There are quite a lot of errors in the text. 
My version of elixir (1.17.3) told me it is not allowed to place the first three functions in chromosome.ex, since the declaration of %{Chromo..} happens there. So I placed them in genetic.

Here is my version:

To Genetic!

def start_link(genotype) do
  Agent.start_link(fn -> genotype.() end)
end

def get_fitness(pid) do
  c = Agent.get(pid, & &1)
  c.fitness
end

def eval(pid, fitness_func) do
  c = Agent.get(pid, & &1)
  Agent.update(pid, fn c ->
     %Chromosome{c | fitness: fitness_func.(c)} end)
end

In module genetic:

def initialize(genotype, opts \\ []) do
  population_size = Keyword.get(opts, :population_size, 4)
  agents_population = for _ <- 1..population_size do
    start_link(genotype)
  end
end

def evaluate(agents_population, fitness_function, _opts \\ []) do
  pids = Enum.map(agents_population, fn {:ok, pid} -> pid end)
  do_tasks =
  Enum.map(pids, &Task.async(fn -> eval(&1, fitness_function) end))
  |> Enum.map(&Task.await(&1))
  agents_population = Enum.sort_by(pids, fn pid -> get_fitness(pid) end)
end

You have to watch the timeout of your agents (maybe set it to :infinity)
To test it you have to comment out all the lines of evolve but the first.
The reason is that you also have to rewrite crossover, mutation and so on for this Agents method!

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Thanks!