Which programming language do you want to learn next?

I plan to stick to python, but learn its functional programming features.

I'm boggling at the concept of doing PHP "the wrong way".

Only small, single-paradigm languages can ever do things "right". With PHP you can do almost anything, in ways that are almost not wrong. ;)

Haskell. I know Scala and occasionally use it work (although my main project is still in Java: it's often difficult to mix languages at the same level in a project and for many reasons, I'd like to keep as few external dependencies for my project as possible) and great enjoy it. I also know OCaml and am doing some hobby projects in it. I've been a Lisp weenie for a long time now (Scheme, Common Lisp, recently Clojure both at work and in hobby projects).

So, why Haskell if I already know lots of other functional languages (both statically and dynamically typed ones)? Due to type classes and lazyness/purity by default. I still find myself thinking imperatively, or thinking at a lower functional level (e.g., just tail recursion and folds/map). I'd like to learn to think in a a lazy and purely functional way and Haskell seems to be a way to get there. Additionally, both Scala (with implicits and higher kindred types) and OCaml (since 3.12 with first class modules) do support type classes, but Haskell seems to be a good way to learn to use them. I've been thinking a great deal about type systems and type safe DSLs, presently working my way through TAPL. One particular field that interests me is the intersection of programming languages and systems: can we do user level systems programing in languages other than C; how can we safely "hint" a garbage collector to avoid memory pressure issues that happen frequently in memory intensive applications written in high level languages; can we use the type systems to ensure application-level code can sustain loss of consistency or availability (Google for CALM Conjecture for a dynamically typed Ruby DSL approach to this)?

Finally, Haskell is a great language in terms of forcing myself to think before I code (Yes, I should already be doing that): you may have a solution that may be short in terms of lines of code but takes hours to come up with. Why is this important? That's a great way of improving myself in other areas of programming: seeing which classes of algorithms solve a particular program, finding the simplest solution vs. one that comes to mind first.

[EDIT: Forgot to say that fun also plays a huge part of it. It's the reason I chose on the site (was the best one of those listed there: Haskell won't get you a job nor is there always a guarantee you'll be more productive with it, especially right away) and a perfectly legitimate one].

Clojure. I've found my code in other languages looking more and more like Lispy as time goes on, and my shame at not actually having learnt to use one well has grown too strong.

Qi / Shen.

Having settled on Common Lisp a good few years ago I haven't explored a new programming language for a looong time (having gone through the phase of trying out a lot of them before that). However, recently I've felt the need to broaden my horizons again and did dabble a little in Haskell.