A not efficient but easy-to-code implementation of Array#flatten

my_flatten.rb

# Flat the array layer by layer 
# 
# Insights:
#
# - Using recursion.
#

class Array
  def my_flatten(depth = nil)
    if depth.nil? || depth.is_a?(Fixnum)
      return flat_all if not depth
      return flat_by_depth(depth)
     else
      raise(ArgumentError, ":depth should be a Fixnum")      
     end 
  end
  
private
  def flat_all
    new_ary = self.dup
    contains_array = false
    tmp_ary = []

    loop do
      contains_array = false
      tmp_ary = []
      new_ary.each do |elem|
        if elem.is_a? Array
          contains_array = true if elem.is_a? Array
          elem.each { |sub_elem| tmp_ary << sub_elem }
        else
          tmp_ary << elem
        end
      end
      new_ary = tmp_ary
      break if not contains_array
    end
    new_ary
  end

  def flat_by_depth(depth)
    new_ary = self.dup
    contains_array = false
    tmp_ary = []

    (1..depth).each do |n|
      contains_array = false
      tmp_ary = []
      new_ary.each do |elem|
        if elem.is_a? Array
          contains_array = true if elem.is_a? Array
          elem.each { |sub_elem| tmp_ary << sub_elem }
        else
          tmp_ary << elem
        end
      end
      new_ary = tmp_ary
      break if !contains_array || n >= depth
    end
    new_ary
  end
end

class ArgumentError < StandardError;  end

s = [ 1, 2, 3 ]           #=> [1, 2, 3]
t = [ 4, 5, 6, [7, 8] ]   #=> [4, 5, 6, [7, 8]]
a = [ s, t, 9, 10 ]       #=> [[1, 2, 3], [4, 5, 6, [7, 8]], 9, 10]
p a.my_flatten            #=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
a = [ 1, 2, [3, [4, 5]]]
p a.my_flatten(1)         #=> [1, 2, 3, [4, 5]]
p a.my_flatten('lol')     #=> :depth should be a Fixnum (ArgumentError)