Writing the First Property

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 can build on that. The base step here looks something like this:

ExpectedPrice = checkout:total(ChosenItems, PriceList, [])

If we write a generator that gives us all known values for these variables, we can use the steps the same way we would with examples. The generator will literally generate test cases so that we don’t have to. So, let’s take a look at the properties, generators, and the implementation of the first test.