This repository accompanies a comprehensive guide to building large, maintainable Haskell systems, focusing on architecture, modularity, and practical design techniques. It presents patterns for separating pure domain logic from side effects, organizing code into layers and components that can be tested in isolation. Readers encounter multiple styles—MTL/typeclass constraints, tagless-final encodings, free and freer monads, ReaderT-style application environments—and learn when to apply each....