Response to nima_m on #ruby IRC asking how to implement this state machine in Ruby: https://pasteboard.co/Izbzhuz.png

heater.rb

# frozen_string_literal: true

class Heater
  STATE = {
    s1: {heater: false, cooler: false},
    s2: {heater: false, cooler: true},
    s3: {heater: true, cooler: false}
  }.freeze

  attr_reader :state
  attr_reader :cooler
  attr_reader :heater

  def initialize(state: :s1)
    @state = state

    start
  end

  def start
    Thread.abort_on_exception = true

    @thread = Thread.new do
      loop do
        STATE.fetch(@state).each do |key, value|
          instance_variable_set "@#{key}", value
        end

        event = wait_for_event
        new_state = public_send @state, event

        @state = new_state if new_state
      end
    end
  end

  def s1(event)
    case event
    when 'T<15'
      :s3
    when 'T>35'
      :s2
    end
  end

  def s2(event)
    :s1 if event == 'T<25'
  end

  def s3(_event)
    :s1 if event == 'T>30'
  end

  def wait_for_event
    sleep Math::E

    %w[T<15 T<25 T>30 T>35].sample
  end
end