The Changing Craft of Programming with John Crepezzi

John Crepezzi has spent his career building tools for other programmers—at Tumblr, at Square, and then at GitHub, where he worked on the merge queue and the early versions of Codespaces. He came to Jane Street to build a code search engine for OCaml. A week into the job, ChatGPT came out, and he’s been working on AI tooling ever since. In this episode, Ron and John talk about how a three-day prototype grew into a firmwide platform for agentic coding, how AI can break the social contract around code review, and how to build tools that support both agent workflows and human understanding.You can find the transcript for this episode on our website.Some links to topics that came up in the discussion:QBasicPirates of Silicon ValleyHoogle — the Haskell API search engine that inspired the original code-search projectGitHub merge queue and CodespacesSorbet — gradual typing for Ruby; Hack — Meta’s typed PHPActive RecordBubblewrap and passt/pasta — sandboxing toolsseccomp BPFClaude Code and OpenAI Codex CLIModel Context Protocol and Agent SkillsPerfetto and magic-traceAntithesisBonsai — Jane Street’s OCaml web UI librarySignals & Threads: Language Design with Leo White

Aaron Bauer is a software engineer and one of Jane Street's few developer educators—a role that splits his time between writing code and teaching other people how to write it. Before joining the firm, he taught computer science at Carleton College, including four straight terms online during the pandemic. In this episode, Ron and Aaron discuss what it takes to teach engineering inside a company with its own language, its own version control, and its own editors, and what changes when an LLM can do the exercise for you. Along the way, they consider the underrated power of a live lecture; why the text editor is still the software engineer's home base; using editor telemetry to find out how AI is actually changing developer workflows; and how Jane Street rebuilt its intern curriculum around testing, design, and code review now that producing the code is the easy part.You can find the transcript for this episode on our website.Some links to topics that came up in the discussion:Foldit and AlphaFoldCarleton College Computer ScienceFlipped classroom, active learning, mastery learning, and cognitive load"The Pen Is Mightier Than the Keyboard" — Mueller & Oppenheimer on handwritten vs. typed notesMerlin and ocaml-lspecaml — writing Emacs extensions in OCamlBonsai — Jane Street's OCaml web UI libraryOxCamlClaude Code output styles — the "learning" mode Aaron describes (note: Anthropic has since moved this to a plugin)Jane Street's tech internship

"Alternative data" is Wall Street's name for information that comes from non-traditional sources: satellite photos of parking lots, credit card panels, weather forecasts. Eric Mannes has spent over a decade at Jane Street, first as a commodities trader and now helping lead the firm's alternative data team. In this episode, Eric and Ron talk about what it takes to turn messy external data into datasets a trading strategy can rely on. Along the way, they cover the day oil futures settled at a negative price and the systems that broke as a result; the years when the commodities desk's risk system was one very large Excel spreadsheet; the hard question of what a company even is; and why better ML models raise the value of careful data engineering.You can find the transcript for this episode on our website.Some links to topics that came up in the discussion:FiggieAdverse selection2020 Russia–Saudi Arabia oil price warDual-listed company"LAMBDA: The ultimate Excel worksheet function"TrinoThe Bitter LessonLearn more about Jane Street’s internship program.Apply for Data Engineering roles at Jane Street

In university Jacob Baskin studied at the intersection of computer science and economics, thinking about systems that incentivize people to express their true preferences. He put those ideas into practice at Google, where he worked on ad serving, before joining Jane Street’s database infrastructure team. In this episode, Ron and Jacob discuss Superstore, a distributed columnar database now central to Jane Street’s tech stack that Jacob began building practically the day he started. How do you support wide-ranging analytical queries while transactional writes stream in at the speed of trading systems? And what’s it like when your first design doc leads to an eight-figure hardware purchase? After building Superstore Jacob has since gone back to his roots, thinking about schemes for bidding on compute time as he works to optimize usage of the Hive, Jane Street’s massive compute cluster for research. You can find the transcript for this episode on our website. Some links to topics that came up in the discussion: Mechanism design, second-price auction MapReduce, BigTable, Google File System Vertica Apache Parquet CockroachDB Paxos BitTorrent

Nate Foster is a professor at EPFL in Switzerland in the Networked Systems Abstractions Lab, and a visiting researcher at Jane Street on the Networking team. In this episode, he and Ron consider what happens when you bring a software mindset to network engineering. Can you use programming language theory and formal methods to realize the dream of software-defined networks? Along the way, they discuss how hyperscalers have shaped networking hardware; the return (or not) of multicast; the ways ML workloads are reshaping the networking layer; and the success Jane Street has had using an early Internet protocol, BGP, together with a more declarative high-level specification language. You can find the transcript for this episode on our website. Some links to topics that came up in the discussion: P4 (Programming language Lenses (bidirectional transformation) OpenFlow Kleene algebra with tests NetKAT End-to-end principle Border Gateway Protocol “Stable Internet routing without Global Coordination,” aka the Gao-Rexford conditions Unison file synchronizer Barefoot Networks

