Writing the First Property
Explore writing your first simple property test in PropEr, starting with a basic sum calculation without specials. Learn to create layered generators for items and prices, and implement the corresponding function in Elixir to validate your property-based tests.
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Getting started
The first property doesn’t have to be advanced. In fact, it’s better if it’s simple. Start with something trivial-looking that represents how we want to use the program. Then, our job as developers is to make sure we can write code that matches or changes our expectations. Of our two properties, the simplest one concerns counting sums without caring about specials.
We’ll want to avoid a property definition, such as sum(ItemList, PriceList) =:= checkout:total(ItemList, PriceList, []), since that would risk making the test similar to the implementation. A good approach to try here is generalizing regular example-based tests. Let’s imagine a few cases:
20 = checkout:total(["A","B","A"], [{"A",5},{"B",10}], []),
20 = checkout:total(["A","B","A"], [{"A",5},{"B",10},{"C",100}], []),
115 = checkout:total(["F","B","C"], [{"F",5},{"B",10},{"C",100}], []),
That’s actually tricky to generalize. It’s possible that to come up with examples we just make a list of items, assign them prices, pick items from the list, and then sum them up ourselves. Even if it’s not really straightforward, we ...