In the last two years at WhatsApp we have developed eqWalizer - a type-checker for Erlang AS IS - in style of Flow, Pyre, Hack. It’s already used in a significant part of our codebase. In this talk we overview the key eqWalizer’s design choices we made and explain the reasons behind them. Moreover, we’ll report about the challenges of using Erlang in large code bases and how static typing has allowed us to scale development through providing fast signal to developers. Finally, we’ll also present the lessons learned from enabling static typing for the code underpinning the largest messaging app in the world.