def_bang.exs

defmodule DefBang do
  defmacro def!(call, do: block) do
    quote bind_quoted: [call: Macro.escape(call, unquote: true), block: Macro.escape(block)], location: :keep do
      {name, args} = Macro.decompose_call(call)

      def unquote(name)(unquote_splicing(args)) do
        unquote(block)
      end

      def unquote(:"#{name}!")(unquote_splicing(args)) do
        case unquote(name)(unquote_splicing(args)) do
          {:ok, result} -> result
          {:error, error} -> raise error
        end
      end
    end
  end
end

defmodule Test do
  import DefBang

  @doc """
  Declares a function and it's bang version.

  ## Examples

      iex> Test.foo("bar")
      {:ok, "bar"}

      iex> Test.foo("baz")
      {:error, "baz"}

      iex> Test.foo!("baz")
      ** (RuntimeError) baz
  """
  def! foo("bar"), do: {:ok, "bar"}
  def! foo("baz"), do: {:error, "baz"}
end