Property-Based Testing with PropEr, Erlang, and Elixir

Re-centering with ?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. You 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(DefaultGenerator, [AlternativeGenerators]) in Erlang, and shrink(default_generator, [alternative_generators]) in Elixir. The DefaultGenerator 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. ...