Negative Testing: Calibrating Negative Properties

Getting started

Gathering statistics is the easiest and best tool we have to check whether a property is good. Let’s take a look again at prop_expected_result(). This time, we’ll look into the type of result we get. Right now, the property has two valid cases:

1. The list of items are all valid, and it will pass.
2. At least some items missing from the price list and it will fail.

If we use the collect/2 function to gather statistics, we get the following:

===> Testing prop_checkout:prop_expected_result()
....................................................................................................
OK: Passed 100 test(s).
91% prices_missing
9% valid