Will Wilson is the founder and CEO of Antithesis, which is trying to change how people test software. The idea is that you run your application inside a special hypervisor environment that intelligently (and deterministically) explores the program’s state space, allowing you to pinpoint and replay the events leading to crashes, bugs, and violations of invariants. In this episode, he and Ron take a broad view of testing, considering not just “the unreasonable effectiveness of example-based tests” but also property-based testing, fuzzing, chaos testing, type systems, and formal methods. How do you blend these techniques to find the subtle, show-stopper bugs that will otherwise wake you up at 3am? As Will has discovered, making testing less painful is actually a tour of some of computer science’s most vexing and interesting problems. You can find the transcript for this episode on our website. Some links to topics that came up in the discussion: Antithesis, Will’s company FoundationDB’s deterministic simulation framework QuickCheck — the original Haskell property-based testing library, by Koen Claessen and John Hughes Hypothesis — property-based testing for Python, created by David MacIver QuviQ — John Hughes’ company commercializing QuickCheck, including automotive testing work Netflix Chaos Monkey Goodhart’s law — “When a measure becomes a target, it ceases to be a good measure” CAP theorem — the impossibility result for distributed systems that FoundationDB claims to have in some sense violated. Paxos — the consensus algorithm FoundationDB reimplemented from scratch Large cardinals, an area Will studied before abandoning mathematics Lyapunov exponent — measure of chaotic divergence Chesterton’s fence The Story of the Flash Fill Feature in Excel Building a C compiler with a team of parallel Claudes Barak Richman, “How Community Institutions Create Economic Advantage: Jewish Diamond Merchants in New York”

Chris Lattner is the creator of LLVM and led the development of the Swift language at Apple. With Mojo, he’s taking another big swing: How do you make the process of getting the full power out of modern GPUs productive and fun? In this episode, Ron and Chris discuss how to design a language that’s easy to use while still providing the level of control required to write state of the art kernels. A key idea is to ask programmers to fully reckon with the details of the hardware, but making that work manageable and shareable via a form of type-safe metaprogramming. The aim is to support both specialization to the computation in question as well as to the hardware platform. “Somebody has to do this work,” Chris says, “if we ever want to get to an ecosystem where one vendor doesn’t control everything.”You can find the transcript for this episode on our website.Some links to topics that came up in the discussion:Democratizing AI compute (an 11-part series)Modular AIMojoMLIRSwift

Daniel Pontecorvo runs the “physical engineering” team at Jane Street. This group blends architecture, mechanical engineering, electrical engineering, and construction management to build functional physical spaces. In this episode, Ron and Dan go deep on the challenge of heat exchange in a datacenter, especially in the face of increasingly dense power demands—and the analogous problem of keeping traders cool at their desks. Along the way they discuss the way ML is changing the physical constraints of computing; the benefits of having physical engineering expertise in-house; the importance of monitoring; and whether you really need Apollo-style CO2 scrubbers to ensure your office gets fresh air.You can find the transcript for this episode on our website.Some links to topics that came up in the discussion:ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers)Some research on CO2’s effects on human performance, which motivated us to look into CO2 ScrubbersThe Open Compute ProjectRail-Optimized and Rail-only network topologies.Immersion cooling, where you submerge a machine in a dielectric fluid!

Ian Henry started his career at Warby Parker and Trello, building consumer apps for millions of users. Now he writes high-performance tools for a small set of experts on Jane Street’s options desk. In this episode, Ron and Ian explore what it’s like writing code at a company that has been “on its own parallel universe software adventure for the last twenty years.” Along the way, they go on a tour of Ian’s whimsical and sophisticated side projects—like Bauble, a playground for rendering trippy 3D shapes using signed distance functions—that have gone on to inform his work: writing typesafe frontend code for users who measure time in microseconds and prefer their UIs to be “six pixels high.”You can find the transcript for this episode on our website.Some links to topics that came up in the discussion:Bauble studioJanet for Mortals, by Ian HenryWhat if writing tests was a joyful experience? 

In Young Cho thought she was going to be a doctor but fell into a trading internship at Jane Street. Now she helps lead the research group’s efforts in machine learning. In this episode, In Young and Ron touch on the porous boundaries between trading, research, and software engineering, which require different sensibilities but are often blended in a single person. They discuss the tension between flexible research tools and robust production systems; the challenges of ML in a low-data, high-noise environment subject to frequent regime changes; and the shift from simple linear models to deep neural networks.You can find the transcript for this episode on our website.