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Matej Cerny

@matejcerny.bsky.social
195 followers 148 following 821 posts

Scala & Functional Programming Engineer. Database/SQL Veteran.

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Matej Cerny @matejcerny.bsky.social · 17h
Arman Bilge from Typelevel is coming to Prague! 🇨🇿 The meetup is happening next Thursday, Oct 8, and it's going to be about #Scala Native and Cats Effect. www.meetup.com/functional-j...
meetup.com
Arman Bilge from Typelevel: Towards Scala Native and optimizing Cats Effect, Thu, Oct 8, 2026, 6:30 PM | Meetup
Arman Bilge from Typelevel is coming to town! He'll talk to us about the TypeLevel road to Scala Native and how they have optimized Cats Effect along the way. Mark the date
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Matej Cerny @matejcerny.bsky.social · 29/09/2026
http4s v0.23.38 is out and this is not routine maintenance! 🚨 11 security advisories landed at once. A few highlights on why you should care: 💣 Trivial remote denial of service 🔀 Request smuggling on Ember 🍪 Cookie leakage across origins 📈 HTTP/2 memory exhaustion
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Matej Cerny @matejcerny.bsky.social · 28/09/2026
Have you upgraded to #Scala 3.9 yet? You should - 3.10 already has three RC versions out. This is one of the features moving from experimental to preview. It's called "relaxed lambda syntax".
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Matej Cerny @matejcerny.bsky.social · 25/09/2026
I missed this talk - it is about SiriusXM, the company I currently work at! Funny enough, Julien works there right now too 😀 And by the way, yes, we do have our own satellites (six if I am not mistaken). www.youtube.com/watch?v=28-v...
youtube.com
Discover SiriusXM new streaming platform with Luka Jacobowitz and Damian Mazurkiewicz
YouTube video by Scala for Fun & Profit
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Matej Cerny @matejcerny.bsky.social · 24/09/2026
AI won't replace developers. It will overwhelm Product. Engineers won't lose jobs because AI writes the code, but because Product will fail to keep up with delivery speed.
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Matej Cerny @matejcerny.bsky.social · 23/09/2026
Just in case #Lean turns out to be the next big thing, this syntax introduction is worth a look. 💡 You will immediately feel at home if you know Scala and a bit of Haskell. overreacted.io/a-lean-synta...
overreacted.io
A Lean Syntax Primer — overreacted
Programming with proofs.
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Matej Cerny @matejcerny.bsky.social · 18/09/2026
Scala CLI spoiled us, but how do other ecosystems handle single-file scripts with external dependencies? Alex Nedelcu put together the exact recipes across 11 languages - from Python and Java to Rust and Haskell. alexn.org/blog/2026/09...
alexn.org
Executable Scripts with Dependencies
On programming and personal projects
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Matej Cerny @matejcerny.bsky.social · 17/09/2026
The new VirtusLab Scala Stack website looks great! I like opinionated tools, but claiming direct-style is "the best choice for long-term system development" sounds a bit too strong. Direct-style has real benefits, but presenting it as an absolute ignores why so many teams rely on effect systems.
vss.virtuslab.com
VirtusLab Scala Stack (VSS)
Direct-style Scala: type-safe code that is easy to comprehend and generate.
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Matej Cerny @matejcerny.bsky.social · 16/09/2026
"The rows aren't wide - they are panoramic." Gosh, I'm glad that I don't do big data anymore 😀 www.youtube.com/watch?v=FG8s...
youtube.com
Interview with Big Data engineer in 2026.
YouTube video by Kai Lentit
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Matej Cerny @matejcerny.bsky.social · 15/09/2026
Oh yeah, imposter syndrome, my biggest fear. Btw, I can't even touch-type when someone is looking over my shoulder (my former colleagues can confirm it 😀). marcelphilipiak.net/blog/why-liv...
marcelphilipiak.net
Why Live Coding Interviews Fail Everyone — A Cognitive Interference Problem | Marcel Philipiak
Live coding rounds feel like Russian roulette even to elite problem-solvers. The answer isn't nerves or lack of prep — it's dual-task interference. A breakdown of why the format breaks down under its ...
