Start a small Elixir server from a script, then query it from inside the same script

plug_demo.exs

#
# to run and restart continuously when the code changes:
#
# find scripts lib mix.* | entr -rc mix run --no-halt scripts/plug_demo.exs
#

defmodule MySimpleServerPlug do
  # https://hexdocs.pm/plug/Plug.Conn.html
  import Plug.Conn
  
  # https://hexdocs.pm/plug/Plug.Conn.html
  def init(options) do
    options
  end
  
  # https://hexdocs.pm/plug/Plug.Conn.html
  def call(conn, _options) do
    conn
    # https://hexdocs.pm/plug/Plug.Conn.html#put_resp_content_type/3
    |> put_resp_content_type("text/plain")
    # https://hexdocs.pm/plug/Plug.Conn.html#send_resp/1
    |> send_resp(200, "Hello!")
  end
end

# https://hexdocs.pm/plug/Plug.Adapters.Cowboy.html
{:ok, _pid} = Plug.Adapters.Cowboy.http MySimpleServerPlug, []

# https://hexdocs.pm/httpotion/HTTPotion.html#get/2
# https://hexdocs.pm/httpotion/HTTPotion.html#request/3
response = HTTPotion.get! "http://localhost:4000"

200 = response.status_code
"Hello!" = response.body