falsify: Internal Shrinking Reimagined for Haskell

Abstract

In unit testing we apply the function under test to known inputs and check for known outputs. By contrast, in property based testing we state properties relating inputs and outputs, apply the function to random inputs, and verify that the property holds; if not, we found a bug. Randomly generated inputs tend to be large and should therefore be minimised. Traditionally this is done with an explicitly provided shrinker, but in this paper we propose a way to write generators that obsoletes the need to write a separate shrinker. Inspired by the Python library Hypothesis, the approach can work even across monadic bind. Compared to Hypothesis, our approach is more suitable to the Haskell setting: it depends on a minimal set of core principles, and handles generation and shrinking of infinite data structures, including functions.