Writing the First Property
Get started with the example by writing the first property.
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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 ...