google_maps.ex

defmodule GeocodingAdapter.GoogleMaps do

  defmodule UnparseableResponse do
    defexception message: "Received a response from Google Maps that contained invalid json"
  end

  def geocode(address) do
    fetch_response
    |> return_empty_unless_success
    |> parse_body
  end

  defp fetch_response do
    response = HTTPotion.get("http://foo.com/bar/baz.json")
    { :ok, response }
  end

  defp return_empty_unless_success({ :ok, response }) do
    if response[:status_code] == 200 do
      { :ok, response }
    else
      { :error, [] }
    end
  end
  defp return_empty_unless_success({ _, response }) do
    response
  end

  defp parse_body({ :ok, response }) do
    try do
      Poison.decode!(response[:body])
    rescue
      Poison.SyntaxError -> raise UnparseableResponse
    end
  end
  defp parse_body({ _, response }) do
    response
  end

end

google_maps_test.exs

defmodule GeocodingAdapter.GoogleMapsTest do
  use ExSpec
  import Mock

  setup do
    street_number = '888'
    route = '3rd St.'
    {
      :ok,
      [
        subject: GeocodingAdapter.GoogleMaps,
        address: [
          name:           "Shiplify HQ",
          street_address: Enum.join([street_number, route], " "),
          city:           "Atlanta",
          state:          "GA",
          postal_code:    "30318",
        ],
      ]
    }
  end

  describe "geocode" do
    it "returns empty array when receiving a non-2XX return code from Google's API", context do
      { subject, address } = { context[:subject], context[:address] }
      response = %{ status_code: 404 }

      with_mock HTTPotion, [get: fn(_url) -> response end] do
        assert subject.geocode(address) == []
      end
    end

    it "raises an exception if Google's API returns invalid json", context do
      { subject, address } = { context[:subject], context[:address] }
      response = %{ status_code: 200, body: "bad-json" }

      with_mock HTTPotion, [get: fn(_url) -> response end] do
        assert_raise Module.concat(subject, :UnparseableResponse), fn ->
          subject.geocode(address)
        end
      end
    end
  end
end