Personally, I'm a big fan of Haskell. It will make your brain hurt, but that's part of the point -- it's very good at easily creating and using mathematically sound abstractions. I'm not a big fan of Lua, though it's a perfectly reasonable choice for its niche of embeddable scripting language. I have no experience with Processing. The most commonly recommended starting language is python, and it's not a bad choice at all.
Toss in another vote for Haskell. It was my first language (and back before Real World Haskell was written); I'm happy with that choice - there were difficult patches, but they came with better understanding.
We've had these for a year, I'm sure we all know what to do by now.
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