Kotlin Meetup: Contract-Based Testing & Kotlin Contracts

Hello, Kotlin enthusiasts! 👋

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 Kotlin’s 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!

📍 Location: Xebia Netherlands - Amsterdam - Wibautstraat

## 🕣 Timeline

• 17:45 — Doors open, food and drinks 🍕
• 18:30 — Opening 💻
• 18:40 — Robust Contract-Based Integration Testing with Kotest and WireSpec in Action — Willem Veelenturf & Jerre van Veluw
• 19:30 — Quick break
• 19:40 — All you ever wanted to know about contracts — Alejandro Serrano Mena
• 20:25 — Closing
• 20:30 — Chats & drinks 👥

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. You’ll discover how a contract-based approach keeps your software stable. From there, they’ll 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, they’ll 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 “pragmatic perfectionist” 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 contracts—in-place contracts, return contracts, and flow contracts—to understand their role in the language. As a bonus, we’ll 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 GHC’s 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!