How WhatsApp Fixed Erlang's Tooling Problem | Roberto Aloi & Michał Muskała

<p>Part one established that Erlang scales. Part two is about what comes after - keeping a codebase that large healthy for years, when most engineers arrive having never written Erlang before.</p><p>WhatsApp&#39;s answer was to invest in the language itself. The formatter, the language server, the type checker. And then to open source almost all of it back to the community - not as charity, but because diverse use cases and external bug reports make the tools better.</p><p>Roberto Aloi and Michał Muskała are back for the half that doesn&#39;t appear on conference posters.</p><p>Topics include:</p><ul><li><p>why the investment in tooling came from a developer survey rather than an engineering instinct - the data showed Erlang developer experience was the weak point</p></li><li><p>what ELP (the Erlang Language Platform) actually is and how it superseded the Erlang LS that Roberto built before joining WhatsApp</p></li><li><p>how eqWAlizer, WhatsApp&#39;s type checker, was rolled out without breaking other teams - the team owning the type checker also owned the task of fixing things up when new checks introduced errors</p></li><li><p>why types and let it crash aren&#39;t in tension: you can never statically verify everything in a dynamic language, so the gap is where supervision and recovery fit</p></li><li><p>the two biggest misconceptions about static analysis tools: that they&#39;re slow (modern tooling is fast) and that you have to commit to them all at once (you can do it incrementally)</p></li><li><p>why WhatsApp open sources tools that took significant engineering investment - keeping them internal would make them worse, not better</p></li><li><p>what&#39;s still unsolved: exhaustiveness checks at this scale remain an open problem</p></li><li><p>how AI is changing the Erlang learning experience - build first, ask AI to review your patterns afterward</p></li><li><p>the word that ties both episodes together: trust. Supervision earns it at runtime. Types earn it at compile time.</p></li></ul><p>Companion episode: Part 1 on scale and operations.</p><p>Recorded June 25, 2026.</p><p><br></p>