It could be useful to learn an "almost" functional version of Lisp, like Scheme/Racket or Clujure. (You can also try to write "almost" functional code in Common Lisp, but the usual idioms are more imperative.)

The idea is that you write "almost" all the code in small functional functions without side effects, but you can use side effect when the functional solution is very difficult.

Most of them are not typed (there is a Typed Racket version, but the type system is not similar to the Haskell type system).