JEG2

input = [ 1, 2, 3, 4, 5, 8, 9, 11, 12, 13, 15 ]

# divide the input into runs of consecutive numbers
last = input.first
s    = input.slice_before(lambda { |i| [i != last + 1, (last = i)].first })

# replace runs of 3 or more with first-last
p s.map {|runs| runs.size < 3 ? runs : "#{runs.first}-#{runs.last}"}
   .flatten
   .join(', ')     # => 1-5, 8, 9, 11-13, 15

JEG2

class Context
  def initialize(variables)
    variables.each do |name, value|
      instance_variable_set("@#{name}", value)
    end
  end
  
  def run(code, start_line)
    context = init_context
    load_variables(context)