flatten.rb

module ArrayUtils
  def self.flatten(a, ret = [])
    raise ArgumentError, 'parameter must be an array' unless a.is_a?(Array)
    a.each do |i|
      if i.is_a?(Array)
        flatten(i, ret)
      else
        ret << i
      end
    end
    ret
  end
end

if __FILE__==$0
  a = [[1,2,[3]],4,[5,[[6]]]]
  puts ArrayUtils.flatten(a).inspect
end