].sum { |i| i }
+    assert_equal Payment.new(0), [].sum(Payment.new(0))
+  end
+  
   def test_index_by
     payments = [ Payment.new(5), Payment.new(15), Payment.new(10) ]
     assert_equal(