Creating a mock HTTPoison client

client.ex

defmodule Client do
  # This doesn't work!
  use Application.get_env(:my_app, :http_client_base, HTTPoison.Base)

  defp process_url(url) do
    url |> URI.parse |> Map.put(:scheme, "https") |> URI.to_string
  end

  defp process_request_body(body) do
    body
  end

  defp process_request_headers(headers) do
    headers
  end
end

client_test.exs

defmodule ClientTest do
  use ExUnit.Case

  doctest Client

  test "uses Test.HTTPClient.Base" do
    assert %{method: :get, url: _, body: _, headers: _} = Client.get("http://example.com")
  end
  
  test "upgrades url to https" do
    assert %{method: :get, url: "https://example.com", body: _, headers: _} = Client.get("http://example.com")
  end
end

test_http_client.ex

# Emulates HTTPoison.Base but returns request maps instead of doing anything
defmodule Test.HTTPClient.Base do
  defmacro __using__(_) do
    quote do
      def get(url, headers \\ [], options \\ []) do
        request(:get, url, "", headers, options)
      end 

      def request(method, url, body \\ "", headers \\ [], options \\ []) do
        %{
          method: method,
          url: process_url(url),
          body: process_request_body(body),
          headers: process_request_headers(headers),
        }
      end 
    end 
  end 
end