I built the RealWorld app in Lean 4
Summary
The article documents building the RealWorld reference app entirely in Lean 4, covering server, database, and frontend implementations and the tooling used (Lean libraries for HTTP, routing, JSON, JWTs, HTML rendering, and PostgreSQL access). It discusses production viability, correctness by construction concepts, and the trade-offs of Lean-based development, including proofs and limitations. It also contrasts Lean-based implementations with other languages and highlights practical testing and tooling considerations.