Elixir: How can I describe mix.exs settings correctly?

Question

I tried to write a web scraping tool by using HTTPoison. As a first step, I wrote a short HTTP accessing code along the steps shown below;

1. Create a project by mix

   $ mix new httptest1

2. Write a short code on lib/httptest1.ex.

   defmodule Httptest1 do
     require HTTPoison
   
     def test1 do
       ret = HTTPoison.get! "http://www.yahoo.com"
       %HTTPoison.Response{status_code: 200, body: body} = ret
       IO.inspect body
     end
   end
   
   Httptest1.test1()
   

3. Modify mix.exs for HTTPoison.

   defmodule Httptest1.Mixfile do
     use Mix.Project
   
     def project do
       [app: :httptest1,
        version: "0.0.1",
        elixir: "~> 1.0",
        build_embedded: Mix.env == :prod,
        start_permanent: Mix.env == :prod,
        deps: deps]
     end
   
     # Configuration for the OTP application
     def application do
       [applications: [:logger, :httpoison]]
     end
   
     # Dependencies can be Hex packages:
     #
     defp deps do
       [
        {:httpoison, "~> 0.6"}
       ]
     end
   end
   

4. Run $ mix deps.get for dependency.

5. Run $ mix run, then compilation failed;

   ==> idna (compile)
   Compiled src/idna.erl
   Compiled src/idna_ucs.erl
   Compiled src/punycode.erl
   (... snip ...)
   Generated httpoison app
   
   == Compilation error on file lib/httptest1.ex ==
   ** (exit) exited in: :gen_server.call(:hackney_manager, {:new_request, #PID<0.154.0>, #Reference<0.0.1.1724>, {:client, :undefined, :hackney_dummy_metrics, :hackney_tcp_transport, 'www.yahoo.com', 80, "www.yahoo.com", [connect_timeout: 5000, recv_timeout: :infinity], nil, nil, nil, true, :hackney_pool, :infinity, false, 5, false, 5, nil, nil, nil, :undefined, :start, nil, :normal, false, false, false, false, nil, :waiting, nil, 4096, "", [], :undefined, nil, nil, nil, nil, :undefined, nil}}, :infinity)
       ** (EXIT) no process
       (stdlib) gen_server.erl:212: :gen_server.call/3
       src/hackney_client/hackney_manager.erl:66: :hackney_manager.init_request/1
       src/hackney_client/hackney_manager.erl:56: :hackney_manager.new_request/1
       src/hackney_connect/hackney_connect.erl:181: :hackney_connect.socket_from_pool/4
       src/hackney_connect/hackney_connect.erl:36: :hackney_connect.connect/5
       src/hackney_client/hackney.erl:319: :hackney.request/5
       lib/httpoison.ex:60: HTTPoison.request/5
       lib/httpoison.ex:60: HTTPoison.request!/5
   

When I used $ iex -S mix instead, the result was same.

But if I moved httptest1.ex to the same directory where mix.exs was placed, like $ mv lib/httptest1.ex . and tried to specify the source file explicitly; $ mix run httptest1, it worked correctly.

Question: I suspect my mix.exs setting goes something wrong, what's that?

Answer 1

All .ex in lib/ are compiled. Since Elixir is meta-programming language when you compile a file you are actually running the code. Which means that Httptest1.test1() is executed when you compile your project.

HTTPoison needs to be started for it work correctly. It is started when your application starts which it is when you do mix run .... But when your project is compiling your project or your dependencies are not started so calls to your dependencies might fail.

Check out this chapter Supervisor and Application in the getting started guide to learn how to run code when your application starts.

Answer 2

Improved! Now I know how to create an executable invoked command line.

program;

defmodule Httptest1 do
  use Application
  require HTTPoison

  def start(_type, _args) do
    import Supervisor.Spec, warn: false
    children = []
    opts = [strategy: :one_for_one, name: Httptest1.Supervisor]
    Supervisor.start_link(children, opts)
  end

  def main(args) do # entry point
    ret = HTTPoison.get! "http://www.yahoo.com"
    %HTTPoison.Response{status_code: _, body: body} = ret # no status code matching now
    IO.inspect body
  end

end

and mix.exs (both created by $ mix new httptest1 --sup supervisor option);

defmodule Httptest1.Mixfile do
  use Mix.Project

  def project do
    [app: :httptest1,
     version: "0.0.1",
     elixir: "~> 1.0",
     build_embedded: Mix.env == :prod,
     start_permanent: Mix.env == :prod,
     escript: escript,  # <<<< Added 
     deps: deps]
  end

  def escript do # define escript 
    [main_module: Httptest1]
  end

  def application do
    [applications: [:logger, :httpoison],
     mod: {Httptest1, []}] # 
  end

  defp deps do
    [
      {:httpoison, "~> 0.6"}
    ]
  end
end

Then in the iex invoked by $ iex -S mix type Httptest.test1, the expected result comes up! Thanks.

Also, to create command line executable,

   $ mix escript.build

then ./httptest1 is generated. Done.