Array Iter in Ruby

array_iter.rb

class ArrayIter < Array

  class Iter < SimpleDelegator
    attr_reader :next
    attr_reader :prev
  end

  def <<(other)
    last.instance_variable_set(:@next, other) if last
    other_iter = Iter.new other
    other_iter.instance_variable_set(:@prev, last) if last
    super other_iter
  end

  def push(other)
    throw 'push not supported'
  end

  def concat(other)
    return if other.empty?

    last.instance_variable_set(:@next, other.first) if last
    other.first.instance_variable_set(:@prev, last) if last

    super other
  end

  def map!
    # not sure
    super
  end

end

array_iter_test.rb

require_relative './test_helper'

require './lib/array_iter.rb'

class TestArrayIter < Minitest::Test

  def test_empty
    assert_empty ArrayIter.new
  end

  def test_one
    array_iter = ArrayIter.new
    array_iter << 'a'
    assert_equal 1, array_iter.size
    assert_equal nil, array_iter.first.prev
    assert_equal nil, array_iter.first.next
  end

  def test_two
    array_iter = ArrayIter.new
    array_iter << 'a'
    array_iter << 'b'

    assert_equal 2, array_iter.size

    assert_equal 'a', array_iter.first
    assert_equal nil, array_iter.first.prev
    assert_equal 'b', array_iter.first.next

    assert_equal 'b', array_iter.last
    assert_equal 'a', array_iter.last.prev
    assert_equal nil, array_iter.last.next

    assert_equal 97, array_iter.first.ord
    assert_equal "apple", array_iter.first.concat('pple')
    assert_equal "apple", array_iter.first
    assert_equal "apple", array_iter.last.prev
  end

  def test_three
    array_iter = ArrayIter.new
    array_iter << 'a'
    array_iter << 'b'
    array_iter << 'c'

    assert_equal 3, array_iter.size

    assert_equal 'a', array_iter.first
    assert_equal nil, array_iter.first.prev
    assert_equal 'b', array_iter.first.next

    assert_equal 'b', array_iter.at(1)
    assert_equal 'a', array_iter.at(1).prev
    assert_equal 'c', array_iter.at(1).next

    assert_equal 'c', array_iter.last
    assert_equal 'b', array_iter.last.prev
    assert_equal nil, array_iter.last.next
  end

  def test_dup
    array_iter = ArrayIter.new
    array_iter << 'a'
    array_iter << 'b'

    array_iter_dup = array_iter.dup

    assert_equal 'a', array_iter_dup.first
    assert_equal array_iter.first, array_iter_dup.first

    assert_equal nil, array_iter_dup.first.prev
    assert_equal 'b', array_iter_dup.first.next
  end

  def test_select!
    array_iter = ArrayIter.new
    array_iter << 'a'
    array_iter << 'b'
    array_iter << 'c'

    array_iter.select! { |x| ['a', 'c'].include? x }

    assert_equal 2, array_iter.size

    assert_equal 'a', array_iter.first
    assert_equal nil, array_iter.first.prev
    assert_equal 'b', array_iter.first.next

    assert_equal 'c', array_iter.last
    assert_equal 'b', array_iter.last.prev
    assert_equal nil, array_iter.last.next
  end

  def test_concat
    array_iter_one = ArrayIter.new
    array_iter_one << 'a'
    array_iter_one << 'b'

    array_iter_two = ArrayIter.new
    array_iter_two << 'y'
    array_iter_two << 'z'

    array_iter_one.concat array_iter_two

    assert_equal 4, array_iter_one.size

    assert_equal 'a', array_iter_one.first
    assert_equal nil, array_iter_one.first.prev
    assert_equal 'b', array_iter_one.first.next

    assert_equal 'b', array_iter_one.at(1)
    assert_equal 'a', array_iter_one.at(1).prev
    assert_equal 'y', array_iter_one.at(1).next

    assert_equal 'z', array_iter_one.last
    assert_equal 'y', array_iter_one.last.prev
    assert_equal nil, array_iter_one.last.next
  end

  def test_map!
    array_iter = ArrayIter.new
    array_iter << 'a'
    array_iter << 'b'

    array_iter.map! { |x| x.upcase }

    assert_equal 2, array_iter.size

    assert_equal 'A', array_iter.first
    assert_equal nil, array_iter.first.prev
    assert_equal 'B', array_iter.first.next

    assert_equal 'B', array_iter.last
    assert_equal 'A', array_iter.last.prev
    assert_equal nil, array_iter.last.next
  end
end

Failing (as expected) on:

  1) Error:
TestArrayIter#test_map!:
NoMethodError: undefined method `prev' for "A":String
    test/array_iter.rb:130:in `test_map!'

I'm just not sure how to re-wrap the updated array with Iter delegates.