arbitrarily nested array of integers flattener

flattener.rb

# frozen_string_literal: true

# Will flatten an arbitrary nested array of Integer
#
# NB: I am not very happy with this solution.
# I preferred to do it in either Scala or Elm.
# I'm out of practice in Scala and still learning Elm.
# I did try to keep this "functinal thinking"
# to have a deterministic function and
# eliminate any side effects.
#
# Usage: Flattener.flattenize(ary)
#
# @param [Array<Integer,Array>] array to flatten
#
# @return [Array<Integer>] flattened array
#
# @raise IllegalArgumentError when Array contains a non-Integer
#
# @example
# puts "Flattening []"
# puts Flattener.flattenize([]).to_s
#
# @example
# puts "Flattening [1, [2]"
# puts Flattener.flattenize([1,[2]]).to_s
#
# @example
# puts "Flattening [1,2,3,4]"
# puts Flattener.flattenize([1,2,3,4]).to_s
#
# @example
# puts "Flattening [[1,2,[3]],4]"
# puts Flattener.flattenize([[1,2,[3]],4]).to_s
#
# @example
# begin
#   puts "Flattening [[1,2,[3]],'c']"
#   puts Flattener.flattenize([[1,2,[3]],'c']).to_s
# rescue Flattener::IllegalArgumentError
#   puts 'This one raised because of a non-Integer argument'
# end
module Flattener

  class IllegalArgumentError < StandardError; end
  
  def self.flattenize(ary)

    head = ary[0]
    rest = ary.drop 1

    puts "ary: #{ary}" if $DEBUG
    puts "head: #{head}" if $DEBUG
    puts "rest: #{rest}" if $DEBUG

    if ary.empty?
      ary
    else
      case head
      when Integer
        if rest.empty?
          rest.unshift(head)
        else
          Flattener.flattenize(rest).unshift(head)
        end
      when Array
        Flattener.flattenize(head) + Flattener.flattenize(rest)
      else
        raise IllegalArgumentError, 'Array must contain only Integer or Array'
      end
    end
  end
end

test_flattener.rb

require "minitest/autorun"
require 'minitest/rg'

require 'flattener'

class TestFlattener < Minitest::Test

  def setup

  end

  def test_that_empty_works
    assert_equal [], Flattener.flattenize([])
  end

  def test_that_simple_works
    assert_equal [1,2,3,4], Flattener.flattenize([1,2,3,4])
  end

  def test_that_nested_works
    assert_equal [1,2,3,4], Flattener.flattenize([[1,2,[3]],4])
  end

  def test_that_non_ints_raise
    assert_raises(Flattener::IllegalArgumentError){ Flattener.flattenize([[1,2,[3]],'c']) }
  end

end