elixir guards and options handling.md

defmodule Test do
  def test(a, opts \\ []) when is_list(opts) do
    with opts <- opts |> Enum.into(%{foo: :a}) do
      test(a, opts)
    end
  end 
  
  def test(a, opts) do
    IO.inspect(a)
    IO.inspect(opts)
    true
  end
end

Test.test(1)
#=> 1
#=> %{foo: :a} 
# default opts match on the guard as a list

Test.test(1, foo: :b)
#=> 1
#=> %{foo: :b}
# values are not overwritten

https://elixirforum.com/t/whats-the-most-idiomatic-way-to-require-a-key-in-a-keyword-list-or-make-it-optional/9577