Gens N’ Roses: Appetite for Reduction

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QuickCheck is a shockingly effective tool for validating the initial and ongoing correctness of production software. One of QuickCheck’s most compelling features is that when a test failure is found, the failing test case is simplified to a minimal counterexample, through a process called shrinking. This makes it significantly easier to understand why a test has failed.

The Haskell version of QuickCheck, and most of its derivatives, tackle shrinking in a type-directed fashion. Quiviq’s Erlang version on the other hand, and a few similar tools for other dynamic languages, couple their shrinking capability directly with their test data generators. Now, types are good, right? So surely the Haskell version is better! Why wouldn’t we want to use types to guide our shrinking process?

In this talk, I will highlight the technical and social problems that we, at Ambiata, experienced with type-directed shrinking. We’ll explore the alternative, integrated shrinking, and we’ll see that even in a statically typed language, integrated shrinking provides enormous benefits for developer productivity. We will see how to implement integrated shrinking, using rose trees, in the context of a strong statically typed language. Finally, we will see how to create a rose tree monad transformer, a tool which will allow us to create effectful test data generators, which still shrink properly!

From this talk, the audience will learn about the difference between type-directed and integrated shrinking, and how integrated shrinking can provide superior counterexamples, while placing a smaller burden on developers writing tests.

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