foldr.rb

require 'minitest/unit'

module Enumerable

  def foldr(*args, &block)

    case args.count
    when 2
      init, method = args
    when 1
      if block_given?
        init = args.first
      else
        method = args.first
      end
    end

    lst = self.dup
    init = lst.pop unless init

    if lst.empty?
      init
    else
      if method
        (lst.shift).send(method, lst.foldr(init, method, &block))
      elsif block_given?
        block.call(lst.shift, lst.foldr(init, &block))
      else
        raise StandardError.new("method argument or block needed")
      end
    end
  end

end

class TestFoldr < MiniTest::Unit::TestCase
  def setup
    @list = [1.0, 2.0, 3.0]
  end

  def test_sum
    assert_equal 6, @list.foldr(0, :+)
    assert_equal 6, @list.foldr(0) { |a, b| a + b }
    assert_equal 6, @list.foldr(:+)
    assert_equal 6, @list.foldr { |a,b| a + b }
  end

  def test_div
    assert_equal 1.5, @list.foldr(1.0, :/)
    assert_equal 1.5, @list.foldr(1.0) { |a, b| a / b }
    assert_equal 1.5, @list.foldr(:/)
    assert_equal 1.5, @list.foldr { |a,b| a / b }
  end

  def test_product
    assert_equal 6, @list.foldr(1.0, :*)
    assert_equal 6, @list.foldr(1.0) { |a, b| a * b }
    assert_equal 6, @list.foldr(:*)
    assert_equal 6, @list.foldr { |a,b| a * b }
  end
end

MiniTest::Unit.autorun

reverse.injectしたら最短ですね。http://www.dzone.com/snippets/foldr-and-foldl-ruby