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Matej Cerny @matejcerny.bsky.social · 14/09/2026
Metals v1.6.9 is out! 🎉 The biggest feature (though still experimental) is the jar filesystem for VS Code! 📂 On my end, glad I could help out with support for kyo-test and a few improvements to the MCP tool to make it much more capable, precise, and token-efficient. 🤖
scalameta.org
Metals v1.6.9 - Osmium | Metals
We're happy to announce the release of Metals v1.6.9, which is mostly a bugfix
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Matej Cerny @matejcerny.bsky.social · 14/09/2026
sbt 2.1.0-M1 is out, but bumping to Scala 3.9 breaks plugin compatibility across minor releases. Any plugin compiled with sbt 2.1 won't run on 2.0 ⚠️ sbt 2.0 should have waited for the Scala 3.9 LTS baseline. Splitting the plugin ecosystem this early in the 2.x cycle is unfortunate. 😟
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Matej Cerny @matejcerny.bsky.social · 10/09/2026
I wrote down the basics of context hygiene, project instructions and progressive disclosure via skills. The concepts are fundamental, but with concrete setup examples for Scala development ✌️ matejcerny.cz/en/agentic-d...
matejcerny.cz
Agentic Development / Context Window
The most common mistake is treating the context window like storage - assuming the more you dump in, the more the agent knows. It is the exact opposite. As conversation history grows, response quality...
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Matej Cerny @matejcerny.bsky.social · 09/09/2026
Scala CLI v1.17.0 is out! 🎉 The big milestone: #Scala 3.9.0 is now the default LTS.
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Matej Cerny @matejcerny.bsky.social · 08/09/2026
http4s v0.23.37 is out! 🚨 A critical security release patching several Ember HTTP/2 DoS vectors. It also brings support for RFC 10008 (the HTTP QUERY method) - a standard way to send read queries with a payload instead of abusing POST. github.com/http4s/http4...
github.com
Release v0.23.37 · http4s/http4s
Security fixes GHSA-gq9p-f254-h286: Ember HTTP/2 buffers a frame's declared payload before checking SETTINGS_MAX_FRAME_SIZE GHSA-3q2f-8v8m-249p: Ember HTTP/2: CONTINUATION frame with END_HEADERS b...
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Matej Cerny @matejcerny.bsky.social · 08/09/2026
I thought the whole point of the Scala 3 macro redesign was to get rid of Shapeless-style libraries. I'm no macro expert, but the more I read about it, the less I believe that goal was achieved. Anyway, a great read on a library that fixes some of Mirror's limitations. 👇
halotukozak.com
Efficient Derivation With No Macros: Extending Scala 3 Mirrors with M&DE - halotukozak
Scala 3 Mirrors give you a type's shape, but not its custom field names, default values, or annotations, and they refuse value classes outright. M&DE derives an enriched mirror that carries all of tha...
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Matej Cerny @matejcerny.bsky.social · 04/09/2026
And here is the official summary of what the new LTS version means for library maintainers, what features landed since #Scala 3.3 and how to migrate cleanly! ​https://www.scala-lang.org/news/3.9/
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Matej Cerny @matejcerny.bsky.social · 03/09/2026
Scala 3.9.0 with a long-term support is out! 🚀 🎯 SIP-71: Implicit conversions with `into` are now stable
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Matej Cerny @matejcerny.bsky.social · 03/09/2026
ADTs are not an academic FP gimmick - every language and programmer should use them! ☝️ Why they matter even more in the AI era? Context windows.
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Matej Cerny @matejcerny.bsky.social · 02/09/2026
Akka and Gradle were removed from the Scala homepage supporters list. 🥲 Akka handled #Scala 2.13 maintenance, but VirtusLab stepped in to take over smoothly. Huge credit to them for keeping production stacks alive!
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Matej Cerny @matejcerny.bsky.social · 01/09/2026
How does SQL database access in #OCaml compare to the #Scala ecosystem? 🤔 Part 3 of the "From Scala to OCaml" series continues with Postgres integration: Doobie and Cats Effect vs Caqti and Miou, connection pooling, and typed queries. gilwath.com/articles/fro...
gilwath.com
Discovering OCaml as a Scala Developer Part 3: Serving data with SQL Database (Postgresql)
Using a SQL Database to serve the data for our CRUD API
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Matej Cerny @matejcerny.bsky.social · 31/08/2026
Martin Odersky talking about #Scala on a non-Scala channel: "Typically I don't need a debugger when I write functional code." 💡 Great conversation covering language design, the limits of Rust, and why strict types matter even more with AI 👇 youtu.be/LdN4sPWM-WY
youtu.be
Creator of Scala: Comparing Languages And How AI Will Impact Them | Martin Odersky
YouTube video by Ryan Peterman
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Matej Cerny @matejcerny.bsky.social · 28/08/2026
Turns out roughly 72% of my audience on X is older than me. How should I interpret this? Should I stop using emojis? 😀
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Matej Cerny @matejcerny.bsky.social · 28/08/2026
More and more people are jumping in, awesome! A bit disappointed that queues on Postgres haven't been mentioned, though 🤷 youtu.be/MZfhhl6ej-k
youtu.be
I replaced my entire tech stack with Postgres
YouTube video by Better Stack
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Matej Cerny @matejcerny.bsky.social · 27/08/2026
Experimenting with Android apps using #Scala for the core and #Kotlin for the UI. Pure wishcraft given my limited Android background 😀 But so far it's working surprisingly well. Also, Metals is super helpful for a polyglot repo like this!
