I use both a lua-configured mail-client, and a lisp-configured emacs. Of the two they're both powerful and extensible, but the Lua config feels more natural to me.
Hmmm but does the mail-client let you load code in without restart?
The Lua also might feel more natural just because of having used imperative languages more.
It can do - there is a trivial eval(read_input()) facility baked in.