nil_array.rb

module NilArray
  refine Array do
    def fetch(index, ...)
      return if index.negative?
      
      super
    end

    def [](left, right = nil)
      if left.is_a?(Range)
        return if left.begin&.negative? || left.end&.negative?
      else
        return if left.negative? || right&.negative?
      end

      super
    end
  end
end

using NilArray

numbers = [*42..50]

p numbers[-1]
p numbers[-3..-1]
p numbers[..-1]
p numbers.fetch(-1)
p numbers.fetch(-1) { rand }
p numbers.fetch(-1, 42)