I don't know about their maturity, but I think Haskell's type system and purity would help with security. It doesn't have anything like `eval` and is much less

I don't know about their maturity, but I think Haskell's type system and purity would help with security. It doesn't have anything like `eval` and is much less tolerant of weird input by default than dynamically typed languages (at least in my experience). Having no or limited state also helps--bad input and other mistakes tend to be more localized when they can't possibly change anything else in the program. If you're really crazy about security, then I suspect the Haskell code would also be much easier to analyze and verify.