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Matej Cerny @matejcerny.bsky.social · 25/08/2026
Cats Effect 3.7.1 is out! 🎉 It brings several perf improvements, bug fixes and documentation updates. Also, support for #Scala 2.12 is coming to its end - not in this release, but soon. ☝️ github.com/typelevel/ca...
github.com
Release v3.7.1 · typelevel/cats-effect
This is the sixty-first release in the Cats Effect 3.x lineage. It is fully binary compatible with every 3.x release and fully source-compatible with the 3.7.x lineage. What's Changed Enhancements ...
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Matej Cerny @matejcerny.bsky.social · 24/08/2026
Kotlin uses coroutines to make deep recursion stack-safe. Can #Scala do the same without built-in suspend? With no monads, no virtual threads, and no effect algebra? Very technical dive into Scala 3's macros 👇 halotukozak.com/posts/scala-...
halotukozak.com
Direct-Style Deep Recursive Functions in Scala 3 - halotukozak
Kotlin's DeepRecursiveFunction turns deep recursion into a heap-allocated trampoline while keeping direct-style code, built on suspend functions. This post explains why that works, then builds a Scala...
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Matej Cerny @matejcerny.bsky.social · 24/08/2026
After more than 4 years of quiet, Monix just released v3.5.0 - dropping support for Scala 2.12 and making JDK 17 the new minimum runtime. Great to see maintenance and updates back on track! github.com/monix/monix/... #scala
github.com
Release v3.5.0 · monix/monix
This version updates the supported runtime and Scala versions, dependencies, and includes several concurrency and scheduler bug fixes. NoteIt does not upgrade Cats-Effect to 3.x, because that's sti...
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Matej Cerny @matejcerny.bsky.social · 21/08/2026
Want to be an exceptional #Java or #Kotlin developer? Want an unfair edge? Learn Scala. Advice that worked 10 years ago - and works just as well today.
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Matej Cerny @matejcerny.bsky.social · 20/08/2026
If you are wondering where the long-awaited #Scala 3.9 LTS is, be aware that six RC versions have already been released. There were some regressions with Pekko, but they are now fixed. The stable version should land in just a week! 🎉
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Matej Cerny @matejcerny.bsky.social · 19/08/2026
A lot has changed from Postgres 10 to 19 🐘 If you haven't followed every release, here's a quick guide on what best practices changed (and what stayed the same): www.crunchydata.com/blog/postgre... #postgres
crunchydata.com
Postgres 19: How Our Advice Has Changed Since We Wrote It | Crunchy Data Blog
Revisiting Crunchy posts on COPY, TOAST, BRIN, covering indexes, and partitioning: what we said then, which Postgres versions changed the story, and what we recommend on Postgres 19.
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Matej Cerny @matejcerny.bsky.social · 17/08/2026
Great new addition to Pi's harness in v0.84.x: native support for rendering Mermaid diagrams! 📊
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Matej Cerny @matejcerny.bsky.social · 14/08/2026
My family was away for the week, so I spent it catching failed Metals MCP calls to prevent unzipping jars. Just small tweaks here and there, but hopefully the overall DX (or AX? 😀) is a bit better now. #scala
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Matej Cerny @matejcerny.bsky.social · 13/08/2026
And by the way, the latest release v2.0.0-M18 from yesterday should be at parity with the 1.x branch! Try it!
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Matej Cerny @matejcerny.bsky.social · 12/08/2026
A background story about Metals v2: How Databricks made editors work for a 26M-line Java & Scala monorepo. 🚀 www.databricks.com/blog/open-so... #scala #java
databricks.com
Open-sourcing Metals v2: Databricks’ Java and Scala language server for multi‑million line codebases
A look at how Databricks moved its JVM development onto lightweight editors: the platform flywheel behind standardizing on one IDE, the adoption data showing the shift, and the engineering inside Meta...
