Lasm brings Lean to web apps

Lasm brings Lean, a programming language with a proof system, into web application development. It compiles Lean 4 to WebAssembly and provides ways to run applications on Node, serverless platforms, edge runtimes and browsers. The point isn’t that every part of an app is automatically proven correct. Developers can prove selected properties of their code, while other parts, including the runtime and some data-handling layers, remain outside those proofs.

The project’s example is a full-stack app written in Lean, with 117 theorems covering areas such as authorization, validation and pagination. The project says its API and browser tests pass on several runtimes, but it also spells out what those results don’t prove. For a business, this is an early option to explore when a software failure would be costly and you want stronger checks around specific rules. It’s not a reason to rebuild an existing system without weighing the learning and integration work. Lasm’s project page has the details. It’s worth tracking tools like this alongside the business questions they might actually help answer.