Use HTTPoison to initialize a module attribute

问题

I am trying to do initialize a module attribute like this

  response = HTTPoison.get! url
  {:ok, response} = Poison.decode(response.body)
  @attr response

I have done it before with a file, something like this:

  @external_resource file = Path.join([__DIR__, "file.txt"])
  Module.register_attribute __MODULE__, :attr, accumulate: true

  for line <- File.stream!(file, [], :line) do
    @attr line
    ...

Is not possible to do the same with HTTPoison and fetching the response of an API? I am receiving this error:

== Compilation error in file lib/module.ex ==
** (ArgumentError) argument error
    (stdlib) :ets.lookup_element(:hackney_config, :mod_metrics, 2)
    /project/deps/hackney/src/hackney_metrics.erl:27: :hackney_metrics.get_engine/0
    /project/deps/hackney/src/hackney_connect.erl:69: :hackney_connect.create_connection/5
    /project/deps/hackney/src/hackney_connect.erl:37: :hackney_connect.connect/5
    /project/deps/hackney/src/hackney.erl:316: :hackney.request/5
    lib/httpoison/base.ex:630: HTTPoison.Base.request/9
    lib/httpoison.ex:66: HTTPoison.request!/5
    lib/module.ex:4: (module)

回答1:

The dependent applications are not started automatically at compile time. You need to explicitly start HTTPoison before using it:

HTTPoison.start()
response = HTTPoison.get! url
{:ok, response} = Poison.decode(response.body)
@attr response

来源：https://stackoverflow.com/questions/50931455/use-httpoison-to-initialize-a-module-attribute