Re-centering with shrink
Explore how to use the shrink macro in property-based testing with PropEr to guide shrinking toward meaningful data points. Understand how to configure default and alternative generators to assist shrinking in finding simpler, relevant counterexamples faster, especially in scenarios like timestamp and timezone generation.
We'll cover the following...
shrink
shrink is conceptually the simplest of the two macros that can be used to impact shrinking. It is best used to pick a custom zero-point toward which PropEr will try to shrink data. We can do this mainly by giving the framework a normal generator for normal cases, and then suggesting it uses other simpler generators whenever an error is discovered.
The macro takes the form shrink(default_generator, [alternative_generators]). The default_generator will be used for all passing tests. Once a property fails, however, shrink lets us tell PropEr that any of the alternative generators in the list are interesting ways to get simpler relevant data. We can give hints about how the framework should search for failures. And if the alternative generators are not fruitful, so be it. The shrinking will continue in other ways ...