Flatten deeply nested Map in Elixir

flatten_nested_json.ex

defmodule Json do
  def flatten(%{} = json) do
    json
    |> Map.to_list()
    |> to_flat_map(%{})
  end
  def flatten(%{} = json) when json == %{}, do: %{}

  defp to_flat_map([{_k, %{} = v} | t], acc), do: to_flat_map(Map.to_list(v), to_flat_map(t, acc))
  defp to_flat_map([{k, v} | t], acc), do: to_flat_map(t, Map.put_new(acc, k, v))
  defp to_flat_map([], acc), do: acc
end

%{id: "1", foo: %{bar: %{qux: "hello world"}, baz: 123}}
|> Json.flatten()
|> IO.inspect()

# %{baz: 123, id: "1", qux: "hello world"}

The second clause will never be matched: def flatten(%{} = json) when json == %{}, do: %{} because just as = will match successfully for any two maps, == returns true for any two maps. Use Map.equal?/2 to find if two maps are identical. This doesn't work in guard clauses though (i.e.when). So you probably have to check for an empty map inside the function body (if that is what the second flatten clause is intended to do).