mix_array_flattener.rb

# Module MixArrayFlattener
#   Add #flatten_array to Array for the purpose 
#   of flatten nested array

module MixArrayFlattener
  InvalidArrayItemFound = Class.new(StandardError)
  
  # Flatten nested array!
  #
  # @return single flat array, 
  #   => Example: converts [1,2,[5,1],3] to [1,2,5,1,3]
  def flatten_array
    return self if empty? # if array is an empty array
    
    # Chwck if there is at least one nested array
    if self.select { |item| item.is_a?(Array) }.nil?
      return self
    end
    
    # iniatializing output array variable!
    new_ary = []
    
    # iterate through an array 
    self.each do |ar|
      case ar
      when Fixnum
        new_ary << ar 
      when Array
        new_ary += ar.flatten_array
      else
        # Raise if invalid type object found, 
        #  considering only Fixnum and Array is valid
        raise InvalidArrayItemFound, 'Not an integer or (nested) array item'
      end
    end
    
    new_ary
  end
end

# Mixing the module with array class!
class Array
  include MixArrayFlattener
end

# Output:
# for invalid array element, this should raise error
# [1,2, [2,3], [7,9,8,78, [78,89,6,8,4,[1,3,2,6]]], [43,89,90,'s']].flatten_array
# => MixArrayFlattener::InvalidArrayItemFound: Not an integer or (nested) array item

## for valid array element, this should flatten the array
#[1,2, [2,3], [7,9,8,78, [78,89,6,8,4,[1,3,2,6]]], [43,89,90,1212121],23232322,0,9,6].flatten_array
# => [1, 2, 2, 3, 7, 9, 8, 78, 78, 89, 6, 8, 4, 1, 3, 2, 6, 43, 89, 90, 1212121, 23232322, 0, 9, 6]

# [1, 3].flatten_array
# => [1, 3]