Property-Based Testing with PropEr, Erlang, and Elixir

Invariants

Some programs and functions are complex to describe and reason about. They could be needing a ton of small parts to all work right for the whole to be correct, or we may not be able to assert their quality because it is just hard to define. For example, it’s hard to say why a meal is good, but it might include criteria like: the ingredients are cooked adequately, the food is hot enough, it’s not too salty, not too sweet, not too bitter, it’s well-presented, the portion size is reasonable, and so on. Those factors are all easier to measure objectively and can be a good proxy for “the customer will enjoy the food.” In a software system, we can identify similar conditions or facts that should always remain true. We call them invariants ...