httpoison_dialyzer.ex

defmodule App.Service do

  @spec parse() :: {:ok, String.t()}, | {:error, String.t()}
  def parse do
    url = "https://example.com/foo.txt"
    
    case HTTPoison.get(url) do
      {:ok, %HTTPoison.response{status_code: 200, body: body}} ->
        body   # Note: Not {:ok, body}
      _ ->
        {:error, "whatever"}
    end
  end

end