Joachim Breitner
Principal Research Software Engineer at Lean FRO
Freiburg im Breisgau, Baden-Württemberg, Germany
Summary
🤩
Rockstar🎓
Top SchoolJoachim Breitner is a Principal Research Software Engineer with 22 years of experience building high-performance language tools, theorem provers, and backend systems from academia to industry. He has deep expertise in functional languages and compilers, contributing to flagship projects like GHC, Lean 4, Cabal, and LiquidHaskell where he focused on core algorithms, performance tuning, and soundness of termination and recursion checks. His career spans research roles (postdoc at UPenn, doctoral work on semantics and interactive theorem proving) and industry positions including staff engineering at DFINITY and a stint as Senior Language Programmer at Epic Games, showing he moves comfortably between formal methods and production systems. Joachim is an active open-source contributor whose work ranges from optimizing Haskell libraries (text, containers) to improving UI/UX of community projects and reverse-engineering hobbyist formats, demonstrating both breadth and attention to detail. Known for obsessing over correctness and pixel-perfect rendering alike, he pairs mathematical rigor with practical engineering—often improving compiler internals and developer-facing documentation. Based in Freiburg, Germany, he blends academic pedigree (distinctions in CS and Math from KIT) with hands-on impact across major, widely used language ecosystems.
22 years of coding experience
8 years of employment as a software developer
Indian Institute of Technology BombayBachelor of Science - BS, Physics, Bachelor of Science - BS, Physics at Albert-Ludwigs-Universität FreiburgDiplom, Informatik, mit Auszeichnung (with distinction), Diplom, Informatik, mit Auszeichnung (with distinction) at Karlsruher Institut für Technologie (KIT)
German, English
Stackoverflow
25,573reputation
461kreached
289answers
114questions
haskell
top-1%
twitter
top-5%
functional-programming
top-5%
lazy-evaluation
top-5%
pattern-matching
top-5%
optimization
top-5%
Github Skills (88)
agda10
restructuredtext10
identity10
compiler-development10
testing10
authentication10
file-format10
type-system10
gtk10
game-development10
haskell10
dependent-types10
proofs10
hackage10
uid10
Programming languages (42)
Github contributions (5)
entropia/tip-toi-reveng
Apr 2013 - Dec 2022
Trying to understand the file format of Tip Toi
Role in this project:
Reverse Engineer
Reverse EngineerContributions:18 releases, 11 reviews, 840 commits in 9 years 9 months
Contributions summary:Joachim's commits focus on reverse engineering the file format of the Tiptoi toy. They wrote and modified shell scripts to fetch and add audio files from the static tiptoi.com server. The user also created and improved Haskell scripts, including those to determine the XOR value and to extract and decode audio files from the GME format.
file-formattoitipmarkup-languagemanual
haskell/containers
Sep 2011 - Feb 2018
Assorted concrete container types
Role in this project:Back-end Developer
Back-end DeveloperContributions:1 review, 16 commits, 5 PRs in 6 years 6 months
Contributions summary:Joachim primarily contributed to the `Data.DenseIntSet` module, a core component of the `containers` library. They focused on optimizing the performance of set operations, including union, intersection, and insertion, and improving memory usage. The user implemented and refined QuickCheck properties for comprehensive testing and bug fixing, ensuring the correctness of the optimized code.
containersconcretepodmandockercontainerization
Find and Hire Top DevelopersWe’ve analyzed the programming source code of over 60 million software developers on GitHub and scored them by 50,000 skills. Sign-up on Prog,AI to search for software developers.