Today, 2025-05-14, at 1830 UTC (11:30 am PDT, 2:30 pm EDT, 7:30 pm GMT, 20:30 CET, …) we are streaming the 44th episode of the Haskell Unfolder live on YouTube.

State-based testing with quickcheck-lockstep (Haskell Unfolder #44)

Many Haskell programmers will be familiar with property based testing of pure functions (for those who are not, various episodes of the Haskell Unfolder have discussed this: #4, #21, #38 and #40). Property based testing for stateful systems (“IO code”) is however much less well-known, which is a pity as it is just as useful! In this episode we will demonstrate how we can use quickcheck-lockstep to verify the responses we get from a simple stateful API; as we will see, all of the lessons from property based testing for pure functions can be applied in this stateful setting also.

About the Haskell Unfolder

The Haskell Unfolder is a YouTube series about all things Haskell hosted by Edsko de Vries and Andres Löh, with episodes appearing approximately every two weeks. All episodes are live-streamed, and we try to respond to audience questions. All episodes are also available as recordings afterwards.

We have a GitHub repository with code samples from the episodes.

And we have a public Google calendar (also available as ICal) listing the planned schedule.

There’s now also a web shop where you can buy t-shirts and mugs (and potentially in the future other items) with the Haskell Unfolder logo.