Python is pretty good, but you'd be amazed at how well Haskell does with Project Euler.Python is great for this kind of thing. I just pull up the shell and hack a quick function together. Great for GCD, LCM, co-primality, etc. Quicker than C++ to compile and run.

I just rewrite the function every session I want to use it, which ends up with me learning the definitions of these things very thoroughly (and degrading my mental arithmetic).

There are tons of problems that it just chews through with no thought. Which is exactly what you'd expect from a purely functional language, really