You can use this module to test your custom Babel.Steps.

It defines two functions:

• apply/2
• apply!/2

The difference to Babel.apply/2 and Babel.apply!/2 is that they return the "raw" result from the step, including in an error case (nothing gets wrapped in a Babel.Error).

apply!/2 raises by asserting on the {:ok, value} shape of the apply/2 result.