ohno.rb

require('yaml')

module Ohno
  def self.replace_index(welp, index)
    case welp
    when String
      welp == "$index" ? index : welp.gsub(/\$index/, index.to_s)
    else
      welp
    end
  end
end

class Loop
  def self.allocate
    k = Class.new(Array)
    k.send(:define_method, :init_with) do |coder|
      (coder.map['from']..coder.map['to']).each do |index|
        push(Ohno.replace_index(coder.map['do'], index))
      end
    end
    k.new
  end
end

puts YAML.load(DATA.read).inspect

__END__
--- !ruby/object:Loop
from: 0
to: 3
do: $index