This is the first in a series of posts about using property-based testing (PBT) within Komposition, a screencast editor that I’ve been working on during the last year. It introduces PBT and highlights some challenges in testing properties of an application like Komposition.

Future posts will focus on individual case studies, covering increasingly complex components and how they are tested. I’ll reflect on what I’ve learned in each case, what bugs the tests have found, and what still remains to be improved.

For example, I’ll explain how using PBT helped me find and fix bugs in the specification and implementation of Komposition’s video classifier. Those were bugs that would be very hard to find using example-based tests or using a static type system!

This series is not a tutorial on PBT, but rather a collection of motivating examples. That said, you should be able to follow along without prior knowledge of PBT.


In early 2018 I started producing Haskell screencasts. A majority of the work involved cutting and splicing video by hand in a non-linear editing system (NLE) like Premiere Pro or Kdenlive. I decided to write a screencast editor specialized for my needs, reducing the amount of manual labor needed to edit the recorded material. Komposition was born.

Komposition is a modal GUI application built for editing screencasts. Unlike most NLE systems, it features a hierarchical timeline, built out of sequences, parallels, tracks, clips, and gaps. To make the editing experience more efficient, it automatically classifies scenes in screen capture video, and sentences in recorded voice-over audio.

If you are curious about Komposition and want to learn more right away, check out its documentation.

Komposition’s timeline mode.

Some of the most complex parts of Komposition include focus and timeline transformations, video classification, video rendering, and the main application logic. Those are the areas in which I’ve spent most effort writing tests, using a combination of example-based and property-based testing.

I’ve selected the four most interesting areas where I’ve applied PBT in Komposition, and I’ll cover one in each coming blog post:

  1. Timeline flattening
  2. Video scene classification
  3. Focus and timeline consistency
  4. Symmetry of undo/redo

I hope these case studies will be motivating, and that they will show the value of properties all the way from unit testing to integration testing.

Property-Based Testing

To get the most out of this series, you need a basic understanding of what PBT is, so let’s start there. For my take on a minimal definition, PBT is about:

  1. Specifying your system under test in terms of properties, where properties describe invariants of the system based on its input and output.
  2. Testing that those properties hold against a large variety of inputs.

It’s worth noting that PBT is not equal to QuickCheck, or any other specific tool, for that matter. The set of inputs doesn’t have to be randomly generated. You don’t have to use “shrinking”. You don’t have to use a static type system or a functional programming language. PBT is a general idea that can be applied in many ways.

The following resources are useful if you want to learn more about PBT:

The code examples will be written in Haskell and using the Hedgehog testing system. You don’t have to know Haskell to follow this series, as I’ll explain the techniques primarily without code. But if you are interested in the Haskell specifics and in Hedgehog, check out “Property testing with Hedgehog” by Tim Humphries.

Properties of the Ugly Parts

When I started with PBT, I struggled with applying it to anything beyond simple functions. Examples online are often focused on the fundamentals. They cover concepts like reversing lists, algebraic laws, and symmetric encoders and decoders. Those are important properties to test, and they are good examples for teaching the foundations of PBT.

I wanted to take PBT beyond pure and simple functions, and leverage it on larger parts of my system. The “ugly” parts, if you will. In my experience, the complexity of a system often becomes much higher than the sum of its parts. The way subsystems are connected and form a larger graph of dependencies drives the need for integration testing at an application level.

Finding resources on integration testing using PBT is hard, and it might drive you to think that PBT is not suited for anything beyond the introductory examples. With the case studies in this blog series I hope to contribute to debunking such misconceptions.

Designing for Testability

In my case, it’s a desktop multimedia application. What if we’re working on a backend that connects to external systems and databases? Or if we’re writing a frontend application with a GUI driven by user input? In addition to these kinds of systems being hard to test at a high level due to their many connected subsystems, they usually have stateful components, side effects, and non-determinism. How do we make such systems testable with properties?

Well, the same way we would design our systems to be testable with examples. Going back to “Writing Testable Code” by Miško Hevery from 2008, and Kent Beck’s “Test-Driven Development by Example” from 2003, setting aside the OOP specifics, many of their guidelines apply equally well to code tested with properties:

  • Determinism: Make it possible to run the “system under test” deterministically, such that your tests can be reliable. This does not mean the code has to be pure, but you might need to stub or otherwise control side effects during your tests.
  • No global state: In order for tests to be repeatable and independent of execution order, you might have to rollback database transactions, use temporary directories for generated files, stub out effects, etc.
  • High cohesion: Strive for modules of high cohesion, with smaller units each having a single responsibility. Spreading closely related responsibilities thin across multiple modules makes the implementation harder to maintain and test.
  • Low coupling: Decrease coupling between interface and implementation. This makes it easier to write tests that don’t depend on implementation details. You may then modify the implementation without modifying the corresponding tests.

I find these guidelines universal for writing testable code in any programming language I’ve used professionally, regardless of paradigm or type system. They apply to both example-based and property-based testing.

Patterns for Properties

Great, so we know how to write testable code. But how do we write properties for more complex units, and even for integration testing? There’s not a lot of educational resources on this subject that I know of, but I can recommend the following starting points:

Taking a step back, we might ask “Why it’s so hard to come up with these properties?” I’d argue that it’s because doing so forces us to understand our system in a way we’re not used to. It’s challenging understanding and expressing the general behavior of a system, rather than particular anecdotes that we’ve observed or come up with.

If you want to get better at writing properties, the only advice I can give you (in addition to studying whatever you can find in other projects) is to practice. Try it out on whatever you’re working on. Talk to your colleagues about using properties in addition to example-based tests at work. Begin at a smaller scale, testing simple functions, and progress towards testing larger parts of your system once you’re comfortable. It’s a long journey, filled with reward, surprise, and joy!

Testing Case Studies

With a basic understanding of PBT, how we can write testable code, and how to write properties for our system under test, we’re getting ready to dive into the case studies:

  1. Introduction
  2. Timeline Flattening
  3. Video Scene Classification
  4. Integration Testing


Thank you Chris Ford, Alejandro Serrano Mena, Tobias Pflug, Hillel Wayne, and Ulrik Sandberg for kindly providing your feedback on my drafts!

Buy the Book

This series is now available as an ebook on Leanpub. While the content is mostly the same, there are few changes bringing it up-to-date. Also, if you’ve already enjoyed the articles, you might want support my work by purchasing this book. Finally, you might enjoy a nicely typeset PDF, or an EPUB book, over a web page.