I am not a programmer. I learned about functional programming through
Mathematica (not really a general purpose language, but it is very
good at what it was designed for). Before that I only knew some
BASIC, Pascal and a little 8086 asm. I enjoyed working with
Mathematica a lot, and after I have used it for a few years, I tried
to find a similar language that could replace C++ for high performance
numerical computing tasks (in particular monte carlo simulation). I
played a little with different languages but couldn't find anything
that seemed suitable. Of course the language itself is not the only
factor, it is important that there already be a community using the
language for the same kinds of tasks. After all the most important
thing is getting one's work done (note that I'm not a programmer), and
having fun by learning a new language is secondary.

But I am still open to suggestions, especially from people who do use
functional language to write long-running numerical simulations.