Do object-scoped from encoding of HTTP parameters

form_encode.ex

defmodule FormEncode do
  def form_encode(params) when is_map(params) do
    form_encode([], params)
  end

  def form_encode(nesting, value) when is_map(value) do
    Enum.map_join(value, "&", fn({k, v}) ->
      n1 = Enum.reverse(Enum.reverse(nesting), [k])
      form_encode(n1, v)
    end)
  end

  def form_encode(nesting, value) when is_binary(value) do
    URI.encode_www_form(form_key(nesting) <> "=" <> value)
  end

  def form_key([h | []]) do
    to_string(h)
  end

  def form_key([h | t]) do
    to_string(h) <> "[" <> Enum.map_join(t, "][", &(to_string(&1))) <> "]"
  end
end

form_encode_test.exs

defmodule FormEncodeTest do
  use ExUnit.Case
  import FormEncode

  test ".form_encode with a flat map, string keys, one pair" do
    assert form_encode(%{"hello" => "ciao"}) == "hello%3Dciao"
  end

  test ".form_encode with a flat map, atom keys, one pair" do
    assert form_encode(%{hello: "ciao"}) == "hello%3Dciao"
  end

  test ".form_encode with a singly nested maps, string keys, one pair" do
    assert form_encode(%{"greetings" => %{"hello" => "ciao"}}) == "greetings%5Bhello%5D%3Dciao"
  end

  test ".form_encode with a singly nested maps, atom keys, one pair" do
    assert form_encode(%{greetings: %{hello: "ciao"}}) == "greetings%5Bhello%5D%3Dciao"
  end

  test ".form_encode with a deeply nested maps, string keys, multiple final pairs" do
    expected = "translations%5Bgreetings%5D%5Bbuon+giorno%5D%3Dgood+day&translations%5Bgreetings%5D%5Bhello%5D%3Dciao"
    result = form_encode(%{"translations" => %{"greetings" => %{"hello" => "ciao", "buon giorno" => "good day"}}})

    assert result == expected
  end

  test ".form_encode with a deeply nested maps, string keys, multiple non-final pairs" do
    expected = "translations%5Bgreetings%5D%5Bhello%5D%3Dciao&translations%5Bnumbers%5D%5Bone%5D%3Duno"
    result = form_encode(%{"translations" => %{"numbers" => %{"one" => "uno"}, "greetings" => %{"hello" => "ciao"}}})

    assert result == expected
  end

  test ".form_key for a single-item string list is the value" do
    assert form_key(["ciao"]) == "ciao"
  end

  test ".form_key for a single-item atom list is the value as a string" do
    assert form_key([:ciao]) == "ciao"
  end

  test ".form_key wraps non-initial values in square brackets" do
    assert form_key(["ciao", "hello", "hi"]) == "ciao[hello][hi]"
  end

  test ".form_key wraps non-initial values in square brackets, mixed values" do
    assert form_key(["ciao", :hello, "hi"]) == "ciao[hello][hi]"
  end
end