Property-Based Testing with PropEr, Erlang, and Elixir

Dividing with ?LETSHRINK

As you use PropEr, you may find yourself stuck with generators creating huge data structures that take a long time to shrink and often don’t give very interesting results back. This often happens when some very low-probability failure is triggered, meaning that the framework had to generate a lot of data to find it, and has limited chances of shrinking things in a significant manner.

Whenever that happens, the ?LETSHRINK([Pattern, ...], [Generator, ...], Expression) is what you need. In practice, we use the generator like this:

Erlang

?LETSHRINK([A,B,C], [list(number()), list(number()), list(number())],
           A ++ B ++ C)

Elixir

let_shrink([
    a <- list(number()),
    b <- list(number()),
 c <- list(number()) ...