About us
Hello, fellow Kotlin enthusiasts! đ
We're thrilled to announce the unification of various Kotlin communities into a vibrant, inclusive, and dynamic collective - the Dutch Kotlin User Group (NLKUG)! Our goal is to build a lively community with lots of interesting events where we can learn and have fun.
Joining NLKUG means you'll have access to all sorts of cool stuff. We're talking about meetups led by community members, free training days, conferences, and more. It's a great way to get better at Kotlin, share your own experiences, and meet people who love coding as much as you do.
But remember, our community thrives on collective wisdom! We encourage you to bring your unique experiences and insights to the table. Because at NLKUG, every member is both a learner and a contributor. đĄ
Ready to join us? Become a member of our community at https://kotlin.nl. It's totally free!
Come and be part of our journey, and let's get better at Kotlin together! đ
Do you want to become a speaker? Fill-in the form:
https://forms.gle/tvsoXbUUC7SVMLtH8
Upcoming events
1
Kotlin Meetup: Contract-Based Testing & Kotlin Contracts
Xebia Amsterdam, Wibautstraat 200/202, Amsterdam, NLHello, Kotlin enthusiasts! đ
Join the Dutch Kotlin User Group on Wednesday, 4 November 2026, for an evening exploring contracts from two perspectives: building reliable integrations and understanding the analyses Kotlinâs compiler performs for you.
Willem Veelenturf and Jerre van Veluw from Flock will demonstrate contract-based integration testing with Kotest and Wirespec. Then Alejandro Serrano Mena from JetBrains will explore Kotlin contracts and how they are evolving into new language features.
Meet fellow Kotlin developers, exchange ideas, and join the discussion. Make sure to RSVP!đ Location: Xebia Netherlands - Amsterdam - Wibautstraat
## đŁ Timeline
- 17:45 â Doors open, food and drinks đ
- 18:30 â Opening đť
- 18:40 â Robust Contract-Based Integration Testing with Kotest and WireSpec in Action â Willem Veelenturf & Jerre van Veluw
- 19:30 â Quick break
- 19:40 â All you ever wanted to know about contracts â Alejandro Serrano Mena
- 20:25 â Closing
- 20:30 â Chats & drinks đĽ
All times are local to the Netherlands (CET).
## Talk 1: Robust Contract-Based Integration Testing with Kotest and WireSpec in Action
Willem Veelenturf & Jerre van Veluw ¡ Flock
In this session, Willem and Jerre will take you on a journey through the Flock architecture and its three pillars. Youâll discover how a contract-based approach keeps your software stable. From there, theyâll dive into Wirespec, a tool that turns API contracts into type-safe code for both sides of an integration.
To bring these concepts to life, theyâll wrap up with a live coding demo, building robust, contract-based integration tests from scratch using Kotest and Kotlin.### About the speakers
Willem Veelenturf is a Software Engineer and co-founder at Flock.community. He is passionate about Kotlin, type-safe integrations, and building scalable software.
Jerre van Veluw is a Software Engineer at Flock.community and creator of Wirespec. Specializing in Kotlin and API design, he is a âpragmatic perfectionistâ who advocates for spec-first development and robust testing pipelines.## Talk 2: All you ever wanted to know about contracts
Alejandro Serrano Mena ¡ JetBrains
Contracts are a small part of Kotlin, but one that unlocks many of the nice analyses that the compiler does for you. In this talk, we look at the major kinds of contractsâin-place contracts, return contracts, and flow contractsâto understand their role in the language. As a bonus, weâll see how those concepts are morphing into new language features.### About the speaker
Alejandro Serrano Mena is passionate about formal methods and functional programming in software development, especially using Kotlin and Haskell. He works as a researcher in the Kotlin Language Evolution team at JetBrains and helps co-maintain the Arrow library.
He enjoys using and improving these tools as well as spreading the word: he has written four books for different experience levels and regularly speaks at conferences and meetups.
Previously, Alejandro worked in academia on compilers and type systems for functional languages. His PhD thesis focused on error messages, and he has contributed to efforts such as improving GHCâs support for impredicativity. He puts this knowledge into practice in open-source projects, many of which use metaprogramming techniques or compiler extensions.## RSVP
Reserve your seat and join us for an evening of Kotlin, practical demos, and conversations with the community. We look forward to seeing you there!
17 attendees
Past events
18
