Scala

Scala mixes object-oriented and functional programming, and many learners come from Java. The Scala Book by Alvin Alexander and A Tour of Scala introduce the syntax, while Creative Scala and Essential Scala by Noel Welsh and Dave Gurnell teach functional thinking through small examples. Scastie runs Scala in the browser without a local build setup.

The Functional Programming in Scala Specialization is a structured next step. For functional libraries, Scala with Cats and The Type Astronaut's Guide to Shapeless go deep into type classes and generic programming. Programming in Scala, First Edition by Martin Odersky and co-authors is free, though it predates Scala 3 and later language changes.