Defines a property as part of an ExUnit test.

The property macro takes at minimum a name and a do-block containing the code of the property to be tested. The property code is encapsulated as ean ExUnit test case of category property, which is released as part of Elixir 1.3 and allows a nice mix of regular unit test and property based testing. This is the reason for the third parameter taking an environment of variables defined in a test setup function.

The second parameter sets options for Proper (see PropCheck ). The default is :quiet such that execution during ExUnit runs are silent, as normal unit tests are. You can change it e.g. to :verbose or setting the maximum size of the test data generated or what ever may be helpful. For seeing the result of wrapper functions PropCheck.aggregate/2 etc, the verbose mode is required.

Counter Examples

If a property fails, the counter example is in a file. The next time this property is checked again, only the counter example is used to ensure that the property now behaves correctly. Additionally, a property with an existing counter example is embellished with the tag failing_prop. You can skip all other tests and property by running mix test --only failing_prop. In this case only the properties with counter example are run. Another option is to use the --stale option of ExUnit to reduce the amount of tests and properties while fixing the code tested by a property.