Haskell

The deep end, and the language that changes how you think even if you never ship it. Haskell is purely functional and lazy, so side effects live in the type system and the compiler catches whole families of bugs before anything runs. Type classes, algebraic data types and pure functions everywhere are the full functional experience, nothing watered down. You do not need category theory to start, whatever the forums imply. GHCup installs the compiler and tools, and the University of Helsinki's Haskell MOOC offers its material and exercises openly. Budget more time than for anything else here: laziness makes performance harder to predict, and the library ecosystem assumes comfort with advanced types sooner than beginners would like.