The evolve signature in this code sample suddenly loses the population argument which I think isn’t intended, the code goes on to use it a little later without an alternate source for it.

Edit: In this same code block, the variable best gets assigned an integer whereas previous it’d been a Chromosome. Other code (like our printing the results) expects best to contain a Chromosome, as it gets returned from evolve().

My amended evolve function looks like this instead:

  def evolve(population, problem, generation, last_max_fitness, temperature, opts \\ []) do
    population   = evaluate(population, &problem.fitness_function/1, opts)
    best         = Enum.max_by(population, &problem.fitness_function/1)
    best_fitness = best.fitness
    temperature  = 0.8 * (temperature + (best_fitness - last_max_fitness))

    IO.write("\rCurrent Best: #{List.to_string(best.genes)} (#{best.fitness})")
    if problem.terminate?(population, generation, temperature) do
      best
    else
      generation = generation + 1
      population
      |> select(opts)
      |> crossover(opts)
      |> mutation(opts)
      |> evolve(problem, generation, best_fitness, temperature, opts)
    end
  end

(Disregard the IO.write, I like to see what’s going on :))