The type mechanism is strongly static and the compiler does all the work to infer types. There's no distinction between primitive types and object types because Scala wants everything to descend from one ur-object call Any. The syntax is much simpler and cleaner than Java; Scala folks call it "low ceremony." You can leave your paragraph-long CamelCase variable names back in Java Land.
Scala offers many of the features expected of functional languages, such as lazy evaluation, tail recursion, and immutable variables, but have been modified to work with the JVM. The basic metatypes or collection variables, like linked lists or hash tables, can be either mutable or immutable. Tail recursion works with simpler examples, but not with elaborate, mutually recursive examples. The ideas are all there, even if the implementation may be limited by the JVM. Then again, it also comes with all the ubiquity of the Java platform and the deep collection of existing Java code written by the open source community. That's not a bad trade-off for many practical problems.
If you must juggle the data in a thousand-processor cluster and have a pile of legacy Java code, Scala is a great solution.
Haskell: Functional programming, pure and simple
For more than 20 years, the academics working on functional programming have been actively developing Haskell, a language designed to encapsulate their ideas about the evils of side effects. It is one of the purer expressions of the functional programming ideal, with a careful mechanism for handling I/O channels and other unavoidable side effects. The rest of the code, though, should be perfectly functional.
Sign up for CIO Asia eNewsletters.