Language recommendations for expanding programming skills (For a semi-experienced software developer) [closed]

Question

I have little (<1 year professional) experience with

• Perl
• Groovy/Java

I have limited (<2 year professional)

• C

I have decent experience (>= 6 years professional) with

• PHP
• SQL

I have hobby experience with

• C++/DX9 (some simple windows games/demos)
• Obj-C (a few iphone app's)
• ASM (http://www.amazon.com/Assembly-Language-Intel-Based-Computers-5th/dp/0132383101/ref=sr_1_1?ie=UTF8&s=books&qid=1263401280&sr=8-1) - I stopped when it got too windows specific.

So my question is what language can I next approach (on my free time) to give me some new insight to programming and problem solving in general - I was looking at maybe LISP - something which would be very foreign to me. I want to tackle something very, very different from the languages listed above.

EDIT: I think I'll investigate Haskell - thanks for the feedback! and maybe possibly Erlang, and I really liked Adrian Kosmaczewski's idea about a Mac App for snow leopard.

Answer 1

Haskell, followed shortly by Python.