Level 3 — Advanced

These lessons cover testing with HUnit, QuickCheck, and Hspec, monad transformers, mtl, concurrency with threads, MVar, and async, software transactional memory, strictness and performance, profiling, language extensions, GADTs and type families, parser combinators, JSON with aeson, web APIs with Servant, and running Haskell in production. Recommended: the Level 2 lessons, especially type classes, monads, and IO.