Functional Languages on the Horizon

The usefulness of functional languages is really starting to go mainstream now. In the past, I've learned Haskell, Scheme, and Ruby, and I'm studying Scheme again now. Trying to learn every technology is impossible so I'm going to stick with Scheme right now because of the great books that are available for learning it.

Practical reasons for learning some functional languages:

Scala

• Lift Web Framework: http://liftweb.net/
• Java interoperability
  

Haskell

• New O'Reilly book, "Real-World Haskell": http://www.realworldhaskell.org/blog/
• Most modern functional language in semi-mainstream use.
  

Scheme/Lisp

• Great books: "Lisp in Small Pieces", "The Scheme Programming Language" (by Kent Dybvig), and "The Structure and Interpretation of Computer Programs".
• Good, cross-platform IDE: DrScheme
• Ability to leverage knowledge for Arc (Paul Graham's language which will almost certainly have a large following due to his popularity)