[icarus127]

> variables don't have types, only individual values do.

It really depends on what language you're using. This is more or less true in Python, Ruby, and Javascript. In Lisp generic methods you can specify the type for the input variables and be guaranteed that if you are inside the method the parameters are of that specific type. For optimization reasons you can also declare to the compiler that variables are a specific type. This is really important when doing numeric computations in a tight loop. I've had 50% - 80% performance improvement by declaring types (amounting to hours of run time). I think Groovy and Clojure also allow this but I don't know much about those two.

There's also the maintenance issue though. After a few years of maintaining a fairly large Lisp project I've become pretty convinced that the only way to stay sane, at least for me, is to treat it as a statically typed language. I have asserts and check-type macros all over the place. If variables may have multiple types they can be checked against '(or type1 type2) but things do become more complicated then.

[frou_dh]

Interesting. Admittedly I have only used (UnCommon) Lisps in a hobbyist way and haven't been concerned with techniques for extra speed/robustness there.