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Matej Cerny @matejcerny.bsky.social · 11/08/2026
🚨 Security Update: sbt 1.12.15 and 2.0.6 are out! This release fixes an RCE vulnerability in TCP mode that was discovered using LLM-assisted code auditing. If your build sets serverConnectionType, upgrade now! www.scala-lang.org/blog/2026/08... #scala
scala-lang.org
Fixing a remote execution vulnerability in sbt
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Matej Cerny @matejcerny.bsky.social · 05/08/2026
Another version of Kyo is out! 🎉 As usual, it ships with plenty of updates, but this release is mostly about kyo-sql: native PostgreSQL and MySQL drivers running directly on Kyo's async stack with their own wire protocol implementations - no JDBC involved! 🔥 github.com/getkyo/kyo/r...
github.com
Release v1.0.0-RC6 · getkyo/kyo
v1.0.0-RC6 ships Kyo's own SQL module. kyo-sql pairs raw SQL with a typed DSL that mirrors SQL syntax, and underneath there is no JDBC: the drivers speak the Postgres and MySQL wire protocols direc...
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Matej Cerny @matejcerny.bsky.social · 04/08/2026
On the latest Rock the JVM, Daniel Spiewak explains how targeting Scala Native for Cats Effect 3.7 improved the entire engine even on the JVM, why DevOps is #Scala 's next big frontier, what it takes to scale your career to Distinguished Engineer, and more.
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Matej Cerny @matejcerny.bsky.social · 04/08/2026
Based in Warsaw 🇵🇱? Netflix is hiring, and this might be for you: explore.jobs.netflix.net/careers/job/... #scala
explore.jobs.netflix.net
Distributed Systems Engineer 5 - Data Platform Poland | Warsaw,Poland | Netflix
Netflix is one of the world's leading entertainment services, with over 300 million paid memberships in over 190 countries enjoying TV series, films and games across a wide variety of genres and langu...
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Matej Cerny @matejcerny.bsky.social · 03/08/2026
There is a new #Scala 3 optics library worth checking out: cats-eo. It unifies optic families under a single trait, features macro derivation backed by Hearth and delivers massive performance gains (such as traversals running up to 11x faster compared to Monocle).
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Matej Cerny @matejcerny.bsky.social · 01/08/2026
⚠️ There is a proposal to change the default Seq implementation from List to Vector. It makes sense, but I personally would rather keep the status quo to avoid unexpected runtime changes while people are still migrating from #Scala 2 to 3.
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Matej Cerny @matejcerny.bsky.social · 31/07/2026
What a beautiful way to start a Friday 🤣
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Matej Cerny @matejcerny.bsky.social · 30/07/2026
Two signs your domain model is incomplete: ❌ You need an 'if' statement ❌ You are adding another optional argument to a function Model your actual domain states with ADTs. Stop patching logic. #scala
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Matej Cerny @matejcerny.bsky.social · 27/07/2026
I started using Herdr about two weeks ago, and I can confirm it's really good! But to be fair, I didn't use tmux - I migrated from a plain Ghostty terminal.
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Matej Cerny @matejcerny.bsky.social · 24/07/2026
Metals v1.6.8 out 🎉 A bit of a curious release - lots of fixes for Java, Bloop and old school editor workflows. Still, a massive effort with 119 merged PRs! scalameta.org/metals/blog/... #scala
scalameta.org
Metals v1.6.8 - Osmium | Metals
We're happy to announce the release of Metals v1.6.8, which includes a few
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Matej Cerny @matejcerny.bsky.social · 22/07/2026
You know what that means, right? Some library is getting a massive performance boost! This time it's Scala Yaml, which is up to 10x faster with ~20x lower memory allocation per operation! 🤯 #scala
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Matej Cerny @matejcerny.bsky.social · 22/07/2026
Is it a case of AI psychosis if I use Claude, Codex, Gemini, and Grok every day? 🤔
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Matej Cerny @matejcerny.bsky.social · 22/07/2026
Could anyone tell me what is wrong with this description? It's concise, includes settings and a complete repro. Also, saying that it violates policy when you have no policy regarding LLMs is pretty ironic. #scala github.com/typelevel/ca...
github.com
Tracing static init fails on the Scala.js WebAssembly backend · Issue #4608 · typelevel/cats-effect
Summary I'm adding WASM support to Kyo and there's at least one blocker to support it in modules that integrate with cats-effect. On the Scala.js WebAssembly backend, constructing any IO throws jav...
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Matej Cerny @matejcerny.bsky.social · 21/07/2026
Hey, #Java developers! Your domain models are leaking bugs and you need to fix them immediately! Start using records + sealed hierarchies. They give you:
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Matej Cerny @matejcerny.bsky.social · 17/07/2026
In case you missed it, Scarf, the analytics platform which you might know from the Sonatype integration, is moving away from #Haskell. 😢
avi.press
After 7 years in production, Scarf has reluctantly moved away from Haskell
After 7 years in production, Scarf has reluctantly moved away from Haskell — Avi Press
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