Today I Learned

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How to assert Elixir doctest raises an error

Today I learned how to assert an Elixir doctest raises an error. Check this out:

defmodule MyModule do
  @doc """
  This function raises ArgumentError.

  ## Examples

      iex> MyModule.my_func()
      ** (ArgumentError) something is wrong
  """
  def my_func() do
    raise(ArgumentError, "something is really wrong")
  end
end

The previous doctest will fail with this message:

  1) doctest MyModule.my_func/0 (1) (MyModuleTest)
     test/my_module_test.exs:3
     Doctest failed: wrong message for ArgumentError
     expected:
       "something is wrong"
     actual:
       "something is really wrong"
     code: MyModule.my_func()
     stacktrace:
       lib/my_module.ex:10: MyModule (module)
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